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Damage evolution of Si particles in a Sr modified cast A356(T6) Al alloy is quantitatively characterized as a function of strain under tension, compression, and torsion. The fraction of damaged Si particles, their size distributions, and orientation distribution of particle cracks are measured by image analysis and stereological techniques. Silicon particle cracking and debonding are the predominant damage modes. Particle debonding is observed only under externally applied tensile loads, whereas particle cracking is observed under all loading conditions. The relative contributions of Si particle debonding and fracture to the total damage strongly depend on stress state and temperature. For all loading conditions and stress states studied, the average size of damaged Si particles is considerably larger than the bulk average size. The rate of damage accumulation is different for different loading conditions. At a given strain level, Si particle damage is lowest under compression and highest under torsion. The anisotropy of the damage is highly dependent on the deformation path and stress state. Under uniaxial tension, the cracks in the broken Si particles are mostly perpendicular to the loading direction, whereas in the compression test specimens they are parallel to the loading direction. The Si particle cracks in the torsion and notch-tension test specimens do not exhibit preferred orientations. The quantitative microstructural data are used to test damage evolution models.

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fringes due to double diffraction from (1-101) M2C reflections is presumably due to their relatively higher intensities resulting in intense double diffraction spots.An examination of the results of the present and previous studies on the secondary hardening high performance steels indicates the following.(1) The black-white strain contrast indicating coherency of the carbides with the ferrite matrix was observed only until the peak aging temperature of 482 ЊC.(2) The appearance of fringes in the M 2 C carbides coincided with the onset of overaging, i.e., aging at or above 510 ЊC. One reasonable explanation for these observations is that, until peak hardening, the ''M 2 C carbides'' are in the form of zones that are coherent with the matrix. The absence of fringes up to peak aging suggests that the precipitates may not have a crystalline structure of their own. It may, however, be argued that the absence of fringes may be due to the smaller size of the carbides and/or poor diffracted intensity rather than the lack of a crystal structure. Based on the present study, it is not possible to unambiguously claim that the fringe formation is the result of ''crystallization'' of the carbides. However, the present results along with the observation in the previous studies that diffraction patterns up to peak aging showed diffused intensity streaks along ͗100͘ ␣ are strongly suggestive that the carbides could be in the form of zones until overaging. [16,17] High resolution studies are underway to analyze the carbide structure in the peak hardened condition. REFERENCES 1.The Al-Si-Mg base cast alloys are widely used for structural applications. Mechanical properties and fracture behavior of these alloys depend on macrodefects such as internal oxide layers and shrinkage macroporosity, [1,2] microporosity, [3,4] dendrite cell size, [5,6] and size and shape of Si particles [7] present in the interdendritic regions. Fracture and debonding of silicon particles is an important aspect of damage evolution in these alloys. [3,[7][8][9] Fracturing/debonding of silicon particles, formation and growth of voids around such particles, and subsequent interlinkage of the voids lead to crack propagation in the interdendritic regions. Figure 1 illustrates this fracture micromechanism. It is observed that silicon particles fracture/debond at stresses significantly below the ultimate tensile strength, but usually above yield stress of the material. [7] There have been numerous studies on effect of tensile strain on the fraction of damaged* par-*''Damaged'' includes fractured and/or debonded silicon particles. ticles in Al-Si-Mg base alloys. [7,10] The damaged silicon particles are observed at very low plastic strain levels, and the fraction of damaged particles monotonically increases with the tensile strain. [7,10] However, in the earlier investigations, the mechanical tests were performed at room temperature only. To the best of the authors' knowledge, no experimental data on the effect of temperature on the microstructural damage exi...

An important aspect of damage evolution in cast Al-Si-Mg base alloys is fracture/cracking of Si particles. This microstructural damage is quantitatively characterized as a function of strain rate in the range 10 Ϫ4 to 3.7 ϫ 10 ϩ3 , at an approximately constant uniaxial compressive strain level (20 to 25 pct). It is shown that the fraction of damaged silicon particles, their average size, and size distribution do not vary significantly with the strain rate, and at all strain rates studied, larger Si particles are more likely to crack than the smaller ones. However, the stress-strain curves are sensitive to the strain rate. These observations have implications for modeling of deformation and fracture of cast components under high strain rate crash conditions.