Compression behavior of porous NiTi shape memory alloy

doi:10.1016/j.actamat.2004.09.029

Acta Materialia

Volume 53, Issue 2, 10 January 2005, Pages 337-343

https://doi.org/10.1016/j.actamat.2004.09.029 Get rights and content

Abstract

Porous NiTi alloy with several different porosities was processed by spark plasma sintering. The compression behavior of the porous NiTi was examined with the aim of using it possibly as a high energy absorbing material. A model for the macroscopic compression behavior of porous shape memory alloy (SMA) is presented in this work, where Eshelby’s inhomogeneous inclusion method is used to predict the effective elastic and superelastic behavior of a porous SMA based on the assumption of stress–strain curve. The analytical results are compared with experimental data for porous NiTi with 13% porosity, resulting in a reasonably good agreement.

Introduction

Over the last two decades shape memory alloys (SMA) have attracted great interest in various applications ranging from aerospace [1] and naval [2] to surgical instruments, medical implants and fixtures [3], [4]. The use of SMAs has promoted extensive research on developing SMA constitutive models.

Among SMAs, NiTi alloy has been used most extensively due to its large flow stress and shape memory effect (SME) strain. Most recently, porous NiTi have attracted increasing attention for possible application in medical implant devices and as high energy absorption structural material. The progress in manufacturing and characterization of the porous NiTi SMA has been reported by a number of researchers. Li et al. [5], [6] fabricated porous NiTi SMA by combustion synthesis method, the stress–strain curves in their work show that the porous NiTi synthesized by this method is brittle. Li et al. [7] also fabricated the porous NiTi from powder sintering; they show that there is no stress plateau in the stress–strain curve and the material is still brittle. Kim et al. [8] produced porous NiTi by self-propagating high temperature synthesis (SHS), and again the porous NiTi fabricated by this method is brittle. Lagoudas et al. [9] used the hot isostatic press (HIP) method. The stress–strain curve in their work exhibits brittle behavior. Since these previous studies on porous NiTi exhibited poor ductility, it is necessary for us to develop a better processing method which provides porous NiTi with higher ductility. Therefore, the spark plasma sintering (SPS) method [10] is introduced in this work. The pre-alloy NiTi raw powders of superelastic grade (Ni 50.9 at.%–Ti 49.1 at.%) are loaded into a graphite die and pressed to the desired pressure and then a huge on-off pulsed current is induced through the die and stacked powder particles. Under the condition of pulsed current heating, powder particles are activated to a high energy state and neck formation easily occurs at low temperature in very short time compared with ordinary sintering processes like hot press (HP), HIP or SHS. Moreover, the effect of spark discharge purifies the surface of powder particles, which guarantees neck formation and high quality of sintered materials. The above features of SPS meet our demand for preparing porous NiTi using NiTi alloy powders.

In order to optimally design the microstructure and properties of the porous SMAs, it is important to build a simple, yet accurate model to describe its microstructure–mechanical behavior relation. If a porous NiTi is treated as a special case of a particle-reinforced composite, one can apply a micromechanical model based on Eshelby’s method with Mori–Tanaka mean-field (MT) theory [11], [12], [13], [14], [15], [16], [17] and self-consistent method [18], [19]. Both methods have been used to model macroscopic behavior of composites with SMA fibers [20], [21]. Young’s modulus of a porous material was modeled by using the Eshelby’s method with MT theory [22].

In this paper, Eshelby’s equivalent inclusion method with Mori–Tanaka mean-field theory is used to predict the stress–strain (SS) curve of a porous NiTi under compression where the superelastic deformation corresponding to the second stage of the SS curve is accounted for. Then the predicted SS curve is compared with the experimental data of the porous NiTi specimen processed by SPS.

Section snippets

Experimental results of NiTi specimens processed by SPS

We have processed three different types of specimens by spark plasma sintering (Dr. Sinter SPS-515S, Sumitomo Coal Mining Co., Japan). Fig. 1 is a schematic drawing of the SPS device. An ingot of NiTi alloy (Ni 50.9 at.% and Ti 49.1 at.%) was made by Sumitomo Metals, Osaka, Japan, which was then shipped to Fukuda Metals, Kyoto, Japan, where the plasma rotating electrode process (PREP) was used to process NiTi powders. The average diameter of the NiTi powders processed by PREP is 150 μm. The

Modeling of the compressive stress–strain curves of porous NiTi

This model assumes piecewise linear SS curve of superelastic NiTi. This idealized SS curve is illustrated in Fig. 7, where the first linear part A_iB_i corresponds to the elastic loading of 100% austenite phase, the second linear part B_iD_i is the stress-induced martensite transformation plateau, D_id_i is unloading of 100% martensite phase, d_ib_i is the reverse transformation lower plateau, final linear part is b_iA_i, elastic unloading of 100% austenite phase. The subscript ‘i’ in Fig. 7 denotes

Conclusion

Porous and solid NiTi specimens are processed by spark plasma sintering where two different porosities are used, 13% and 25%. The 13% porosity NiTi appears to possess a sound microstructure with high ductility while the 25% porosity NiTi specimens exhibit much lower stress flow than that of the 13% porosity.

Then the compressive stress–strain curve of the 13% porosity NiTi is simulated by a model which is based on piecewise linear stress–strain curve. This model predicts the piecewise SS curve

Acknowledgements

The present work was supported by a sub-grant from ONR-MURI project via University of California at San Diego where PI is Prof. S. Nemat-Nasser N-000140210666. The program monitor at ONR is Dr. R. Barsoum.

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Compression behavior of porous NiTi shape memory alloy

Abstract

Introduction

Section snippets

Experimental results of NiTi specimens processed by SPS

Modeling of the compressive stress–strain curves of porous NiTi

Conclusion

Acknowledgements

Mater. Sci. Forum

Key Eng. Mater.

Intermetallics

Cell. Met. Met. Foaming Technol.

Acta Metall.

Int. J. Eng. Sci.

Phys. Solids

Smart Mater. Struct.

J. Phys. IV

Metallography