Unnotched fatigue behavior of an austenitic Ni–Ti shape memory alloy

Abstract

Constant stress amplitude fatigue life of an austenitic Ni (55.88 wt.%)–Ti shape memory alloy (SMA) within the stress amplitude range of 180–450 MPa was evaluated. The stress–strain hysteresis loops were monitored throughout the fatigue loading. They reveal that with the increasing number of fatigue cycles, the critical stress required for the stress-induced martensitic transformation, width of the hysteresis loop, recoverable and frictional energies of each cycle, all decrease while accumulated plastic strain increases. Post-mortem characterization of the fatigued specimens by employing differential scanning calorimetry (DSC), X-ray diffraction (XRD), and fractography were carried out, in order to understand the fatigue micromechanisms. Results indicate that the progressive accumulation of stress-induced martensite in the alloy is the source for the fatigue failure. Implications of these observations are discussed within the context of fatigue performance of SMAs and other materials that undergo stress-induced transformations.

Introduction

Shape memory alloys (SMAs) undergo a diffusionless martensitic transformation, either upon cooling from the austenitic phase or upon the application of stress. As a result, SMAs exhibit two distinct mechanical responses depending on the temperature of deformation. The super-elasticity effect (SE) arises due to the formation of stress-induced martensite (SIM) from austenite during mechanical loading. One of the primary mechanisms responsible for the shape memory effect (SME), which occurs when the SMA is deformed in the martensitic condition, is detwinning (i.e., an energetically favored martensitic variant growing at the expense of the others) of the martensite. Both these processes can be reversed, either by increasing the temperature and releasing the stress, respectively.

Because of their unique thermo-mechanical behavior, SMAs are being used for both structural and functional applications such as orthodontic guide-wires, deployable satellite antennas, micro-electro-mechanical systems (MEMSs), biomedical devices and implants, and foundations for earthquake protection, etc. Success in some of these applications hinges on the ability of these alloys to perform multiple shape memory or pseudo-elastic transformation cycles over a long service period. For example, biomedical applications of Ni–Ti SMAs (Nitinol) such as aortic and coronary stents have to meet the stringent requirement of a working life of 10 years (about 400 million pulsative cycles) [1]. Consequently, it is important to understand and predict the effect of cyclic loading on the transformation behavior of Nitinol SMAs.

In ductile metals, the back and forth motion of dislocations during a fatigue cycle and the associated kinematic irreversibility of displacements lead to permanent damage accumulation. This in turn results in crack nucleation and growth, leading to fatigue fracture. For the occurrence of either SME or SE in a given SMA, it is essential to avoid dislocation motion and have nominally reversible processes. However, fatigue does indeed occur in SMAs due to the accumulation of defects and structural changes, like the change in order of β(austenite)-phase or martensite [2]. This manifests as not only mechanical fatigue but also functional fatigue, seen as loss of shape memory or superelastic effects over number of cycles. Due to the nature of phase transformation in SMAs, the mechanical and functional fatigue processes in SMAs are interrelated. Since the issue of fatigue is crucial to reliability of SMA components, investigation of SMA behavior under cyclic loading is of paramount importance. Considerable amount of research has gone into studying the mechanical response of NiTi plates, bars and wires to uniaxial tensile loading, mechanical cycling (mostly pull–pull cycling), thermal cycling, etc., and their underlying mechanisms. Systematic studies of fatigue in pseudo-elastic NiTi were made by Eggeler and co-workers [3], [4], [5], Miyazaki et al. [6], and Melton and Mercier [7]. Thermo-mechanical response of pseudo-elastic NiTi wires was studied by Tsoi et al. [8]. Pappas et al. [9] have investigated functional degradation in thin NiTi wires subjected to thermo-mechanical load. While several studies have been conducted on this topic, many aspects of fatigue in SMAs are still not well understood and need further investigations. Some of the important aspects will be briefly discussed in the following.

Generally, the thermo-elastic transformation in SMAs manifests as hysteresis in stress–strain as well as thermal response. This is because the forward and reverse transformation occurs over different span of temperature and/or stress. In general, upon cycling loading, the span of transformation temperatures and stresses widens and the domain of the hysteresis associated with the forward and reverse transformations diminishes [10]. The range of transformation temperatures and strains are important material parameters and are sensitive to the material composition, heat treatment, service temperature, pre-strain, etc. Ren et al. [11] have investigated hysteretic behavior in superelastic SMAs and show that a shakedown or stabilization behavior is observed with the narrowing of the hysteresis. A small change in the total transformation strain in the material over cycling is also seen in their studies. They have proposed a phenomenological model to predict the change in stress–strain hysteresis during cycling.

Only a limited amount of work is available in the open literature on the effect of these variables on the fatigue performance of SMAs. In particular, the role of SIM transformation in strain accumulation during fatigue of SMAs needs detailed investigation. While the strain controlled fatigue of NiTi has been investigated in the past [3], [4], [5], [11], fatigue under stress controlled mode has not been reported so far in the open literature. The differences in the mode of cycling could influence the kinematic irreversibility and hence the fatigue performance of SMAs.

In the broader context, an additional scientific issue is the following. In general, it is recognized that a stress-induced transformation in front of a growing crack can be beneficial in enhancing the toughness as in, for example, transformation-induced plasticity steels and transformation-toughened zirconia [12], [13], [14], [15], [16]. This in-turn enhances the fatigue life of the materials. The formation of SIM from austenite is accompanied by a small volume decrease of ∼0.5% [17]. This suggests the possibility of changes in fatigue properties associated with the phase transformation [18]. For example, in 304 stainless steel, a volumetric increase due to the stress-induced martensitic transformation is associated with an increase in the fatigue threshold. This effect, occurring at low load ratios, is primarily attributed to internal stresses [19]. Holtz et al. [20] have investigated the effect of SIM transformation in Nitinol on the fatigue threshold. They conclude that the internal tensile stress generated at the crack tip due to the decrease in volume associated with the SIM transformation superimposes on the applied stress, thereby causing an increase of the nett stress intensity acting at the crack tip. This in turn manifests as a shift in the fundamental threshold curve towards lower values. McKelvey and Ritchie [21] show that fatigue crack growth thresholds are higher and crack-growth rates slower in martensite compared to stable austenite and superelastic austenite. They attributed the similarity of fatigue crack propagation behavior in stable and superelastic austenite to the fact that the austenite–martensite phase transformation was suppressed by the tensile hydrostatic state of stress that exists ahead of a plane-strain fatigue crack. Such effect, if true, is likely to significantly affect the fatigue crack initiation and thereby the fatigue life. This is due to the fact that ∼80% of unnotched fatigue life is spent in initiating a dominant micro-crack [18]. However, this has not yet been sufficiently examined.

Moumni et al. [22] have investigated fatigue of SMAs and using an energy approach, they showed that the dissipated energy in the stabilized cycle is a relevant parameter for the estimation of the fatigue life. In a recent work, Roy et al. [23] have investigated austenitic fatigue in thin NiTi wires which has significant two-way shape memory due to thermo-mechanical training. They show that at stress levels much higher than the critical stress for the onset of SIM (σ_SIM), there is a rapid deterioration in fatigue life compared to those cycled at stresses lower than the critical stress. They hypothesize that this could be due to the population of austenite–martensite interfaces which form during cycling which act as stress concentrators and dislocation mediated plastic deformation. This interferes with the two-way memory effect and leads to the functional fatigue of the wires.

This work was initiated with the objective of gaining a better understanding of the issues raised above. Stress controlled fatigue tests were conducted on an austenitic SMA at room temperature to estimate the strain accumulation due to kinematic irreversibility. Progressive damage accumulation in the material with mechanical cycling was monitored. This was complemented with detailed post-mortem characterization of the samples, to ascertain micromechanical causes of failure.