Microstructural Evolution and Room Temperature Mechanical Properties in Additively Manufactured Mar M 509 with Short Cycle Heat Treatment.

Abstract

The Co-based superalloy Mar M 509, known for its high-temperature oxidation and hot corrosion resistance, is processed via laser powder bed fusion (LPBF). Microstructure and mechanical properties of Mar M 509 in as-printed (As-P) and heat-treated (HT) states are compared based on two build orientations (longitudinal (L) and transverse (T)) to establish structure-property links with heat treatment. The As-P condition displays a distinct cellular microstructure (500-600 nm) with 50-60 nm carbide particles adorning cell boundaries. Longitudinal (L) build has columnar grains (8-35 μm along major axis) with a grain aspect ratio of 4, while transverse (T) orientation exhibits equiaxed, bimodal microstructure (5-10 μm and 15-25 μm grain sizes). Strong <001> texture is noted in L. Mechanical properties at room temperature differ between L and T; T (569±12HV) has 15% higher hardness compared to L (489±18HV) and 34% higher 0.2% yield strength (YS), but 30% lower elongation than L. Post a short heat treatment cycle at 1250°C, weld bead structure and cell boundaries break down. Both L (25-33 μm along major axis) and T orientations (5-42 μm) experience grain growth, and carbides coarsen (250-350 nm). Post-heat treatment, dislocation density decreases, indicating recrystallization; lattice parameter of matrix reduces, implying solute depletion contributing to carbide enrichment. Yield strength drops from 860 MPa to 740 MPa in L and from 1150 MPa to 840 MPa in T, with ductility rising from 14% to 23% in L.