M Kalyan Phani
@opju.ac.in
Director
OP Jindal University
RESEARCH, TEACHING, or OTHER INTERESTS
Metals and Alloys, Waste Management and Disposal, General Engineering, Materials Science
FUTURE PROJECTS
Additive Manufacturing: Laser Metal Deposition
Applications Invited
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
M Kalyan Phani, Victor Pfahl, Chengfu Ma, Anish Kumar, W Arnold, and K Samwer
IEEE
Contact-resonance atomic force microscopy (CR-AFM) has emerged as an advanced scanning probe microscopy technique for characterizing materials with lateral resolutions down to 5 nm. In CR-AFM the AFM cantilever may be excited either by an ultrasonic transducer beneath the sample (AFAM mode) or by the piezoelement on the cantilever chip (UAFM mode). The contact-resonance frequency can be used to derive the tip-sample contact stiffness and in-turn the elastic modulus of the material under examination after proper calibration. The width of the resonance curve can be used to measure the damping of the tip-sample contact. In addition to the internal friction in the material and the air damping, partial slip or sliding of the tip at the surface also contributes significantly to the measured contact damping. The CR curves become nonlinear at high excitation amplitudes while under relatively low contact forces. The transition from linear to nonlinear behavior indicates the onset of mechanical yielding of the contact from sticking to sliding. The sliding depends on the friction coefficient of the tip material, usually Si, against the material examined. The present study aims at understanding the influence of excitation amplitude (causing the tangential force) and applied load (normal force) on the appearance of nonlinearities in the CR curves. The softening behavior of the CRs is analyzed in terms of a Duffing oscillator.
C. Ma, V. Pfahl, Z. Wang, Y. Chen, J. Chu, M. K. Phani, A. Kumar, W. Arnold, and K. Samwer
AIP Publishing
Contact-resonance atomic force microscopy (CR-AFM) has been used to measure the viscoelastic loss tangent of soft materials such as polymers. Usually, the damping is attributed to the dissipation in the contact volume due to internal friction and air damping of the cantilever. However, partial slip or even full sliding can exist in the contact zone under tangential loading, and thus, the accompanying energy dissipation must be taken into account when measuring the damping constant Qloc−1. Here, the stick-to-sliding transition of the tip-sample contact in CR-AFM was studied. Amplitude drops were observed in the resonance curves caused by such stick-to-sliding transitions. The results show that the stick-to-sliding transition arises under small contact forces and large excitation amplitudes. Extra energy loss from full sliding induces large contact damping. The critical lateral displacement needed for the stick-to-sliding transition varies linearly with the contact radius.
V. Pfahl, M. K. Phani, M. Büchsenschütz-Göbeler, A. Kumar, V. Moshnyaga, W. Arnold, and K. Samwer
AIP Publishing
We report on friction measurements on a La0.6Sr0.4MnO3 (LSMO) thin film using atomic force microscopy cantilever contact-resonances. There is a contribution to the damping of the cantilever oscillations, which is caused by micro-sliding of the cantilever tip on the surface of the thin film. This frictional part decreases with temperature parallel to the increase in the resistivity of the thin film. The LSMO is well-known for a ferromagnetic to paramagnetic phase transition that occurs without changes in the rhombohedral (R-3c) crystalline structure. The magnetic transition at the Curie temperature TC ∼ 360 K is accompanied by a metal-to-metal transition with a large increase in electrical resistivity. The behavior of the cantilever damping constant demonstrates that there is a direct coupling between mechanical friction and the mobility of the electrons in the LSMO film.
M Kalyan Phani, Anish Kumar, T Jayakumar, Walter Arnold, and Konrad Samwer
Beilstein Institut
The distribution of elastic stiffness and damping of individual phases in an α + β titanium alloy (Ti-6Al-4V) measured by using atomic force acoustic microscopy (AFAM) is reported in the present study. The real and imaginary parts of the contact stiffness k* are obtained from the contact-resonance spectra and by using these two quantities, the maps of local elastic stiffness and the damping factor are derived. The evaluation of the data is based on the mass distribution of the cantilever with damped flexural modes. The cantilever dynamics model considering damping, which was proposed recently, has been used for mapping of indentation modulus and damping of different phases in a metallic structural material. The study indicated that in a Ti-6Al-4V alloy the metastable β phase has the minimum modulus and the maximum damping followed by α′- and α-phases. Volume fractions of the individual phases were determined by using a commercial material property evaluation software and were validated by using X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) studies on one of the heat-treated samples. The volume fractions of the phases and the modulus measured through AFAM are used to derive average modulus of the bulk sample which is correlated with the bulk elastic properties obtained by ultrasonic velocity measurements. The average modulus of the specimens estimated by AFAM technique is found to be within 5% of that obtained by ultrasonic velocity measurements. The effect of heat treatments on the ultrasonic attenuation in the bulk sample could also be understood based on the damping measurements on individual phases using AFAM.
Lei Gu, Bulbul Chakraborty, P. L. Garrido, Mohan Phani, and J. L. Lebowitz
American Physical Society (APS)
We have studied the compressible antiferromagnetic Ising Model on a triangular lattice using Monte Carlo simulations. It is found that the coupling to the strain removes the frustration of the rigid model and the simulations show a transition from the disordered to an ordered, striped phase at low temperatures. This transition involves two broken symmetries: the Ising symmetry and a three-state Potts symmetry characteristic of the triangular lattice. In the absence of bond fluctuations, this transition is always strongly first order. Using finite-size scaling analysis, we find evidence that, in the presence of fluctuations, the transition becomes weakly first order and possibly second order when the coupling to the lattice is increased. We discuss the relevance of this model to certain phase transitions in alloys. @S0163-1829~96!04918-1#
Daniel F. Styer, Mohan K. Phani, and Joel L. Lebowitz
American Physical Society (APS)
Joel L. Lebowitz, Mohan K. Phani, and Daniel F. Styer
Springer Science and Business Media LLC
M K Phani and D Dhar
IOP Publishing
It is shown that for continuum percolation with overlapping discs having a distribution of radii, the net areal density of discs at percolation threshold depends non-trivially on the distribution, and is not bounded by any finite constant. Results of a Monte Carlo simulation supporting the argument are presented.
B M Arora, M Barma, D Dhar, and M K Phani
IOP Publishing
The authors study the bulk conductivity of a random network of diodes and insulators as a function of the concentration of diodes p near the percolation threshold. The network considered is a square lattice and the diodes are oriented along the positive x or y axes. The results of the analogue and digital simulations show that the conductivity in the easy direction varies as (p-pc)t, where pc is the critical probability for directed percolation, and t=0.73+or-0.10.
M K Phani and D Dhar
IOP Publishing
The authors investigate the applicability of the real-space renormalisation group technique to the directed percolation problem in two dimensions. They formulate decimation transformations that take into account the existence of two length scales in the problem. It is found that the values of critical parameters obtained are unreliable, as different approximate renormalisations give widely differing values.
D Dhar, M K Phani, and M Barma
IOP Publishing
Studies the problem of directed site animals on the square, triangular and hexagonal lattices. Closed form expressions are proposed for A(s), the number of animals of size s, on the square and triangular lattices. These expressions have been checked for s<or=33 and s<or=10 for the square and triangular lattices respectively by explicit enumeration. They imply that A(s) varies as lambda 2 s-0 for large s, where lambda =3 for the square lattice, and lambda =4 for the triangular, and theta =1/2 for both. For the hexagonal lattice, A(s) is found for s<or=48. The results are consistent with lambda =2.0252+or-0.0005 and theta =1/2.
Kurt Binder, Joel L. Lebowitz, Mohan K. Phani, and Malvin H. Kalos
Elsevier BV
Deepak Dhar, Mustansir Barma, and Mohan K. Phani
American Physical Society (APS)
It is shown that the percolation problem with blocked, one-way, or two-way bonds is self-dual on a square lattice. The usual directed percolation (blocked or one-way bonds) is dual to a simpler process with one- or two-way bonds. The results of a Monte Carlo simulation of the latter are reported and an improved bound on the critical probability is derived. The corresponding resistance problem in which circuit elements have different forward and backward resistances is also shown to be self-dual.
Paramdeep S. Sahni, Gregory Dee, J. D. Gunton, M. Phani, Joel L. Lebowitz, and M. Kalos
American Physical Society (APS)
We present results of a Monte Carlo study of the time development of a two-dimensional order-disorder model binary alloy following a quench to low temperature from a disordered, high-temperature state. The behavior is qualitatively quite similar to that seen in a recent study of a three-dimensional system. The structure function exhibits a scaling of the form K/sup 2/(t)S(k,t) = G(k/K(t)) where the moment K(t) decreases with time approximately like t/sup -1/2/. If one interprets this moment as being inversely proportional to the domain size, the characteristic domain growth rate is proportional to t/sup -1/2/. Additional insight into this time evolution is obtained from studying the development of the short-range order, as well as from monitoring the growth of a compact ordered domain embedded in a region of opposite order. All these results are consistent with the picture of domain growth as proposed by Lifshitz and by Cahn and Allen.
M. K. Phani, Joel L. Lebowitz, M. H. Kalos, and O. Penrose
American Physical Society (APS)
Novel computer simulations are described of the time evolution of an ordering model binary alloy following quenching from a disordered state at a high temperature. The results are interpreted with use of ideas of Lifshitz and of Cahn and Allen; the ordering process is described by a kinetic equation for the motion of the walls separating domains with different orderings. The characteristic length increases as t/sup 1/2/, and the structure function scales as kt/sup 1/2/.
M. K. Phani, Joel L. Lebowitz, and M. H. Kalos
American Physical Society (APS)
We have investigated, via Monte Carlo computations, the phase diagram of an ordering binary alloy-equivalent to an Ising spin system-on an fcc lattice with nearest- and next-nearest-neighbor pair interactions $H=J\\ensuremath{\\Sigma}{\\mathrm{nn}}^{}{\\ensuremath{\\sigma}}_{i}{\\ensuremath{\\sigma}}_{j}\\ensuremath{-}\\ensuremath{\\alpha}J\\ensuremath{\\Sigma}{\\mathrm{nnn}}^{}{\\ensuremath{\\sigma}}_{i}{\\ensuremath{\\sigma}}_{j}$, ${\\ensuremath{\\sigma}}_{i}=\\ifmmode\\pm\\else\\textpm\\fi{}1$, $Jg0$. Our studies indicate that this system undergoes a first-order transition; i.e., there is a discontinuity in the energy and order parameters as a function of temperature, for values $\\ensuremath{-}1\\ensuremath{\\lesssim}\\ensuremath{\\alpha}\\ensuremath{\\lesssim}0.25$. For larger values of $|\\ensuremath{\\alpha}|$ the transition appears to be continuous, without any metastable states. Our results are in good agreement with Kikucki's cluster variation method at the two values of $\\ensuremath{\\alpha}$ at which it has been applied, namely, 0 and -0.25. For $\\ensuremath{\\alpha}\\ensuremath{\\lesssim}\\ensuremath{-}0.5$ renormalization-group arguments strongly indicate that the transition is first order. If this is so, then our results indicate that the discontinuities for $\\ensuremath{\\alpha}l\\ensuremath{-}1$ must be very small. The nature of the ground states changes at $\\ensuremath{\\alpha}=0 \\mathrm{and} \\ensuremath{-}0.5$. At these values of $\\ensuremath{\\alpha}$ the ground states are infinitely degenerate. The structure of the low-temperature phases, at all values of $\\ensuremath{\\alpha}$, is discussed.
Mohan K. Phani, Joel L. Lebowitz, M. H. Kalos, and C. C. Tsai
American Physical Society (APS)
M. J. Sablik, M. Cook, N. Vuong, L. Phani, M. Zawonski, and M. Phani
AIP Publishing
The cubic model, described by the Hamiltonian ⋅=−JΣ&lt;ij≳ σiσjδαiδαj where σi=±1 and αi=x, y, or z, is subjected to a Monte Carlo simulation. For an 8×8×8 fcc lattice, it is found that the system exhibits a first order transition at kBTo/J=3.45±0.01 in fairly close agreement with results obtained previously by the Kikuchi approximation. Discontinuities at the phase transition and stability limits for the metastable states on either side of the phase transition are also investigated.
Mohan K. Phani, Joel L. Lebowitz, M. H. Kalos, and C. C. Tsai
American Physical Society (APS)
We report results of computer simulations of a binary alloy on an fcc lattice, equivalent to an Ising system with a nearest-neighbor antiferromagnetic interaction J > 0 and a next-nearest-neighbor ferromagnetic interaction -..cap alpha..J, ..cap alpha.. > 0. Our data indicate the existence of a discontinous change in energy and in sublattice magnetization as a function of temperature, for small ..cap alpha... For ..cap alpha.. > or approx. = 0.25, the transition appears to be continuous suggesting a tricritical point at some intermediate ..cap alpha...