Fatigue crack initiation at Nd-rich particles in an Nd containing high temperature titanium alloy
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Fatigue crack initiation at Nd-rich particles in an Nd containing high temperature titanium alloy J. F. Lei, Q. J. Wang, Y. Y. Liu, S. X. Guan, Z. G. Wang, D. Li, and Z. Q. Hu State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China (Received 20 February 1996; accepted 9 May 1997)
Scanning electron microscopy (SEM) and surface replica method have been employed to study the micromechanism of fatigue crack initiation at Nd-rich phase particles in a high temperature titanium (Ti-55) alloy. It was found that the microcrack initiates near the equator of Nd-rich particles in the matrix. The microcrack grows first at an angle of about 45± with respect to the tensile axis, and then its growth direction becomes approximately normal to the tensile axis. The experimental results are analyzed in terms of the elastic stress distribution around soft particles imbedded in the matrix to account for the experimental findings of particle cracking and the associated surface microcrack initiation near the particle “equator.” A model of fatigue crack initiation at a soft surface particle is proposed.
I. INTRODUCTION
Constituent particles are often preferential sites for fatigue crack initiation. Modes of fatigue crack initiation depend on the matrix material, type of particle, and properties of the interface between particles and the matrix. Particle-related fatigue crack initiation has been studied extensively.1–8 Tanaka and Mura7,8 theoretically treated this problem by using a dislocation dipole model and an energy criterion, and successfully explained the fatigue strength reduction due to inclusions. Several microscopic observations have been made to clarify the micromechanisms of fatigue crack initiation. Lankford and Kusenberger2 presented a typical model of fatigue crack initiation at inclusions in 4340 steel, based on particle-matrix debonding. However, almost all particles studied so far are harder than the matrix. The fatigue crack initiation at particles softer than the matrix has been less well reported. The particles studied in this paper are soft Ndrich particles in a newly developed near a high temperature titanium alloy containing Nd (hereafter called Ti-55 alloy), the nominal composition of which is Ti–5Al–4Sn–2Zr–1Mo –0.25Si–0.7Nd (wt. %). Ti55 alloy shows excellent tensile strength and creep resistance up to 823 K as well as improved fatigue strength. The addition of the rare earth element Nd to the alloy is very important to ensure excellent thermal stability and high temperature properties of the alloy.9 Our previous work10 revealed that fatigue crack initiation related to Nd-rich particles occurred at high cyclic stress levels. Since the elastic stress distributions around particles softer or harder than the matrix are almost opposite, the model of fatigue crack initiation at soft J. Mater. Res., Vol. 12, No. 10, Oct 1997
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