The effect of matrix strength on void nucleation and growth in an alpha-beta titanium alloy, CORONA-5
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I.
INTRODUCTION
T H E R E have been a number of papers on void formation and growth, which have recently been reviewed by Van Stone, Cox, Low, and Psioda. ~ This work has largely been concerned with hard particles in a ductile matrix. However, a number of studies have been concerned with two ductile phase structures, such as a-/3 titanium. 2-6 These studies have been concerned primarily with void behavior at constant yield strength and varying microstructures. The present study is concerned with the effect of matrix yield strength on void nucleation and growth at a series of constant microstructures, where the matrix is martensite, aged and unaged, and retained beta, aged and unaged.
II.
EXPERIMENTAL PROCEDURE
A. Material The material chosen for this study was CORONA 5, containing by weight 4.73 pct Mo, 4.75 pct AI, 1.43 pct Cr, 0.071 pct Fe, 0.007 pct N, and 0.133 pct O. The material was prepared as a 200 lb, 20.3 cm diameter ingot. The ingot was forged in the beta field at 1038 ~ (1900 ~ to 11.4 cm square and then forged to 7.6 cm round at 885 ~ in the a + /3 field. The beta transus was found to lie between 921 ~ and 927 ~ (1690 ~ and 1700 OF). Subsequent T.V. VIJAYARAGHAVAN, Graduate Student, and HAROLD MARGOLIN, Professor, are with the Department of Metallurgy and Materials Science, Polytechnic University, 333 Jay Street, Brooklyn, NY 11201. Manuscript submitted March 20, 1987. METALLURGICALTRANSACTIONS A
working was from 855 ~ (1570 ~ The 7.6 cm material was press forged from 7.6 cm to 5.7 cm. It was then hot rolled with a series of reheatings to rods 1.52 and 2.08 cm diameters.
B. Heat Treatment It was desired to produce a series of equiaxed a microstructures with a martensite matrix in one instance and a/3 matrix in the other. The microstructures and required heat treatments are given in Table I.
C. Tensile Testing Button head specimens with a 2.54 cm gage length and 0.65 cm diameter were tensile tested at room temperature on an Instron tensile machine at an initial strain rate of 0.0076 min -1. For true strain measurements, the diameter was monitored throughout the plastic region of the stressstrain curve.
D. Metallography Specimens were electropolished after grinding through 600 grit and were etched with R-etch. 6 Microconstituent measurements were determined by the linear traverse method. A minimum of 250 particles was measured to determine alpha particle size, d~, volume fraction,f~, and inter"particle spacing, h. For measurements of longest voids for a given strain, the specimens were serially sectioned. For purposes of maintaining void sizes unaltered by preparation procedure the specimens were epoxied prior to electropolishing and etching. The procedure is described elseVOLUME 19A, MARCH 1988--591
Table I.
Equiaxed Microstructures and Heat Treatments
Matrix* and H. T. I.D.
Heat Treatment
M-F-1 868 ~ 24 hours; WQ. M-I-1 899 ~ 72 hours; furnace cooled over 24 hours to 868 ~ 24 hours, 868 ~ WQ. M-C-1 899 ~ 332 hours; furnace cooled over 24 hours to 868 ~ 24 hours, 868 ~ WQ. B-F-1 790 ~ 24
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