Microstructure and sound velocity of Ti-N-O synthetic inclusions in Ti-6Al-4V
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I.
INTRODUCTION
NITROGEN and oxygen stabilize the alpha phase in titanium, tll Regions within Ti alloy parts containing high local levels of N and O can stabilize a hard-alpha phase. In an investigation of hard alpha-stabilized defects in Ti-6A1-4V (Ti64), the hardness at the center of the alpha-stabilized regions ranged from Rockwell C 50 to 80, compared with base alloy values of Rockwell C 32 to 36; and the alpha-phase composition at the defect center was measured to be 0.4 to 5.0 wt pct A1 and 0.5 to 3.1 wt pet V, compared to 5.5 to 7.2 wt pct A1 and 1.4 to 2.8 wt pct V in the alpha phase of the base alloy, t21 These defects were measured to contain 3.5 to 14.8 wt pct N and up to 2.5 wt pct O. t3J The presence of hardalpha-stabilized defects is extremely undesirable because these defects can seriously degrade materials performance.t4.5] Nondestructive inspection of Ti parts includes ultrasonic techniques to find hard alpha. 161 Extensive work has related the microstructure of the Ti matrix alloy to ultrasonic behavior. [7-1~ The work, described subsequently, focused on the nature of the hard-alpha inclusion; it evaluates the role of hard-alpha composition on the physical properties associated with ultrasonic reflectance and correlates the detectability of hard alpha with its composition.
II.
EXPERIMENTAL PROCEDURE
A. Preparation of Titanium-Nitrogen-Oxygen Alloys Titanium-nitrogen samples were prepared by three techniques: (a) hot isostatic pressing (HIP) consolidation of blended Ti and TiN powders; (b) arc melting of Ti sponge in a N-containing atmosphere; and (c) arc melting of sponge Ti plus TiN powder. Each technique produced Ti-N alloys containing a certain amount of oxygen carried in with the constituent raw materials. M.F.X. GIGLIO]q'I, Metallurgist, R.S. GILMORE, Physicist, and L.C. PEROCCHI, Specialist, are with Corporate Research and Development General Electric Company; Schenectady, NY. Manuscript submitted February 17, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
1. HIP consolidation of Ti plus Ti-powder blends Five compositions were prepared by mixing 16-g sampies of powder. The powders used were Baker Ti powder, Lot 512350, and CERAC* TiN, Lot 2332. The *CERAC is a trademark of Cerac, Inc., Milwaukee, WI.
five nominal compositions prepared were 12.8, 6.8, 4.7, 3.5, and 1.8 wt pct (33.3, 20.0, 14.3, 11.1, and 5.9 at. pct) N. The powders were blended by rolling in jars for 20 hours. The powders were "hip ped" at 1400 ~ and 210 MPa for 3 hours. The HIP cans consisted of drilled rods of Ti64.
2. Arc melting of Ti sponge in a N-containing atmosphere Two alloy compositions were made by melting Ti sponge in a N atmosphere. First, 333 g of Ti sponge was arc melted in a static environment of Ar-15 vol pct N. The metal was melted into a small cylindrical button, about 75 mm (3 in.) in diameter. The alloy was melted three times in the Ar-N atmosphere. Then, the ingot was melted three times in an Ar atmosphere (without nitrogen) to homogenize the composition. The metal was weighed before and after
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