Ambient-Temperature Indentation Creep of Shape Memory NiTi Alloys: Additively Manufactured versus Cast

M.r Islam ,Mohammadreza Nematollahi,Mohammad Elahinia,Parisa Bayati,Ahmadreza Jahadakbar,M Haghshenas

doi:10.3390/jmmp5030087

M.r Islam , Mohammadreza Nematollahi + Show 4 more

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https://doi.org/10.3390/jmmp5030087

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Abstract

In this study, depth-sensing indentation creep response of cast and additively manufactured (laser powder bed fusion) NiTi alloys in heat-treated conditions have been investigated at ambient temperature. Indentation creep tests were evaluated with the help of a dual-stage approach comprising a loading segment with a subsequent constant load-holding stage and an unloading phase afterward. The investigation was carried out at a maximum load of 50 mN along with a holding time of 600 s. Different creep parameters comprising indentation creep displacement, creep strain rate, creep stress exponent as well as the indentation size effect have been analyzed quantitatively for the employed materials. In addition, microstructural analysis has been performed to ascertain the processing–microstructure–creep property correlations. A substantial indentation size effect was seen for both cast and printed NiTi samples in heat-treated conditions. According to the creep stress exponent measurements, the dominant mechanism of rate-dependent plastic deformation for all NiTi samples at ambient temperature is attributed to the dislocation movement (i.e., glide/climb). The outcome of this investigation will act as a framework to understand the underlying mechanisms of ambient-temperature indentation creep of the cast and printed NiTi alloy in conjunction with heat-treated conditions.

Highlights

Shape memory alloys (SMAs) such as NiTi possess the capability to retrieve their original shape despite a significant degree of deformation under temperature and stress conditions
We found that printed samples experienced more creep displacement
The ambient temperature creep behavior of cast and additively manufactured NiTi alloy in heat-treated conditions has been investigated by a depth-sensing indentation testing technique

Summary

Introduction

Shape memory alloys (SMAs) such as NiTi possess the capability to retrieve their original shape despite a significant degree of deformation under temperature and stress conditions. They are being used extensively as a functional material in engineering and medical applications [1,2]. The LPBF process starts with spreading a thin layer of NiTi powder; a high-power density laser is used to melt and fuse the metallic powder This cycle is repeated layer-by-layer with the help of a spreader and adjustable build platform until a complete part has been fabricated [8]

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