Enhanced Fatigue Strength of Commercially Pure Ti Processed by Rotary Swaging

Hasan Alkhazraji,Ehab El-Danaf,Lothar Wagner,Manfred Wollmann

doi:10.1155/2015/301837

Abstract

Fully reversed bending fatigue tests were performed on polished hour-glass specimens of commercially pure titanium grade 1 with three different grain sizes, that were produced by severe plastic deformation (rotary swaging) and subheat treatments, in order to examine the effect of grain size on fatigue. An improvement in fatigue strength was observed, as the polycrystal grain size was refined. The endurance limit stress was shown to depend on the inverse square root of the grain size as described empirically by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. It was found that postdeformation annealing above recrystallization temperature could additionally enhance the work-hardening capability and the ductility of the swaged material, which led to a marked reduction in the fatigue notch sensitivity. At the same time, this reduction was accompanied with a pronounced loss in strength. The high cycle fatigue performance was discussed in detail based on microstructure and mechanical properties.

Highlights

In conventional fatigue design, the fatigue limit obtained at a number of stress cycles of 107 has been used to determine the allowable stress level for design against high cycle fatigue (HCF)
(1) ultrafine grained (UFG) material can be produced by rotary swaging (RS) with average grain size of about 0.1 μm; Fine grain (FG) and Coarse grain (CG) with average grain size of 0.36 and 90 μm, respectively, are produced by postdeformation annealing
Work-hardening capability and ductility are achieved by postdeformation annealing, namely, in FG and CG materials

Summary

Introduction

The fatigue limit obtained at a number of stress cycles of 107 has been used to determine the allowable stress level for design against high cycle fatigue (HCF). Lukaˇs et al [24] have demonstrated the H-P effect for the high cycle fatigue behavior of copper for two grain sizes of 0.3 and 70 μm. The inverse square root of grain size dependence reported in pioneering researches by Hall and Petch for the yield and cleavage fracture stresses of iron and steel materials demonstrated extension of this grain size dependent relationship to other material properties such as the complete stress strain behavior of diverse metals and alloys, assessments of the ductile-brittle transition, hardness, fatigue, fracture mechanics, shear banding, and strain rate sensitivity properties. In this work, rotating bending fatigue of three-grainsized commercially pure (cp) Ti grade 1 processed by rotary swaging (RS) was conducted on smooth and notched samples. Fatigue performance of various grain sizes is compared and contrasted

Experimental Procedure

Results and Discussion

Conclusions