Design of a V–Ti–Ni alloy with superelastic nano-precipitates

Abstract

Stress-induced martensitic transformations enable metastable alloys to exhibit enhanced strain hardening capacity, leading to improved formability and toughness. As is well-known from transformation-induced plasticity (TRIP) steels, however, the resulting martensite can limit ductility and fatigue life due to its intrinsic brittleness. In this work, we explore an alloy design strategy that utilizes stress-induced martensitic transformations but does not retain the martensite phase. This strategy is based on the introduction of superelastic nano-precipitates, which exhibit reverse transformation after initial stress-induced forward transformation. To this end, utilizing ab-initio simulations and thermodynamic calculations we designed and produced a V45Ti30Ni25 (at%) alloy. In this alloy, TiNi is present as nano-precipitates uniformly distributed within a ductile V-rich base-centered cubic (bcc) β matrix, as well as being present as a larger matrix phase. We characterized the microstructure of the produced alloy using various scanning electron microscopy (SEM) and transmission electron microscopy (TEM) methods. The bulk mechanical properties of the alloy are demonstrated through tensile tests, and the reversible transformation in each of the TiNi morphologies were confirmed by in-situ TEM micro-pillar compression experiments, in-situ high-energy diffraction synchrotron cyclic tensile tests, indentation experiments, and differential scanning calorimetry experiments. The observed transformation pathways and variables impacting phase stability are critically discussed

Thermodynamic and Kinetic Simulations Used for the Study of the Influence of Precipitates on Thermophysical Properties in NiTiCu Alloys Obtained by Spark Plasma Sintering

“…In contrast, Povoden-Karadeniz et al [ 26 ] simplified the description of B2 (BCC_B2), still with a good reproduction of phase boundaries but without the requirement of substitutional vacancies associated with questionable compound energies, proposing the description (Cu, Ni, Ti) 0.5 (Cu, Ni, Ti) 0.5 (Va) 3 . This model was also used recently to describe phase stabilities in the Ni-Ti-V system in agreement with experiments [ 28 ] and can be used as the matrix phase in thermokinetic simulations. Alternatively, for equilibrium and metastable thermodynamic computations, we propose a 4 sublattice description (Ni) 2 (Ti) 2 (Ni) 1 (Ti, Va) 1 for the same phase, allowing for highly Ti-defective regions in the Ti-understoichiometric Ni-Ti base SMA matrix, as proposed by first-principles analysis, the phase named “FP_B2” in the developed open-source thermodynamic database mc_sma [ 29 ].…”