Evolution of precipitates in Ni-Co-Cr-W-Mo superalloys with different tungsten contents

The Ni-Co-Cr-W-Mo system is critical for the design of nickel-based superalloys.This system stabilizes different topologically close-packed(TCP)phases in many commercially superalloys with high W and Mo contents.Scanning electron microscopy(SEM),transmission electron microscopy(TEM)and thermodynamic calculations were applied to investigate the thermodynamics of the precipitates in two different W-contained Ni-Co-Cr-WMo superalloys(Alloy 1 and Alloy 2).Computational thermodynamics verifies the experimental observation of theμphase formation as a function of temperature and alloy chemistry,but the kinetics for the precipitation of the M6 C phase do not agree with the experimental findings.The major precipitates of Alloy 1 at temperatures of700℃and 750℃during long-time exposure are M23 C6,γ′phase and MC;for Alloy 2,they are M23 C6,γ′phase,MC,M6 C andμphase.W addition is found to promote the precipitation of M6 C andμphase during exposure.M6 C has higher W and lower Ni content thanμphase,whereas M6 C is an unstable phase that would transform into M12 C after 5000-h exposure at 750℃.A great quantity of needle-likeμphases precipitated after exposure at 750℃for5000 h,which have no effect on the impact properties of Alloy 2.

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, A1 1.3, and Nb-kV 0.13 （mass%） with annealed martensite （TAM steel） were investigated using optical microscopy, field emission gun scanning electronic microscope （FEG SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and mechanical testing. The mierostructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix （TPF steel）, the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain harden- ing behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.

Microstructural change and stress rupture property of Nimonic 105 superalloy for advanced ultra-supercritical power plants

Microstructural change,stress rupture property,deformation and fracture mechanisms of Nimonic 105 superalloy at 750℃have been studied.Experimental results showed that the stress rupture strength of the alloy at 750℃ for 10^(5)h is about 200 MPa.γ'precipitates and M_(23)C_(6)carbides grew gradually with prolonging the rupture time,while no significant change was observed in MC carbide morphology.After stress rupture test at 750℃ and 250 MPa for 23,341 h,a transition from spherical to cuboidal morphology of γ'precipitates was found,and nearly continuous chains of M_(23)C_(6)carbides formed on the grain boundary.Orowan looping and strongly coupled dislocation pairs cutting and microtwinning were the dominant deformation mechanisms at 750℃ and 350-450 MPa,while the main deformation mode was Orowan looping at 750℃ and 250 MPa.The failure of the alloy was mainly attributed to the nucleation,growth and interlinkage of voids.

Effect of W on formation and properties of precipitates in Ni-based superalloys

The formation and properties of precipitates in wrought Ni-based superalloys with different W contents during long-term exposure to high temperatures were investigated.The scanning electron microscope,transmission electron microscope,and chemical phase analysis were used to investigate the formation and properties of precipitates.It is found that with increasing W content,the quantity and thermal stability of MC carbide in Ni-based superalloys increased,while the quantity of M_(23)C_(6)carbides decreased.As the results show,W has a higher partition coefficient in γ'-and γ-matrix,and the addition of W promotes the precipitation of γ'phase.W content has no significant effect on the morphology,size,crystal structure,and coarsening rate of γ'precipitates.The influence of W content on high-temperature tensile and creep properties of the alloys was investigated.The results showed that W content has no obvious influence on the high-temperature yield strength,but the elongation and area reduction decreased significantly when the addition of W was more than 4 wt.%.Because of the similar volume fractions of γ'phase,the creep fracture strengths in the tested alloys with lower W concentrations were not significantly different after long-term exposure at 700℃.