Temperature and microstructural dependence of the deformation of a high Nb Ti-Al alloy
- PDF / 5,766,450 Bytes
- 13 Pages / 598 x 778 pts Page_size
- 100 Downloads / 175 Views
I.
INTRODUCTION
EXTENSIVE investigation has been carried out on intermetallic compounds in an effort to find high strength materials that can retain their high strength at elevated temperatures. Among the candidates of interest is TiA1, which has a face centered tetragonal (Llo) structure. Unfortunately, neither the ambient temperature ductility nor the ambient temperature fracture toughness of TiA1 or its ternary alloys is particularly high. Improvements have been made, however, by the additions of Nb and Nb alloys. II-4~ These investigations generally consider alloys with small amounts of alloying elements such as 2 at. pct. Recently, though, Perepezko e t al. rsj have discovered a stable phase of Ti-44Al-11Nb existing above 1200 ~ called /'2, which is based on the B2 (CsCI) structure. By proper cooling, this phase transforms to a lamellar structure consisting of alternating laths of gamma (TiAI) and alpha-2 (Ti3A1).t61 The orientation relationships between the two phases are (111)3,1J(0002)c~2 and [11013,~111120]c~2.tT~ Since scant work has been done on this specific alloy, the authors propose to do a mechanical properties/microstructural study. Typically, alloys of the L12 or DOI9 type have one type of superdislocation (and its partials) responsible for their deformation properties. For example, in the case of Ni3A1, the core of a (101) superdislocation transforms from the {111} plane, where it is glissile, to the {100} plane, where it is sessile. Thermal activation is needed to move the dislocation, resulting in an anomalous temperature dependence of the flow stress. Briefly, the reason for the core transformation is as follows. A [101] superdislocation has a higher energy on the (111) plane than it does on the (010) plane. This is because the antiphase boundary (APB) energy on the (111) plane produced by 1/21101] shear is higher than that produced on J.T. KANDRA, ONR Postdoctoral Fellow, E.W. LEE, Materials Engineer, are with the Naval Air Warfare Center, Aircraft Division, Warminster, PA 18974. Manuscript submitted May 10, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
the (010) plane, t81 Takeuchi and Kuramoto proposed a model in which the driving force for thermally activated cross slip (core transformation) of screw dislocations from their glissile configuration on the (111) plane to their sessile configuration on the (010) plane is the anisotropy of the APB energy, the elastic anisotropy, and the applied s t r e s s . 191 For examples of this, one is directed to References 10 and 11. A similar phenomenon takes place in Mn3Sn and some other hexagonal-based DO19 intermetallic compounds, except that the superdislocation of importance is the (1120) type. The high energy glissile configuration is expected to occur on the (0001) basal plane, while the lower energy sessile configuration is expected to occur on the {10i0} prismatic plane, t~2'13] Cross slip from the basal plane to the prismatic plane could then be driven by two factors: first, cross slip onto the prismatic plane creates a low energy APB tha
Data Loading...
