Microstructural and fracture characterization of Nb-Cr-Ti mechanically alloyed materials
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INTRODUCTION
VERY high fracture toughness values (KIC ⬇ 80 to 100 MPa冪m) have been measured for the composition Nb13Cr-37Ti (at. pct), which is a large grained, single-phase bcc solid solution alloy.[1] The alloy exhibited a yield strength of 900 MPa and a fatigue crack growth threshold of ⬇ 7 MPa冪m at ambient temperature. If the same Nb/Ti ratio is maintained (⬇1.3) as more Cr is added to this composition, Cr2Nb forms in situ, which is advantageous because this intermetallic is stable at high temperatures, giving the composite better creep and oxidation resistance.[2] However, gaining control of the size and dispersion of the intermetallic has proven to be difficult.[1,3,4] Cast materials that were relatively slowly cooled resulted in large particles of Cr2Nb that, in some cases, were well dispersed, but in other cases were contiguous in the grain boundaries.[1,3] Rapid solidification of these material compositions resulted in completely different microstructures that had Cr2Nb particles with high contiguity.[4] Contiguity allows cracks to grow through the brittle intermetallic Cr2Nb, with minimum growth through the high toughness matrix. Large Cr2Nb particles break at lower stress than smaller particles because of flaws in the intermetallic, which means that there is probably an optimum size for these particles. For the same volume fraction of intermetallic, there are fewer large particles, but they are susceptible to cracking, which limits fracture toughness. There are many more small particles for a given volume fraction, if the particles are small. However, small particles, which do not break, can be detrimental to fracture toughness because of constraint exerted by the particles on matrix deformation.[5,6] Thus, the achievement of maximum fracture toughness requires the creation of optimum microstructure, where the size and volume fraction of intermetallic particles is controlled. The failure of rapid solidification to provide a suitable D.L. DAVIDSON, formerly with the Southwest Research Institute, San Antonio, TX 78238, is retired. K.S. CHAN, Institute Scientist, is with the Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted June 26, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructure indicated that further attempts to control cooling rates would be very costly, and it was not at all clear that the desired result would be achieved. The techniques that have been developed in mechanical alloying (MA) showed promise that a second phase of controlled size would be well dispersed.[7] It has been shown that dispersal of the second phase by mechanical alloying results in increased creep resistance and enhancement of yield stress in other composites containing intermetallics.[7] Possibly, the materials made by mechanical alloying would be amorphous,[8] which could be converted to the desired structures in a very controlled way by heat treatment. With these benefits of MA as promised, coupled with past manufacturing and processing experience, a series of NbCr-Ti alloys was fabrica
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