Experimental and numerical investigations of the effects of the spatial distribution of α phase on fracture behavior in hypoeutectic Al–Si alloys

Abstract

A uniform, fine -phase microstructure enhances the mechanical properties of Al-Si alloys, however, it is an open question how the  phase affects the crack-growth behavior. This paper addresses the effects of the morphology and distribution of  phase on the fracture behavior in a model dual-phase Al-7%Si alloy with different microstructures. The influences of microstructural factors on crack-growth behavior are examined using in-situ experiments. The results show that a globular -phase microstructure produces a straight crack-growth path, whereas a dendritic, orientational -phase microstructure leads to a deflected crack profile. Finite-element modeling is performed to simulate the fracture behavior, and to rationalize the observed phenomena. The near-tip J-integral based fracture criterion is used to predict the fracture path. Numerical results indicate that a variation in the morphology and distribution of  phase changes the symmetry and intensity of the near-tip stress, strain and displacement fields due to the strong mismatch in elastic-plastic properties of the  phase and eutectic phase, which have major influences on both crack-growth direction and crack-tip driving force.

In situ observation of ductile fracture using X-ray tomography technique

“…9b by being dragged by crack segment G. A similar crack deflection angle was held for crack segment G and surrounding regions during the subsequent crack propagation. Similar to the reported tendency [29], this is probably because the crack was repelled by particle A shown in Fig. 11a due to the existence of an asymmetrical near-tip field around this coarse particle.…”

Effects of Austempering Temperature on Strength, Ductility and Toughness of Low-C High-Al/Si Carbide-Free Bainitic Steel

“…The other is the embrittling effect ( Ref 22), and it is caused by the large blocky martensite or martensite/ austenite (MA) islands formed during impact deformation. The reason for this is that the newly formed martensite is untempered and brittle, and the fracture strength of the brittle martensite depends on its size, i.e., the martensite with a larger size tends to fracture at a smaller fracture strength ( Ref 23,24). It means that large blocky martensite or MA islands favor the formation of microcracks and provide easy fracture paths, thus reducing impact toughness (Ref 9).…”

“…In contrast, the majority of the retained austenite in the specimens austempered at lower temperatures is small in size and stable during deformation, playing a toughening role by blunting the crack tips because of its own tough character ( Ref 25). And, even if small blocks or thin films of retained austenite do transform into martensite, they are not easy to crack due to their higher fracture strength associated with their small sizes and thus may not have a detrimental effect on toughness ( Ref 23,24). Therefore, it is the two-fold effect, i.e., the toughening and embrittling effects of strain-induced martensitic transformation that occurs during impact deformation of the notched-specimens that lead to the highest impact toughness obtained at an intermediate austempering temperature of 320°C-but neither at the lowest temperature of 300°C nor at the highest temperature of 350°C.…”

Evaluating Spatial Distribution Randomness of Overlap Permissive Second Phase in Three- and Two-Dimensions

“…Over the last decade, experimental and numerical studies [1][2][3][4][5][6][7][8][9][10][11] have been indicated that the spatial distribution of second phase particles played an important role on the mechanical properties. Tessellation techniques, 12,13) correlation functions, [14][15][16] nearest neighbor distances [17][18][19] contact distribution 20) and counting methods 21) have been suggested to describe the spatial distribution of second phase particles.…”