Temperature Dependent Deformation Mechanisms of Alloy 718 in Low Cycle Fatigue
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T. H. SANDERS, JR. is Research Scientist, Fracture & Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, G A 30332. R. E. FRISHMUTH is Engineer in Program Support and G. T. EMBLEY is Engineer in Mechanics of Materials at General Electric Company, Gas Turbine Division, Schenectady, NY 12345. Manuscript submitted September 22, 1980. METALLURGICAL TRANSACTIONS A
MATERIALS AND TEST PROCEDURES The material used in this text program was wrought alloy 718 in the form of 1.905 cm (0.75 in.) diam barstock, which received the heat treatment shown in Table I. The chemical composition and mechanical properties are provided in Table II. An ASTM grain size of 10 was measured for this material. The specimen design used was a 0.635 cm (0.25 in.) diam circular specimen with a uniform gage and button-head ends as recommended in ASTM E606-77T. All tests were conducted at Mar-Test, Inc., Cincinnati, Ohio. Testing was performed in air under total axial strain control using a closed loop, servocontrolled, hydraulically activated test machine. Sample heating was accomplished by means of an induction heater. Throughout each test, axial force and plastic strain signals were continuously monitored. The plastic strain signal was obtained from the force and total strain through the use of an analog strain computer. Initiation in this program was defined to be the cycles to obtain the first noticeable increase in plastic strain after the initial strain softening had occurred. Detailed descriptions of the test procedures used are contained in Refs. 2 and 3. All testing was conducted at a frequency of 20 cycles/min. TEST RESULTS LCF test results obtained for temperatures up to 647 ~ (1200 ~ are listed in Table III. Examination of the recorded plastic strain and load signals indicated that the material cyclically softened and that all softening had occurred by the time N I / 2 was reached (where N l / 2 is defined as a number of cycles to specimen separation. The relationship between total axial strain, A~, cycles to initiation, N,, and temperature is plotted in Fig. 1. At cyclic lives of less than about 104 cycles the lifetime
ISSN 0360-2133/81/0611-1003500.75/0 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 12A, JUNE 1981--1003
Table I. Heat Treatment ot Wrought Alloy 718 Used In LCF Testing Solution treat at 968 ~ for 1 h and oil cool. Age at 718 ~ hold 8 h, furnace cool at rate of 55.5 ~ to 622 ~
decreases with temperature. Above 104 cycles this trend begins to reverse. Above t05 cycles the strain required to produce failure decreases with temperature from 538 to 204 ~ (1000 to 400 ~ Figure 2 is a plot of plastic strain range, Acp, vs cycles to crack initiation, N,. This figure indicates that for temperatures up to 427 ~ (800 ~ dependence of N,
Table II. Composition and Tensile Properties of Alloy 718 C
Mn
P
S
Si
Cr
Ni
0.05
0.01
0.005
0.004
0.12
18.28
51.93
Cu
Mo
Ti
Fe
AI
Co
B
(Cb + Ta)
0.03
2.88
1.03
20.06
0.47
0.12
0.005
5.01
Tensile Properties: Tempera
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