Effect of thermomechanical treatments on size and distribution of suicides and tensile properties of alloy Ti-6Al-5Zr-0.
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Effect of Thermomechanical Treatments on Size and Distribution of Silicides and Tensile Properties of Alloy Ti-6AI-5Zr-0.5Mo-0.25Si C. RAMACHANDRA and VAKIL SINGH Alloy Ti-6A1-5Zr-0.5Mo-0.25Si (685) is an important near a titanium alloy designed for high temperature applications in jet engines. The authors ~ have studied the precipitation and characteristics of silicides under differently heat treated conditions of alloy 685 and have clearly demonstrated that silicides have a deleterious effect on tensile ductility of the alloy at room temperature. 2 Similar detrimental effect of silicide precipitation on tensile ductility has been observed also in alloy 6843 and 829. 4 However, Tien et al. 5 have observed considerable improvement in ductility of a/3 titanium alloy by manipulation of the silicide morphology with Widmanst~itten plates to evenly distributed cuboids through thermomechanical treatment. In most of the silicon bearing titanium alloys, there is heterogeneous precipitation of silicides in the thermally heat treated conditions. 3,4.6-9 The present work is concerned with the effect of thermomechanical treatment on size and distribution of silicides and their consequent effect on the tensile properties of alloy 685. The alloy 685 used in the present investigation was obtained from Titanium International, U . K . , in the form of 25 mm diameter rods in the /3 forged and mill-annealed condition. It contains by wt pct 6.18A1, 5.27Zr, 0.5Mo, 0.28Si, 0.01N, 0.024Fe, 0.140, and 30 ppm H. Flat pieces (55 x 12 x 3.5 mm) were sectioned, encapsulated in evacuated silica tubes with titanium getter, and solution treated in the/3 range at 1323 K for 0.5 hour using a Kanthal wound resistance heating vertical tube furnace. The /3 transus temperature for alloy 685 has been reported l~ to be nearly 1298 K. Following solution treatment, the pieces were quenched in water at room temperature and were subjected to aging treatments under vacuum (~10 -5 MPa). The as-quenched blanks aged without any cold reduction are designated as WQ-A, and those aged following cold reductions of 6, 12, and 15 pct are designated as WQ-6CR-A, WQ-12CR-A, and WQ-15CR-A, respectively. The reported temperatures are accurate to within • K. Flat tensile specimens with gage dimensions of 15 x 5 x 2.3 mm were prepared with adequate shoulder radius and head portion for convenient gripping in a coaxial manner. Tensile tests were conducted at room temperature at a nominal strain rate of =1.11 x 10 -4 s -1 using a 5 ton Instron. Thin foils for electron microscopy were prepared from the head portion of the tensile samples by twin jet electropolishing technique described elsewhere j or by window technique. The foils were examined using Philips EM 300 or JEOL 200 CX transmission electron microscopes. Several dark-field transmission electron micrographs were
C. RAMACHANDRA, Lecturer, and VAKIL SINGH, Reader, are with the Department of Metallurgical Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India. Manuscript submitted Septembe
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