Textural and microstructural gradient effects on the mechanical behavior of a tantalum plate
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I.
INTRODUCTION
DESPITE results which clearly show that inhomogeneities in texture are often present in rolledt2-4J and annealedts,61 materials, or both, m the texture at the centerline of plate material is often assumed to be representative of the bulk. Accounting for the spatial distribution of texture in the prediction of plastic anisotropy has generally been considered impractical both in terms of modeling and measuring spatial texture variations. However, the spatial distribution of texture has recently been implemented in a number of investigations by Dawson and co-workers, starting with the research described in Reference 7. Through-thickness variations in texture are generally measured by preparing samples for X-ray diffraction at different depths in the plate. Recent advances in the measurement of single orientations by automatic analysis of electron backscatter diffraction patterns have made it practical:to directly measure the spatial distribution of texture from through-thickness sections of plate material. This technique is employed to investigate the effects of the texture gradient in a tantalum plate on the mechanical behavior, along with the effects of grain size. II.
EXPERIMENTAL DETAILS
A. Compression Testing The material for this study was rolled and annealed vacuum arc-remelted tantalum plate. The nominal chemistry is reported in Table I. Cylindrical compression test specimens (5-mm diameter, 6.2-mm length) were electrodischarge machined (EDM) from the plate. Specimens were prepared with center axes parallel to the principal directions of the plate. A minimum of two specimens in each orientation were compressed at strain S.I. WRIGHT, G.T. GRAY, III, and A.D. ROLLETT, Staff Members, are with the Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted June 16, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
rates of 0.001 s -~ at 25 ~ The stress-strain curves of the through-thickness specimens exhibited an unusual double yield drop, as shown in Figure 1. Yield drops in body-centered cubic (bcc) metals are often associated with interstitial unlocking phenomena, t81 However, in this case, examination of the throughthickness compression samples after straining revealed that the samples were slightly hourglass shaped rather than remaining cylindrical or barreling slightly as commonly observed in compression samples. In contrast, samples with axes parallel to the longitudinal and transverse sections remained cylindrical and did not show double yield drops. Repeated tests performed using a variety of lubricants confirmed the unusual response of the through-thickness sample. These observations indicate that the yield response of this material cannot be completely attributed to interstitial effects. The influence of textural and microstructural inhomogeneities on the nonuniform plastic response must also be considered.
B. Metallography Optical metallography of the as-received material revealed an equiaxed microstructure. The average grain diameter was measured using a line-interc
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