Structural Characteristics of Multilayered Ni-Ti Nanocomposite Fabricated by High Speed High Pressure Torsion (HSHPT)

Gheorghe Gurau,Carmela Gurau,Vedamanickam Sampath,Mihaela Marin,Petrica Alexandru,Bogdan Mihai Galbinasu,Francisco Manuel Braz Fernandes

doi:10.3390/met10121629

Abstract

It is generally accepted that severe plastic deformation (SPD) has the ability to produce ultrafinegrained (UFG) and nanocrystalline materials in bulk. Recent developments in high pressure torsion (HPT) processes have led to the production of bimetallic composites using copper, aluminum or magnesium alloys. This article outlines a new approach to fabricate multilayered Ni-Ti nanocomposites by a patented SPD technique, namely, high speed high pressure torsion (HSHPT). The multilayered composite discs consist of Ni-Ti alloys of different composition: a shape memory alloy (SMA) Ti-rich, whose Mf > RT, and an SMA Ni-rich, whose Af < RT. The composites were designed to have 2 to 32 layers of both alloys. The layers were arranged in different sequences to improve the shape recovery on both heating and cooling of nickel-titanium alloys. The manufacturing process of Ni-Ti multilayers is explained in this work. The evolution of the microstructure was traced using optical, scanning electron and transmission electron microscopes. The effectiveness of the bonding of the multilayered composites was investigated. The shape memory characteristics and the martensitic transition of the nickel-titanium nanocomposites were studied by differential scanning calorimetry (DSC). This method opens up new possibilities for designing various layered metal-matrix composites achieving the best combination of shape memory, deformability and tensile strength.

Highlights

Multilayered composites have attracted much attention in engineering design as a promising technique to develop a novel combination of physical and mechanical properties acquired from the individual characteristics of the incorporated materials [1,2,3,4,5]
The aim of our research is to study the structure and phase transformations of smart Ni-Ti multilayered composites obtained using the high speed high pressure torsion (HSHPT) technique
The details of the HSHPT procedure and the ad hoc machine used in the present work are given in our earlier papers [19,21]

Summary

Introduction

Multilayered composites have attracted much attention in engineering design as a promising technique to develop a novel combination of physical and mechanical properties acquired from the individual characteristics of the incorporated materials [1,2,3,4,5]. Bimetallic shape memory composites are among the most widely investigated class of composites, offering better shape memory properties for the design of new decomplex applications [6]. Shape memory alloys are stimulus-responsive materials with two universal properties: superelasticity and shape memory effect [7,8]. The occurrence of martensite-to-austenite and austenite-to-martensite transitions gives rise to shape memory and superelastic responses [9,10]. Among these alloys, Ni-Ti SMAs are among the most interesting thermo-responsive SMAs that are Metals 2020, 10, 1629; doi:10.3390/met10121629 www.mdpi.com/journal/metals. Research on Ni-Ti shape memory alloys has been revived by controlling the “size-effect”

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