Influence of foreign particles on fatigue behavior of Ti-6Al-4V prealloyed powder compacts
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INTRODUCTION
T w o decades of intensive research and development activities have resulted in methods, processes, equipment, and substantial experience to produce complex-shaped high integrity titanium alloy powder metallurgy components for demanding airframe and gas turbine engine applications, x'2'3 Plasma Rotating Electrode Process (PREP) prealloyed spherical powders are now commercially produced and handled in a dedicated clean room facility,4 giving the users the ability to produce components almost free of contarninants. However, the vast experience with nickel-base prealloyed powders 5-8 has shown that producers and users of high performance rotating engine components would like to establish the effect of defects on fatigue strength of compacts. In spite of all precautions, it is impossible to obtain completely inclusion-free prealloyed titanium powders. But, drawing a parallel with nickel-base alloys, it is likely that only "large" particles will degrade properties; those below a certain size will be innocuous. If the inclusions are too small to affect mechanical properties, they can be accepted; if they degrade mechanical properties, four options suggest themselves: change to a more defect-tolerant alloy, manipulate the microstructure so that it becomes more accommodating to the foreign particles, 8 remove the contaminants by cleaning, or define the extent to which properties are degraded, and determine whether the resulting levels are acceptable. 5'6'8 It should be noted that even ingot metallurgy products contain a low level of small defects and airframe and engine manufacturers accept from experience a certain level of probability of component failure from these defects. 9 Past work ~~ showed that material with defects suffers a loss in fatigue strength, while contaminant-free powder compacts display fatigue strength equal to or exceeding defect free wrought alloy levels due to finer grain size and lack of texture. 10 S. W. SCHWENKER, Materials Engineer, and E H. FROES, Technical Area Manager, Titanium Programs, are with Air Force Wright Aeronautical Laboratories, Materials Laboratory, AFWAL/MLLS, Wright-Patterson AFB, OH 45433. D. EYLON is Principal Investigator and Group Manager, Metcut-Materials Research Group, P.O. Box 33511, Wright-Patterson AFB, OH 45433. Manuscript submitted April 1, 1985. METALLURGICALTRANSACTIONSA
The objective of the present work was to study systematically the effect of various types and size ranges of contaminants on the room temperature fatigue strength of PREP Ti-6A1-4V compacts. This was accomplished by artificially seeding an ultra clean powder with three species of contaminants in three size ranges, a total of nine conditions. A detailed fractographic characterization and precision sectioning ~6'19'2~were employed to assist in analyzing the fatigue results. The selection of contaminant species and their sizes was based on earlier work l~ and represents the types and sizes of inclusions that in the past have been introduced into Ti-6A1-4V powder compacts by crosscon
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