Korean Journal of Materials Research ISSN:1225-0562(Print) 2287-7258(Online) 한국재료학회지

Tensile tests and creep tests were carried out at high temperatures on an Al-Al₄C₃ alloy prepared by mechanical alloying technique. The material contains about 2.0 % carbon and 0.9 % oxygen in mass percent, and the volume fractions of Al₄C₃ and Al₂O₃ particles are estimated at 7.4 and 1.4 %, respectively, from the chemical composition. Minimum creep rate decreased steeply near two critical stresses, σ_cl (the lower critical stress) and σ_cu (the upper critical stress), with decreasing applied stress at temperatures below 723 K. Instantaneous plastic strain was observed in creep tests above a critical stress, σci, at each test temperature. σ_cu and σci were fairly close to the 0.2% proof stress obtained by tensile tests at each test temperature. It is thought that σ_cl and σ_cu correspond to the microscopic yield stress and the macroscopic yield stress, respectively. The lower critical stress corresponds to the local yield stress needed for dislocations to move in the soft region within subgrains. The creep strain in the low stress range below 723 K arises mainly from the local deformation of the soft region. The upper critical stress is equivalent to the macroscopic yield stress necessary for dislocations within subgrains or in subboundaries; this stress can extensively move beyond subboundaries under a stress above the critical point to yield a macroscopic deformation. At higher temperatures above 773 K, the influence of the diffusional creep increases and the stress exponent of the creep rate decreases.