Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof
 c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf
 c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef
 cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness.

Metallurgical factors affecting fracture toughness of aluminum alloys

Microstructural processes of plastic instabilities in strengthened metals

On the influence of microstructure on crack propagation mechanisms and fracture toughness of metallic materials

Differential scanning calorimetry (DSC) in conjunction with transmission electron microscopy (TEM) are used to characterize the matrix precipitate structure of high strength and overaged tempers of three 7000 series aluminum alloys. Excellent consistency exists between the DSC results, based on the dissolution behavior of existing precipitates, and TEM observations. Comparison is made between matrix precipitate constituency and mechanical properties. A significantly high GP zone particle density was observed in a high strength 7050 alloy temper, but this temper did not have higher strength than other predominantly GP zone matrix tempers. Maximum strength was observed in a 7050 alloy temper that contained approximately equal amounts of GP zones and ή phase precipitates. Strengthening appears to be based on the contribution of both coherent GP zones and semicoherent ή precipitates. Use of the DSC approach and the free energy of activation for precipitate dissolution are recommended as rapid and quantitative means of precipitate identification.

Calorimetric studies of 7000 series aluminum alloys: II. Comparison of 7075, 7050 and RX720 alloys

With the advent of linear elastic fracture mechanics, the detailed effects of processing and microstructure on toughness can be evaluated. The effect of microstructure on plane stress and plane strain fracture toughness is considered in detail together with strength, fatigue behavior and corrosion resistance. It is concluded that second phase particles in all size ranges can influence toughness. Increasing the size and amount of particles or decreasing precipitate coherency all lead to decreases in toughness. Grain structure is also shown to play a prominent role in determining plane stress fracture toughness ; at a given strength level, a fibrous grain structure and the prevention of recrystallization are desirable. The ability to influence fatigue crack propagation by control of processing is more remote though relatively little systematic work has been carried out in this field. Thermomechanical processing is considered to offer another possible route to achieving a desirable balance of toughness, strength and corrosion resistance.

Metallurgical factors affecting high strength aluminum alloy production

The mechanical behavior of a wrought high strength aluminum alloy is examined as a function of the concentration of second-phase microconstituents. These second-phase particles are located in grain boundaries and interdendritic sites. Their concentration was varied by thermal-mechanical processing. In the areas of strengths, ductility, crack toughness, fatigue, and stress corrosion cracking, empirical results are presented and discussed. Observations are made on changes in the anisotropic character of this category of material with lower concentrations of second phases.

Joseph H. Mulherin

Papers

Influence of nonequilibrium second-phase particles formed during solidification upon the mechanical behavior of an aluminum alloy