Фазовые изменения в композиционном сплаве АК12ММгН-18%SiCp после интенсивной пластической деформации и отжига

Abstract

The effect of severe plastic deformation by high pressure torsion (HPT) on the structure of AK12MMgN-18%SiCp composite alloy was investigated. A liquid-forged billet was taken as an initial material. Samples 8 mm in diameter and 0.45 mm in thickness were deformed by torsion in a Bridgman anvil at room temperature up to 5 turns under the pressure of 4 GPa. After deformation the samples were annealed in the temperature range of 300-500°C for 5 minutes. The average area and volume fraction of particles in the initial, HPT-processed and annealed samples were evaluated. The change of the concentration level of a solid solution in the states under study was analyzed. It was shown that HPT led to a fragmentation of particles and a reduction of their average area: silicon particles from 4.4±0.1 to 0.32±0.02 μm2, intermetallic particles from 5.2±2.6 to 0.20±0.02 μm2, and SiC from 37.6±0.9 to 3.9±0.02 μm2. At the same time, a partial dissolution of excess phases occurred that resulted in the formation of a supersaturated solid solution. Post-HPT annealing led to a dissolution of the supersaturated solid solution with a separation of secondary phases of different morphology not typical to the alloy: globular silicon, rod shaped phase enriched by Ni, Si and Fe and a more compact phase containing Si and Cu. The higher the annealing temperature, the more intensive was the process of supersaturated solid solution decomposition and the higher was the volume fraction of silicon and intermetallic particles. Their average area increased with annealing temperature. No changes in the average area and volume fraction of SiC particles during annealings of HPT-processed alloy were observed.