Influence of grain size on fatigue crack growth in FCC and HCP metals

Abstract

The influence of grain size on fatigue crack growth was studied for several single-phase materials representing the face-centered-cubic (FCC) and hexagonal-close-packed (HCP) crystal systems. These materials furthermore represented various levels of stacking fault energy (SFE). The materials employed in this work are pure aluminum (high SFE) and an austenitic stainless steel (low SFE) from the FCC crystal system, and commercially pure titanium (low SFE) and a titanium-8% aluminum binary alloy (low SFE) from the HCP system. Mechanical property tests were conducted at room temperature to determine monotonic tensile properties and fatigue crack growth properties. A metallographic survey and grain size determination was made for each material. Transmission electron microscopy work was carried out to characterize the plastically deformed substructure of each material. Examination of fatigue fracture surfaces by scanning electron microscopy and of replicas of fatigue fracture surfaces by transmission electron microscopy characterized the features of fatigue fracture and microplastic behavior of each material.

The tensile properties were analyzed with respect to grain size. The fatigue crack growth properties were also analyzed with respect to grain size but within the framework of a constant state of stress at the tip of the fatigue crack. This analysis technique is suggested by the observations of other researchers concerning the effects of material yield strength and test specimen thickness on fatigue crack growth and also by the macroscopic behavior of test specimens during fatigue testing. The fatigue crack growth properties were correlated with the deformed substructures and the fracture surface features of the materials studied.

The results of this work indicate grain size has an appreciable influence on fatigue crack growth for constant states of stress in materials of low stacking fault energy. Materials of high stacking fault energy exhibit little or no grain size influence on crack growth. The sensitivity of grain size influence in low SFE materials is related to the state of stress, i.e., higher states of stress result in greater sensitivity. There is little or no stress state sensitivity for the high SFE materials. The grain size dependency of fatigue crack growth and the stress state sensitivity is related to the stacking fault energy and subsequent deformation substructure morphology of the materials.