Martensitic Transformations in a NiTi Fiber Reinforced 6061 Aluminum Matrix Composite

Approximately 19.5 volume percent, 50.7 at % Ni-Ti shape memory alloy fiber reinforced 6061-T6 aluminum alloy matrix composite (SMA-MMC) and homogeneous 6061-T6 control materials were produced by vacuum hot pressing. The test materials were initially subjected to a 5.8% tensile elongation at room temperature. During this process, the shape memory alloy reinforced composite materials exhibited a bimodal yield. The initial yield was due to plastic flow in the aluminum matrix, the subsequent yield was due to the initiation of a stress induced austenite to martensite (SIM) transformation in the shape memory alloy fibers. The SMA-MMC specimens exhibited unusual positive curvature hardening during the final stages of the 25°C tensile loading resulting from the saturation of the SIM transformation. During the subsequent 25 °C to 75 °C unconstrained heating process, the SMA-MMC exhibited a nonlinear thermal contraction resulting from the martensite to austenite shape memory transformation of the NiTi reinforcement. At the conclusion of the unconstrained heating process the temperature was held constant at 75°C as the loading grips were reapplied and the test materials were tensile loaded to failure. The elevated temperature flow strength of the SMA-MMC was significantly higher than the final SMA-MMC room temperature flow strength; this increase was due to the imposition of a large longitudinal compressive stress in the composite matrix by the NiTi fiber shape memory response, and due to the increase in NiTi fiber transformation stress with increased temperature. The elevated temperature ultimate strength and final failure strain of the SMA-MMC materials were both significantly higher than that of the 6061-T6 control materials. The experimental results are analyzed by a nonlinear, one-dimensional composite constitutive model. The model provides a compact quantitative description of the SMA-MMC thermal mechanical response as a function of temperature and applied stress, and correctly expresses important experimental features.