Fatigue crack-growth in shape-memory NiTi and NiTi-TiC composites

An experimental study was conducted to examine the room-temperature Fatigue crack-growth characteristics of shape-memory NiTi matrix composites reinfo rced with 10 and 20 vol.% of TIC particles. Microstructural characterizatio n of these hot-isostatically-pressed materials shows that the TiC particles do not react with the NiTi matrix and that they lack any texture. Overall fatigue crack-growth characteristics were found to be similar for the unrei nforced and reinforced materials. However, a slight increase in the thresho ld for fatigue crack initiation was noted for the composites. The fracture toughness, as indicated by the failure stress intensity factor range, was f ound to be similar for all materials. Neutron diffraction studies near the crack-tip of the loaded fracture NiTi specimen detected no significant deve lopment of texture at the crack-tip. These results are explained by recours e to fractographic observations. Finally, a comparison is made between the micromechanisms of fracture of metal matrix composites, which deform by dis location plasticity, and those of the present NiTi-TiC composites, which de form additionally by twinning. (C) 2000 Elsevier Science S.A. All rights re served.