Deformation enhanced grain growth in a superplastic Sn-1% Bi alloy

Ultrahigh high-strain-rate superplasticity in a nanostructured high-entropy alloy

Superplasticity describes a material’s ability to sustain large plastic deformation in the form of a tensile elongation to over 400% of its original length, but is generally observed only at a low strain rate (~10⁻⁴ s⁻¹), which results in long processing times that are economically undesirable for mass production. Superplasticity at high strain rates in excess of 10⁻² s⁻¹, required for viable industry-scale application, has usually only been achieved in low-strength aluminium and magnesium alloys. Here, we present a superplastic elongation to 2000% of the original length at a high strain rate of 5 × 10⁻² s⁻¹ in an Al₉(CoCrFeMnNi)₉₁ (at%) high-entropy alloy nanostructured using high-pressure torsion. The high-pressure torsion induced grain refinement in the multi-phase alloy combined with limited grain growth during hot plastic deformation enables high strain rate superplasticity through grain boundary sliding accommodated by dislocation activity.

Superplasticity at high strain rates is challenging to achieve in high strength materials. Here, the authors show superplastic elongation in excess of 2000% in a high entropy alloy nanostructured by high-pressure torsion.

A Mechanism of Superplastic Deformation and Deformation Induced Grain Growth Based on Grain Switching

Grain growth depending on strain is generally observed during structural superplasticity. A new deformation model is proposed in order to explain this grain growth.

This model is based on grain switching by Ashby and Verrall. The modification is to introduce grain size irregularity. The grain structure with minimum irregularity consists of one 5–7 pair in hexagonal array, which corresponds to edge dislocation. While the tensile stress oblique to the pair results the grain switching described by dislocation glide, the stress parallel to it results the grain switching which transforms the pentagon into the quadrilateral and then removes it. The latter can be described by dislocation climb and results the enhancement of grain growth. The expansion into three dimensions is discussed.

Dynamic Growth of Ultra-Fine Austenite Grains Deformed above Ad3 Temperature in a Nb-V-Ti Steel

Generally, three typical behaviors are recognized in hot-deformation of austenite. However, considering that those austenite grains involved in single-pass deformation are mostly on the scale of dozens of micrometers or even much larger than that, it is meaningful to investigate hot-deformation behaviors of austenite grains smaller than 10μm. In the current paper, austenite grains of different sizes were prepared through repetitive treatment of rapid reheating and quenching with changing the holding temperature and/or holding time. Kinds of true stress-true strain curves at 900oC and 950oC indicate that austenitic deformation can be gradually coordinated by grain boundary behaviors, such as grain boundary sliding and/or diffusion. Simultaneously, the macroscopic deformation is more likely to be dominated through co-operation grain boundary sliding (CGBS).