Materials Week '97: Wednesday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday afternoon, September 17.

HIGH TEMPERATURE DEFORMATION: General Abstract Session

Session Chair: Rajiv S. Mishra, University of California, Davis, CA

GRAIN SIZE AND TEMPERATURE DEPENDENCE OF SUPERPLASTIC DEFORMATION BEHAVIOR IN AN Al-Mg ALLOY UNDER NEAR-ISOSTRUCTURAL CONDITIONS: D.H. Bae, A.K. Ghosh, Dept. of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109

To understand superplastic deformation mechanisms, stress-strain rate behavior of a fine grain processed Al-Mg alloy has been characterized by conducting an improved multi-step strain rate change test method to preserve near isostructural conditions during the test over a temperature range of 400-550°C. The grain size of the alloy was varied in the range of 8-30µm by varying thermomechanical processing conditions. A sigmoidal relationship between stress and strain rate is observed unequivocally in each case. Activation energy (Q), stress exponent (n) and grain size exponent (p) are calculated as a function of strain rate. In high strain rate region, n is 4.55, p is 0.37 and activation energy is close to that of lattice self diffusion. These results are quantitatively in agreement with coupled dislocation glide-climb process, except for a small grain size dependence which has not been clearly identified in the literature. In low and intermediate strain rate region, using an effective stress versus strain rate relationship, n is 1.7, p is 2.07 and activation energy is somewhat higher than that of grain boundary diffusion. In this strain rate region, grain boundary sliding accommodated by the climb of dislocations within the mantle region is found to provide good agreement.

2:20 pm

HOT TORSION OF CREEP RESISTANT RAPIDLY SOLIDIFIED ALLOYS FVS 1212 AND FVS 0812 (8009): D. Shimansky, H.J. McQueen, Mechanical Engineering, Concordia University, 1455 de Maisonneuve Blvd. W., H-549-34, Montreal, Quebec H3G 1M8, Canada

Isothermal hot torsion tests were performed on the rapidly solidified (RSP) aluminum alloys FVS1212 (Al-12.4Fe-1.2V-2.3Si) and FVS 0812 8009) over the T range of 300 to 600°C and at strain rates from 0.1 to 4.0 s^-1. Due to deformation heating and possible localized dynamic. recrystallization, stress peaks feature prominently in the - curves. Fracture strain increased with rising T, decreasing ; ductility is low compared to conventional Al alloys. Flow stresses decreased gradually with increasing deformation T and declining . The activation energies were much higher than those of pure Al due to the high volume fraction of very fine nearly spherical Al₁₃(Fe,V)₃Si dispersoids which present major obstacles to dislocation motion and which are very resistant to coalescence. In the (sinh)ⁿ function, there is a linear correlation between coefficient and slope s in the Arrhenius plots and an inverse correlation between n and ; as a net result of these relations activation energy is independent of for which the optimum value is 0.02 MPa^-1. Comparisons are made to high T deformation of other RSP alloys.

2:40 pm

APPLICATION OF SURFACE DISPLACEMENT MAPPING TO DUCTILITY ISSUES IN -TiAl BASED ALLOYS: N.E. Biery, M. DeGraef, T.M. Pollock, Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213-3890

Gamma TiAl-based alloys are good candidates to replace nickel-based alloys in some turbine engine components. However, the constraints that should be imposed on the design of turbine components to account for the lower ductility of these materials are not completely clear. One of the major concerns is the ability of these alloys to blunt stress concentrators through plastic deformation. To study this behavior we have developed a fast and inexpensive technique to measure surface strains. We will show that strain contours at the root of sharp notches in -based alloys agree well with FE solutions if the local microstructure is taken into account. The local strain at the notch root is considerably higher than that obtained in bulk tensile tests, suggesting that smooth bar testing underestimates the ability of these alloys to blunt stress concentrators.

3:00 pm BREAK

3:10 pm

MECHANICAL BEHAVIOR OF DEFENSE WASTE PROCESSING FACILITY SURROGATE GLASS AT ELEVATED TEMPERATURES: Brian M. Powers, Eric M. Taleff, University of Texas at Austin, Austin, TX 78712-1085

An experimental study of high-temperature mechanical properties was conducted on Defense Waste Processing Facility (DWPF) surrogate glass. DWPF glass is currently used to vitrify nuclear wastes for safe disposition. The DWPF surrogate glass used in this investigation differs from DWPF glass only in the use of nonradioactive materials as surrogates for radioactive waste products. The disposal process involves pouring molten DWPF glass, during which pour instabilities cause void formation. These voids result in stress concentrations as the molten glass hardens, often leading to crack formation. The mechanical properties of DWPF surrogate glass have been studied experimentally in order to better understand the fracture process. Mechanical tests have been conducted in a range of temperatures, up to 1050°C, through which the viscoelastic behavior of the DWPF surrogate glass varies greatly. The data accumulated are expected to lead to better mechanical integrity in future DWPF glass castings, and thus safer disposition of nuclear wastes.

3:30 pm

FRACTURE TOUGHNESS OF COPPER-BASE ALLOYS FOR FUSION ENERGY APPLICATIONS: D.J. Alexander, S.J. Zinkle, A.F. Rowcliffe, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6151

The fracture toughness of several high-strength copper alloys was measured from room temperature to 250°C to determine their suitability for structural applications in fusion energy applications. Oxide-dispersion strengthened copper alloys and precipitation-hardened copper-nickel-beryllium alloys showed a significant reduction in toughness at elevated temperatures (250°C). This decrease in toughness was much larger than would be expected from the relatively modest changes in the tensile properties. However, copper-chromium-zirconium alloys strengthened by precipitation showed only a small decrease in toughness at the higher temperatures. The embrittled alloys showed a transition in fracture mode from transgranular microvoid coalescence at room temperature to intergranular with localized ductility at high temperatures. The Cu-Cr-Zr alloy maintained the ductile microvoid coalescence failure mode at all test temperatures.