Annealing behaviour of nanocrystalline NiTi (50 at% Ni) alloy produced by high-pressure torsion

Abstract

International audienceAn equiatomic nanocrystalline NiTi alloy, deformed by high-pressure torsion (HPT) was investigated. The as-prepared bulk NiTi alloy consists of both amorphous and nanocrystalline phases. Crystallization and structural changes during annealing were investigated by differential scanning calorimetry (DSC), X-ray diffraction and transmission electron microscopy. The DSC thermograms and X-ray analyses reveal stress relaxation and partial crystallization below 500 K while grain growth of the nanocrystals occurs predominantly after heating to temperatures above 573 K. Along with the amorphous phase crystallization, a continuous growth of pre-existing nanocrystals that are retained after HPT is observed. The DSC signals observed during continuous heating experiments indicate an unusually large separation between the crystallization and the growth stages. A detailed analysis of the evolution of the enthalpy release upon annealing reveals reproducibly non-monotonous trends with annealing temperature that cannot be explained solely with nucleation and growth of crystalline volume fractions. Instead, the results can be rationalized by assuming a reverse amorphization process occuring during annealing at 523 K. This behavior, that also causes a large variation of the nanocrystals size after annealing at higher temperatures, is discussed with respect to the nanoscale microstructural heterogeneity after initial deformation processing

Shape memory characteristics of a nanocrystalline TiNi alloy processed by HPT followed by post-deformation annealing

“…It was shown also that grain refinement and the formation of a nanocrystalline microstructure suppresses the austenitic to martensitic (B19') phase transformation and promotes the formation of an intermediate martensitic phase of R in the range of ~60-150 nm [12]. The amorphization of TiNi is also significant because the grain size of specimens obtained via an amorphous state followed by crystallization due to PDA may be much smaller by comparison with those fabricated by conventional methods and this may improve their mechanical properties [13][14][15][16][17][18][19][20].…”

“…There are many reports describing the application of HPT processing to these alloys [9][10][11][12][13][14][15][16][17][18][19][20][21] but only very limited information is at present available on the shape memory behavior for nanocrystalline TiNi alloys prepared by HPT processing followed by PDA [22,23]. In principle, obtaining a fully high strength martensitic microstructure is a very important requirement for achieving good SME.…”

Structure and Properties of TiNi Alloy Subjected to Severe Plastic Deformation and Subsequent Annealing

“…To form a crystalline structure in the alloy, samples subjected to HPT are usually heated up to a particular temperature to initiate the crystallization process. It is well-known that the characteristics of the crystalline structure such as grain size, grain distribution and phase composition are determined by the parameters of crystallization and other accompanying processes that occur on heating the alloy, for instance, structural relaxation, grain growth, etc [2][3][4][5][6][7]. Thus, to control crystalline phase formation, it is necessary to clearly understand the sequence of the processes taking place on heating of an amorphous alloy and the influence of these processes on the structure of the alloys and their properties.…”