Mechanical properties and fracture mechanisms of ultrafine grained Cu-9.6% Zn alloy processed by multiaxial cryoforging

Aim of the present study is to investigate mechanical properties and fracture mechanisms of low stacking fault energy ultrafine grained Cu-9.6 wt% Zn brass processed by multiaxial forging (MAF) at cryogenic temperature. The alloy was developed by metal mold casting followed by homogenized annealing at 800 °C for 4 h before cryoforging. The forging was carried out for 5, 9 and 12 cycles which are equivalent to cumulative strains of 3.0, 5.4 and 7.2, respectively. The brass deformed with a cumulative strain of 7.2 showed more than 9 times improvement in the yield strength (YS=680 MPa) compared to that of the homogenized annealed sample (70 MPa). However, the ductility decreased to a very low range after deforming to 12 cycles because of extreme work hardening at the cryogenic temperature due to suppression of dynamic recovery. Cryoforging followed by short-annealing (225-300 °C for 20 min) was found to be effective to enhance the ductility without much affecting its YS. Formation of ultrafine grains along with nanotwins (confirmed by TEM) results a simultaneous improvement in the YS and ductility in the cryoforged+short-annealed samples. Macroscopic tensile fracture mechanisms and fractography analysis were carried out for the cryoforged and cryoforged+short-annealed samples. The unified tensile fracture criterion (Ellipse criterion) in terms of modification of the stress state in the fracture zone suggested that reduction of necking angle is resulted from the geometrical hardening effect due to the change of stress state. As number of forging cycles increases, which correspondingly increases critical normal stress (σo) and shear stress (τo), the tensile shear fracture angle decreases indicating in the increase of shear mode fracture. This is ascertained to the grain size refinement, which increases the hardness of the material. Fractography analysis also highly corroborated with the analysis of the unified tensile fracture criterion and %elongation. The dimple size as well as %elongation was found to gradually decrease with decrease in the shear fracture angle, which is equivalent to increase in the cumulative strain.