Vijay Kumar Sutrakar

Coupled effect of size, strain rate, and temperature on the shape memory of a pentagonal Cu nanowire

A body-centered pentagonal nanobridge structure with lattice constants c = 2.35 and a = 2.53 A has been observed under high strain rate tensile loading on an initially constrained [Formula: see text] Cu nanowire at various temperatures. Extensive molecular dynamics (MD) simulations have been performed using the embedded atom method (EAM) for cross-sectional dimensions ranging from 0.723 x 0.723 to 2.169 x 2.169 nm(2), temperature ranging from 10 to 600 K, and strain rates of 10(9)-10(7) s(-1). Formations of such pentagonal nanowire are observed for a temperature range 200-600 K for particular cross-sectional dimensions and strain rates. A large inelastic deformation of approximately 50% is obtained under both isothermal loading and adiabatic loading. With very high degree of repeatability of such pentagonal nanowire formation, the present findings indicate that the interesting stability property and high strength of elongated nanowires have various potential applications in nanomechanical and nanoelectronic devices. Further, we demonstrate a novel thermomechanical unloading mechanism by which it is possible to impart recovery from a pentagonal nanowire following a hysteresis loop: [Formula: see text].

Formation of stable ultra-thin pentagon Cu nanowires under high strain rate loading

Molecular dynamics (MD) simulations of � 100� /{100} Cu nanowires at 10 K with varying cross-sectional areas ranging from 0.3615 × 0.3615 nm 2 to 2.169 × 2.169 nm 2 have been performed using the embedded atom method (EAM) to investigate their structural behaviors and properties at high strain rate. Our studies reported in this paper show the reorientation of � 100� /{100} square cross-sectional Cu nanowires into a series of stable ultra-thin pentagon Cu nanobridge structures with diameter of ∼1 nm under a high strain rate tensile loading. The strain rates used for the present studies range from 1 × 10 9 to 0.5 × 10 7 s −1 . The pentagonal multi-shell nanobridge structure is observed for cross-sectional dimensions <1. 5n m. From these results we anticipate the application of pentagonal Cu nanowires even with diameters of ∼1 nm in nano-electronic devices. A much larger plastic deformation is observed in the pentagonal multi-shell nanobridge structure as compared to structures that do not form such a nanobridge. It indicates that the pentagonal nanobridge is stable. The effect of strain rate on the mechanical properties of Cu nanowires is also analyzed and shows a decreasing yield stress and yield strain with decreasing strain rate for a given cross-section. Also, a decreasing yield stress and decreasing yield strain are observed for a given strain rate with increasing cross-sectional area. The elastic modulus is found to be ∼100 GPa and is independent of strain rate effect and independent of size effect for a given temperature. (Some figures in this article are in colour only in the electronic version)

Stress-induced phase transformation and pseudo-elastic/pseudo-plastic recovery in intermetallic Ni―Al nanowires

Extensive molecular dynamics (MD) simulations have been performed in a B2-NiAl nanowire using an embedded atom method (EAM) potential. We show a stress induced [Formula: see text]-centered-tetragonal (BCT) phase transformation and a novel temperature and cross-section dependent pseudo-elastic/pseudo-plastic recovery from such an unstable BCT phase with a recoverable strain of approximately 30% as compared to 5-8% in polycrystalline materials. Such a temperature and cross-section dependent pseudo-elastic/pseudo-plastic strain recovery can be useful in various interesting applications of shape memory and strain sensing in nanoscale devices. Effects of size, temperature, and strain rate on the structural and mechanical properties have also been analyzed in detail. For a given size of the nanowire the yield stress of both the B2 and the BCT phases is found to decrease with increasing temperature, whereas for a given temperature and strain rate the yield stress of both the B2 and the BCT phase is found to increase with increase in the cross-sectional dimensions of the nanowire. A constant elastic modulus of approximately 80 GPa of the B2 phase is observed in the temperature range of 200-500 K for nanowires of cross-sectional dimensions in the range of 17.22-28.712 A, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in the temperature.

Comment on “Surface stress induced structural transformations and pseudoelastic effects in palladium nanowires” [Appl. Phys. Lett. 93, 093108 (2008)]

Recently, Lao and Moldovan have reported that 100 Pd nanowire of cross-sectional dimensions 2.18 2.18 nm2 undergoes spontaneous reversible phase transformation from face-centered-cubic fcc to body-centeredtetragonal bct structure, provided that temperature is above a critical value of 22.5 K. The results reported by Lao and Moldovan have been re-examined in the present paper, which is due to inconsistent results reported for Pd nanowires in their study as compared to other FCC metal nanowires. For example, Diao et al. have shown that surface stress alone causes fcc→bct phase transformation in 100 gold nanowire of cross-sectional dimensions 2.0 2.0 nm2 during the energy minimization process i.e., T =0 K . Lao and Moldovan reported that Pd nanowire of cross-sectional dimensions 2.18 2.18 nm2 do not show fcc→bct phase transformation during energy minimization, instead a critical temperature of 22.5 K is reported for such phase transformation. We believe that Pd nanowire of smaller cross-sectional size 2.0 2.0 nm2 approximately should be unstable in nature instead of metastable as reported by Lao and Moldovan due to very high surface stresses and it should get transforms into a stable structure via spontaneous phase transformation during the energy minimization process, as observed in other fcc metal nanowire. Further to confirm this, we have performed extensive molecular dynamics simulations using embedded-atommethod potential of Foils et al. The same potential was considered by Lao and Moldovan. We found that Pd nanowires of cross-sectional size 2.0 2.0 nm2 show phase transformation during energy minimization process itself, as shown in Figs. 1 a and 1 b . The results shown here are contradicting the result reported in Ref. 1, whereas our present results show similar trends as reported by Diao et al. for gold nanowires. Snapshot a in Fig. 1 shows an initial 100 / 100 Pd nanowire, which show phase transformation during the energy minimization process leading to Fig. 1 b . Interestingly, we observed that the newly transformed structure is hexagonally closed packed hcp instead of bct structure that was reported by Lao and Moldovan see Fig. 1 and 2 of Ref. 1 . It is also important to mentioned here that some of the nanowires having cross-sectional dimensions less than 2.0 2.0 nm2 shows 100 / 100 to 110 / 111 lattice reorientation during the energy minimization process. Whereas Lao and Moldovan have mentioned that such lattice reorientation is only possible above cross-sectional dimensions of 2.18 2.18 nm2, which contradict our present results. Such 100 / 100 to 110 / 111 lattice reorientation for an initial cross-sectional dimensions of 0.9875 0.9875 nm2 i.e., 2.5 2.5 atomic lattice units is shown in Fig. 1 c . Lao and Moldovan have also mentioned that Pd nanowire with larger cross-sectional dimensions such as 2.57 2.57 nm2 shows 100 to 110 / 111 reorientation at 100 K. In other studies, i.e., in gold and copper nanowires, 100 to 110 / 111 reorientation is also reported by Diao et al. and Liang et al., respectively. In fact, it has been established now that 100 to 110 / 111 spontaneous reorientation de-

Comment on "Pseudoelasticity of Cu-Zr nanowires via stress-induced martensitic phase transformations" †Appl. Phys. Lett. 95, 021911 "2009…‡

Recently, a novel stress-induced phase transformation in an initial ⟨100⟩/{100} B2-CuZr nanowire has been reported for the first time [Sutrakar and Mahapatra, Mater. Lett. 63, 1289 (2009)]. Following this, a martenisitic phase transformation in Cu–Zr nanowire was shown [Cheng et al., Appl. Phys. Lett. 95, 021911 (2009)] using the same idea [Sutrakar and Mahapatra, Mater. Lett. 63, 1289 (2009)]. The pseudoelastic recovery of the bct phase of Cu–Zr by unloading has also been shown [Cheng et al., Appl. Phys. Lett. 95, 021911 (2009)]. They also tested the epitaxial bain path [Alippi et al., Phys. Rev. Lett. 78, 3892 (1997)] and reported that the bct phase in the nanowire is metastable, whereas the bulk counterpart is unstable. This aspect is re-examined in this comment with corrected results.

Modeling of Cohesive Zone and Crack Growth in Ni-Al Thin-Film Using MD-XFEM Based Approach

Intermetallic alloys of Ni-Al have important applications in high temperature anti-corrosive coatings, engine and turbine related materials, and shape memory devices. Predicting failure behavior of these materials is difficult using purely continuum model, since several of the material constants are complicated functions of micro and nano-scale details. This includes solid-solid phase transformation. In the present paper, a framework for analyzing fracture in two-dimensional planar domain is developed using a molecular dynamic (MD) simulation and extended finite element method (XFEM). The framework is then applied to simulate fracture in Ni-Al thin-film. Effect of Ni Al crystallites of various sizes on the mechanical properties is analyzed using direct MD simulations. Initiation and growth of crack under slow (quasi-static) tensile loading in mode-I condition is considered. Mechanical properties at room temperature are estimated via MD simulations, which are further used in the XFEM at the continuum scale. A cohesive zone model for the macroscopic XFEM model is implemented, which directly bridges the molecular length-scale via MD framework. Numerical convergence studies are reported for mode-I crack in initially single crystal B2 Ni-Al thin film.Copyright