The flow and fracture of a Ti 3 Al-Nb alloy
- PDF / 3,267,892 Bytes
- 9 Pages / 594 x 774 pts Page_size
- 70 Downloads / 191 Views
I.
INTRODUCTION
IN a response to the need for advanced alloys possessing high strength-to-density ratios at elevated temperatures, new families of alloys based on intermetallic compounds are being developed. In particular, there has been considerable developmental activity directed toward the use of alloys based on the titanium aluminide phase, alphatwo (Ti3AI), as a means of extending the use of titanium alloys to temperatures as high as 650 ~ A generic problem with titanium aluminide alloys, as well as most other aluminide-base alloys, is their limited tensile ductility and poor toughness at low temperatures. Research on Ti3A1 and single-phase alloys based on Ti3A1 indicates that while planar slip with Burgers vectors contained within the basal plane readily occurs, it is very difficult to activate slip or twinning, which provides caxis extension or compression at low temperatures. ~.2,3J As a result, large strain incompatibilities occur which cause local normal stresses sufficient to nucleate and propagate cracks. Thus, single-phase Ti3A1 is brittle at low temperatures. At elevated temperatures (->600 ~ Ti3A1 exhibits ductile behavior, which is believed to be due to the onset of climb and c-component slip. tl,2'31 Considerable developmental effort has been directed at exploring alloy additions which might impart some low-temperature ductilities to the alpha-two phase. Perhaps the most significant is the recognition that the addition of Nb in sufficient amounts to retain a small amount of beta phase can impart ductility at room temperature.ln~ On that basis, alloy development programs have concentrated on alloys based on alpha-two phase but which contain small volume fractions of the beta phase. The Ti-24Al-llNb (Ti-14AI-21Nb in weight percent) alloy, hereafter referred to as Ti-24-11, serves as a model system for this new family of alloys. Although this alloy can be processed to several microstructural conditions with complex microstructures, t5 9] of particular interest is a fine-scale (beta + alpha-two) "basket-weave ~ microstructure consisting of laths of the alpha-two phase in a continuous matrix of a "beta" phase. The alpha-two phase,
D.A. LUKASAK, formerly with The Pennsylvania State University, is Senior Engineer, ALCOA Technical Center, Alcoa Center, PA 15069. D.A. KOSS, Chairman, is with the Metals Science and Engineering Program, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted December 29, 1988. METALLURGICAL TRANSACTIONS A
which occupies more than 90 pct of the structure (despite being discontinuous), contains a significant amount of Nb (roughly 8 to 10 at. pct), most of which remains in solution. With the processing used in the present study, the alpha-two phase retains the D0~9 structure of Ti3A1; the "beta" phase is Nb-rich (38 to 40 at. pct Nb) and remains a disordered bcc phase after the processing treatments, f~~ It should be recognized that the beta phase is frequently ordered in alloys similar to Ti-24-11, especially after rapid cooling. [5~6,8,H1Studies
Data Loading...
