Aidang Shan

Direct observation of shear deformation during equal channel angular pressing of pure aluminum

Equal Channel Angular (ECA) pressing is a method through which intense plastic strain can be introduced into materials by simple shear. It is suggested that during ECA pressing, only simple shear deformation is introduced into the specimen. The degree of shear deformation can be well predicted by theory and is assumed to be uniform across the specimen except the top and end part. The theory had been proved to be correct by observation of ECA pressing of plasticine with a transparent plexiglass tool and by finite element modeling. However, direct observation of shear deformation had not yet been conducted in metallic materials. One difficulty in observing the shear deformation is that marks or scratches on the surface of the specimen will be erased or destroyed by severe surface deformation caused by friction. In this research, a special method is employed to eliminate the surface friction effect so that a clear shear deformation figure can be observed.

Shear deformation and grain refinement in pure Al by asymmetric rolling

Asymmetric rolling(ASR), as one of severe plastic deformation(SPD) methods, was widely used to make ultra-fined materials with enhanced performance. Internal marks were used to show the shear deformation during asymmetric rolling with pure aluminium as a model material. Effects of reduction ratio and mismatch ratio on the shear deformation were studied. With the observed shear deformation results, equivalent strain was calculated. For lager shear deformation, rolling equipment was modified to increase friction between specimen and the rollers. Consequently, extremely fine grains with size of 500 nm are obtained in pure aluminium. With improved asymmetric rolling, the ability of grain refinement of ASR is greatly improved.

Microstructure and mechanical properties of AlMgSi alloys after equal channel angular pressing at room temperature

Equal channel angular (ECA) pressing up to four passes was conducted on as-solution-treated AlMgSi alloy at room temperature. The alloy work piece was failed after the second pass owing to rapid hardening. Four ECA pressing passes could be successfully performed without any cracking by introducing intermediate annealing at 200 °C for an hour after the second pass. Ultrafine microstructure with grain size of about 0.5 μm was obtained after four ECA pressing passes. Tensile strength was significantly improved after two passes of ECA pressing while reasonable ductility was maintained. Tensile strength decreased with intermediate annealing, and additional two ECA pressing passes after intermediate annealing did not increase the tensile strength. High density of dislocations was found inside grains after four repetitive ECA pressings. It is suggested that the high strength of the alloy was resulted from both the high density of dislocation and fine grain sizes.

Estimation of friction during equal channel angular (ECA) pressing of aluminum alloys

Abstract Equal channel angular (ECA) pressing is a novel process to obtain an ultra-fine grain size in alloys and compounds through severe plastic deformation. However, severe friction occurs during such pressing. In this report, pure aluminum and an AlMgSi alloy were processed by ECA pressing and the load was recorded. The mechanical properties of the alloys before and after different passes during processing were measured by the tensile test. The relationship between the flow stress and the material properties are discussed. The results show that a properly designed die will give less friction. From this experiment, it is suggested that the exit channel of the ECA pressing die should be designed to be as short as possible, as long as the channel is strong enough to ensure ECA pressing. These results confirm that ECA pressing is a promising process that can be conducted under low pressure.

Austenite grain growth prediction coupling with drag and pinning effects in low carbon Nb microalloyed steels

Abstract A metallurgical model has been developed to predict the austenite grain growth in Nb microalloyed steels. The mutual effects of Nb(CN) particle pinning and Nb solute drag on grain growth kinetics are studied. The particle dissolution, the undissolved particle coarsening and the changes in Nb solute in solution during reheating or isothermal heat treatment process are taken into account in the model. It is shown that, besides the pinning exerted by the NbC precipitates, the solute drag of Nb in solid solution plays an important role in the inhibition of austenite grain growth in Nb microalloyed steels. The Nb solute drag effect on grain growth decreases with increasing temperature because the grain boundary can gradually break away from the solute atmosphere in the higher velocity region at high temperature. The mean austenite grain size sluggishly increases with temperature in the low temperature region, while it significantly increases in the relative high temperature region. The predicted austenite grain size concerning the combined effect of Nb drag and Nb(CN) pinning is in good agreement with the experimental results from the literature.

Quantitative analysis of homogenization treatment of INCONEL718 superalloy

Quantitative analysis was employed to establish reasonable and practical homogenization model of INCONEL718 superalloy. Metallographic method was applied to determining the incipient melting temperature. The result shows that the incipient melting temperature of d406 mm INCONEL718 ingot is situated between 1 170 °C and 1 180 °C. In order to predict the elimination process of Laves phase in quantity, a time and temperature dependent homogenization model was proposed. Among all the elements in the as-cast microstructure, Nb and Ti are the most positive segregated elements. The diffusion coefficients of alloying elements at 1 140 °C were obtained by fitting the linear relationship between ln d (d: residual segregation index) and time. The calculation results of diffusion coefficients were compared with other two commercial Nb-bearing superalloys.

Microstructure and solidification behaviour characterisation of phosphorus and boron doped IN718 superalloy

Abstract The additions of 0·022–0·025 mass-% phosphorus and 0·010–0·012 mass-% boron could enhance the creep properties and increase the service temperature of IN718 superalloy markedly. However, the as cast microstructure and solidification behaviour of modified IN718 with P and B additions are not completely recognised. In the present paper, differential scanning calorimeter (DSC) technique accompanied by scanning electron microscope and electron probe microanalysis characterisation is applied to understand the effects of P and B additions. It is found that P and B additions may promote the formation of big blocky Laves phase and result in the existence of B bearing phase enriched in Nb and Mo. In the meantime, another endothermic peak appearing before the Laves phase peak in the DSC heating curve is related to the melting of the B bearing phase. After the thermodynamics calculation, this new endothermic peak detected may be involved with the melting of M3B2. Furthermore, the endothermic peak appears before the Laves phase peak in the DSC heating curve, suggesting that the incipient melting temperature is greatly reduced as a result of P and B additions. In addition, both melting and solidification temperature ranges are significantly widened, indicating that the solidification process becomes more complex for the P and B doped IN718 superalloy.

Effects of P and B addition on as-cast microstructure and homogenization parameter of Inconel 718 alloy

Abstract The effects of phosphorus and boron addition on the as-cast microstructure and homogenization parameters of Inconel 718 were studied. The results indicate that the addition of phosphorus and boron promotes the formation of blocky Laves phase. Due to the strong segregation behavior of boron in the final residual liquid, a low melting B-bearing phase enriched in Nb, Mo and Cr is observed. According to the differential scanning calorimeter results and electron probe micro-analysis characterization, the solidification sequence of Inconel 718 with phosphorus and boron addition in best combination is determined as L → L +γ → L +γ+ MC → L +γ+MC+Laves → γ+MC+Laves+B-bearing phase. Accordingly, the homogenization temperature is recommended to be adjusted at least 40°C lower than that of standard Inconel 718 due to the existence of low melting B-bearing phase.

Strengthening and stress drop of ultrafine grain aluminum after annealing

Abstract Ultrafine grain pure aluminum was produced by equal channel angular pressing and cold rolling, the deformed aluminum was annealed at 200 °C for 1 h. The tensile curves of deformed and annealed aluminum show that yield strength of deformed aluminum increases by 100%–300% and its elongation decreases by about 20%. After low temperature annealing, strength of annealed aluminum increases by 20% and elongation decreases by over 50%, the recovery of dislocations may be the main cause of annealing strengthening. In addition, there is an abrupt stress drop in the tensile curves of annealed aluminum and the formation of shear band is responsible for it.

Solidification process of conventional superalloy by confocal scanning laser microscope

The solidification process of a conventional superalloy, IN718, was investigated by confocal scanning laser microscope (CSLM). The liquid fraction during solidification was obtained as a function of real time and temperature in reference with the in-situ observation. The characteristics of L→γ transformation were analyzed and the γ growing rate of each stage was also calculated. Scheil equation was employed to predict the segregation behavior, and the predict results are in consistence with the experimental results. As a result, the confocal scanning laser microscope shows a great potential for solidification process research.