1998 Volume 84 Issue 9 Pages 678-684
The present authors have previously shown that in the Ti-6Al-2Sn-4Zr-2Mo/10TiB particulate composites the fatigue fracture originated neither from the interior of particulate nor from the particulate/matrix interface, but rather from the matrix area of the composites. This finding strongly suggested that the high cycle fatigue strength is related with the matrix microstructure. Therefore, the present study aimed at improving the high cycle fatigue strength by the modification of matrix microstructure.
The Ti-6Al-2Sn-4Zr-2Mo/10TiB particulate composites were produced by the blended elemental P/M method using extra low chlorine titanium powder smaller than 45 μm in diameter. The smooth axial fatigue tests were carried out at an R ratio of 0.1 in the load-controlled mode.
The matrix microstructure was highly dependent on the cooling rate after the sintering treatment. The faster cooling rate of 10K/sec produced the well-defined colony microstructure. The massive colony microstructure was obtained by the cooling rate slower than 0.3K/sec. The modification of the matrix microstructure from a colony into a fine acicular α microstructure resulted in the increase in high cycle fatigue strength. For example, fatigue strength at 107 cycles was increased from 490 MPa to 550 MPa. The three main contributions to the fatigue strength were suggested as the contribution from the increase in Young's modulus (18%), tensile strength (39%) and microstructural modifi-cation (43%), respectively.
