Dissolution precipitation mechanism of TiC/Ti composite layer produced by laser cladding

Abstract

TiC reinforced Ti matrix composite layer was fabricated by laser cladding of Ti and TiC powder mixture on Ti–6Al–4V alloy. Dissolution precipitation mechanism was speculated to illustrate the formation of TiC dendrite in the composite layer. Microstructure evolution of the composite layer has been explained by this mechanism. The composite incorporates the advantages of external particle composites and in situ synthesised particle composites. This mechanism offers an alternative novel idea for the design of bulk composites as well as composite layers. The composite layer exhibits high hardness and excellent wear resistance.     

Drying conditions and their effect on ceramic shell investment casting process

Abstract

The ceramic shell investment casting process has gained an important position in the family of precision casting techniques owing to the scientific advances in many of its aspects, such as binder solutions, refractory materials, pattern materials, and in the manufacturing process. In this endeavour an effort was made to investigate the effect of drying conditions on the performance of ceramic shell moulds. The binder used in this work was an acetone based polysilicic acid, which had been considered as an alternative to the conventionally used binders (namely, ethyl silicate, colloidal silica aquasols etc.). The effects of drying conditions were observed on ceramic wafers, prepared from the slurry, made of fused silica and the binder. The wafers were dried at different temperatures and relative humidity conditions. The effect of drying conditions was observed through the measurement of compressive strength of the ceramic shell wafers. The binder was also subjected to various drying conditions and the characteristic changes of the binder solutions were investigated.     

Phosphorus grain boundary segregation under different applied tensile stress levels in a Cr-Mo low alloy steel

Phosphorus grain boundary segregation during 100 MPa stress ageing at 520 °C in a 2.25Cr1Mo steel is observed using Auger electron spectroscopy. In the segregation kinetics there is only a depletion trough of phosphorus below its thermal equilibrium level. Combined with preceding experimental results for other stress levels, the evolution of segregation kinetics with stress is addressed. The basic features of the segregation kinetics include two segregation peaks and one depletion trough. The first peak, the second peak and the trough may be related to the vacancy-solute complex effect, the boundary diffusion effect and the creep deformation inhomogeneity effect, respectively.     

Phosphorus grain boundary segregation under an intermediate applied tensile stress in a Cr–Mo low alloy steel

Phosphorus grain boundary segregation under an intermediate applied tensile stress of 200 MPa at 520 °C in a 0.025 wt.% P-doped 2.25Cr1Mo steel, which has already been thermally equilibrated, is observed using Auger electron spectroscopy. The segregation during stress ageing has a non-equilibrium nature. There are two phosphorus segregation peaks over its thermal equilibrium level, and between them there is a depletion trough of phosphorus below its thermal equilibrium level. The first segregation peak is mainly caused by the vacancy-phosphorus complex effect and the second one by the diffusional creep effect. The depletion trough of phosphorus at the initial creep stage may be attributed to the strain hardening bias effect.     

Non-equilibrium grain boundary segregation of phosphorus under a high applied tensile stress in a 2.25Cr1Mo steel

Grain boundary segregation of phosphorus under a 350 MPa tensile stress at 520 °C in a 0.025 wt.% P-doped 2.25Cr1Mo steel, which has already been thermally equilibrated, is examined using Auger electron spectroscopy. The segregation of phosphorus during stress ageing has a non-equilibrium characteristic, which has two phosphorus segregation peaks over its equilibrium segregation level, one of which is mainly due to the vacancy–phosphorus complex effect and the other due to the diffusional creep effect.     

A model for intergranular segregation/dilution induced by applied stress

A model for the effects of low applied stress on grain boundary segregation/dilution of solute has been suggested in the present paper. This model is based on the following assumptions: (1) The grain boundary is a weaker region on strength than the perfect crystalline in the interior of gain and will preferentially be deformed when a polycrystalline is exerted by an low applied stress. (2) Grain boundaries will work as sources of vacancies to emit vacancies when a compression stress is exerted on them and as sinks to absorb vacancies when a tension stress is exerted; (3) Oversaturated vacancies induced by the applied stress will be combined with the solute atoms to form vacancy-solute atom complexes, the diffusion rate of which is far greater than that of solute atoms in matrix; (4) The effects of applied stress on grain boundary segregation/dilution of solute will be controlled by the balance between the complex diffusion and the reverse solute atom diffusion. According to this model, there will be a critical time during stress aging, at which a maximum level of grain-boundary segregation/dilution will occur. This model can be corroborated by Shinoda and Nakamura's observation for phosphorus and Misra's observation for sulfur in steels. It can be expected that a new basis for understanding the low ductility intergranular fracture induced by applied stress will result from this new model.