First-principles characterization of reversible martensitic transformations

Abstract

Reversible martensitic transformations (MTs) are the origin of many fascinating phenomena, including the famous shape memory effect. In this work, we present a fully ab initio procedure to characterize MTs in alloys and to assess their reversibility. Specifically, we employ ab initio molecular dynamics data to parametrize a Landau expansion for the free energy of the MT. This analytical expansion makes it possible to determine the stability of the high-and low-temperature phases, to obtain the Ehrenfest order of the MT, and to quantify its free energy barrier and latent heat. We apply our model to the high-temperature shape memory alloy Ti-Ta, for which we observe remarkably small values for the metastability region (the interval of temperatures in which the highand low-temperature phases are metastable) and for the barrier: these small values are necessary conditions for the reversibility of MTs and distinguish shape memory alloys from other materials.A martensitic transformation (MT) [1] is a diffusionless phase transition, triggered by temperature or stress, that changes the symmetry of a high-temperature phase (austenite) and forms variants of a low temperature phase (martensite). Most of the MTs are irreversible, as dislocations, shear, and plastic deformation accumulate during the transformation. However, if the symmetry of martensite is lower than that of austenite and if the variations in lattice parameters and atomic volumes are small, the MT can be reverted, that is, the system can be switched between the two phases with small latent heat [2][3][4][5]. Reversible MTs in metals or polymers are appealing as they often result in the shape memory effect, the ability to recover a predetermined shape upon heating, and pseudoelasticity, the capacity to accommodate large deformations without plasticity [6][7][8]. Other examples in which reversible MTs are important include the recently discovered gum metals [9], where metastable phases have been observed to form via reversible transformations [10].An urgent technological challenge for actuator and biomedical applications is to identify alloys that exhibit reversible MTs that are stable during operational cycles. With very few exceptions [11], first principles investigations aiming to clarify the mechanisms underlying a MT generally rely on static, T = 0 K calculations. These, however, are often inadequate to describe the atomistic processes responsible for the dynamic and/or thermodynamic stabilization of the austenite phase at finite temperatures, as well as the interval of temperatures in which austenite and martensite are metastable (metastability region), the free energy barrier, the latent heat, and even the Ehrenfest order of a MT.To overcome these limitations we have employed ab initio molecular dynamics (aiMD) simulations to access structural properties at finite temperature, and combined our ab initio data with a 2-4-6 Landau-Falk expansion of the free energy [12, 13] to characterize the nature of reversible MTs and suggest necessary conditions to dis...

Discovery ofω-free high-temperature Ti-Ta-Xshape memory alloys from first-principles calculations

“…To determine the region in the (c Ta , c X ) space where the transformation temperatures are higher than 100 • C, we have calculated the 0 K energy difference between the β and α phases ∆E (β−α ) (c Ta , c X ). In Ti-Ta based alloys, the entropy difference between the two phases depends very weakly on c Ta and c X 32 and can be assumed to be constant. Therefore, the 0 K energy difference is usually sufficient to estimate the much more computationally expensive free energy difference between austenite and martensite.…”

“…A possible alternative to Ni-Ti as high-temperature shape memory alloys (HTSMAs) 14,15 are Ti-Ta alloys [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] . In these alloys, the transformation temperatures A s and M s increase with decreasing Ta concentration c Ta , and can be as high as 430 • C when c Ta is reduced to 20 at.% 30 .…”

A general model for the crystal structure of orthorhombic martensite in Ti alloys

“…A wide array of experimental (Li et al, 2013;Gutié rrez Moreno et al, 2017;Bö nisch et al, 2014;Dobromyslov & Elkin, 2006;Moffat & Larbalestier, 1988;Elmay et al, 2017;Hao et al, 2018;Kim et al, 2013) and theoretical (ab initio) (Li et al, 2019;Ferrari et al, 2019;Gutié rrez Moreno et al, 2017;Li et al, 2013) studies bear witness to the relevance of the $ 00 MT in Ti alloys. However, despite the large amount of data gathered, a unified understanding of the crystal structure of 00 across different Ti alloys has not been reached to date.…”

First-Principles Calculations of High-Pressure Physical Properties of Ti0.5Ta0.5 Alloy

“…Behera et al [13] measured the enthalpy change of Ti-xTa alloys at the temperatures ranging 463 to 1257 K, and then clearly revealed that the enthalpy came mainly from the contributions of two parts: (i) one part from untransformed α and coexisting β phases, (ii) another part from the diffusional phase transformation from α phase to β phase. Furthermore, plenty of studies on TiTa alloys were also carried out by others [14][15][16][17].…”