Thermal aging effects on the mechanical properties of as-cast Ni 3Al-based alloy

Abstract

A series of tensile tests were conducted at room and elevated temperatures on specimens of the alloy known as IC221M (Ni–8.2Al–7.6Cr–1.5Mo–2.1Zr, wt.%). Specimens were tested in the as-cast condition or after aging in either air or Ar for up to 1000 h at 900–1100 °C. Room temperature yield strength decreased continuously with aging time at 900 °C from the as-cast value of 530–320 MPa after 1000 h. A similar trend was found for hardness. The strength reductions with aging time at 900 °C were similar for aging in either air or Ar. For aging at 1050 and 1100 °C, aging in air caused significant strength loss that was attributed to aggressive oxidation. The room temperature yield strength of specimens aged in Ar increased for aging at 1050 and 1100 °C. Microhardness testing also showed that specimens aged at 1100 °C had higher hardness than those aged at 900 °C. Analysis using equilibrium thermodynamics suggested that the increases of yield strength and hardness resulted from increasing the amount of fine γ′ precipitation in the alloy matrix. For testing at elevated temperatures, the as-cast alloy had room temperature yield strength of 530 MPa that increased continuously up to a value of 650 MPa at 700 °C. Aging in Ar for 1000 h at 900 °C resulted in a room temperature yield strength of 320 MPa that increased to 560 MPa at 700 °C and decrease slightly to 550 MPa at 900 °C. Aging in Ar for 1000 h at 1100 °C produced room temperature yield strength of 435 MPa that increased to 550 MPa at 500 °C and decreased continuously at high test temperatures. The behavior of these specimens was also rationalized using equilibrium thermodynamics to estimate the fractions of coarse and fine γ′ particles in the microstructures.