The effect of processing on the hot workability of Ti-48Al-2Nb-2Cr alloys
- PDF / 924,674 Bytes
- 11 Pages / 612 x 792 pts (letter) Page_size
- 5 Downloads / 192 Views
TRODUCTION
TWO-phase, near-g titanium aluminides, such as Ti48Al-2Nb-2Cr (at. pct), are being considered for high-temperature structural applications in the aerospace and automotive industries due to their high strength and good oxidation resistance at elevated temperatures.[1–4] Near-g alloys with useful engineering properties have been developed by understanding the effect of alloying additions on their microstructure and properties. A variety of microstructures, such as duplex, near-gamma, near-lamellar, and lamellar structures, can be generated by heat treatment. Fine microstructures, though, can only be achieved by thermomechanical processing.[4,5] Although hot working by isothermal forging and extrusion has been widely used by many investigators to study microstructural development, there are only a limited number of studies dealing with hot working behavior of g-titanium aluminides.[6–20] These reports on hot workability have focused on ingot metallurgy (I/M) processing and typically have emphasized one material starting condition.[14,21– 24,26,27] The purpose of this study is to evaluate the effect of processing on the hot workability of a single alloy composition. II.
EXPERIMENTAL PROCEDURES
Materials with a nominal composition of Ti-48Al-2Nb2Cr (unless otherwise noted, all compositions are in atomic percent) were prepared by both I/M and powder metallurgy (P/M) techniques. The actual compositions and processing steps are presented in Tables I and II, respectively. The I/M samples were prepared from a single 20-kg vacuum induction skull melted ingot prepared by Duriron Co., Inc. (Dayton, OH). The ingot (ID TA-22) measured 70 mm in diameter by 750 mm. The ingot was hot isostatically pressed (‘‘Hipped’’) at 1260 7C/172 MPa/4 h to eliminate any residual casting shrink or porosity. The ingot was then sectioned into 250-mm lengths and heat treated to homog-
G.E. FUCHS, Advisory Engineer, is with Lockheed Martin Corporation, Schenectady, NY 12301-1072. Manuscript submitted February 7, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
enize the as-cast 1 hipped structure. The effect of various homogenization heat treatment cycles on the microstructure of this alloy was previously examined.[24] Two of the I/M material starting conditions selected for this study were based on these results. Ingot metallurgy samples were homogenized to produce either a duplex microstructure after heat treatment at 1200 7C for 96 h (ID TA-22A) or a lamellar microstructure after heat treatment at 1440 7C for 20 minutes (ID TA-22C). The TA-22A sample exhibited a near-g microstructure with equiaxed g grains with a small amount of equiaxed a2 and b grains (Figure 1). A significant amount of coarse twins were observed in the g grains. A limited amount of g/a2 lamellar grains were observed in the TA-22A microstructure. These lamellar grains were remnants of the prior a-dendrites present in the as-cast microstructure. Due to the low homogenization heat treatment temperature (i.e., 1200 7C), these lamellar grains were not eliminated during
Data Loading...
