Yielding and deformation behavior of the single crystal superalloy PWA 1480
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I.
INTRODUCTION
NICKEL-base superalloys are being used in single crystal form due to improvements in creep, rupture, and fatigue resistance over conventionally cast alloys.l PWA 1480 is a modem single crystal alloy which is being used in gas turbine engines as a turbine blade material, 1 and is being considered for use in the space shuttle main engine (SSME) as the turbopump blade material. 2 In this study, mechanisms of monotonic deformation in PWA 1480 were investigated as a function of temperature, strain level, and strain rate. This information provides a basis for the next phase of the project, which is the development of monotonic and cyclic constitutive models. In addition to the specific goal of characterizing the material as a part of the SSME program, a further goal was to add to the fundamental understanding of deformation processes in high 3/' volume fraction superalloys.
II.
EXPERIMENTAL PROCEDURES
A. Material
The composition of the alloy is given in Table I. Single crystal bars measuring 2.5 cm in diameter and 15 cm in length were produced by TRW, Inc. The crystal orientations were determined by the Laue back reflection X-ray technique. All bars whose tensile axes were within 10 deg of (001) were accepted. The bars were subjected to a heat treatment procedure consisting of a solutionizing treatment at 1285 ~ for 4 hours followed by rapid cooling, an aging WALTER W. MILLIGAN, Graduate Research Assistant, and STEPHEN D. ANTOLOVICH, Director and Professor, are with the Fracture and Fatigue Research Lab, School of Materials Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245. Manuscript submitted February 10, 1986. METALLURGICALTRANSACTIONSA
Table I.
Element A1 Ti Ta Cr Co W C Ni
Alloy Composition
Weight Percent 4.8 1.3 11.9 10.4 5.3 4.1 (42 ppm) bal.
treatment at 1080 ~ for 4 hours followed by rapid cooling, and a final aging treatment at 870 ~ for 32 hours followed by air cooling. Test specimens were designed and machined in accordance with ASTM Specification E-8. Two specimens were machined from each bar, and specimens had a 6 mm diameter and a 25 mm gage length. B. Mechanical Testing
Interrupted tensile tests were conducted in order to study deformation mechanisms which operate during yielding. Tests were conducted at 20,705,760, 815,871,927, 982, and 1093 ~ and at strain rates of 8.33 x 10 -5 s -1 (0.5 pct/min) and 8.33 x 10-3 s -l (50 pct/min). Testing was done under total strain control using a SATEC computerized test system which was equipped with a high temperature extensometer. Specimens were induction heated, and a temperature gradient not exceeding • ~ was maintained over the gage length. After the preprogrammed plastic strain level was reached (nominally 0.1 or 0.3 pct), the specimens were immediately returned to zero load and cooled to room temperature. Samples of the same alloy which were tested to failure were supplied by Pratt&Whiteney Aircraft. Although VOLUME 18A, JANUARY 1987--85
the P & W material has a slightly higher A1 + Ti content than the TRW materi
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