The role of alpha and beta phases in fatigue crack propagation of Ti-Mn alloys
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I.
INTRODUCTION
IN the course of an investigation of the
low cycle fatigue behavior of c~-/3 Ti-Mn alloys, it was shown that the slope, C, of a log A ep/2 vs log Ni curve decreased with increasing average Bauschinger strain (ABS). 1 This implied that the Bauschinger effect might play a role in crack propagation. It was also noted 1 that fatigue life, in the low cycle region, was largest for the alloys with the highest cyclic strain hardening rate. In a parallel investigation 2 it was found that marked strain partitioning occurred between the a and /3 phases, with strain higher in the c~ phase. These two sets of observations suggested that crack propagation studies of ~-/3 Ti-Mn alloys would be worthwhile.
II.
EXPERIMENTAL PROCEDURE
A. Melting and Processing Six Ti-Mn ingots with the nominal compositions of 0.4, 2, 4, 6, 8, and 10 wt pct Mn were obtained commercially. The actual compositions are shown in Table I. The ingots were initially soaked at 1094 ~ upset forged 35 pct, then rolled to 10.2 x 27.9 x 50.8 cm. Each piece was cut in half and rolled from a thickness of 9.84 cm to 3.50 cm at 704 ~
JONG SOO PARK, formerly Graduate Student in the Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York, is now Research Engineer, Imperial Clevite, Inc., Technology Center, 540 East 105 Street, Cleveland, OH 44108. HAROLD MARGOLIN is Professor in the Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, NY, 11201. Manuscript submitted August 24, 1982.
METALLURGICAL TRANSACTIONS A
B. Heat Treatment Pieces 85.7 x 82.6 x 35 mm, to be used for compact tension specimens, were heat-treated to produce equiaxed structures. The final heat treatment was 24 hours at 700 ~ followed by a water quench, for all alloys except for the 10.0 Mn alloy, which was heat-treated at 670 ~ This lower temperature was used because at 700 ~ the 10 Mn alloy is too close to the/3 transus to avoid going above and below this temperature during heat treatment. The specific heat treatments used are shown in Table I.
C. Specimen Preparation Following heat treatment 1.6 mm was machined off the 85.7 x 82.6 mm surfaces of the specimen, and 3.2 mm was removed from each of the other surfaces. The compact tension specimens were then machined according to ASTM specifications E-399, 1974. 3 A strip approximately 6 mm wide was then electropolished on the specimen in the region where the crack was expected to propagate. To prepare the notch for crack propagation a slot 0.09 mm in radius and 1.5 mm in length was produced by a spark discharge machine. The specimen was then fatigued at 30 Hz at 21,128N at 0.1 R-ratio for 1.5 x 105 to 5 x 105 cycles, depending on the specific alloy. Near the end of this cycling a fatigue crack formed and began to grow.The crack at this point was approximately 0.5 mm in length. The loading and crack propagation directions are indicated by the use of two symbols. For example, in the LT designation, L refers to the loading direction and T to the
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