On the possibility of applying severe plastic deformation by high pressure torsion for the manufacture of Al-Nb metal matrix composites

The article discusses the possibility of fabrication an aluminium based matrix composite of the Al-Nb hybrid system using severe plastic deformation by high pressure torsion (HPT). Disks of aluminum (with diameters from 6 to 12 mm) and niobium (12 mm in diameter), were stacked in the form of a three-layer Al-Nb-Al package, and subjected to deformation. HPT was carried out at room temperature on Bridgman anvils under a pressure of 5 GPa at N =10, 25 and 30 revolutions, at a strain rate of ω =1 and 2 rpm. Regardless of the choice of the aluminum disk diameter and the deformation modes, monolithic and defect-free samples were fabricated. However, the most effective fragmentation and distribution of niobium in the aluminum matrix was observed when a diameter of aluminum discs was 10 mm and deformation modes N = 25 and 30 revolutions and ω = 2 rpm were applied. Three structural areas were found in the processed samples: the central one with wide curved layers of niobium in aluminum, a finely dispersed lamellar structure in the mid radius area, and a uniform distribution of niobium particles in aluminum matrix at the periphery. The observed heterogeneity of the structure in the samples correlated well with changes in microhardness, the values of which varied nonmonotonically: the minimum level (about 100 HV) was observed in the center, the maximum values (about 280 and 300 HV) in the middle of the radius and about 130 HV at the periphery of the sample, for N = 25 and 30 revolutions, respectively. In addition, it was shown by X-ray diffraction methods that at a higher strain rate and N = 25 and 30, strain-induced aging occurred in the composite material with the synthesis of the hardening intermetallic phase Al3Nb, the volume fraction of which increased from 2.8 to 3.1 % with increasing number of revolutions.