This study investigated the stress-strain behavior of additively manufactured IN939 over a temperature range from room temperature to 1000？C. The results reveal a systematic decrease in ultimate tensile strength (UTS) above 600？C, accompanied by a non-monotonic variation in yield strength and an abrupt reduction in strain at 800？C. After completion of the mechanical tests, one side of each fractured specimen was sectioned near the necked region for cross-sectional analysis, while the opposite side was prepared for fractographic examination; both analyses were conducted using scanning electron microscopy (SEM). Failure below 600？C was characterized by a high γ’ precipitate density and predominantly intergranular fracture. The anomalous strain response observed at 800？C is examined in detail, and a transition in deformation mechanisms associated with temperature-induced changes in the alloy’s microstructure is proposed. At 800？C, fracture surfaces exhibited microcracks, reduced γ’ density, micro-scale voids, and small discontinuities. At 1000？C, incipient melting zones were observed, and no ductile dimples were present.
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