Superplastic behavior of two-phase titanium aluminides
- PDF / 887,184 Bytes
- 5 Pages / 613 x 788.28 pts Page_size
- 2 Downloads / 234 Views
I.
INTRODUCTION
THE titanium aluminides, TiA1 (gamma) and Ti3AI (alpha-2), have considerable potential for application in the aerospace industry because of their attractive elevated temperature strength and low density, t~-61 Both intermetallic compounds, however, normally have low ductility at room temperature which restricts their application. Thus, methods to improve their ductility, as well as retain their strength, are a major challenge. It is the purpose of the present study to investigate the mechanical properties of a fine-grained Ti-AI alloy consisting of the two phases TiAI and Ti3A1 distributed uniformly throughout the material. This fine microstructure is readily achieved if a composition is selected in the two-phase region where approximately 50 vol pct of the two aluminides, TiAI and Ti3AI, coexist. An examination of the Ti-AI phase diagram t6"71reveals that such an alloy composition has many characteristics of a hypereutectoid steel. That is, the alloy can be heated into a single-phase region at high temperature where the aluminum is completely in solution in the hexagonal closepacked structure of titanium (equivalent to austenite in steel); upon cooling, proeutectoid TiAI will precipitate followed by a eutectoid reaction, wherein a lamellar structure of TiAI and Ti3A1 is obtained (equivalent to pearlite in steel). If the alloy is mechanically worked below the eutectoid temperature, the lamellar structure will break up to form two equiaxed fine-grained phases (equivalent to strain-enhanced spheroidization in eutectoid steels). Such a Ti-A1 alloy can be expected to have good strength and good ductility at room temperature and exhibit superplastic behavior at elevated temperature. The processing and mechanical properties, especially the superplastic characteristics, of a Ti(57 at. pct)-Al(43 at. pct) alloy are described in this article. II. M A T E R I A L , P R O C E S S I N G , AND EXPERIMENTAL PROCEDURES The two-phase Ti-A1 alloy used in the present study was prepared by Lipsitt and colleagues, t~-3'51 Powders of S.C. CHENG, Graduate Student, and O.D. SHERBY, Professor, are with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. J. WOLFENSTINE, Assistant Professor, is with the Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92717. Manuscript submitted August 26, 1991. METALLURGICAL TRANSACTIONS A
two different composition Ti-Al alloys were initially prepared by a rotating electrode process. One composition was at 50.5 at. pet Ti, and the other composition was at 73.2 at. pct Ti. The two powders were mixed in proportion to achieve a composition of Ti(57 at. pet)A1(43 at. pet). The powders were packed in a can, evacuated, sealed, and then hot extruded above 1200 ~ with a reduction of 26: l, to a diameter of 9.52 mm. The microstructure of the extruded bar is shown in Figure 1. A lamellar structure is evident as a result of the eutectoid transformation after extrusion. (The rapid cooling suppressed the formation of pr
Data Loading...
