Mechanisms of Microstructure Evolution in TiNi-Based Alloys under Warm Deformation and its Effect on Martensite Transformations

Abstract

In the work, we studied the regularities and mechanisms of microstructure formation in binary TiNi alloys with 50.2 and 50.8 at.% Ni under warm abc pressing with a stepped decrease in strain temperature (873, 673, 623, and 573 К) and isothermal (723 К) multipass caliber rolling. In the TiNi alloy with 50.2 at.% Ni at all abc pressing stages, microstructures inhomogeneous in grain size were formed due to faster dynamic recrystallization and hence faster formation of finer grains and subgrains in strain localization bands compared to microvolumes bounded by these bands. After final abs pressing at 573 К with a total true strain е = 7.7, a microstructure composed of submicroand nanocrystalline grains and subgrains was found. In the TiNi alloy with 50.8 at. % Ni subjected to warm rolling, three stages of grain structure evolution were revealed. At the first stage with low strains, the average grain size ‹d› increased due to collective dynamic recrystallization. At the second and third rolling stage, the average grain size ‹d› decreased steeply due to discontinuous and continuous dynamic recrystallization. On rolling at е = 2.0, a microstructure composed of microand submicrocrystalline grains was formed. An algorithm was proposed for estimating the critical strain for the onset of dynamic recrystallization from dependences of grain sizes on true strain. The sequences and temperatures of martensite transformations from a cubic В2 phase to rhombohedral R and monoclinic В19′ martensite phases were studied depending on the strain accumulated in abc pressing and rolling.

Deformation Behavior, Structure, and Properties of an Aging Ti-Ni Shape Memory Alloy after Compression Deformation in a Wide Temperature Range

“…4 , [11][12][13][14][15][16][17][18][19] To achieve the highest special (functional) properties, severe plastic deformation (SPD) can be applied based on methods such as equal-channel angular pressing, MaxStrain, abcpressing, and cold multipass rolling, since it can form an ultrafine-grained (UFG) structure. [20][21][22][23][24][25][26][27][28] The expansion of the application scope of these alloys as well as the complexity of their structure and intelligent devices based on SME and SE is accompanied by increased requirements on their properties. This leads to the necessity to optimize their manufacturing processes, which requires more detailed knowledges about the deformation and structure formation features of the material.…”

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

“…The TiNi-based alloys exhibit shape memory effects (SME) and superelasticity and belong to the class of intelligent materials that have found application in engineering and medicine [3]. 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.…”

Effect of abc Pressing at 573 K on the Microstructure and Martensite Transformation Temperatures in Ti49.8Ni50.2 (at%)

“…Research data are also available on the grain-subgrain structure of Ti 49.8 Ni 50.2 specimens exposed to abc pressing (multi-axial forging) with their true strain set from 2.2 to 1.5 or equal to ≈3.6 at each step of the strain temperature successively decreased from 873 K to 573 K [15][16][17][18]. Actually, such pressing conditions mean that each temperature step of abc pressing is applied to specimens with a different initial grain-subgrain structure.…”