Precipitation and Growth Simulation of γ′ Phase in Single Crystal Superalloy DD6 with Multiphase-Field Method and Explicit Nucleation Algorithm

The microstructure evolution of Ni-based superalloys during heat treatment is of great significance for obtaining better service performance. However, heat treatment experimentation is costly and time-consuming, and sometimes fails to reveal physical mechanisms well. In the present study, a multiphase-field model coupled with an explicit nucleation algorithm was established to simulate the precipitation and growth of γ′ phase in DD6 superalloy, which can be applied to a multicomponent elastic-inhomogeneous system. The PanNickel© database was used to calculate thermodynamic and kinetic data in multicomponent superalloys. First, the coupling method of multiphase-field model and explicit nucleation algorithm was introduced. The coupled model was used to simulate the precipitation of γ′ phase under isothermal and non-isothermal conditions. It was found that a unimodal microstructure was formed under isothermal conditions and there was a “soft impingement” phenomenon, while a bimodal distribution composed of cuboidal γ′ precipitates and fine secondary γ′ precipitates was formed during a cooling process of 25–125 °C/min. The effect of cooling rate was studied. Then, the chemical and elastic driving forces were analyzed. It was found that Al and Ta contributed most to the chemical driving force, while Re and W gathered at the γ/γ′ interface and inhibited the growth of γ′ phase. γ′ precipitates had a cuboidal shape under the influence of elastic driving force. Finally, the growth and coarsening process of γ′ phase was studied and compared with the well-known Lifshitz−Slyosov−Wagner (LSW) theory. The growth of γ′ phase can be divided into rapid growth, coarsening and quasi-static coarsening stages according to the simulation results, among which the coarsening stage is basically consistent with the LSW theory. The current model can be used to simulate the precipitation and growth of single crystal superalloys where multicomponent and elastic effects are considered.

COARSENING KINETICS IN Al ALLOY CAST THROUGH RAPID SLURRY FORMATION (RSF) PROCESS

<p class="AMSmaintext"><span lang="EN-GB">In the present investigation,<strong> </strong>the rate of coarsening kinetics was discussed on conventional diffusional Lifshitz, Slyozov and Wagner (LSW) theory, coarsening by convective mass flow and coalescence by stirring speed. The rate of coarsening kinetics was found to be faster than predicted by diffusional LSW theory, but slower than predicted by the convective mass flow. The effect of flow pattern with baffles in a crucible at different holding time was also examined. The results showed that use of baffles produced fine and more globular primary α-Al phase when compared to sample processed without baffles having the same holding time. </span></p>

Precipitation and Growth of Intermatallic Phases in an Fe-Ni Superalloy during Prolonged Ageing

The influence of prolonged aging on the precipitation process of intermetallic phases, as well as carbide and boride in an Fe-Ni superalloy has been studied. The samples were subjected to a solution heat treatment at 980°C for 2 h and water quenched, and then aged at 715, 750 and 780°C for 0.5-500 h. The samples were analysed using transmission electron microscopy (TEM) and X-ray diffraction. Direct measurements on the electron micrographs allowed to calculate the mean diameter () of the γ’ phase. The growth step of the γ’ phase particles has been analysed on the basis of the LSW theory, as well as the activation energy of the γ’ phase coagulation has been estimated.

Quantitative Phase Field Study on the Precipitation Dynamics of γ′ Phase in Ni-Based Superalloys with Different Concentration

A quantitative phase field simulation was performed on the dynamics evolution of γ′ (L12-Ni3X) phase in Ni-based superalloys, the microstructure, the volume fraction and the particle size distribution (PSD) of γ′ phase for Ni-Al alloys aged at 1173K with the Al concentration c=0.178, 0.180 and 0.182 were investigated, and the results were compared with Lifshitz-Slyozov-Wagner (LSW) theory and Brailsford-Wynblatt (BW) theory. As the Al concentration increases the γ′ phase morphology changed from the separated cuboidal shape to the connected rectangle shape, the nucleation and growth of γ′ phase became faster and the volume fraction of the γ′ phase increased. The average particle radius <r> of γ′ phase and the aging time t has a exponent relationship <r> ~ tn at the coarsening stage with the exponents n=0.313, 0.235 and 0.204 for c=0.178, 0.180 and 0.182, respectively. The peaks of the fitted PSDs are less than the predicted value from the LSW theory and the fitted PSDs are wider than that of LSW predicted, while they are similar to that of the BW theory as the Al concentration increases. The peaks appear at a smaller r/<r> than the predictions of the LSW and BW theories.

Ostwald Ripening of the Platinum Nanoparticles in the Framework of the Modified LSW Theory

An analysis of the experimental data related to the mechanism of Pt particles sintering has been carried out using the modified LSW theory. The size distribution for the Pt nanoparticles at the stage of Ostwald ripening fits the generalized Lifshitz-Slyozov-Wagner model calculated with the assumption of two parallel mechanisms involved in the nanoparticles growth (dissolution): diffusion and Wagner’s (controlled by the chemical reaction rate). Comparison between the experimental histograms and the curves calculated theoretically proves the governing role of the Wagner’s mechanism (chemical reaction) in the Pt nanoparticles growth.

Magnetic Susceptibility of Quasi-two-dimensional Cuprate Antiferromagnets

Parallel magnetic susceptibility temperature dependence of the high-TC superconducting parent compound La2CuO4 is calculated in both antiferromagnetic (AFM) and paramagnetic phase. By making use of the quantum Heisenberg three-dimensional AFM model including the in-plane spin anisotropy, the calculation is performed within the framework of three different theories: Green’s function theory in random-phase approximation (RPA), linear spinwave (LSW) theory and mean-field (MF) theory. The results suggest that at low temperatures quantum spin fluctuations play an important role, while at the temperatures above the critical one short-range correlations have a great impact on the behavior of the system. This leads to the discrepancy between RPA and MF results, since the later neglects the above phenomena. Further, LSW theory expectedly agrees with RPA results only at low temperatures where the magnon interactions are negligible. Comparison to the theoretical and experimental results quoted in literature confirms that RPA method presents the most appropriate method among the applied ones, suggesting that this approach is satisfactory in the case of the parallel magnetic susceptibility, while in order to reproduce the transversal one, spin-orbit coupling must be included.

Influence of Friction Stir Processing on Coarsening Dynamics of Fe-Rich Phase in Semi-Solid Al-Si-Fe Alloy

The as-cast and friction stir processed (FSP) hypereutectic AlSi29Fe3 were reheated to the semi-solid state, the coarsening dynamics of the Fe-rich phase during the process of remelting were investigated by the means of quantitatively metallurgical analysis. The results indicated that the coarsening of the Fe-rich phase in semi-solid state for both microstructures followed the Lifshitz-Slyozov-Wagner (LSW) theory:. However, the coarsening exponent n of the Fe-rich phase in the as-cast microstructure was 3, in the FSP microstructure was 2. Therefore, the growth rate of the Fe-rich phase in the FSP microstructure was larger than in the as-cast microstructure. In the meantime, the morphology of the Fe-rich phase changed to short bar from granular in the FSP microstructure.

Coarsening Process of Decomposed Phases in Cu-Ni-Cr Alloys

A study of the coarsening process of the decomposed phases was carried out in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100> Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the morphology of the Ni-rich phase changed to ellipsoidal. The variation of mean radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the modified LSW theory for ternary alloys.