Mechanical Properties of Smart Metal Matrix Composite by Shape Memory Effects
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ABSTRACT The thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix smart composite is investigated experimentally and analytically. The yield stress of the composite is observed to increase with prestrain given to the composite. Analytical model is developed by utilizing a shape memory alloy constitutive model of exponential type for the thermomechanical behavior of the composite. The model predicts that the composite yield stress increases with increasing prestrain, and the key parameters in affecting the composite yield stress are prestrain and matrix heat treatment. The model predicts reasonably well the experimental results of the enhanced composite yield stress. INTRODUCTION A smart composite, which utilizes the shape memory alloy (SMA) fiber as a reinforcement, has been designed and successfully developed [1-6]. The design concept of smart composite is shown in our previous study [2]. SMA fibers in composite are loaded at room temperature, making the austenitic phase of TiNi SMA fiber changed to martensitic phase (forward transformation). Then they are heated so as to induce the martensite to austenite phase transformation (called reverse transformation). With this reverse transformation, the SMA fibers shrink in the composite, that induces tensile stress in the fibers and compressive stress in the matrix. This compressive stress in the matrix is the main source of the tensile properties enhancement of the smart composite. The internal stress analysis is the key step in the precise evaluation of the smart composite. Recently we have developed two types of smart composite utilizing TiNi SMA fiber as a reinforcement, and demonstrate successfully the enhancement of the yield stress of TiNi/aluminum composite and fracture toughness of TiNi/epoxy composite at high temperature [1, 3, 5]. In our previous studies [2, 4, 6], pure aluminum was used for the matrix due to easiness of processing into a composite and demonstrating the enhancement of the composite yield stress. For applications of smart composites to structural composite, use of pure aluminum as a matrix metal is not advantageous due to its low flow stress (yield stress). Hence in the present study, use of 6061 aluminum alloy and T6 heat treatment are adopted. For the processing of metal matrix smart composite, we used pressure casting of molten aluminum matrix previously [4], while here, we will use a vacuum hot press method to enhance strong interfacial bonding between TiNi fibers and aluminum alloy matrix through a solid-solid diffusion bonding process. In our previous model, the martensite phase induced in the prestrain process is assumed to show reverse transformation to austenite above austenite starting temperature, Af, and the TiNi fiber is fully austenitic at temperatures higher than Af [2, 6]. In this paper, we develop a model including stress effects, especially, on the transformation during the heating process, because this transformation is strongly effected by temperature and stress [7]. 143
Mat. Res. Soc.
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