H. C. Burghard

Bio: H. C. Burghard is an academic researcher from Southwest Research Institute. The author has contributed to research in topics: Coalescence (physics) & Microvoid coalescence. The author has an hindex of 1, co-authored 1 publications receiving 23 citations.

The influence of precipitate morphology on microvoid growth and coalescence in tensile fractures

Abstract: A study of the fine-scale topography of tensile fractures in selected aluminum alloys was made to provide information on the influence of second-phase microconstituents on microvoid initiation, growth, and coalescence. The test materials included three commercial aluminum alloys and three high-purity Al-Cu binary alloys heat-treated to provide a wide range of precipitate morphology. The fracture surfaces of notched-tensile specimens were examined, and the fine-scale topographic features compared with microstructural parameters. The principal observations made were: 1) for a dual precipitate morphology, void initiation first occurs at the larger precipitates, 2) fracture may occur by growth and coalescence of voids initiated at only a small fraction of precipitate sites, 3) void initiation can occur independent of precipitate particles, and 4) intergranular fracture may occur by growth and coalescence of microvoids within the grain-boundary zone. These observations established that the detailed aspects of fracture by microvoid coalescence are intimately related to precipitate morphology, but no simple, uniform correlation of fracture surface topography with precipitate size and distribution was evident.

The influence of compositional and microstructural variations on the mechanism of static fracture in aluminum alloys

Abstract: The object of the paper is to examine the effects of alloy purity and state of aging on the fracture mechanism and resultant toughness of pure Al-Cu alloys, and commercial duralumin. In pure alloys, the transition from a shear to an intergranular mode of fracture with overaging is associated with changes in the nature and size of the matrix precipitate, which affect the slip character. In the corresponding commercial purity alloys, no such fracture mode transition occurs. The presence of second-phase dispersoids inhibits planar slip, and in the overaged state inclusion-matrix interfaces present a suitable alternative site to the grain boundaries for strain accumulation, resulting in debonding leading to the initiation of voids, which subsequently grow and coalesce. The fracture toughness, as conventionally measured, indicates the material’s resistance to crack initiation rather than propagation and is effectively independent of fracture mode. The work hardening capacity has a marked effect on void size, and is shown to be a sensitive indicator of fracture toughness in both pure and commercial alloys. Based on a simple model, good agreement is obtained between experimental results of toughness and those predicted from a knowledge of the tensile properties.

Ductile fracture topography: Geometrical contributions and effects of hydrogen

Abstract: A volume element which is deformed becomes elongated and narrowed with increasing strain, for reasons of constant plastic volume; inclusion spacings experience the same changes. Equivalent plasticity expressions permit a description of this effect as a function of fracture strain. Such an effect must be taken into account in comparison of initial inclusion spacings with dimple sizes observed in tensile or toughness specimen fractures. It also plays a role in the assessment of dimple size changes in hydrogen-assisted ductile fracture. The most convenient way to display the effect in the hydrogen case is on a plot of ductility lossvs dimple size ratio. This comparison demonstrates that the geometrical effect generally should lead to large increases in size ratio as ductility decreases; thus the common observations of dimple sizereduction with hydrogen evidently represent large increases in dimple nucleation, while dimple sizeincreases may represent microvoid growth effects.