Structure and Multistage Martensite Transformation in Nanocrystalline Ti-50.9Ni Alloy

Tamara M. Poletika,Svetlana L. Girsova,Aleksander I. Lotkov,Andrej N. Kudryachov,Natalya V. Girsova

doi:10.3390/met11081262

Abstract

An electron microscopic study of the evolution of the size, morphology, and spatial distribution of coherent Ti3Ni4 particles with a change in the aging temperature in a nanocrystalline (NC) Ti-50.9 at % Ni alloy with an inhomogeneous grain–subgrain B2-austenitic nanostructure has been carried out. It was found that with an increase in the aging temperature, along with a change in the size and shape of Ti3Ni4 nanoparticles, their spatial distribution changes from location at dislocations to precipitates at subboundaries. Research has shown that the presence of different types of internal interfaces in the nanostructure contributes to the heterogeneous distribution of coherent Ti3Ni4 nanoparticles in the volume of the B2 matrix, which is associated with the precipitation of particles in the region of low-angle subboundaries and the suppression of the Ti3Ni4 precipitation in nanograins with high-angle boundaries. The difference in the structural-phase state of nanograins and subgrains regions is the main reason for the implementation of the anomalous R-phase transformation effect in the sequence of multistage martensitic transformations B2↔R↔B19′.

Highlights

Nanocrystalline TiNi alloys with unique properties of shape memory effect and superelasticity, due to the reversible transformation between B2-austenite and B190 martensite, have high strength and functional stability and are widely used in medicine [1,2]
There are two types of nanograins: (i) dislocation-containing nanograins formed under severe deformation and further annealing and (ii) dislocationfree nanograins crystallized from amorphous state [11]
Aging at different temperatures is found to provide coherent precipitation of Ti3 Ni4 particles differing in size, morphology, and spatial distribution in grain–subgrain structure and cause different variations to the transformation behavior of NC Ti-50.9Ni alloy

Summary

Introduction

Nanocrystalline TiNi alloys with unique properties of shape memory effect and superelasticity, due to the reversible transformation between B2-austenite and B190 martensite, have high strength and functional stability and are widely used in medicine [1,2] Their representatives in biomedicine are Ni-rich TiNi alloys, which are aged with the formation of coherent Ti3 Ni4 particles [1,2,3]. Such precipitates in TiNi polycrystals can change the path of martensitic transformations from B2→B190 to the sequence of transformations through the transition R-phase: B2→R→B190 [3,4,5]. The study of NC TiNi alloys is complicated by the necessity of accounting for grain refinement and associated increase in critical martensite shear stresses [8], possible suppression of B2-austenite solid solution decomposition in nanograins [9], and high defect density in the nanostructure after severe deformation [10]

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