Correlation of fracture toughness with microstructural features for ultrafine‐grained 6082 Al alloy

Abstract

In the present work, cryorolling (CR) and room temperature rolling (RTR) followed by annealing (AN) at 200°C were carried out to investigate the effects of grain size, precipitates (Mg‐Si‐phases), and AlFeMnSi‐phases on the fracture toughness of 6082 Al alloy. Using the values of the conditional fracture toughness, (KQ), in the critical fracture toughness (KIC) validation criteria, it was found that the sample size is inappropriate, which implies that the conditional fracture toughness obtained cannot be considered as the critical fracture toughness. Therefore, to establish the relative improvement in fracture toughness, the equivalent energy fracture toughness (Kee) and J‐integral were calculated and used. The results show that the values of Kee (89.91 MPa √m) and J (89.86 kJ/m2) obtained for the sample processed via CR followed by AN (CR + AN) are the highest when compared with the other samples processed through CR, RTR, and RTR followed by AN, RTR + AN. Microstructural features such as high fraction of low Taylor factor, high fraction of kernel average misorientation, Si‐rich particles, small size AlFeMnSi‐phases, and mixed mode of failure (transgranular shear and micro‐void coalescence) also support the high fracture toughness in the CR + AN sample. It was also observed that the effect of residual stresses on the fracture toughness of CR and RTR samples is minimal. Therefore, the correlation between microstructure and residual stresses is not considered in the present work due to very small values of the residual stresses for CR and RTR samples and the absence of residual stress from the heat‐treated samples.

Effect of crack path and high angle grain boundaries on fracture toughness and fatigue behaviour of cryorolled AA2219

“…The micrographs of T87‐AA2219 alloy show equiaxed grains with clear boundaries whereas every other rolled show elongated and fragmented grains with ill‐defined grain boundaries. The grain structure obtained are similar to the structure observed in Al–Mg sheets in the work of Dommeti et al 14 The grains of the RTR50U and LNR50U samples are columnar, stretched and fragmented with few grains broken into smaller grains during material processing 15 . The grain boundaries of the LNR50U and LNR50C alloy are not clearly visible, as in the T87 condition, due to the heavy deformation of the alloy during the rolling process.…”

“…The coarser precipitates in cryorolled samples are one of the reasons for the better fracture toughness values than alloy rolled at room temperature. The better plane stress fracture toughness of cryorolled alloy than the RTR alloy is due to the formation of dense dislocation structure, coarser second phase particles and high mobility of dislocations 4,15 …”

“…From the misorientation angles, the energy stored in the material after rolling can be determined using Equation 6 15,18 . The average values of the KAMs determined for a step size of 0.1 μm and Burgers vector value of 2.86 × 10 −10 give an insight into the energy stored in the material after rolling.…”

“…The fine grains in rolled material stop the crack nucleation by limiting the stress raisers and the amount of crack tip branching 15,21 . Moreover, ultra‐fine grain materials have higher grain boundary and dislocation density, which acts as barriers for further crack propagation 4,22,34 .…”

Structure–Property Correlation of Al–Mg–Si Alloys Micro-alloyed with Sn

“…This suggests that the Fe-rich Al(Fe, Mn)Si intermetallic and Si particles are responsible for crack propagation. 35 Furthermore, from the EDS analysis of 0.04 wt% Sn and 0.08 wt% Sn peak-aged rolled alloys in Figures 18 and 19 19, EDS spots 1-2)], pointing out that the fracture development involves the cracking of Al(Fe, Mn)Si intermetallic. Once the particles cracks, a micro-void is formed and tends to grow.…”