Shock-induced martensitic transformations in near-equiatomic NiTi alloys

Abstract

Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M s ) temperatures. The NiTi-I alloy (M s Ϸ ϩ27 ЊC) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of ''stress-assisted'' martensite. The NiTi-II alloy (M s Ϸ Ϫ45 ЊC) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensilestress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of ''strain-induced'' martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low M s temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.

Mechanisms of Shock Strength Exhibited by a Nickel‐Rich Nickel‐Titanium‐Hafnium Alloy

“…This hypothesis is further supported by considering that in previous work studying NiTi alloys that were not precipitate strengthened, martensite was retained within similar intragranular microband features. [28] This hypothesis is also consistent with the bilinear release response seen in Figure 3b. Initially, elastic unloading of the loaded stress-induced martensite structure occurs, then, elastic unloading is accompanied by reverse martensitic transformation and/or plastic release.…”

Atomistic study of temperature and strain rate-dependent phase transformation behaviour of NiTi shape memory alloy under uniaxial compression

“…It exhibits SE while the test temperature is close to the austenite finish temperature (A f ) [8][9][10][11][12][13][14]. A considerable body of experiments has been conducted on NiTi alloys to investigate its strain rate and temperature-dependent phase transformation behaviour [11,12,[14][15][16][17][18][19]. Nemat-Nasser et al [8,9] studied the dynamic behaviour of NiTi at various strain rates in a range of 10 −3 to about 2 × 10 4 s −1 and at temperatures in a range of 77-400 K. The results showed that the SE behaviour of NiTi was more sensitive to temperature than to strain rate.…”

Shock compression of reactive powder mixtures

“…martensitic transformations and crystal to amorphous phase transitions have also been studied. [11][12][13][14][15][16][17][18][19][20][21] Solid state structural phase transformations from low to high or from high to low density states have been considered to occur via reconstructive or displacive mechanisms. 22 Such displacive phase transformations involve coordinated shifts of atoms comprising of homogeneous strain and shuffle, and hence, are favoured under high pressure, while reconstructive transformations are less likely to occur at high pressures due to the reduced mobility of atoms.…”

“…15 SEM micrographs of recovered Nb+Si compacts revealing (a) an unreacted portion, (b) the transition zone between unreacted and partially-reacted material, (c) a partially-reacted region, and (d) the transition zone between partially-reacted and fully-reacted material. The reaction product consists of NbSi 2 nodules, which are concentrated near the Nb particle interfaces (from Vecchio et al 100 ).…”