Influence Of Heat Treatment Regimes On Microstructure And Creep Properties Of FGH95Alloy

In the paper, by means of the heat treatment at the different temperatures, the creepproperties measuring and microstructure observation, an investigation has been made into theinfluence of heat treatment regimes on the microstructure and creep properties of FGH95nickel-based superalloy, and discussing the deformation and fracture mechanisms of the alloyduring creep. In the further, the precipitating temperature, the driving force of the phasetransformation and nucleation of the carbides are calculated by thermodynamic method forpredicting the precipitation sequence of them.Results show that the microstructute of FGH95alloy consists of Jmatrix J cphase andcarbides. After treated by HIP at1120, the coarser J cphase is precipitated in the regions ofPPB, and significant amount of fine J cphase rea distributed within the grains along the samedirection. After treated by HIP at1180, the coarser J cphase is fully dissolved, and the sizeof the grains is obviously grown up. In the further, after the HIP alloy is solution treated at1155, the fine J cphase and carbides are dispersedly precipitated in the alloy. And with theenhancement of the solution temperature, the quantity of the coarser J cphase diminishs andthe one of the carbides increases. When the solution temperature enhances to1160, thecoarser J cphase is fully dissolved, significant amount of the fine J cphase are dispersedlyprepicatated within the grains, and the particle-like MC carbides are precipitated alongboundaries. But after solution treated at1165, the size of the grains is obvious grown up,and the slice-like carbides are precipitated along boundaries. After the HIP alloy is solutiontreated at1140and aging treated in the molten salt at480, a few coarser J cphase isprecipitated along the boundaries, and some particle-like carbides are distributed alongboundaries to improve the cohesive strength between the grains, which is thought to be themain reason of the alloy possessing a better creep resistance and longer creep lifetimes.In the ranges of the applied temperatures and stresses, the creep activation energies and stress exponents of the alloys cooled in molten salts at450and480are calculated to beQ1=519.8kJ/mol and Q2=579.4kJ/mol. And the precipitated carbides in the alloly duringheat treatment are identifies as NbC, TiC phase, thereinto, the precipitating temperatures ofNbC and TiC carbides are defined to be1353K and1090K, respectively. Compared to theelements Nb and Ti, the Cr atom is a weaker former of the carbides. Therefore, the (Nb, Ti)Ccompound phase may be precipitated in the alloy, during heat treatment, due to thecomparability in the crystal structure of the Nb, Ti atoms. In the further, the driving forcevalues of the phase transformation for TiC, NbC and WC carbides precipitated from the alloyat583are calculated to be-9.77J/mol,-11.23J/mol and-13.48J/mol, respectively, and thenucleating driving force of TiC, NbC and WC carbides precipitated from the alloy at583are calculated to be-80.69J/mol,-59.95J/mol and-36.01J/mol, respectively. Thedeformation mechanisms of the alloy during creep is that the slipping of dislocations in theJ matric and dislocation shearing into the J cphase, thereinto, the dislocations shearing intothe J cphase may be decomposed to form the configuration of the Shockleys partials plusstacking fault, which may hinder the cross-slipping of dislocations to improve the creepresistance of alloy.