Microstructural and Mechanical Stability of a Ti-50.8 at.% Ni Shape Memory Alloy Achieved by Thermal Cycling with a Large Number of Cycles

Abstract

The influence of thermal cycling (TC) with a large number of cycles on the microstructure, the parameters of martensitic transformations (MTs), and the mechanical properties of a Ti-50.8 at.% Ni shape-memory alloy in coarse-grained (CG) and ultrafine-grained (UFG) states was investigated. The effect of microstructural and mechanical stability was found in both coarse-grained and ultrafine-grained states starting from the 100th cycle of martensitic transformations. In addition, an unusual temperature change was observed in martensitic transformations occurring with the formation of an intermediate R phase.

Severe Plastic Deformation and Thermomechanical Processing: Nanostructuring and Properties

“…Thirteen research papers have been published in this Special Issue of Metals. The papers cover a wide spectrum of topics, including (i) deformation mechanisms, the mechanical properties and fatigue behaviour of SPD-processed materials [1][2][3][4][5][6][7], (ii) microstructural and mechanical stability at elevated temperatures [8,9], phase formations in alloys [10], the improvement of physical properties (electrical conductivity [11], magneto-resistance [12]) and biomedical properties (biocorrosion resistance [6], biocompatibility [13]) by SPD.…”

“…However, a similar decomposition pathway was found. Churakova and Gunderov [9] analysed the influence of thermal cycling on the microstructural and mechanical stability of a Ti-Ni shape memory alloy, which was processed by ECAP, and compared it to a coarse-grained, undeformed counterpart. It was found that the ECAP-processed alloy is more attractive for applications, due to its higher level of properties compared to the coarse-grained state, and its higher stability during thermal cycling.…”

Crystal Structure Defects in Titanium Nickelide after Abc Pressing at Lowered Temperature

“…The abc pressing [4,5] and equal channel angular pressing (ECAP) [1,[6][7][8] are promising methods for obtaining massive semifinished products from TiNi-based alloys with an ultrafine-grained (UFG) structure. The regularities and features of microstructure evolution in TiNi-based alloys under the action of SPD at 623-773 K are studied in a number of works: [9][10][11][12][13][14][15][16][17][18][19][20][21] after ECAP and [4,5,22,23] after abc pressing. The effect of grain-subgrain structure refinement on the mechanical and inelastic properties of these alloys after ECAP was studied in [9][10][11][12][13][14][15][16][17][18][19][20][21] and after abc pressing in [22][23][24][25][26].…”

“…The regularities and features of microstructure evolution in TiNi-based alloys under the action of SPD at 623-773 K are studied in a number of works: [9][10][11][12][13][14][15][16][17][18][19][20][21] after ECAP and [4,5,22,23] after abc pressing. The effect of grain-subgrain structure refinement on the mechanical and inelastic properties of these alloys after ECAP was studied in [9][10][11][12][13][14][15][16][17][18][19][20][21] and after abc pressing in [22][23][24][25][26]. It was found that these methods can achieve a refinement of the grain-subgrain structure up to 100-500 nm, depending on the pressing temperature.…”

“…The MT temperatures during the cooling and heating of samples of these alloys decrease by 20 degrees or more after ECAP at 623-773 K [1,8,14,15,27], and after abc pressing, they either do not change or change only slightly [22][23][24][25]. It is shown in [11] that the dislocation density in Ti 50 Ni 50 and Ti 49.2 Ni 50.8 alloys after ECAP equals 10 15 -5.3•10 15 m −2 . A higher dislocation density (10 17 -10 18 m −2 ) was found in the transition region between crystalline and amorphous phases after cold rolling of Ti 49.2 Ni 50.8 alloy [28].…”