Void growth and coalescence in single crystals

Abstract

a b s t r a c tVoid growth and coalescence in single crystals are investigated using crystal plasticity based 3D finite element calculations. A unit cell involving a single spherical void and fully periodic boundary conditions is deformed under constant macroscopic stress triaxiality. Simulations are performed for different values of the stress triaxiality, for different crystal orientations, and for low and high work-hardening capacity. Under low stress triaxiality, the void shape evolution, void growth, and strain at the onset of coalescence are strongly dependent on the crystal orientation, while under high stress triaxiality, only the void growth rate is affected by the crystal orientation. These effects lead to significant variations in the ductility defined as the strain at the onset of coalescence. An attempt is made to predict the onset of coalescence using two different versions of the Thomason void coalescence criterion, initially developed in the framework of isotropic perfect plasticity. The first version is based on a mean effective yield stress of the matrix and involves a fitting parameter to properly take into account material strain hardening. The second version of the Thomason criterion is based on a local value of the effective yield stress in the ligament between the voids, with no fitting parameter. The first version is accurate to within 20% relative error for most cases, and often more accurate. The second version provides the same level of accuracy except for one crystal orientation. Such a predictive coalescence criterion constitutes an important ingredient towards the development of a full constitutive model for porous single crystals.

Grain Scale Representative Volume Element Simulation to Investigate the Effect of Crystal Orientation on Void Growth in Single and Multi-Crystals

“…The RVE simulations with a rate-dependent crystal plasticity model could reproduce the orientation dependent single crystal flow stress behavior of the bcc metal under two distinct constant stress triaxialities as reported in the previous work by Yerra et al [13] and as observed in general bcc metals. In other words, the crystal with the highest flow stress resulted in the earliest localization (or least uniform elongation corresponding to UTS point), while delayed localization for the lower flow stress when a void is included in the RVE.…”

“…The number of the activated slip systems increases as the stress triaxiality increases. For example, additional six slip systems (2,3,8,9,13,19 slip systems in Table 1) are activated when the stress triaxiality is increased to 1. The activation of the slip system is further complicated when the void is embedded in the single crystal or at the boundary of bi-crystal.…”

“…For example, Yerra et al [13] investigated the behavior of void in the material under different stress states (or different stress triaxiality) in a single crystal level. They simulated void coalescence behavior in the single crystal with different characteristic orientations using the crystal plasticity finite element method.…”

“…However, even with this limitation, the current work is still meaningful considering recent advances in the computational approach based on crystal plasticity, which accurately predicts the anisotropic deformation response of cubic crystals under complex loading conditions [7,[13][14][15][16][17][18][19].…”

Anisotropic failure modes of high-strength aluminium alloy under various stress states

“…In addition to anisotropic yielding (Hill, 1948;Barlat et al, 2005;Rousselier et al, 2012), some alloys also show anisotropic failure (Chen et al, 2009). A numerical representation of the microstructure coupled with damage models enabled Steglich et al (2008) to represent the anisotropic ductile fracture of an aluminium alloy, while Yerra et al (2010) numerically described the fracture inside a grain using a crystal plasticity material model around a spherical void. Recently, Luo et al (2012) proposed an anisotropic failure criterion based on a linear transformation of the plastic strain-rate tensor.…”