Adiabatic Shear Banding in Nickel and Nickel-Based Superalloys: A Review

Abstract

This review paper discusses the formation and propagation of adiabatic shear bands in nickel-based superalloys. The formation of adiabatic shear bands (ASBs) is a unique dynamic phenomenon that typically precedes catastrophic, unpredicted failure in many metals under impact or ballistic loading. ASBs are thin regions that undergo substantial plastic shear strain and material softening due to the thermo-mechanical instability induced by the competitive work hardening and thermal softening processes. Dynamic recrystallization of the material’s microstructure in the shear region can occur and encourages shear localization and the formation of ASBs. Phase transformations are also often seen in ASBs of ferrous metals due to the elevated temperatures reached in the narrow shear region. ASBs ultimately lead to the local degradation of material properties within a narrow band wherein micro-voids can more easily nucleate and grow compared to the surrounding material. As the micro-voids grow, they will eventually coalesce leading to crack formation and eventual fracture. For elevated temperature applications, such as in the aerospace industry, nickel-based superalloys are used due to their high strength. Understanding the formation conditions of ASBs in nickel-based superalloys is also beneficial in extending the life of machining tools. The main goal of the review is to identify the formation mechanisms of ASBs, the microstructural evolutions associated with ASBs in nickel-based alloys, and their consequent effect on material properties. Under a shear strain rate of 80,000 s−1, the critical shear strain at which an ASB forms is between 2.2 and 3.2 for aged Inconel 718 and 4.5 for solution-treated Inconel 718. Shear band widths are reported to range between 2 and 65 microns for nickel-based superalloys. The shear bands widths are narrower in samples that are aged compared to samples in the annealed or solution treated condition.

A Review on the Adiabatic Shear Banding Mechanism in Metals and Alloys Considering Microstructural Characteristics, Morphology and Fracture

“…Moreover, torsion tests have been carried out in torsional Hopkinson bar [203], as depicted in Figure 35c, or impact torsional loading, allowing for strain rates of up to 10 3 and 10 4 s −1 , respectively. Solid and hollow cylinders have been used as examined specimens, with the latter benefitting from a more homogeneous stress field due to their lower stress/strain gradients with the radius in contrast to solid specimens.…”

Effects of Quasi-Static Strain Rate and Temperature on the Microstructural Features of Post-Processed Microstructures of Laser Powder Bed Fusion Ti6Al4V Alloy

“…It is seen in Figures 8-10 that the test data for the three different forms of LPBF Ti6Al4V(ELI) show at least two of the three known stages of strain hardening. Stage I exhibits a sharp decline in the rate of strain hardening, which is ascribed to the multiplication of dislocations, with an increase in plastic strain at a decreasing rate and therefore a piling-up of dislocations at grain boundaries, resulting in an increase in flow stress [46,47]. Immediately after this stage, the strain hardening rate decreases to another region with a constant value of strain hardening and is referred to as stage II.…”

“…The localization of deformation is enhanced when the resistance to deformation decreases with strain. It is further enhanced by strain rates, weakening regions where stress concentration is high [45][46][47]. Therefore, flow stress localization, which appears in the form of ASBs, may fail to occur in harder materials due to their high resistance to softening and the attendant formation of localized shear bands [47].…”