Low cycle fatigue of Ti-Mn alloys: Microstructural aspects of fatigue crack growth
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INTRODUCTION
THE effect of microstructural parameters on mechanical properties of titanium alloys, including low cycle fatigue properties, has been the subject of a large number of investigations. In Ti-17, an a-/3 titanium alloy, processed to produce three types of microstructure, namely, equiaxed a in aged/3 matrix, Widmansthtten a in aged/3 matrix, and a mixed microstructure containing fl flecks ~ multiple crack nucleation and crack link-up was observed in all cases. In microstructures containing Widmanst~itten c~, multiple fatigue cracks formed early in life, and they were along a plate boundaries. In the equiaxed a structure, cracks nucleated at a interfaces or along slip bands within a particles, and they occurred later in life. In Ti-6-2-4-6, another commercial Ti alloy, the effect of two types of a morphologies, i.e., Widmanst/itten plus grain boundary and equiaxed on crack nucleation and early growth was studied. 2 At lower strains in both microstructures, cracks initiated at a-/3 interfaces and in the aged /3 matrix. At higher strains cracks occurred predominantly at slip bands within the a phase. For a given cycling condition, Widmanst~itten a and grain boundary a plates provided easier crack nucleation sites compared to the equiaxed a particles. This is the third in a series of articles in which Ti-Mn alloys have been used as model materials to study the effect of volume fraction of phases, morphology of a phase, and grain and particle size on fatigue properties of two phase a-/3 Ti alloys. In the first two of these papers 3'4 the cyclic stress-strain behavior and fatigue life of these alloys were examined. It was shown that a hardens cyclically,/3 softens, while a-fl alloys are intermedia'de in behavior. Grain size or morphology had no strong effect on the cyclic stress-strain response of the alloys. Based on total strain, the a alloy had the longest life at high strains; the/3 alloy had the longest life at low strains, and a-/3 alloys had the highest fatigue resistance at intermediate strains. Alloys with equiaxed (E) alpha microstructure showed longer life than those with Widmanst~itten ( W ) a . Increasing grain and particle size decreased fatigue life. The purpose of this paper is to show how the fatigue crack path is affected by microstructure. Y. SALEH is Research Scientist with Olin Corporation, Metals Research Laboratories, 91 Shelton Avenue, New Haven, CT 06511. H. MARGOLIN is Professor, Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, NY 11201. Manuscript submitted February 8, 1982.
METALLURGICALTRANSACTIONS A
EXPERIMENTAL PROCEDURES
Five binary Ti-Mn alloys containing 0.4 to 10.2 pct Mn were obtained from TIMET. The ingots were processed in several steps before being finally swaged at 700 ~ in the a-/3 field, to 20.1 mm diameter. Blanks for fatigue testing were heat treated under vacuum to produce several particle (or grain) sizes of E and Wa morphologies. The final heat treatment temperature in all cases was 700 ~ f
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