<title>Strengthening mechanisms of TiNi shape memory fiber/Al matrix composite</title>

Abstract

Metal matrix composites (MMCs) have been studied intensively with expectation of applying MMCs to various structural and machine components. A recent summary of the thermomechanical behavior of MMCs has been given elsewhere.1'2 One of the important findings in the recent studies is that strengthening of MMCs is identified by two mechanisms: back stress strengthening and dislocation punching.3'4 In most MMC systems, the coefficient of thermal expansion (CTh) of reinforcement is smaller than that of metal matrix, resulting in tensile residual stress in the matrix at room temperature. This tensile residual stress in the matrix reduces the tensile flow stress (particularly yield stress) of a MMC.5 If the residual stress in the matrix in a MMC is controlled to be compressive, the tensile flow stress of the MMC is expected to be increased. A shape memory alloy (SMA) fiber, after its shape is memorized and presirained at martensitic phase, can shrink to its original length upon heating to austenitic finish temperature. If such shrinkable SMA fibers are embedded in a metal matrix to form a MMC, compressive residual stress in the matrix is induced at austenitic stage, resulting in enhanced tensile flow stress of the MMC, see the process of inducing such compressive residual stress in the matrix in Fig. 16. Motivated by the above idea depicted in Fig. 1 ,we are led to process TiNi SMA fiber/i 100 Al matrix (TiNi/Al) composite by pressure casting route, the details of which has been given elsewhere6. As-processed TiNi/Al composite was machined to tensile specimen of flat bar type and given tensile prestrain c at martensitic stage, then heated to 363K just above the austenitic finish temperature (337K), followed by tensile testing at 363K. The results of the stress-strain curves of TiNi/Al composite with and without prestrain, and unreinforced Al are shown in Fig. 2. It is obvious from Fig. 2 that the flow stress of the TiNi/Al composite with prestrain is higher than that of the composite without it, which in turn is higher than the unreinforced metal due to back stress strengthening mechanism. The strengthening due to prestrain is the main subject of this paper. The analytical model similar to the present one has been developed for the analysis of a short fiber SMA fiber MMC.7 Analytical modeling will be stated in section II and numerical results and comparison with the experimental results will be discussed in Section III, followed by the conclusion in Section IV.