Abstract
Phase transitions in nickel-titanium shape-memory alloys are investigated by means of atomistic simulations. A second nearest-neighbor modified embedded-atom method interatomic potential for the binary nickel-titanium system is determined by improving the unary descriptions of pure nickel and pure titanium, especially regarding the physical properties at finite temperatures. The resulting potential reproduces accurately the hexagonal-close-packed to body-centered-cubic phase transition in Ti and the martensitic $\mathrm{B}2\text{\ensuremath{-}}\mathrm{B}{19}^{\ensuremath{'}}$ transformation in equiatomic NiTi. Subsequent large-scale molecular-dynamics simulations validate that the developed potential can be successfully applied for studies on temperature- and stress-induced martensitic phase transitions related to core applications of shape-memory alloys. A simulation of the temperature-induced phase transition provides insights into the effect of sizes and constraints on the formation of nanotwinned martensite structures with multiple domains. A simulation of the stress-induced phase transition of a nanosized pillar indicates a full recovery of the initial structure after the loading and unloading processes, illustrating a superelastic behavior of the target system.
Highlights
Shape-memory alloys are a class of materials with the property of recovering their original shape upon heat treatment and of sustaining large elastic strains
We have developed a potential based on the second nearest-neighbor (2NN) modified EAM (MEAM) model [14,15,16] and applied it to study the phase transitions of NiTi shape-memory alloys
We found that a stabilization of the hexagonal (ω) phase, a ground state phase predicted by density functional theory (DFT) [24], over the hcp (α) phase always resulted in a significant worsening of the properties of the hcp (α) phase in particular in a negative vacancy formation energy
Summary
Shape-memory alloys are a class of materials with the property of recovering their original shape upon heat treatment (shape-memory effect) and of sustaining large elastic strains (superelasticity). We have developed a potential based on the second nearest-neighbor (2NN) MEAM model [14,15,16] and applied it to study the phase transitions of NiTi shape-memory alloys. The most serious problem of the previously developed Ti potential [21] is that it cannot reproduce the phase transition between body-centered-cubic (bcc) Ti (β-austenite phase) and hexagonal-close-packed (hcp) Ti (α-martensite phase), which is closely related to the transition mechanism in the NiTi shape-memory alloy Both systems (pure Ti and NiTi) have very similar structures of entropically stabilized austenite phases (bcc in Ti and B2 in NiTi) and similar transition paths related to the imaginary phonon modes of the austenite phases [22,23,24,25]. The development of potentials in the present paper proceeds in a systematic manner: Accurate potentials for the pure Ni and Ti systems are developed first, and the development of a binary potential for the Ni-Ti system is subsequently addressed
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