Microstructure and mechanical properties of the (Fe,Ni)Al-based alloy produced by SLM and HIP of spherical composite powder

Abstract

The microstructure and properties of spherical composite powders produced from the β-(Fe, Ni)Al-based alloy by plasma rotating electrode process (PREP) using a steel-shelled electrode were studied. Parameters of the selective laser melting (SLM) mode (laser power density, laser intensity, scan speed and regime) were optimized to ensure the minimal porosity of < 0.08% as well as the absence of cracks or structural defects. Pore distribution and pore volume fraction in the fabricated samples were determined by 3D microCT visualization. The evolution of structural transformations in the matrix of the as-SLM (Fe, Ni)Al alloy was observed as the alloy was heated to the aging temperatures of 850 and 1250 °C. At 1250 °C, the secondary α-Fe precipitates are transformed to coherent cuboids with a cube-on-cube orientation relationship limited by the {100} plane. The in situ transmission electron microscopy (TEM) studies of structural phase transformations occurring upon heating in the temperature range of 20–850 °C demonstrated that coherent α-precipitates with a cubic morphology are homogenously nucleated via the mechanism of spinodal decomposition of an oversaturated solid solution of the B2 phase. Segregation of the pre-precipitation zones < 3 nm in size was observed at 450–650 °C. At the initial stage of ageing, α-Fe precipitates have a near-spherical shape. The positive effect of ageing at 850 and 1250 °C on properties of the alloy produced by SLM of spherical composite powders was demonstrated. Ageing at 1250 °C enhances the ultimate compressive strength (σb) and ductility (ε) from σb = 1490 ± 34 MPa at ε = 1.2% to σb = 1650 ± 41 MPa at ε = 8.0%. The maximum mechanical properties were observed in the samples produced by HIP: conditional ultimate compressive strength: 2120 ± 60 MPa (which corresponds to the true ultimate compressive stress σb = 1765 ± 50 MPa) and plastic yield ε = 16.25% (e = 0.1774) at an offset yield point (true yield stress) σ0.2 = 872 ± 21 MPa. Thermomechanical properties of the (Fe,Ni)Al- based alloy at 873–1373 K was studied. The temperature-force dependence of the steady-state creep rate has been analyzed. The true yield stress equal to 566 MPa was measured at 873 K, logarithmic strain rate 0.001 s−1, and degree of induced plastic strain 0.2%. It was shown that high-temperature mechanical properties of (Fe,Ni)Al- based alloy has intermediate values in between intermetallic alloy Ni41Al41Cr12Co6 and Fe-Ni-Gr-Al alloy (EP747).