Journal of Intelligent Material Systems and Structures

The micro and macromechanical transformation behavior of single and polycrystalline shape memory alloys was investigated using in situ optical microscopy. An interference filter was used on the microscope to enhance observation of grain boundaries and martensitic plate formation and growth. Experiments on CuAlMnZn polycrystals as well as CuAlNi single crystal shape memory alloys were performed and results are reported in this paper. Cyclic behavior, temperature effects and strain rate effects were studied in detail. Excellent correlation between stress relaxation and temperature decay at several strain rates was achieved. At high strain rates, a dramatic redistribution of martensitic variants was seen in the single crystal specimens after loading to a fixed strain. Tests were performed at different temperatures to help identify the mechanism for variant redistribution.

Recent theoretical advances in the development of functional representations for the free energy of mixing associated with solid-solid phase transformations has made possible the development of a new class of constitutive models based directly on bounds to the relaxed free energy. The present work, which continues that of Govindjee et al. (2002) and Hall and Govindjee (2002), demonstrates the application of the theory to the simulation of the complex behavior exhibited by shape memory alloys. The first three case studies undertaken herein support the validity of the theoretical approach by analyzing subtle aspects of the physics associated with such phase transformations. The final simulation, a torsion tube under combined loading, provides impetus for future experimental and theoretical advancements.

Pseudoelasticity is a useful regime of behavior for industrial applications of shape memory alloys. The complexity of the physical phenomena of pseudoelastic shape memory behavior requires micromechanical methods of scale transition to model at higher length scales using crystallographic information. Using a self-consistent model to transition from single crystal to polycrystal behavior, we present results for computed surfaces of constant effective transformation strain for random, drawn, tension, compression and rolled textures of a polycrystalline Cu-Zn-Al shape memory alloy in the pseudoelastic regime. These results are relevant to the development of macroscopic transformation surfaces in stress space. A feature of the results is that normality of transformation strain rate to surfaces of constant macroscopic transformation strain is observed for the most part, apart from regions of high curvature that bridge different segments of the transformation surface.

The Multivariant model describing shape memory alloy (SMA) constitutive behavior is further developed in this paper for improved prediction of thermomechanical response. In the original formulation of the Multivariant model, an interaction energy, representative of the incompatibility of an inclusion to the matrix, was calculated by a micromechanics model in which every variant group (inclusion) was embedded in the austenite phase with numerous other inclusions. However, experiments show that martensite variants tend to form large plates most of which have an invariant plane interface with the austenite and reach the grain boundary. Hence, to better simulate material behavior, in the revised model the micromechanical interaction energy is replaced by a small, constant term. The predictions by the new model for different uniaxial tension directions on a single CuAlNi crystal have excellent agreement with the experimental results. Furthermore, the counterintuitive results for a polycrystal CuZnAl under triaxial loading are also well captured by the new model. As the revised model removes an iterative procedure for interaction energy, calculations are simplified, making the Multivariant model more suitable for larger scale computations.

An extensive experimental program on a 9% Ni, 12% Cr, 2% Mo steel is introduced. This material transforms from the austenitic (γ) phase into the martensitic (α′) phase at a low temperature level around 150°C upon cooling on air only, thus making it especially suitable for testing purposes since it exhibits practically no creep effects during and after transformation (Fischer et al., 1996). Dilatometric tests are carried out for two types of specimens (longitudinal specimens (LSs) with a rolling texture and radial specimens (RSs)). Interestingly, the dilatation loops do not close after cooling down to room temperature. For an increasing annealing temperature the gap becomes smaller and closes for RSs. It turns out that the dilatometric loops close for preloaded specimens, pointing to an initial backstress in the material in the same order of magnitude as the load stress so it must not be neglected. Monitoring the martensite start (M_s ) and finish temperature (M_f ) for different loading conditions and stress levels reveals a strong influence of the type of loading (tension, shear, compression) on M_s . The overall yield stress as a function of temperature is noticeably disturbed immediately after transformation starts. As the temperature approaches M_f the composite effect in the α′-γ region prevails. Experiments including mixed, nonproportional loading paths for cooling as well as at constant temperature have been performed. The orientations of the martensitic variants account for the different deformation behavior for different loading types but equal global equivalent stresses (see also Fischer et al., 2000a). A comprehensive micromechanical modeling concept is presented based on the numerical implementation of a transformation condition (Fischer and Reisner, 1998) governing the variant selection into a finite element algorithm (see Reisner et al., 1998). Finally, the issue of an improved constitutive law for the TRIP strain rate has been tackled. A novel approach for an evolution law including the backstress has first been presented by Videau et al. (1996) and has later been refined by Fischer et al. (2000b).

In this paper vibration damping of a shape memory alloy rod is studied. The mathematical model includes the heat conduction equation, the stress wave equation, a kinetic law and a constitutive law. As kinetic law the Likhachev model is used, which describes the one-way effect, pseudoelasticity properties and thermal loading cycles. It is observed that the vibration amplitude of the rod decreases in this model very quickly. Results are in good agreement with earlier investigations, where other kinetic laws have been used.

A new continuum model for the description of the general mechanical behavior of polycrystalline Shape Memory Alloys (SMA) is introduced in order to represent some particular 3D-effects not described by usual plasticity-like macroscopic models (e.g., reorientation under general loadings). The model describes the one-way and the pseudoelasticity shape memory effects as well as their transient behavior under reorientation of the martensite by arbitrary thermo-mechanical loadings. The main idea for the description of phase change and martensite reorientation consists of the assumption that the driving forces of the phase transformation depend on both the actual stress state and the transformation induced strain (TIS) tensor. In order to describe the change of the martensite structure both, the TIS as a tensorial internal variable together with a scalar valued variable which describes the phase fraction of thermally induced martensite, are used. The evolution of the TIS tensor is described by a new evolution equation without plastic analogy. The problem of modeling the evolution direction and the thermodynamical consistency are discussed in details.

Currently, three methods are commonly used for producing porous NiTi shape memory alloys (SMAs) from elemental powders. These include conventional sintering, Self-propagating High temperature Synthesis (SHS), and sintering at elevated pressure via a Hot Isostatic Press (HIP). Conventional sintering requires long heating times and samples are limited in shape and pore size. SHS is initiated by a thermal explosion ignited at one end of the specimen, which then propagates through the specimen in a self-sustaining manner. One of the difficulties with SHS is the inability to control intermetallic phases. This work will focus on the fabrication and characterization of porous NiTi SMA material produced from elemental powders via HIPping.

Porous NiTi SMA was produced from elemental Ni and Ti powders at elevated temperature and pressure using a HIP. Small and large pore specimens containing average pore sizes ranging from 20 μm up to 1 mm have been produced by slightly varying the HIPping sintering temperatures and times.

Quasi-static and dynamic loading experiments are conducted on various samples produced using the presented methodology and their shape recovery and energy absorption characteristics are measured during the forward and reverse phase transformation and detwinning. Their phase transformation characteristics were found using calorimetric measurements and their composition has been studied using optical and electron microscopy and microprobe X-ray analysis.