Creep and Stress Rupture Behavior of Ti-24Al-11Nb
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CREEP AND STRESS RUPTURE BEHAVIOR OF Ti-24A1-1 1Nb
WEGO WANG* AND LI-TUSNG CHIEN** *U.S. Army Research Laboratory, Materials Directorate, Watertown, MA 02172-0001, USA **Du Pont Taiwan Limited, Taiwan, ROC.
ABSTRACT This work concerns the temperature and stress dependencies of the steady-state creep rate of a Ti-24A1-l lNb alloy in the temperature range between 593 and 760'C (1,100 and 1,400'F) and stress levels 137.9 to 275.8 MPa (20 to 40 ksi). Tests were conducted on both as-extruded and heat-treated specimens. A transition occurs in the creep curves at 704'C (1,300*F) and 206.9 MPa (30 ksi), indicating a creep mechanism change. Generally speaking, the fracture surface of as-extruded specimens shows a ductile transgranular fracture mechanism; a more ductile fracture mode is observed for heat-treated specimens.
INTRODUCTION The relatively low density and superior high-temperature strength of titanium aluminide alloys have made them attractive candidates for aircraft engine materials. However, data on the high-temperature creep and stress rupture properties of the Ti-24A1-1 1Nb alloy are still sparse, and little systematic analysis has been reported on the high-temperature deformation characteristics of this alloy. The major objective of the current study is to investigate the fundamental creep/stress rupture characteristics of this alloy and to establish the stress and temperature dependencies of the steady-state creep rate.
MATERIAL The material for this study was supplied by Nuclear Metals, Inc. Raw material powders atomized by the Plasma Rotating Electrode Process (PREP) were first compacted in two low carbon steel canisters and then extruded with an area reduction ratio of 13.76. The extrusion die had a diameter of 1.40 cm (0.55 inch) and the maximum force applied during this extrusion was 270 tons. The two extruded rods were 93.7 and 96.3 cm (36.9 and 37.9 inches) long, respectively, and had the low carbon steel canning material as an intimate cladding. Slices of materials were machined from the extruded rods for heat treatment and hardness tests. The heat treatment was conducted at 1,000'C (1,832*F) for 4 hours in nitrogen, followed by a furnace cool to room temperature. The microstructures of specimens prior to and after heat treatment are shown in Figs. 1 and 2, respectively. A mixed microstructure of primary oa2 phase and ot2+13 matrix is observed in Fig. 1. The largest primary oC2 size is about 10 gm in the as-extruded sample. The a2+p matrix shows a moderately deformed microstructure resulting from the prior extrusion process. The cz2 +D3
Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
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matrix that has been annealed and transformed to the coarse equiaxed a 2 phase as a result of the heat treatment is shown in Fig. 2. A fine Widmanstatten structure is also observed within the aC 2 grains. The average size of a2 increases to about 15 ýtm after the sample had been heat treated. The microstructural difference is also reflected by their respective hardness values. The Rockwe
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