Mechanical Properties of NiTi-TiC Shape-Memory Composites
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esses between matrix and reinforcement upon the deformation and subsequent shape-memory recovery of the composites. Since the allotropic transformation responsible for the unique properties of NiTi is thermoelastic, these mismatch stresses can be expected to strongly affect the mechanical response of the matrix. This review summarizes recent work done at MIT in the area of mechanical properties of TiCreinforced metal matrix composites with a martensitic NiTi matrix. More details can be found in the original articles describing the thermal transformation behavior [7, 8], the bulk mechanical properties in compression [9], the subsequent shape-memory recovery [10] and the study by neutron diffraction of twinning deformation and shape-memory recovery [11, 12]. EXPERIMENT While the detailed experimental procedures can be found in the original publications [7-12], a brief summary is given in the following. Billets were fabricated by hot-pressing (followed in some cases by hot-isostatically pressing) of blended powders of prealloyed NiTi (51.4 at % Ti) and TiC particles (between 44 gim and 100 pim in size) with volume fractions 0%, 10% or 20%. Compression samples (respectively labeled in what follows NiTi, NiTi-0OTiC and NiTi-20TiC) were cut from the billets by electro-discharge machining and annealed at 930 'C for 1 hour. Mechanical testing was performed in compression between graphite-lubricated carbide plattens with strain measured by strain gauges, a linear variable displacement transducer or an extensometer. Specimens were thermally recovered in air in a Orton dilatometer outfitted with quartz sample-holder and push-rod. Heating occurred from 20'C to a maximum temperature of at least 275°C at a rate of 1 K/min., followed by cooling to 20'C at the same rate. 131 Mat. Res. Soc. Symp. Proc. Vol. 459 0 1997 Materials Research Society
tronNeutron source of diffraction the Los Alamos measurements Neutronwere Science performed Center.in time-of-flight Two samples mode (NiTi atand theNiTi-20TiC) pulsed neuwere deformed in compression up to a stress of -280 MPa, unloaded and subsequently recovered above Af and cooled to room temperature. Strains parallel and perpendicular to the loading axis were calculated from the shift of individual Bragg reflections at a series of stress values, using two detectors forming an angle with respect to the incident beam of -90* (scattering vector Q parallel to the load) and +90* (Q perpendicular to the load). The unstressed, annealed bulk samples were used as a stress-free reference. Changes in the intensities of individual reflections, which correspond to changes in the fraction of NiTi variants in the diffraction condition, were quantified with a Normalized Scale Factor defined as the ratio of a peak intensities under load and prior to loading. The average material response was determined using the Rietveld approach, where the intensities and positions of all Bragg peaks are predicted from an assumed crystal structure. RESULTS Compression Behavior A typical stress-strain curve for NiTi is sh
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