Quantitative Characterization of Heat Treatment Response in a Single Crystal Nickel-Based Superalloy

To study quantitatively the effect of heat treatment on the microstructure, composition and mechanical property in a new single crystal nickel-based superalloy for industrial gas turbine (IGT) applications, the eutectic fraction, carbide fraction, and the fraction, size, shape and distribution of the γ ́ phase was characterized by quantitative metallographic method, the evolution of chemical composition and hardness between core and inter dendrite was tested through EMPA and nanoindentation. The experimental results indicate that: The eutectic fraction decreases from (0.52±0.08) % to (0.03±0.01) %. The carbides fraction decreases from (0.23±0.04) % to (0.12±0.03) %, and Feret ratio decreases from 3.21±2.54 to 2.14±0.98. The γ ́ fraction increases from (55.66±4.18) % to (73.78±3.24) % in core dendritic region, from (64.82±1.44) % to (70.11±3.10) % in inter dendritic region. The γ ́-size is 406±111(nm) in core dendritic region and 918±384(nm) in inter dendritic region before heat treatment, 359±69(nm) in core dendritic region and 361±57(nm) in inter dendritic region after heat treatment. The γ ́-cuboidal degree is 1.08±0.20 in core dendritic region and 1.14±0.23 in inter dendritic region before heat treatment, 1.08±0.19 in core dendritic region and 1.02±0.14 in inter dendritic region after heat treatment. The solidification segregation coefficient of main segregation elements, such as Re, W, Hf, Ta, Al, and Mo, is closer to 1, with an average decrease of 27% after heat treatment. The hardness and modulus increase in core and inter dendritic, and their inhomogeneity is reduced between cores and inter dendritic. The improvement of properties result from the improvement of size uniformity and cuboidal degree of γ ́, and the reduction of carbides and eutectic through element homogeneity during heat treatment. The solidification segregation coefficient of main segregation elements, such as Re, W, Hf, Ta, Al, and Mo, is closer to 1, with an average decrease of 27% after heat treatment. With the addition of refractory elements, some elements partition to the dendrite core, while other elements tend to accumulate in the interdendritic liquid and then solidify as the interdendritic and eutectic regions during solidification. The hardness and modulus increase in core and inter dendritic, and their inhomogeneity is reduced between cores and inter dendritic. The improvement of properties result from the improvement of size uniformity and cuboidal degree of γ ́, and the reduction of carbides and eutectic through element homogeneity during heat treatment.

Experimental Study of High Temperature Phase Equilibria in the Iron-Rich Part of the Fe-P and Fe-C-P Systems

During the solidification of steel, phosphorus strongly segregates in the interdendritic liquid phase. In the continuous casting process, even low levels of P may have a detrimental effect on the final product quality. However, phosphorus is partly added up to 0.10 wt pct to improve the mechanical properties of advanced steel grades nowadays, e.g., High-Strength Interstitial-Free (HSIF). To provide new experimental data for the development of thermodynamic databases and solidification models for P alloyed steel grades, phase equilibria in the Fe-P and Fe-C-P key systems were studied up to 1550 °C using differential scanning calorimetry (DSC) and high temperature laser scanning confocal microscopy (HT-LSCM). Special focus was placed on solid/liquid equilibrium temperatures in the Fe-rich part of the binary Fe-P system between 0.025 and 9 wt pct P. In the ternary system, three isoplethal sections with 0.10 mass pct. P, 0.20 mass pct. C and constant mass percent ratio P/C of 2 were investigated. In the latter section, HT-LSCM observations were linked with DSC signals to optically identify present phase stabilities. Particularly at [pct P] < 1, significant differences between performed measurements and calculated phase equilibrium temperatures using thermodynamic assessments from the literature were identified. In all ternary sections, the experiments indicate less influence of P on the hypo-peritectic range compared to the thermodynamic calculations.

Formation of Nb(C,N) Carbonitride in Cast Austenitic Heat-Resistant Steel during Directional Solidification under Different Withdraw Rates

It is recognized recently that primary “Chinese-script” Nb(C,N) carbonitride is critical to the development of cast austenitic heat-resistant steels for ultra-high temperature applications. In this paper, the precipitation behavior of Nb(C,N) carbonitride in a novel creep and fatigue resistant steel was investigated by the use of the liquid metal cooling directional solidification (LMC-DS) method under different withdraw rates. Thermodynamic calculations were also performed to aid in the understanding of the solidification behavior. Microstructural characterization and thermodynamic calculation agreed that the alloy solidified in the path of primary austenite, eutectic Nb(C,N) carbonitride, and secondary ferrite, regardless of the withdraw rate. However, the primary and secondary dendrite arm spacing decreased significantly with an increase in the withdraw rate, and a quantitative relationship was established. Furthermore, the eutectic reaction range increased at a higher withdraw rate, due to the rapid increase of the solid phase fraction and the accumulation of solutes in the interdendritic liquid phase. This gave rise to a decline in the interlamellar spacing of primary Nb(C,N) carbonitride sheets and rods for the higher withdraw rate. Therefore, a fine “Chinese-script” Nb(C,N) carbonitride in this type of alloys can be achieved through increasing the withdraw rate or the cooling rate during casting.

Permeability in the Mushy Zone

When modeled at macroscopic length scales, the complex dendritic network in the solid-plus-liquid region of a solidifying alloy (the “mushy zone”) has been modeled as a continuum based on the theory of porous media. The most important property of a porous medium is its permeability, which relates the macroscopic pressure gradient to the throughput of fluid flow. Knowledge of the permeability of the mushy zone as a function of the local volume-fraction of liquid and other morphological parameters is thus essential to successfully modeling the flow of interdendritic liquid during alloy solidification. In current continuum models, the permeability of the mushy zone is given as a deterministic function of (1) the local volume fraction of liquid and (2) a characteristic length scale such as the primary dendrite arm spacing or the reciprocal of the specific surface area of the solid-liquid interface. Here we first provide a broad overview of the experimental data, mesoscale numerical flow simulations, and resulting correlations for the deterministic permeability of both equiaxed and columnar mushy zones. A extended view of permeability in mushy zones which includes the stochastic nature of permeability is discussed. This viewpoint is the result of performing extensive numerical simulations of creeping flow through random microstructures. The permeabilities obtained from these simulations are random functions with spatial autocorrelation structures, and variations in the local permeability are shown to have dramatic effects on the flow patterns observed in such microstructures. Specifically, it is found that “lightning-like” patterns emerge in the fluid velocity and that the flows in such geometries are strongly sensitive to small variations in the solid structure. We conclude with a comparison of deterministic and stochastic permeabilities which suggests the importance of incorporating stochastic descriptions of the permeability of the mushy zone in solidification modeling.