Superplasticity usually occurs in fine-grained materials and occurs at low strain rates (10-4~10-3/s) and relatively high temperatures (>0.5Tm, where Tm is the melting temperature). In the past few decades, researchers have achieved superplasticity at high strain rates (>10-2/s) and gained wide attention, which can shorten the superplasticity formation time and have great advantages in practical applications. However, high strain rate superplasticity (HSRS) is extremely rare in high-strength materials, especially in a class of high-strength materials that has recently appeared, namely high-entropy alloys (HEA), lack of superplastic forming ability, which seriously hinders HEA in Potential applications of complex engineering structures. Therefore, the realization of HSRS in HEA will mark a huge breakthrough in advanced materials science.
Researchers at Pohang University of Science and Technology in South Korea have manufactured Al9 (CoCrFeMnNi) 91 high-entropy alloy with nano-scale FCC grains and B2 phase through hot processing and high-pressure torsion (HPT) treatment, achieving a high strain rate of 1073K ( 5×10-2/s) has 2000% ultra-high superplasticity, which is the largest elongation of HEA ever reported. Related papers were recently published in Nature Communications under the title "Ultrahigh high-strain-rate superplasticity in a nanostructured high-entropy alloy".
Paper link:
https://www.nature.com/articles/s41467-020-16601-1
In this study, vacuum induction smelting was used to prepare Al9 (CoCrFeMnNi) 91 (at%) high-entropy alloy. The ingot was homogenized at 1473K for 12 hours and water quenched. The thickness of the ingot was rolled from 7mm to 1.5mm by cold rolling, and then annealed in argon gas at 1273K×15min and water quenched. The high-pressure torsion treatment was carried out at a pressure of 6 GPa at room temperature, and rotated 5 times at a speed of 1 r/min.
The study found that before the superplastic tensile test, the initial microstructure showed FCC and Al-Ni-rich B2 phase. The annealed samples show fine FCC grains (average size is about 2μm), there are relatively large B2 precipitates (800nm-1m) at the boundary of FCC grains, and small B2 inside the FCC grains Precipitate (200-400nm). During the HPT process, the soft FCC grains experienced severe strain, resulting in grain refinement, while the hard B2 phase flowed with the soft FCC phase and maintained its size. This results in the size of the B2 phase ranging from hundreds of nanometers to several micrometers, giving the nanoscale FCC matrix a unique dual microstructure.
Superplastic tensile tests were carried out under different test conditions. At a temperature of 1073K and a strain rate of 5×10-2/s, the maximum elongation reached 2000%. The main mechanism of superplastic deformation is related to grain boundary slip (GBS). HPT treatment can produce nano-scale grains with large-angle grain boundaries, which can promote GBS. Analysis of tensile specimens revealed that in addition to the presence of FCC and B2 phases, σ phases (rich in Cr) were also formed during the superplasticity test. The high strain rate and the presence of B2 and σ phases restricted the grains during superplastic deformation During growth, the σ phase acts as a hard domain in the current HEA, and can generate the necessary dislocations (GND) in the softer domain to maintain interphase boundary compatibility. Maintaining ultrafine grains (UFG) and similar morphology during superplastic deformation confirms the existence of GBS, which adapts to grain rotation through GBS. The occurrence of high-density dislocations occurs in FCC and B2 grains, indicating that intragranular dislocations play an important role in the adjustment of deformation during plastic deformation. In the FCC and B2 phases, the interconnection of vacancies and the movement of dislocations reduces the stress concentration and plays a vital role in accommodating GBS.
In summary, ultra-high strain rate superplasticity (HSRS) can be achieved in high-entropy alloys (HEA) through microstructure engineering. This multiphase microstructure can obtain an ultra-high HSRS record of 2000% elongation, which is significantly greater than Results observed in other studies. This study helps to use the multi-phase ultrafine crystal structure to obtain practical improvements in superplasticity in HEA materials. The unique performance of HEA and this ultra-high HSRS make HEA more widely used in aerospace and automotive industries.