Effects of microstructure evolution on the deformation mechanisms and tensile properties of a new Ni-base superalloy during aging at 800℃

Tensile properties at room temperature of a new casting Ni-base superalloy during aging at 800℃ for0-1000 h were investigated.During aging,granular M23C6 carbides presented at grain boundaries and kept growing from dispersed particles to continuous networks.The γ’ phase significantly coarsened,with the morphology of some γ’ phase changed from spherical to rounded cubic shape after 1000 h.Three deformation mechanisms in relation to the γ’ diameter(dγ’) were identified:(ⅰ) weakly coupled dislocations(WCD) connected by anti-phase boundary(APB) traveled in pair across the γ/γ’ structure when dγ’ was small in the under-aged alloys;(ⅱ) strongly coupled dislocations(SCD) with reduced spacing compared to(ⅰ) sheared γ’ phase when dγ’ increased in the over-aged alloys;(ⅲ) dislocations occasionally by-passed γ’ phase when dγ’ was larger than 97 nm after aging for more than 300 h.The alloy obtained the peak strength when 20 h-aged with dγ’=44 nm which was in the transition between(ⅰ) and(ⅱ).The aginginduced variation in yield strength was correlated to the coarsening of γ’ phase using a theoretical model of precipitation strengthening in terms of the formation of APB.The calculated results suggested that the γ’ phase with a volume fraction of 23% contributed more than 61% of the peak-aged yield strength.Observation after fracture revealed that the alloys usually fractured at grain boundaries.High stress concentration around carbides resulted in cracks by carbides self-cracking and the initiation of cavities.The undesirable agglomeration of M23C6 at grain boundaries was harmful to the properties of the overaged alloys.

Effect of grain boundary precipitates on the stress rupture properties of K4750 alloy after long-term aging at 750℃for 8000 h

In K4750 alloy,the evolution of grain boundary(GB)precipitates,including the degradation of blocky MC carbide particles and the precipitation of granular/needle-likeηphase particles,were observed after longterm aging(LA)at 750℃for 8000 h.During MC degradation,the Ti and C released from the MC carbide combined with Ni and Cr,respectively,in theγ’matrix to formη-Ni_(3)Ti phase and Cr-rich M_(23)C_(6)carbide.Large amounts of granularηphase precipitated at GBs and the needle-likeηphase grew gradually from GBs toward the grain interior.Because of the growth of theηphase through absorbingγ’phase,γ-depleted zones were formed around theηphase.The evolution of the MC carbide andηphase was primarily responsible for the decrease of the stress rupture life and the increase of elongation.After an LA sample was tested at 750℃and 360 MPa,the residual strain distribution was investigated by electron backscatter diffraction(EBSD).The results showed that the residual strain mainly distributed at GBs,especially in the region of MC degradation and at the edges ofηphases,which was closely related to the appearance of phase interfaces.Microvoids/cracks easily initiated at phase interfaces,then easily extended along theγ-depleted zones,thus the stress rupture life of LA samples was substantially shorter than that of samples subjected to the standard treatment.In particular,because of large amounts of fine degraded MC,granular M_(23)C_(6)and granularηphase particles distributed at GBs after 750℃/8000 h LA and microvoid/crack formation could be hindered by the formation of dimples,which led to an increase of elongation.