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The influence of phosphorus on hot deformation microstructure of GH984G alloy was investigated. The results indicate that phosphorus accelerates the process of dynamic recrystallization (DRX) nucleation and prolongs the time of DRX grain growth. The alloy containing more phosphorus shows greater flow stress and dislocation density in the early stage of hot deformation, while the stress and dislocation density decrease with the increase of phosphorus after further deformation. Phosphorus inhibits the formation of Σ3 twin boundary at low strain rate or high deformation temperature but promotes it at high strain rate and low deformation temperature. The microstructure characterizations reveal that more blocky MC carbides with larger size form in the alloy with higher phosphorus content and serve as strong obstacles to the dislocation movement, thus increasing the flow stress and dislocation density in the early stage of hot deformation. The higher dislocation density promotes the nucleation of dynamic recrystallization and then reduces the flow stress and dislocation density at higher strain. The formation of Σ3 boundaries depends on growth accident during the process of DRX grain growth. More MC carbides in phosphorus-containing alloy decrease the rate of grain boundary migration and the frequency of growth accident, then reduce the fraction of Σ3 boundary. The change of twin boundary with phosphorus content at high strain rate and low deformation temperature is related to the critical grain size of twin formation.

The mechanism of primary MC decomposition and its influence on the rupture behaviors in the hot-corrosion resistant superalloy K444 are investigated by the combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that the blocky primary MC mainly distributes at interdendritic regions and grain boundaries after the standard heat treatment. During long-term thermal exposure, primary MC decomposes gradually, which can be summarized into three stages. Firstly, the primary MC reacts with the gamma matrix and produces the M23C6 particles and gamma' film, which can be described as MC + gamma -> M23C6 + gamma'. In the second stage, the un-decomposed primary MC arrests the Ni element and forms the M6C particles and eta phase, which can be expressed as MC + gamma -> M6C + eta. In the third stage, the remaining primary MC reacts with the diffused Ni element and forms the eta phase with M6C and M23C6 particles inside, which can be described as MC + gamma -> M6C + M23C6 + eta. The reaction of the second and third stages should be ascribed to the high (Ti + Nb + Hf)/Al ratio and segregated W, Mo and Cr element in the decomposed region. In addition, the platelet-like sigma phase is found in the third stage. The primary MC decomposition has different effects on the rupture behaviors between tensile and stress-rupture test. (C) 2012 Elsevier B.V. All rights reserved.

Composite powder, which is a mixture of Al/Al 2 O 3 composite particles and nanometer, submicron and micron sized aluminum powder, was prepared by semi-solid mechanical stirring method with addition of Al 2 O 3 ceramic particles. The ceramic particles have an average diameter of 80 μm and a volume fraction of 15% in the slurry. The methods used to measure the size distribution of particles greater than 50 μm and less than 50 μm were sieve analysis and photosedimentation, respectively. The surface morphology and transverse sections of the composite powder of different sizes were examined by scanning electron microscope (SEM), optical microscope and auger electron spectroscopy (AES). The results indicate that the composite powder prepared in present work have a wide size distribution ranging from less than 50–900 μm, and the aluminum particles and Al/Al 2 O 3 composite particles are separated and isolated. The particles greater than 200 μm and less than 50 μm are almost pure aluminum powder. The rate of conversion of ingot aluminum into particles less than 1 μm containing nanometer and submicron sizes is 1.777 wt.% in this work. The aluminum powder of different sizes has different shape and surface morphology, quasi-spherical in shape with rough surface for aluminum particles of micron scale, irregular in shape for aluminum particles of submicron scale, and quite close to a globular or an excellent globular in shape for aluminum particles of nanometer size. On the other hand, the surface of ceramic particle was coated by aluminum particles with maximum thickness less than 10 μm containing nanometer and submicron sizes as a single layer. It is suggested that the surface of ceramic particles can provide more nucleation sites for solidification of liquid aluminum and the nucleation of liquid aluminum can take place readily, grow and adhere on the surface of ceramic particles, although it is poorly wetted by the liquid aluminum and the semi-solid slurry can convert into composite powder owing to the addition of ceramic particles and the shear force of mechanical stirring during solidification.