Structure and properties of severely cold-rolled and annealed Ti–Ni shape memory alloys

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Abstract

The substructure and structure formation as well as the mechanical and functional properties of thermomechanically treated Ti–50.7 at.%Ni and Ti–50.0 at.%Ni alloys were studied using transmission electron microscopy and mechanical testing. A low-temperature thermomechanical treatment is carried out by rolling at room temperature in a true strain range e = 0.3–1.9. It is shown that the severe plastic deformation of Ti–Ni alloys results in a partial material amorphization and in the subsequent formation of a nanocrystalline austenite structure during post-deformation annealing. As a result, the completely recoverable strain and recovery stress become much higher than the values reachable using traditional low-temperature thermomechanical treatment with post-deformation annealing which creates a polygonized dislocation substructure.

Introduction

The unique functional properties of Ti–Ni-based shape memory alloys (SMAs) make it possible to obtain service characteristics of structures and devices which cannot be achieved using traditional materials and technologies [1], [2], [3]. Improving these functional properties is thus a continual concern for modern metals science.

A combination of the main functional properties of SMAs, e.g. recovery strain and recovery stress [1], [2], [3], can be radically improved using low-temperature thermomechanical treatment (LTMT), including cold plastic deformation with a 20–40% thickness reduction followed by post-deformation annealing (PDA) [1], [2], [3]. Such treatment results in the formation of a well-developed polygonized substructure in the austenite [4]. Using severe plastic deformation and post-deformation annealing to produce an ultrafine-grained structure formation [5], [6], [7], [8], [9], [10] presents new possibilities for improving the functional properties of SMAs. This paper describes the interrelations existing between the Ti–Ni SMAs structure, mechanical and functional properties obtained from the LTMT comprising conventional (true strain of e = 0.3) and severe (e = 1.7–1.9) plastic deformation followed by post-deformation annealing.

Section snippets

Experimental procedure

The 0.9 mm diameter wires of Ti–50.0 at.%Ni (Alloy 1) and Ti–50.7 at.%Ni (Alloy 2) were subjected to LTMT by rolling at room temperature. The original alloys were solution treated at 973 K, 3.6 ks and then quenched in water. The martensite-start temperature after quenching was Ms = 333 K for Alloy 1 and Ms = 265 K for Alloy 2.

Alloy 1 was cold-rolled to different thickness reductions corresponding to true strains e = 0.3 and 1.9, while Alloy 2 was cold-rolled to e = 0.3 and 1.7. Both alloys were subjected to

Structure changes

After LTMT, e = 0.3, a well-developed dislocation substructure of cold-deformed martensite and austenite forms in both alloys as shown in (Fig. 1a). In selected-area electron diffraction (SAED), positions of B2-austenite “rings” are shown and other reflections belong to the B19′-martensite. After a PDA in the 473–673 K temperature range, recovered and then polygonized (since 573 K) dislocation substructures are observed in Alloy 1. After PDA at 723 K and higher temperatures, recrystallized austenite

Conclusions

  • (i)

    For Ti–Ni alloys, severe cold plastic deformation in rolling (e = 1.7–1.9) results in the formation of a mixed amorphous and nanocrystalline austenite structure, while a well-developed martensite and austenite dislocation substructure forms under conventional moderate plastic deformation (e = 0.30). Under post-deformation annealing, the amorphous structure transforms into a nanocrystalline structure which coarsens at higher temperatures, while recovery, polygonization and recrystallization take

Acknowledgements

This work was carried out under partial support from Project No. 2.1.2.6604 of the “Development of Scientific Potential of Higher School” Program of the Russian Ministry of Education and Science and from the Natural Science and Engineering Research Council of Canada.

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