| This paper analyzed microstructure of different parts of spray-formed superalloy LSHR, focusing on the formation and long-term aging nucleation mechanism of previous particle boundaries (PPBs) in near the edge region, combined with spray forming process characteristics. High temperature tensile test was carried out on the alloy, and the microstructure and mechanical properties of the alloy were studied by means of SEM, XRD and TEM in order to research the microstructure and mechanical properties of the alloy during long-term aging process.JmatPro thermodynamic phase diagram calculation indicates that equilibrium precipitated phases of spray-formed LSHR alloy mainly consists of y’, MC, M23C6 and M3B2; it may as well precipitate σ and μ phase when exposed to heat or long-term working. Porosity, grain size and density decreased from the center region to the edge. Microstructure of the desposited alloy is mainly equiaxed grains except the the peripheral region where appears many original powder particles composed of fine dendrite and cellular crystal as a result of the higher cooling rate. PPBs and flow linesis are also observed in near the edge region.PPB in near the edge region was systematically studied by heat treatment and analytical instruments (such as SEM, TEM, XRD) combined with thermodynamic phase diagram. Analysis shows that PPB in spray-formed LSHR is composed of large size y’, M23C6 carbide a small amount of carbon nitride,MC, borides, no obvious oxide is observed. Results prove the formation of spray-formed LSHR alloy PPB has nothing to do with oxygen rather than of rapid solidification and inadequate droplet edge region near the liquid content, latent heat of crystallization due to low energy. PPB in spray-formed LSHR can be mostly dissolved through high-temperature solution by dissolving large size γ’ and M23C6.However, But it caused the weakening of grain boundary strength. The alloy is directly long-term aged without solution in 704℃ up to 1200h. With the aging-time prolonged, PPB in near the edge region increased, γ’,carbide and grain boundary roughed. The centre region without PPBs originally wasn’t observed obvious PPB nucleation. No TCP phases Precipitated in all region through out the long-time aging, indicating the alloy is of great stability. That the alloy occurred a reaction as "y(supersaturation)â†'γ (balanced)+γ’+ MC’during the long-term aging is the main reason for the roughed γ’,carbide and grain boundaries and the increasing PPBs.After a standard heat treatment as" solution in 1140℃ for 2h (oil cooling, followed by aging in 815℃ for 8h"the alloy was long-term aged in 750℃ for 0h/200h/500h/800h/1200h, and analized their microstructure and mechanical properties. Results showed that with the increase of aging time, the grain size of the alloy is basically unchanged, the grain boundary carbides by non continuous distribution into the continuous chain distribution, intragranular carbides precipitated and the quantity increased. γ’ coarsed from spherical to square, the volume fraction of γ’ increased, γ/γ’ mismatch increased as well. The large amounts precipitation of γ’ caused Cr, Co, Mo excessing and precipitated aphase. And a larger y/y’ coherent stress made γ’ a worse stability, splitting and precipiting ηphase at the same time. When aged 1200h, high temperature strength and plasticity of the alloy made a sharp decline, due to the needle or plate strip distribution of η and σ phase. |
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