Yaw Wang CHAI

Verified email at gmail.com

School of Materials and Chemical Technology
Tokyo Institute of Technology



                             

https://researchid.co/ywchai

> Highly caliber MATERIALS SCIENTIST with a good metallurgical research background.
> Experienced in phase transformations, interfacial defects, microstructure, and crystallographic characterization in a
range of structural and functional materials;
- shape memory alloys: Ti-Nb & CuZnAl,
- thermoelectric materials: half-Heusler, -FeSi2, Mg2(Si, Sn)
- heat resistant materials: High-Cr ferritic stainless steels, Co3AlC and Ni3Al, Ti alloys
> Skilled in a wide range of microstructure and properties characterization techniques: SEM, EPMA, TEM, HRTEM, STEM-
EDS, STEM-HAADF, in-situ heating TEM, twin-jets electropolishing, focus ion beam, ion-milling, XRD, DTA, DSC, laser flash,
ZEM-3.
> Good communication and presentation skills, team player, practical approach to problem-solving, self-motivated,
positive thinker, inquisitive, and working independently.

EDUCATION

Ph.D.: The University of Liverpool, United Kingdom

RESEARCH INTERESTS

Phase transformations in solid materials, interfacial structure analyses & crystallography, thermoelectric materials, shape memory alloys

26

Scopus Publications

626

Scholar Citations

11

Scholar h-index

12

Scholar i10-index

Scopus Publications

  • Effect of Half-Heusler Interfacial Structure on Thermal Transport Properties of (Ti, Zr)NiSn Alloys
    Mizuki Sato, Yaw Wang Chai, and Yoshisato Kimura

    ACS Applied Materials and Interfaces, ISSN: 19448244, eISSN: 19448252, Published: 2021 American Chemical Society (ACS)
    The microstructure of the half-Heusler phase separation in half-Heusler (HH) MNiSn(M = Ti, Zr) intermetallic compounds has been investigated systematically in this study. Scanning electron microscopy observations from a range of (Tix, Zr1-x)NiSn have revealed the HH single phase at high temperature formed into many HH domains of various HH compositions with different Ti/Zr concentration ratios when x > 0.1. The formation of Ti-rich and Zr-rich HH domains with rather large size (up to several hundred μm in diameter) is thought to originate from a combination of the liquid solidification process and followed by an HH phase decomposition process within a miscibility gap between the TiNiSn and ZrNiSn HH phases. We have noticed that in addition to the mass and size difference based phonon scattering, sharp interfaces between the Ti-rich and Zr-rich HH domains containing high density of misfit dislocations could provide additional phonon scattering centers and reduced thermal conductivity of the alloys. Moreover, the cyclic heat treatment process at temperatures near the HH phase-decomposition's critical temperature could modify the HH domains' microstructure to become more diffuse, coherent with a more comprehensive length scale, and globular shape. These diffuse and coherent Ti-rich HH1/Zr-rich HH2 interfaces can provide an additional enhancement of phonon scattering and thereby result in a more considerable reduction of thermal conductivity than those of relatively less diffuse ones. We anticipate a similar approach of using cyclic heat treatment to modify the microstructure and consequently lead to further enhancement of phonon scattering can also apply to many other thermoelectric alloy systems possessing a miscibility gap.

  • Disconnections and Laves (C14) precipitation in high-Cr ferritic stainless steels
    Yaw Wang Chai, Ko Kato, Chieri Yabu, Shin Ishikawa, and Yoshisato Kimura

    Acta Materialia, ISSN: 13596454, Volume: 198, Pages: 230-241, Published: 1 October 2020 Elsevier BV
    Abstract Even though it is widely accepted that the Laves phase is a promising strengthener for chromium-containing stainless steels, there is little information available on its microstructural evolution and growth. Therefore, in this study, we analysed the behaviour of Laves (C14) precipitates in high-Cr ferritic stainless steel, Fe-20Cr-2Mo-0.5Nb (at%), annealed at 1073 K for different time periods. A high density of (Fe,Cr)2(Mo,Nb) precipitates was formed in the α matrix during annealing. Morphological evolution in the Laves phase followed the sequence of spheroidal → faceted ellipsoidal → faceted needle-like → faceted plate-like. The Laves phase is related to the α matrix by two sets of orientation relationships, viz. OR-1: [ 1 ¯ 1 ¯ 2 ] α / [ 1 2 ¯ 10 ] L , [ 111 ] α / [ 0 1 ¯ 10 ] L , and ( 1 ¯ 10 ) α / ( 000 1 ¯ ) L and OR-2: [ 1 1 ¯ 0 ] α / [ 0001 ] L , [ 113 ] α / [ 1 ¯ 1 ¯ 20 ] L , and ( 3 ¯ 3 ¯ 2 ) α / ( 1 1 ¯ 00 ) L . The habit plane of the Laves phase was found to deviate slightly from the ( 3 ¯ 3 ¯ 2 ) α / ( 1 1 ¯ 00 ) L plane. Interfacial defects, namely disconnections (bD, h) and super disconnections (mbD, mh), observed experimentally along the habit plane, were consistent with theoretical results based on OR-2 as the reference coordinate frame. The Burgers vector bD exhibited an edge component b y D (0.1539 nm) parallel to Y / [ 113 ] α / [ 1 ¯ 1 ¯ 20 ] L and a small normal component b z D (0.0747 nm) perpendicular to the terraces; meanwhile, the overlap step height h =  d ( 1 1 ¯ 00 ) L . The morphological evolution of Laves precipitates was described based on the lateral motion of disconnections and super disconnections on { 3 ¯ 3 ¯ 2 } α / { 1 1 ¯ 00 } L terraces in the 〈 113 〉 α / 〈 1 ¯ 1 ¯ 20 〉 direction. Arrays of the disconnections (bD, h) were found to be capable of accommodating the misfit strain on { 3 ¯ 3 ¯ 2 } α / { 1 1 ¯ 00 } L terraces.

  • Reduced Thermal Conductivity of Mg<inf>2</inf>(Si, Sn) Solid Solutions by a Gradient Composition Layered Microstructure
    Zhifang Zhou, Yaw Wang Chai, Yu Ikuta, Yonghoon Lee, Yuanhua Lin, and Yoshisato Kimura

    ACS Applied Materials and Interfaces, ISSN: 19448244, eISSN: 19448252, Pages: 19547-19552, Published: 29 April 2020 American Chemical Society (ACS)
    Solid solutioning of Mg2(Si, Sn) has been a promising approach in reducing thermal conductivity and leads to improvement of thermoelectric performance. In addition to the Mg2(Si, Sn) solid solutions, we have noticed layered structure with gradient composition, which is formed by non-equilibrium solidification and peritectic reaction process, can provide further reduction of thermal conductivity of the Mg2(Si, Sn) solid solutions. All the layers of the layered structure have the same face centered cubic (FCC)-based structure but varying Sn/Si concentration ratios in each layer. The interfaces between the layers are semi-coherent reticulating with different numbers of misfit dislocations. Such interfacial structure brings large numbers of phonon scattering sources, resulting in the further reduction of thermal conductivity in the Mg2(Si, Sn) solid solutions. Consequently, the undoped Mg2Si0.75Sn0.25 containing higher density of the layered structure has relatively lower thermal conductivity, 1.9 W m -1 K -1 at 523 K than Mg2Si0.25Sn0.75 with much lower density of the layered structure, 2.3 W m -1 K -1 at 523 K.

  • Nanoscale hierarchical structure of twins in nanograins embedded with twins and the strengthening effect
    Haochun Tang, Tso-Fu Mark Chang, Yaw-Wang Chai, Chun-Yi Chen, Takashi Nagoshi, Daisuke Yamane, Hiroyuki Ito, Katsuyuki Machida, Kazuya Masu, and Masato Sone

    Metals, eISSN: 20754701, Published: September 2019 MDPI AG
    Hierarchical structures of 20 nm grains embedded with twins are realized in electrodeposited Au–Cu alloys. The electrodeposition method allows refinement of the average grain size to 20 nm order, and the alloying stabilizes the nanoscale grain structure. Au–Cu alloys are face-centered cubic (FCC) metals with low stacking fault energy that favors formation of growth twins. Due to the hierarchical structure, the Hall–Petch relationship is still observed when the crystalline size (average twin space) is refined to sub 10 nm region. The yield strength reaches 1.50 GPa in an electrodeposited Au–Cu alloy composed of 16.6 ± 1.1 nm grains and the average twin spacing at 4.7 nm.

  • Stabilizing austenite via a core-shell structure in the medium Mn steels
    Xinhao Wan, Geng Liu, Ran Ding, Nobuo Nakada, Yaw-Wang Chai, Zhigang Yang, Chi Zhang, and Hao Chen

    Scripta Materialia, ISSN: 13596462, Volume: 166, Pages: 68-72, Published: June 2019 Elsevier BV
    Abstract Retained austenite plays a significant role in the strength-ductility balance of medium Mn steels. In this contribution, a new processing route (Flash-Austenite Reversion Treatment, Flash-ART) was proposed to design retained austenite with a compositional core-shell structure in an Fe-0.20C-7.76Mn-1.99Al (wt%) steel. It was interestingly found that the austenite core, which should transform into martensite based on its composition and size, was stabilized by the Mn enriched shell. Flash-ART allows us to obtain more retained austenite compared with the conventional ART.

  • Effects of Si on Phase Stability and Precipitation Behavior of C14 Laves Phase (Fe,Cr)<inf>2</inf>(Nb,Mo) in High Cr αfe-base Alloys
    Yoshisato Kimura, Ko Kato, and Yaw Wang Chai

    MRS Advances, eISSN: 20598521, Issue: 25-26, Pages: 1477-1483, Published: 2019 Springer Science and Business Media LLC

  • Thermoelectric properties of nearly single-phase ¢-FeSi<inf>2</inf> alloys fabricated by gas-atomized powder sintering
    Yoshisato Kimura, Masashi Yamada, and Yaw Wang Chai

    Materials Transactions, ISSN: 13459678, Pages: 652-661, Published: 2019 Japan Institute of Metals
    Thermoelectric alloys having nearly ¢-FeSi2 single-phase microstructure were fabricated by sintering gas-atomized powders using the hot pressing. Since the ¢-FeSi2 phase is formed by the peritectoid reaction between 3⁄4-FeSi and ¡-Fe2Si5 phases, the reaction rate for the completion of ¢-FeSi2 phase transition strongly depends on the diffusion path length which is governed by the morphology and size of solidified microstructure consisting of 3⁄4 and ¡ phases. It has been indicated by the wedge drop cast using arc melting that producing fine and fully eutectic microstructure by rapid solidification is quite effective for the completion of ¢-FeSi2 phase transition. An argon gas atomization process was chosen as a rapid solidification technique to produce fine and homogeneous alloy powder having fully 3⁄4 and ¡ eutectic microstructure, which was turned out to be beneficial for the formation of ¢-FeSi2 single-phase microstructure by a short time annealing even within 30 minutes at 1073K for the gas-atomized powders with the averaged particles size of 20 μm and under in diameter. Thermoelectric properties were evaluated for these nearly single-phase ¢-FeSi2 sintered alloys with the addition of doping elements, n-type Co and p-type Mn, 1.67 at% respectively. The absolute value of Seebeck coefficient and electrical conductivity are higher in a p-type Mn alloy than an n-type Co alloy. [doi:10.2320/matertrans.MB201805]

  • Evaluation of Microstructure Formation and Phase Equilibria for Thermoelectric β-FeSi<inf>2</inf> Composite Alloys
    Yoshisato Kimura, Hiroaki Otani, Ayaka Mori, and Yaw-Wang Chai

    MRS Advances, eISSN: 20598521, Pages: 1369-1374, Published: 2017 Springer Science and Business Media LLC
    Thermoelectric composite alloys consisting of the β-FeSi2 matrix and SiO2 particles dispersion were fabricated by a so-called combined reactions sintering process using reduction and oxidation reactions between eutectoid Si decomposed from α-Fe2Si5 and added Fe-oxide powder. Typical microstructure may include some of residual eutectoid Si particles, intermediate product Fe2SiO4 particles, and/or remaining reduced Fe particles depending on the composite alloy compositions and the process conditions. Partitioning of doping element, n-type Co or p-type Mn, during the process plays an important role to control the optimum carrier concentration of the composite alloys. Thermal conductivity can be reduced, as expected, by the dispersion of SiO2 particles. The solubility of doping elements, Co, Mn, Al, and Ru was evaluated in α-Fe2Si5 at 1373 K and in β-FeSi2 at 1073 K being based on the isotherm determination. It is suggested that suitable dopants for the present process are n-type Co and p-type Mn, since they have sufficiently large solubility around 10 at% in both α-Fe2Si5 and β-FeSi2 phases.

  • The effect of an isoelectronic Ti-Zr substitution on Heusler nanoprecipitation and the thermoelectric properties of a (Ti<inf>0.2</inf>,Zr<inf>0.8</inf>)Ni<inf>1.1</inf>Sn half-Heusler alloy
    Yaw Wang Chai, Toshinori Oniki, Takahiro Kenjo, and Yoshisato Kimura

    Journal of Alloys and Compounds, ISSN: 09258388, Volume: 662, Pages: 566-577, Published: 25 March 2016 Elsevier BV
    Abstract The microstructure and thermoelectric properties of an off-stoichiometric quaternary (Ti 0.2 ,Zr 0.8 )Ni 1.1 Sn half-Heusler (HH) alloy was investigated in three heating cycles. A high density of coherent nanoprecipitates with an average diameter of ∼13 nm and an interprecipitate spacing of ∼6 nm was observed in the alloy. Formation of the extremely fine nanoprecipitates, most likely to be the ‘full’ Heusler (FH) phase, was not only strongly related to the degree of excess Ni concentration ratio in the alloy, but also appeared to be affected by the Ti–Zr substitutions. We noticed the behaviour of both electrical resistivity ( ρ ) and Seebeck coefficient ( S ) of the alloy was closely associated with the microstructure evolution of the FH-nanoprecipitates, which depended on their phase instability at elevated temperature and the cyclic heating process. The ρ and S reduced after the 1st heating cycle and stabilised thereafter in the subsequent heating cycles. Despite of the presence of the metallic FH-nanoprecipitates, the stabilised S maintained similar magnitudes to S of the ZrNiSn (without FH-nanoprecipitates) and did not show degradation of S as previously seen in the ZrNi 1.1 Sn containing relatively much larger FH-nanoprecipitates. The high density of FH-nanoprecipitates and the presence of Ti–Zr point defects were responsible for the significant reduction of thermal conductivity ( κ ) of the alloy, about 30% and 20% less than κ of the ZrNiSn and ZrNi 1.1 Sn alloys, respectively. Moreover, further reduction of κ was noticed due to formation of the diffuse HH/FH interfaced FH-nanoprecipitates from the cyclic heating process. Consequently, the alloy has shown a maximum dimensionless figure of merit ( ZT ) up to 0.81 at 870 K.

  • Microstructure and thermoelectric properties of a ZrNi<inf>1.1</inf>Sn half-Heusler alloy
    Yaw Wang Chai, Toshinori Oniki, and Yoshisato Kimura

    Acta Materialia, ISSN: 13596454, Pages: 290-300, Published: 15 February 2015 Elsevier BV
    Abstract The microstructure and thermoelectric properties of a ZrNi 1.1 Sn half-Heusler (HH) alloy have been investigated. The alloy was found to contain a high density of nanosized Heusler precipitates that appeared as platelets and discs. These Heusler nanoprecipitates were responsible for a large improvement of the Seebeck coefficient of the alloy at room temperature compared with the ZrNiSn, which contained no Heusler nanoprecipitates. However, the improvement reduced with increasing temperature due to the phase instability of the Heusler nanoprecipitates. Nevertheless, the reduction of the Seebeck coefficient could be stabilized after heating and cooling the alloy from beyond the precipitate dissolution temperature to below the phase decomposition temperature due to stabilization of the Heusler nanoprecipitates. The size of the stabilized precipitates was significantly reduced to ZT of the ZrNi 1.1 Sn was 0.75 at 900 K, which was an improvement of ∼40% compared with that of the ZrNiSn.

  • Solid solution behavior and thermoelectric properties of half-heusler (Zr<inf>x</inf>, Nb<inf>1-x</inf>)(Ni<inf>y</inf>, Co<inf>1-y</inf>)Sn alloy
    Kentaro Yoshioka, Yaw Wang Chai, and Yoshisato Kimura

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, ISSN: 00214876, Pages: 683-690, Published: 2015 Japan Institute of Metals

  • Fabrication of β-FeSi<inf>2</inf>-based thermoelectric composite alloys by oxidation and reduction reactions during sintering of eutectoid si and iron oxide powder
    Yoshisato Kimura, Koichiro Takeno, Ayaka Mori, and Yaw-Wang Chai

    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals, ISSN: 00214876, Pages: 613-620, Published: 2015 Japan Institute of Metals
    bFeSi2 is an ecofriendly thermoelectric material for hightemperature applications. In the present work, we demonstrate the validity of a new proposed fabrication process for compositetype thermoelectric alloys comprising a bFeSi2 matrix and dispersed SiO2 particles (including Fe2SiO4 particles). The starting materials were singlephase aFeSi2 alloy powder and Fe2O3 powder. We propose that the following reaction sequence occurs during the sintering process: (1) aFeSi2 decomposes into b FeSi2 and Si via the eutectoid reaction, (2) SiO2 is formed by the oxidation of Si, and (3) bFeSi2 is additionally formed by the solidphase reaction between eutectoid Si and reduced Fe that is formed by the reduction of Fe2O3. The microstructure of the composite alloys formed by the combined reactions during the sintering process was observed and characterized mainly using scanning transmission electron microscopy in conjunction with energydispersive Xray spectroscopic chemical analyses and X ray diffraction. The electrical and thermoelectric properties of the composite alloys were measured at temperatures from 300 to 1073 K. High Seebeck coefficient values were observed for ntype Codoped composite alloys from -150 to -250 mV・K-1 and for ptype Mndoped alloys from 200 to 500 mV・K-1. The partitioning of the Co and Mn dopants from the aFeSi2 phase to the b FeSi2 phase throughout the process is important for controlling the Seebeck coefficient. The electrical resistivity is lowered by the dispersed SiO2 particles that are expected to reduce the lattice thermal conductivity of the composite alloys. [doi:10.2320/jinstmet.JA201518]

  • Thermoelectric properties control of half-Heusler compounds by lattice defects and interfaces introduced based on the close relationship with Heusler compounds
    Yoshisato Kimura, Yaw-Wang Chai, Toshinori Oniki, Takahiko Itagaki, and Shinya Otani

    Materials Research Society Symposium Proceedings, ISSN: 02729172, Volume: 1760, Pages: 151-156, Published: 2015 Springer Science and Business Media LLC
    ABSTRACTHalf-Heusler MNiSn (M=Ti, Zr, Hf) compounds are well-known, excellent n-type thermoelectric materials. The n-type Seebeck coefficients of ZrNiSn are reduced because of the precipitation of the metallic Heusler ZrNi2Sn phase. An excellent n-type Seebeck coefficient can be converted to p-type based on the vacancy site occupation by the solute Co atoms in the half-Heusler TiNiSn phase as well as ZrNiSn. The Heusler phase precipitates, including their precursor nano-structure in the half-Heusler matrix and the vacancy site occupation of the half-Heusler phase, are regarded as lattice defects based on the crystallographically and thermodynamically close relationship between half-Heusler and Heusler phases.

  • Ordered Structures and Thermoelectric Properties of MNiSn (M = Ti, Zr, Hf)-Based Half-Heusler Compounds Affected by Close Relationship with Heusler Compounds
    Yoshisato Kimura and Yaw-Wang Chai
    ISSN: 10474838, eISSN: 15431851, Pages: 233-245, Published: January 2015 Springer Science and Business Media LLC
    Half-Heusler compounds are excellent thermoelectric materials. A characteristic of the half-Heusler–type ordered structure is the vacancy site that occupies one-fourth of all the lattice points. Therefore, a half-Heusler ABX phase (where A and B are typically transition metal elements, such as Ti, Zr, and Hf, and X represents a half-metal element such as Sn or Sb) has a crystallographically close relationship with a Heusler AB2X phase in the sense that the vacancy site in the half-Heusler phase is filled with B atoms in the Heusler phase. The thermoelectric properties are improved or affected by point lattice defects related to the vacancy site and the B site, such as the antisite atom B in the vacancy site, vacancies in the B site, and vacancy-site occupancy by quaternary C atoms. A modulated-like nanostructure due to point defects regarding vacancies and Ni atoms is formed for an instance in ZrNiSn alloys even close to the stoichiometric composition. Ni-rich nanoclusters are locally formed by excessive Ni antisite atoms in the vacancy site, which work as precursors of Heusler precipitates (TiNi2Sn, ZrNi2Sn, and so forth). The vacancy-site occupation in ZrNiSn with Co and Ir results in the drastic conversion of thermoelectric properties from n type to p type, and the effective reduction of the lattice thermal conductivity.

  • Intrinsic point defects in thermoelectric half-Heusler alloys
    Yaw Wang Chai, Kentaro Yoshioka, and Yoshisato Kimura

    Scripta Materialia, ISSN: 13596462, Pages: 13-16, Published: 15 July 2014 Elsevier BV
    The single-phase half-Heusler microstructure of ZrNiSn, (Zr 0.5 , Hf 0.5 )NiSn and Zr(Ni, Co 0.2 )Sn alloys was found to contain a high density of Heusler lattice bands 2–7 nm wide. These nanoscale Heusler lattice bands originated from clustering of Ni and/or Co antisites via occupation of structural vacancies by excess Ni or (Ni+Co) concentrations. The presence of these lattice point defects and their subsequent clustering could result in the reduction of thermal conductivity of the half-Heusler alloys.

  • Effect of interstitial carbon atoms on phase stability and mechanical properties of E2<inf>1</inf> (L1<inf>2</inf>) Ni<inf>3</inf>AlC<inf>1-x</inf> single crystals
    Yoshisato Kimura, Masato Kawakita, Hiroyasu Yuyama, and Yaw-Wang Chai

    Materials Research Society Symposium Proceedings, ISSN: 02729172, Volume: 1516, Pages: 243-248, Published: 2013 Springer Science and Business Media LLC
    ABSTRACTSingle crystals of E21 (L12) Ni3AlC1-x were prepared by the unidirectional solidification using the optical floating zone melting method to determine their mechanical properties. Particularly the effects of interstitial carbon atoms on mechanical properties were evaluated by compression tests at room temperature. Operative slip system of E21 Ni3AlC is {111}&lt;011&gt; type which is the same as that of L12 Ni3Al. Strength of Ni3AlC single crystals increases with carbon concentration due to the solid solution effect, though the stress relief of yielding behavior is enhanced at the intermediate carbon content at around 3at%. A large gap appears in the carbon concentration dependence of critical resolved shear stress (as well as yield stress) at almost the same carbon content. This discontinuity in strengthening is attributed to the interaction between multiple solute carbon atoms and mobile dislocations.

  • Microstructure evolution of nanoprecipitates in half-Heusler TiNiSn alloys
    Yaw Wang Chai and Yoshisato Kimura

    Acta Materialia, ISSN: 13596454, Pages: 6684-6697, Published: October 2013 Elsevier BV
    Abstract The microstructures of thermoelectric TiNiSn half-Heusler alloys have been studied in detail. For concentration ratios that are slightly rich in Ni, a high density of Heusler-phase nanosized precipitates tended to precipitate within a half-Heusler matrix. The morphology and average size of the Heusler nanoprecipitates were very sensitive to the Ni concentration ratio in the half-Heusler matrix of the alloys. Smaller Heusler nanoprecipitates with coherent ellipsoidal ( HH orientations. Interfacial defects between the Heusler and half-Heusler phases, as well as lattice point defects, Ni antisites and vacancies, were found to be closely related to the formation of the Heusler nanoprecipitates. A mechanism has been proposed in this study to describe the coarsening of the Heusler nanoprecipitates via the formation of lattice point defects and interfacial defects.

  • Nanosized precipitates in half-Heusler TiNiSn alloy
    Yaw Wang Chai and Yoshisato Kimura

    Applied Physics Letters, ISSN: 00036951, Volume: 100, Published: 16 January 2012 AIP Publishing
    The microstructure of half-Heusler TiNiSn alloy has been investigated in this study. A high density of coherent nanosized Heusler precipitates was found within the half-Heusler matrix. Formation of these Heusler precipitates occurs by a phase separation process, where a single phase solid solution (half-Heusler-Heusler) decomposes into an equilibrium two-phase mixture of half-Heusler and Heusler regions. These Heusler nanoprecipitates improve the thermoelectric properties of the alloy.

  • Effect of vacancy-site occupation in half-Heusler compound ZrNiSn on phase stability and thermoelectric properties
    Yoshisato Kimura, Toshiyasu Tanoguchi, Yasuhiro Sakai, Yaw-Wang Chai, and Yoshinao Mishima

    Materials Research Society Symposium Proceedings, ISSN: 02729172, Volume: 1295, Pages: 335-340, Published: 2011 Springer Science and Business Media LLC
    ABSTRACTThe half-Heusler compound ZrNiSn has a quite small solubility for Ni from the stoichiometric composition towards the Ni-rich direction since Ni atoms are not supposed to occupy the vacancy-site. Nevertheless, Co and Ir atoms preferably occupy the vacancy-site of ZrNiSn, which is contrary to the prediction that they would substitute for Ni sites. This implies that the phase stability of the compound gradually changes toward that of the Heusler compound Zr(Ni,M)2Sn (M = Co, Ir). It has been confirmed that there exists a two-phase field between half-Heusler Zr(Ni,Cox)Sn and Heusler Zr(Ni,Co)2Sn. The n-type thermoelectric property of ZrNiSn can be converted to p-type by the addition of Co and Ir within the compositional range of the half-Heusler phase. The occupation of vacancy sites by Co and Ir atoms leads to a drastic reduction in the thermal conductivity owing to the enhancement of phonon scattering. With further Co addition, the Heusler phase Zr(Ni,Co)2Sn alloys show metallic behavior.

  • Thermoelectric performance of half-Heusler TiNiSn alloys fabricated by solid-liquid reaction sintering
    Yoshisato Kimura, Chihiro Asami, Yaw Wang Chai, and Yoshinao Mishima

    Materials Science Forum, ISSN: 02555476, eISSN: 16629752, Volume: 654-656, Pages: 2795-2798, Published: 2010 Trans Tech Publications, Ltd.
    A new fabrication process was proposed for half-Heusler type TiNiSn thermoelectric alloys. Based on the result that the TiNiSn phase can be formed easily at the Sn(Liquid)/TiNi(Solid) interface, the liquid-solid reaction-sintering process was developed using TiNi and Sn powders. The TiNi compound powders were prepared by the atomization method using argon gas atmosphere. We have fabrictaed nearly single-phase TiNiSn alloys and evaluated their thermoelectrical properties; the presnt TiNiSn alloys have large electrical power factor of about 3.5 mWm-1K-2, and the maximum value of dimensionless figure of merit, ZT = 0.67, can be achieved at around 700 K even without tuning of the carrier concentration through alloying elements.

  • Improvements in honeycomb abradable seals
    D.J. Potter, Y.W. Chai, and G.J. Tatlock

    Materials at High Temperatures, ISSN: 09603409, Pages: 127-135, Published: 2009 Informa UK Limited
    Abstract The concept of utilising honeycomb abradable seals to improve gas turbine engine performance has been under development for many years. Engine operating temperatures, in the region of the seals, have been restricted to below 950°C by the reliance on a chromia scale for degradation protection. The introduction of nickel brazed FeCrAlY based alloys within the honeycomb seal could facilitate a safe increase in operating temperatures to over 1100°C. This is aided by the formation of a more stable, α-alumina scale. These Fe–20Cr–5Al–0.5Y foils, including the commercially produced variant designated MI2100, have been designed for a service lifetime of up to 24,000 hours. However, burner tests and isothermal oxidation tests in laboratory air at 1100 and 1200°C have shown them to fail after much shorter times. The major degradation of the foils occurs adjacent to the brazed region and limits the lifetime of the honeycomb seal. Cross-sectional analysis in a scanning electron microscope of seals manufactured from MI2100 foils, after oxidation testing at 1200°C, has shown that voids form beneath the protecting α-alumina scale. In some cases, these voids are filled with silica, with some chromia present, and may be the origin of the subsequent degradation process. The results contrast with tests on free-standing thin foils (100 μm thickness) of MI2100 and other FeCrAlY alloys, where a continuous layer of chromia is formed below the alumina outer scale, once the aluminium content of the alloy drops below a critical composition. Although there is a small amount of silicon in MI2100, the main source of the high level of silicon found in the honeycombs is most likely to be the brazing alloy, since both nickel and silicon from the braze are very mobile in FeCrAlY alloys at high temperatures. The formation and filling of voids with silica may be associated with the subsequent failure of the protective alumina scales on these brazed alloys, and this mechanism will be developed further in this paper.

  • Self-accommodation in Ti-Nb shape memory alloys
    Y.W. Chai, H.Y. Kim, H. Hosoda, and S. Miyazaki

    Acta Materialia, ISSN: 13596454, Pages: 4054-4064, Published: August 2009 Elsevier BV
    Abstract Self-accommodation morphology of the α ″ martensites in Ti–Nb shape memory alloys with Nb content ranging from 20 to 24 at.% was investigated. Hollow and solid triangular morphologies consisting of three α ″ variants were found to be the α ″ self-accommodation morphologies. The α ″ variants are related to each other by twinning on { 1 1 ¯ 1 } α ″ . A V-shaped morphology consisting of two α ″ variants coupled with a solid triangular morphology consisting of three α ″ variants was found to be another type of self-accommodation morphology. The α ″ variants from the V-shaped morphology are related by twinning on { 1 1 1 } α ″ , whereas the α ″ variants from the solid triangular morphology are related to each other by twinning on { 1 1 ¯ 1 } α ″ . Pairs of crystallographically equivalent { 1 1 ¯ 1 } α ″ twinning dislocations with steps of opposite signs were found to cause the overall orientation of the twinning terraces and steps to be parallel to the { 1 1 ¯ 1 } α ″ twinning interface.

  • Interfacial defects in Ti-Nb shape memory alloys
    Y.W. Chai, H.Y. Kim, H. Hosoda, and S. Miyazaki

    Acta Materialia, ISSN: 13596454, Pages: 3088-3097, Published: August 2008 Elsevier BV
    Abstract The structure of the martensite (α″)/martensite (α″) and parent (β)/martensite (α″) interfaces in a series of binary Ti–Nb alloys with Nb content ranging from 20 to 24 at.% was investigated. Both the α″/α″ and β/α″ interfaces comprised a series of { 2 1 ¯ 1 } β / / { 1 1 0 } α ″ terraces and steps when viewed edge-on (close to [ 0 0 1 ] α ″ ). Interfacial defects, particularly the transformation disconnections (b, h) superimposed along the terrace–step interface structure, have been identified. They were responsible for accommodating most of the transformation strain along the α″/α″ and β/α″ interfaces. Using the parameters b and h, the prediction of the α″ habit plane based on the topological model agreed well with the prediction from the phenomenological theory as well with experimental observations. The α″ habit plane in Ti–20Nb alloy is close to { 7 5 ¯ 5 ¯ } β and moves towards { 4 3 ¯ 3 ¯ } β in Ti–22Nb and Ti–24Nb alloys. The remaining transformation strain along the β/α″ interface was found to be accommodated by Type 1 twinning on ( 1 1 1 ) α ″ with Burgers vector bLIS close to 〈 2 ¯ 1 ¯ 3 〉 α ″ ( ≡ 〈 1 ¯ 1 ¯ 2 ¯ 〉 β ) .

  • Chapter 74 Topological Modelling of Martensitic Transformations
    R.C. Pond, X. Ma, Y.W. Chai, and J.P. Hirth

    Dislocations in Solids, ISSN: 15724859, Pages: 225-261, Published: 2007 Elsevier
    Publisher Summary The topological model provides a physical understanding of the structure of martensitic interfaces and the diffusionless nature of transformations consistent with experimental observations, including recent high-resolution transmission electron microscopy studies. Martensitic interfaces are modeled as coherent terraces with superimposed disconnections and lattice-invariant deformation (LID) that relieve any coherency strains. Additionally, lateral motion of disconnections across the terraces propagates the transformation without long-range diffusion. Such conservative motion imposes severe topological constraints on the nature of admissible terraces; terrace coherency is revealed as a key feature in this context, and feasible terraces are restricted to those with modest coherency strains. Moreover, admissible disconnections are limited to those that can move conservatively on the terraces and the active LID must not form sessile intersections with them. In this model, the habit plane deviates from the terrace plane because each disconnection exhibits step character, and for minimal long-range strain, the angle between the two planes can be found.

  • Martensitic transformations in 'unfamiliar' systems
    R.C. Pond, Y.W. Chai, and S. Celotto

    Materials Science and Engineering A, ISSN: 09215093, Volume: 378, Issue: 1-2 SPEC. ISS., Pages: 47-51, Published: 25 July 2004 Elsevier BV
    Abstract In stiff engineering materials, martensitic transformations are shear-dominant diffusionless processes, and their crystallographic features can be successfully predicted using the phenomenological theory of martensite crystallography (PTMC). Recently, an alternative approach has been developed wherein the interface structure is modelled in terms of interfacial dislocations. This model, referred to as the topological model (TM), provides insight into the transformation process and identifies a set of five criteria that must be met for a transformation to be diffusionless. In the case of stiff engineering materials, the crystallographic predictions of the PTMC and TM are very similar. However, the TM enables a broader range of transformations to be treated, and two examples of ‘unusual’ martensites are presented here. The first is a diffusionless transformation in a small elastically soft protein crystal. The second is a transformation in a prospective high-temperature engineering alloy which exhibits the characteristic crystallographic features of martensite but where concomitant diffusion occurs. In this case, four of the five criteria mentioned above that relate to conservation of substitutional atomic sites are satisfied.

RECENT SCHOLAR PUBLICATIONS

  • Effect of Half-Heusler Interfacial Structure on Thermal Transport Properties of (Ti, Zr) NiSn Alloys
    M Sato, YW Chai, Y Kimura
    ACS Applied Materials & Interfaces 13 (21), 25503-25512 2021

  • Disconnections and Laves (C14) precipitation in high-Cr ferritic stainless steels
    YW Chai, K Kato, C Yabu, S Ishikawa, Y Kimura
    Acta Materialia 198, 230-241 2020

  • Reduced Thermal Conductivity of Mg2(Si, Sn) Solid Solutions by a Gradient Composition Layered Microstructure
    Z Zhou, YW Chai, Y Ikuta, Y Lee, Y Lin, Y Kimura
    ACS Applied Materials & Interfaces 12 (17), 19547-19552 2020

  • Multiscale microstructure engineering and the development of high performance, environmental friendly, low cost and thermally stable half-Heusler thermoelectric materials
    YW Chai, Y Kimura
    Impact 2020 (1), 65-67 2020

  • Nanoscale hierarchical structure of twins in nanograins embedded with twins and the strengthening effect
    H Tang, TFM Chang, YW Chai, CY Chen, T Nagoshi, D Yamane, H Ito, ...
    Metals 9 (9), 987 2019

  • Stabilizing austenite via a core-shell structure in the medium Mn steels
    X Wan, G Liu, R Ding, N Nakada, YW Chai, Z Yang, C Zhang, H Chen
    Scripta Materialia 166, 68-72 2019

  • Thermoelectric Properties of Nearly Single-Phase β-FeSi2 Alloys Fabricated by Gas-Atomized Powder Sintering
    Y Kimura, M Yamada, YW Chai
    Materials transactions 60 (5), 652-661 2019

  • Effects of Si on Phase Stability and Precipitation Behavior of C14 Laves Phase (Fe,Cr)2(Nb,Mo) in High Cr αFe-base Alloys
    Y Kimura, K Kato, YW Chai
    MRS Advances 4 (25-26), 1477-1483 2019

  • Evaluation of Microstructure Formation and Phase Equilibria for Thermoelectric β-FeSi2 Composite Alloys
    Y Kimura, H Otani, A Mori, YW Chai
    MRS Advances 2 (26), 1369-1374 2017

  • The effect of an isoelectronic Ti–Zr substitution on Heusler nanoprecipitation and the thermoelectric properties of a (Ti0. 2, Zr0. 8) Ni1. 1Sn half-Heusler alloy
    YW Chai, T Oniki, T Kenjo, Y Kimura
    Journal of Alloys and Compounds 662, 566-577 2016

  • Microstructure and thermoelectric properties of a ZrNi1. 1Sn half-Heusler alloy
    YW Chai, T Oniki, Y Kimura
    Acta Materialia 85, 290-300 2015

  • Solid Solution Behavior and Thermoelectric Properties of Half-Heusler (Zr-x, Nb1-x)(Ni-y, Co1-y) Sn Alloy
    K Yoshioka, YW Chai, Y Kimura
    JOURNAL OF THE JAPAN INSTITUTE OF METALS AND MATERIALS 79 (12), 683-690 2015

  • Fabrication of beta-FeSi2-Based Thermoelectric Composite Alloys by Oxidation and Reduction Reactions during Sintering of Eutectoid Si and Iron Oxide Powder
    Y Kimura, K Takeno, A Mori, YW Chai
    JOURNAL OF THE JAPAN INSTITUTE OF METALS 79 (11), 613-620 2015

  • 共析 Si と酸化鉄の酸化還元反応焼結による β-FeSi2 基コンポジット熱電材料の創製
    木村好里, 岳野洸一郎, 森彩花
    日本金属学会誌 79 (11), 613-620 2015

  • Ordered structures and thermoelectric properties of MNiSn (M= Ti, Zr, Hf)-based half-Heusler compounds affected by close relationship with Heusler compounds
    Y Kimura, YW Chai
    Jom 67 (1), 233-245 2015

  • Thermoelectric properties control of Half-Heusler compounds by lattice defects and interfaces introduced based on the close relationship with Heusler compounds
    Y Kimura, YW Chai, T Oniki, T Itagaki, S Otani
    MRS Online Proceedings Library 1760 (1), 151-156 2014

  • Intrinsic point defects in thermoelectric half-Heusler alloys
    YW Chai, K Yoshioka, Y Kimura
    Scripta Materialia 83, 13-16 2014

  • Microstructure evolution of nanoprecipitates in half-Heusler TiNiSn alloys
    YW Chai, Y Kimura
    Acta materialia 61 (18), 6684-6697 2013

  • Effect of Interstitial Carbon Atoms on Phase Stability and Mechanical Properties of E21 (L12) Ni3AlC1-x Single Crystals
    Y Kimura, M Kawakita, H Yuyama, YW Chai
    MRS Online Proceedings Library (OPL) 1516, 243-248 2013

  • Nanosized precipitates in half-Heusler TiNiSn alloy
    Y Wang Chai, Y Kimura
    Applied Physics Letters 100 (3), 033114 2012

MOST CITED SCHOLAR PUBLICATIONS

  • Self-accommodation in Ti–Nb shape memory alloys
    YW Chai, HY Kim, H Hosoda, S Miyazaki
    Acta materialia 57 (14), 4054-4064 2009
    Citations: 136

  • Interfacial defects in Ti–Nb shape memory alloys
    YW Chai, HY Kim, H Hosoda, S Miyazaki
    Acta Materialia 56 (13), 3088-3097 2008
    Citations: 92

  • Nanosized precipitates in half-Heusler TiNiSn alloy
    Y Wang Chai, Y Kimura
    Applied Physics Letters 100 (3), 033114 2012
    Citations: 69

  • Microstructure evolution of nanoprecipitates in half-Heusler TiNiSn alloys
    YW Chai, Y Kimura
    Acta materialia 61 (18), 6684-6697 2013
    Citations: 63

  • Topological modelling of martensitic transformations
    RC Pond, X Ma, YW Chai, JP Hirth
    Dislocations in solids 13, 225-261 2007
    Citations: 61

  • Ordered structures and thermoelectric properties of MNiSn (M= Ti, Zr, Hf)-based half-Heusler compounds affected by close relationship with Heusler compounds
    Y Kimura, YW Chai
    Jom 67 (1), 233-245 2015
    Citations: 39

  • Microstructure and thermoelectric properties of a ZrNi1. 1Sn half-Heusler alloy
    YW Chai, T Oniki, Y Kimura
    Acta Materialia 85, 290-300 2015
    Citations: 36

  • Intrinsic point defects in thermoelectric half-Heusler alloys
    YW Chai, K Yoshioka, Y Kimura
    Scripta Materialia 83, 13-16 2014
    Citations: 27

  • The effect of an isoelectronic Ti–Zr substitution on Heusler nanoprecipitation and the thermoelectric properties of a (Ti0. 2, Zr0. 8) Ni1. 1Sn half-Heusler alloy
    YW Chai, T Oniki, T Kenjo, Y Kimura
    Journal of Alloys and Compounds 662, 566-577 2016
    Citations: 24

  • Stabilizing austenite via a core-shell structure in the medium Mn steels
    X Wan, G Liu, R Ding, N Nakada, YW Chai, Z Yang, C Zhang, H Chen
    Scripta Materialia 166, 68-72 2019
    Citations: 17

  • Improvements in honeycomb abradable seals
    DJ Potter, YW Chai, GJ Tatlock
    Materials at High Temperatures 26 (2), 127-135 2009
    Citations: 14

  • Disconnections and Laves (C14) precipitation in high-Cr ferritic stainless steels
    YW Chai, K Kato, C Yabu, S Ishikawa, Y Kimura
    Acta Materialia 198, 230-241 2020
    Citations: 10

  • Reduced Thermal Conductivity of Mg2(Si, Sn) Solid Solutions by a Gradient Composition Layered Microstructure
    Z Zhou, YW Chai, Y Ikuta, Y Lee, Y Lin, Y Kimura
    ACS Applied Materials & Interfaces 12 (17), 19547-19552 2020
    Citations: 7

  • Nanoscale hierarchical structure of twins in nanograins embedded with twins and the strengthening effect
    H Tang, TFM Chang, YW Chai, CY Chen, T Nagoshi, D Yamane, H Ito, ...
    Metals 9 (9), 987 2019
    Citations: 7

  • Thermoelectric Performance of Half-Heusler TiNiSn Alloys Fabricated by Solid-Liquid Reaction Sintering
    Y Kimura, C Asami, YW Chai, Y Mishima
    Materials Science Forum 654, 2795-2798 2010
    Citations: 7

  • Martensitic transformations in ‘unfamiliar’systems
    RC Pond, YW Chai, S Celotto
    Materials Science and Engineering: A 378 (1-2), 47-51 2004
    Citations: 7

  • Effect of vacancy-site occupation in half-Heusler compound ZrNiSn on phase stability and thermoelectric properties
    Y Kimura, T Tanoguchi, Y Sakai, YW Chai, Y Mishima
    MRS Online Proceedings Library (OPL) 1295 2011
    Citations: 4

  • Effect of Half-Heusler Interfacial Structure on Thermal Transport Properties of (Ti, Zr) NiSn Alloys
    M Sato, YW Chai, Y Kimura
    ACS Applied Materials & Interfaces 13 (21), 25503-25512 2021
    Citations: 3

  • Effects of Si on Phase Stability and Precipitation Behavior of C14 Laves Phase (Fe,Cr)2(Nb,Mo) in High Cr αFe-base Alloys
    Y Kimura, K Kato, YW Chai
    MRS Advances 4 (25-26), 1477-1483 2019
    Citations: 1

  • Evaluation of Microstructure Formation and Phase Equilibria for Thermoelectric β-FeSi2 Composite Alloys
    Y Kimura, H Otani, A Mori, YW Chai
    MRS Advances 2 (26), 1369-1374 2017
    Citations: 1