Low Cycle Fatigue Behavior of Ti-6AI-2Sn-4Zr-6Mo: Part I. The Role of Microstructure in Low Cycle Crack Nucleation and E
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I.
INTRODUCTION
THE present study was undertaken to determine the effect of microstructure on the cyclic deformation behavior, LCF crack initiation, and early crack growth in an a + / 3 alloy, Ti-6A1-2Sn-4Zr-6Mo (Ti-6-2-4-6). This alloy was selected because of its extensive application in aircraft engines as compressor discs. The effect of the following microstructural parameters on LCF behavior, namely, morphology, ~ particle size, and prior/3 grain size, were studied. Results will be presented in two papers. This paper will describe the observations on nucleation and growth of cracks. In the following paper strain-life data and cyclic deformation behavior will be discussed.
II.
MATERIALS
The alloy was received in the form of as-extruded and swaged bars 20.7 mm in diameter. The chemistry was as follows: 6.0A1-2.0Sn-4.1Zr-6.0Mo-0.068 Fe-0.10 0-.012N. In general, processing resulted in a uniform fine E microstructure. The beta transus was determined by carrying out a series of heat treatments spaced at 2 to 3 ~ intervals. The heat treatment time was three hours. The beta transus was established as - 9 3 5 ~ To produce desired equiaxed (E) c~ structures, the asreceived 6-2-4-6 alloy was solution treated at 927 ~ in a vacuum of 1.3 • 10 -] N/m 2 for times ranging from two to 180 hours to obtain various sizes of c~ particles. Following the solution treatment, the alloy was step cooled to 871 ~ held for two to five hours, and water quenched. Aging was carried out at 621 ~ for 10 hours, and was followed by air
YASHWANT MAHAJAN is Development Manager, Development Group, at Design Bureau, Hindustan Aeronautics Ltd., Bangalore, 560017 India. HAROLDMARGOLINis Professor, Departmentof Physical and Engineering Metallurgy, Polytechnic Institute of New York, Brooklyn, NY 11201. Manuscript submitted September 17, 1980. METALLURGICALTRANSACTIONSA
cooling. These heat treatments produced a 0.2 pct yield strength of 1158 MPa (168 KSI). To produce Widmanstiitten plus grain boundary alpha (W + GBot) structures, the specimens were held at 954 ~ for one hour and water quenched. They were then reheated to 927 ~ for one hour and water quenched. They were then reheated to 927 ~ and the same procedure, employed for the Ec~ structure, was utilized for the remainder of the heat treatment. To vary the prior/3 grain size, the specimens were held at 954 ~ for various lengths of time, which ranged from 15 minutes to several days. The W + GBoz microstructure had a 0.2 pct yield strength of about 1103 MPa (160 KSI). Following heat treatment, several tensile specimens were machined to standard 0.635 cm diameter, 2.54 cm gage length test specimens. Tensile testing was carried out on a Tinius-Olsen testing machine at a strain rate in the range of 0.007 to 0.01 minute -l. The drawing of the fatigue specimens is shown in F i g ure 1. Specimens prior to testing were mechanically polished on a lathe through 600 grit paper and then electropolished at - 4 5 ~ in a perchloric acid electrolyte recommended by Williams and Blackburn. ] The fatigue tests we
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