Effects of Ti addition on cleavage fracture in Nb-Cr-Ti solid-solution alloys
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RECENT investigations based on the Nb-Cr-Ti system[1,2,3] have demonstrated that Ti addition is beneficial for improving the fracture toughness of solid-solution alloys and in situ composites. This beneficial effect, which is illustrated in Figure 1, is manifested as a marked increase in fracture toughness in alloys containing 30 to 40 at. pct. Ti.[2,3] The amount of toughness improvement achieved by Ti addition is more modest in the in situ composite, and the maximum toughness value appears at a Ti content of about 30 at. pct. While the enhancement is well documented, the mechanism by which Ti improves the fracture resistance in these materials is not fully understood. Correlation of fracture and flow properties revealed a complex relationship between fracture toughness and yield stress (Table 1). A plot of fracture toughness against yield stress shows a maximum toughness value at '765 MPa, which corresponds to the alloy with 37 at. pct. Ti and a tensile ductility greater than 6.5 pct. The lack of understanding is the result of the complex fracture process in the Nb-Cr-Ti alloys. In most of these alloys, the fracture toughness increases with Ti content even though the observed fracture mode is entirely or mostly cleavage, as illustrated in Figure 2, which shows the fracture appearances of Nb-Cr-Ti alloys as a function of Ti content and fracture toughness.[2,3] In the 30 to 40 at. pct. Ti range, where the maximum toughness occurs, several fracture modes including slipband decohesion, grainboundary cracking, cleavage, and dimpled fracture were observed.[1] The fractographic result, therefore, does not provide an unambiguous explanation for the toughness KWAI S. CHAN and DAVID L. DAVIDSON, Institute Scientists, are with the Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted April 6, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
enhancement by Ti addition. Neither the controlling fracture mechanism nor the source of toughness has been fully established. For example, it remains unclear which is the toughness-limiting mechanism among the observed fracture processes, which include slipband decohesion, grain-boundary cracking, cleavage, and dimpled fracture. Although dimpled fracture is present in Nb-Cr-Ti alloys, the size of the microvoids is small and they cover only a small percentage of the fracture surface.[1] Therefore, the microvoids should have a negligible influence on toughness. The implication is that the toughness increase in the Nb-Cr-Ti alloys is due to the deformation accompanying the other aforementioned fracture processes. An experimental correlation revealed that the fracture toughness of both the Nb-Cr-Ti solution alloys and the in situ composites increases with decreasing number of d 1 s electrons per atom in the material system.[2] This correlation is illustrated in Figure 3, which shows a log-log plot of fracture toughness vs the sum of the d and s electrons, per atom, in the alloys.[2] The result suggests that Ti addition affects the fracture toughness by changing th
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