Effect of Structural Refinement on the Mechanical Properties of Fe–28 Al Intermetallic Alloy Produced by Direct Powder Forging IV. Analysis of the Structural Sensitivity of Low-Temperature Strengthening, Heat Resistance, and Creep Resistance

Abstract

The influence of structural refinement in iron aluminide on low-temperature strengthening, heat resistance, and creep resistance in the temperature range 650–750°C was studied. The phase and structural states in which the studied materials demonstrated the best combination of properties were determined. At room temperature, the greatest contribution to low-temperature strengthening was provided by grain size reduction. Compression test results showed that the highest strengthening was exhibited by the samples forged from ground powders. The alloy reinforced with 0.3 at.% CrB₂ demonstrated the highest yield stress at room temperature (1176 MPa) resulting from the formation of the finest structure. Dynamic recrystallization under optimal thermokinetic conditions promoted ultrafine grains (1–2 μm) and fine boride particles (~100 nm). The temperature dependences of yield stress showed abnormal values for all samples. Over the entire test temperature range, the greatest strengthening was exhibited by the samples produced by triple deformation. Significant softening was observed above 600°C. The alloys reinforced with boride particles softened most slowly. At 700°C, the yield stress remained at the level of ~450 MPa. With an increase in temperature to 750°C, the alloy containing 1.5 at.% CrB₂ had higher yield stress than the alloy with 0.3 at.% CrB₂ (207 MPa and 120 MPa, respectively). Over the temperature range 650–750°C, structural refinement favorably influenced creep resistance. At 700°C, under a load of 120 MPa, the best creep resistance was demonstrated by the two-component Fe–28 Al alloy produced by forging from ground powders under dynamic recrystallization conditions and by the alloy reinforced with 1.5 at.% CrB₂. In both cases, the creep rate was ε′ ~ 3.3 · 10^–7 sec^–1. A comparative analysis of the creep resistance of Fe–28 Al matrix composites with 0.3 and 1.5 at.% CrB₂ at 700 and 750°C under loads of 90 and 120 MPa showed that, under all selected deformation conditions, the alloy with the higher boride content exhibited better creep resistance, since fine incoherent particles provided the main contribution to high-temperature strengthening.