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To accurately predict the microstructure evolution of competitive columnar grain growth, the influence of the potential dominant factors of competition growth such as thermal field, solute field and flow field on the overgrowth behavior of trinary-crystal samples during directional solidification was systematically investigated, with the preferred orientation of the experimental grains orienting parallel and at a limited misorientation angle with respect to the temperature gradient direction. It was found that the grain overgrowth rate was weakly dependent on the temperature gradient, which was inconsistent with the classical theoretical assumption that the grain overgrowth rate was determined by the difference of the tip undercooling between the competing grains. In contrast, the grain overgrowth rate was sensitive to the solute field around the dendrite tips. Additionally, with increasing the natural convection, the grain overgrowth rate tended to be promoted at low withdraw speed. These phenomena were attributed to the mechanisms of solute interaction in the converging case and sidebranching events in the diverging case, while the solute field was the dominant factor to govern the overgrowth behavior of the competing grains. Moreover, a new model of competitive grain growth based on the solute field was proposed to predict the microstructure evolution of the Nickel-based superalloys during the directional solidification process. In this new model, the primary spacing of the well-oriented grain in the converging case was smaller than that in the diverging case due to the sidebranching events and dendrite lateral motion, which highlighted a new mechanism of primary spacing evolution in the directional columnar solidification structure.

Effects of boron (B) addition on microstructure and high-temperature stress rupture properties of a heat-treated high chromium polycrystalline Nickel-based superalloy were investigated by adjusting B content (0, 0.016 wt%, 0.048 wt%, 0.09 wt%). The results indicated that the grain boundary characteristics were altered by B addition, such as the morphologies of the MC carbides, M23C6 carbides and precipitation of γ′ particles along grain boundaries. Meanwhile, it was confirmed that B existed in the form of Cr-rich M5B3 borides in the boron-containing alloys. Moreover, boron addition aggravated the inhomogeneity of microstructures, and reduced the volume fraction of secondary γ′ particles. Additionally, the rupture life was remarkably improved as the B content was increased to 0.016 wt%, whilst the elongation tended to improve as the B content was increased to 0.048 wt%. The improvement of stress rupture properties was mainly contributed to the modification of the grain boundary characteristics which retarded crack nucleation and propagation at grain boundaries. However, with excessive addition of B, the stress rupture properties were reduced significantly, as the solid solution strengthening and precipitation strengthening tended to be weakened by B addition, in addition the incipient melting regions and lager size borides promoted the intragranular crack nucleation and propagation.

A new casting Ni-based superalloy is used in industrial and aircraft turbine because of its high strength and excellent hot corrosion resistance at high temperatures(about 900℃).The effect of heat treatment process on its microstructure and mechanical properties of the experimental casting Ni-based superalloy was studied in the present investigation in order to improve its application level.The results showed that the stress-rupture lives were changed with the increase of heat treatment temperatures.After solution treated for 2 h,then cooling by air cooling,the stress-rupture life under the conditions of 760℃and 660 MPa was the highest in the case of heat treatment temperature at 1220℃,while under the testing conditions of 980℃and 180 MPa the stress-rupture life was the highest in the case of heat treatment temperature at 1180℃.The stress-rupture life decreased when being heat-treated at temperatures above 1220 or 1180℃.The tensile strength of the alloy was improved with the increase of heat treatment temperatures from 1120 to 1220℃,and was decreased in the case of heat treatment temperature at 1240℃.The experimental results also showed that with the increment of holding time from 2 to 8 h at 1120℃the stress-rupture life increased under the conditions of 760℃and 660 MPa,while it was decreased under the conditions of 980℃and 180 MPa. It was found that the tensile strength was higher for the alloy being held for 2 and 4 h than that for the alloy being held for 6 and 8 h,and the stress-rupture lives changed with the different cooling ways. The mechanical properties of the alloy may be determined by the size,shape,distribution and volume fractions ofγ' phase andγ/γ' eutectic.