Mechanisms of Surface Degradation and Mechanical Property Deterioration in Haynes 230 Under High-Temperature Ultrahigh Vacuum

Haynes 230 alloy is widely used as a high-temperature structural material in hot-end components. To investigate the mechanisms underlying surface-defect evolution and mechanical-property degradation under high-temperature ultrahigh-vacuum conditions, exposure tests were conducted for up to 2000 h. Tensile tests were performed on as-received and exposed specimens at room temperature and 900 ℃. Scanning electron microscopy (SEM) revealed selective volatilization of alloying elements from the material surface at high temperature, accompanied by M 6 C carbide precipitation and grain-boundary sinking. Voids were also observed beneath the M 6 C carbide precipitates, while M 23 C 6 carbides precipitated along internal grain boundaries. After exposure, the yield strength and elongation decreased slightly at room temperature. This degradation was primarily attributed to preferential cracking of brittle M 6 C carbides and debonding of M 23 C 6 carbides from the matrix during loading, both of which promoted microcrack initiation. By contrast, pronounced mechanical degradation occurred during tensile testing at 900 ℃. Cr depletion inhibited the formation of a protective oxide scale, leaving the surface susceptible to continued oxidation. Concurrently, local stress concentrations associated with grain-boundary sinking promoted preferential cracking along grain boundaries. Once initiated, the cracks propagated readily, while oxygen diffused rapidly along the grain boundaries; together, these processes severely degraded the overall mechanical performance of the alloy.