Low cycle fatigue behavior of Ti-Mn alloys: Fatigue life
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I.
INTRODUCTION
RESULTSof fatigue life tests are usually plotted as number of cycles to failure, IV/, vs stress, total strain or plastic strain, depending on the control mode of the test. In total strain control, the total strain amplitude Ae,/2 is expressed as the sum of the elastic, Aee/2, and plastic, Aep/2, amplitudes. This relationship can be written as:~'2 t
A e , / 2 = Age~2 + Aep/2 = E (2N:)~ + e[ (2N:) c
where o'~, ef', b, and c are material constants. This equation is known as the Coffin-Manson relationship. By using the above equation, it can be demonstrated that fatigue life is governed by stress for long lives and plastic strain for short lives) A log-log plot of Aep/2 vs N: has been shown to fall on a straight line. However, such plots do not always yield straight lines. 3-7 The effect of microstructural parameters on fatigue properties of titanium alloys have been the subject of numerous studies. Sattar, et al 8 reported that a-fl forged titanium alloys had a better resistance to crack initiation than /3 forged. Ashton and Chambers 9 studied the mechanical properties of forged Ti alloys. Microstructures consisting entirely of Widmanst~itten alpha (Wa) had the highest strength and the lowest ductility and had a much shorter life (2 • l04 vs 1.6 • l06 cycles) compared to a microstructure containing a globules and stringers, at a given stress level. Hoo m showed that in a Ti-5.2AI-5.5V-0.9Fe-0.5Cu alloy high cycle fatigue life increased with decreasing a particle size. This was consistent with the results of Lucas and Konieczny ~j who found fatigue strength increased with a decrease in a grain size in Ti-6A1-4V and Ti-6A1-6V-2Sn. Low cycle fatigue life depends primarily on crack growth, since cracks which propagate to failure, nucleate in the early cycles. It is of interest, therefore, to consider the effect of microstructure on fatigue crack growth (FCG) rate. Irving and Beevers ~2'13 studying the FCG rate of Ti-6A1-4V with five different microstructures, found that in the microY. SALEH is Senior Research Scientist with Olin Corporation, 91 Shelton Avenue, New Haven, CT 06511. H. MARGOLIN is Professor, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, NY ll201. Manuscript submitted November 17, 1980. METALLURGICAL TRANSACTIONS A
structure sensitive range (AK -- 10 to 12 M N m -3/2) the growth rate was lowest for the /3 annealed condition. Williams and Rauscher ~4 also showed that the /3 annealed material was related to the more frequent secondary cracking which reduced the striation spacing. Another work ~5 also indicated a slower FCG rate for/3 forged Ti alloys. Most available work on fatigue properties of titanium alloys has been carried out on alloys with commercial processing in which microstructures produced are not always well defined and uniform. A systematic study of the effect of microstructural parameters involving two basic morphologies of the a phase (equiaxed (E) and Widmanstatten) seemed necessary. The Ti-Mn system was chosen as a model for this study, since the phase diag
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