Influence of Grain Size and Age-Hardening on Dislocation Pile-Ups and Tensile Fracture for a Ti-AI Alloy
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INTRODUCTION
I N many alloys the precipitation of particles is a very effective hardening mechanism and is therefore widely used. Beside the yield stress precipitates can also affect the fracture behavior of such alloys. For a microscopical study it appears useful to consider the interaction of moving dislocations with particles as well as the resulting slip distribution during plastic deformation. The two most widely observed reactions are first, bypassing of hard nonshearable particles by dislocations and second, shearing of coherent particles by dislocations. This study will deal only with the latter dislocation-particle interaction. If the moving dislocations can shear the particles, these are destroyed, the slip plane weakens, and the slip occurs preferentially on these planes. ~-4The dislocations form intense slip bands and pile up against grain boundaries which results in an inhomogeneous slip distribution. Tensile specimens with such a slip distribution often show a considerable loss in ductility resulting in macroscopically brittle fracture when particle hardening is increased. 5'6"7 Microscopically, crack nucleation occurs in many cases by shear stress concentrations due to the pile-ups at grain boundaries which are able to crack the boundaries and cause fracture. 5.7 If the pile-ups are very intense, cracking can occur by the first pile-up after having reached the elastic limit. Macroscopically brittleness is observed, although there has been extensive deformation in the slip bands on a microscopic level. In addition to the degree of particle hardening an influence of grain size on the tensile fracture behavior has been observed for such an inhomogeneous slip distribution. 7-11 Investigations by Gysler and Ltitjering on a Ti-Mo alloy (/3-alloy) hardened by coherent o)-particles showed a drastic decrease of ductility with increasing grain size.S A variation of the grain size from 30/x to 120 tz caused a decrease of the true fracture strain from eF = 0.4 to macroscopic brittleness. Similar results have been found on a G. TERLINDE is Research Engineer at GKSS-Forschungszentrum Geesthacht, West Germany. G. LUETJERING is Professor at Technische Universitat Hamburg-Harburg, West Germany. Manuscript submitted February 27, 1981.
METALLURGICAL TRANSACTIONS A
precipitation hardened Fe-Ni-AI alloy 9 and on a Ti-A1-Si alloy hardened by coherent Ti3AI particles. ~0 Up to now no microscopical studies of the pile-up fracture mechanism have been performed to our knowledge. The goal of this work therefore was to investigate this fracture mechanism on a quantitative basis and to explain from the microscopic results the macroscopic mechanical properties. Two parameters were changed: first, the grain size, which determines the length of the dislocation pile-ups, and second, the degree of age hardening, which influences the intensity of the slip bands, since with an increasing age hardening degree more dislocations pile up against the grain boundaries at the onset of plastic deformation. For several values of these pa
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