A multi-barrier model assisted CAFE method for predicting ductile-to-brittle transition with application to a low-carbon ultrahigh-strength steel
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The conventional micromechanical approaches today are still not able to properly predict the ductile-to-brittle transition (DBT) of steels because of their inability to consider the co-operating ductile fracture and cleavage mechanisms in the transition region, and simultaneously to incorporate the inherent complexity of microstructures. In this study, a complete methodology with coupled cellular automata finite element method (CAFE) and multi-barrier microcrack propagation models is presented to advance the prediction of DBT. The methodology contains three key elements: (i) a multiscale CAFE modelling approach to realize the competition between ductile damage and cleavage fracture and embrace the probabilistic nature of microstructures, (ii) a continuum approach to estimate the effective surface energy for a microcrack to penetrate over particle/matrix interface, and (iii) a method to calculate the effective surface energy for the microcrack to propagate across grain boundaries. The prediction of DBT therefore needs only (1) the stress-strain curves tested at different temperatures, (2) the activation energy for DBT, (3) the ratio between the size of cleavage facets and cleavage-initiating defects, and (4) key statistical distributions of the given microstructures. The proposed methodology can accurately reproduce the experimental DBT curve of a low-carbon ultrahigh-strength steel.Keywords:
Cleavage (geology)
Brittleness
Strain energy
Bone possesses excellent mechanical properties, which are closely related to its favorable microstructures optimized by nature through millions of years. In this work, a scanning electron microscope (SEM) was used to observe the microstructures of a shankbone. It showed that the bone is a kind of bioceramic composite consisting of hydroxyapatite layers and collagen protein matrix. The hydroxyapatite layers are further composed of long and thin hydroxyapatite sheets. The hydroxyapatite sheets in different hydroxyapatite layers distribute along different orientations, which composes a kind of intersectant microstructure. The maximum pullout force of the intersectant microstructure was investigated and compared with that of 0° microstructure with their representative models. The result indicated that the maximum pullout force of the intersectant microstructure is markedly larger than that of the 0° microstructure, which was experimentally verified.
Bioceramic
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In order to make clear the formation process, the property and the effect on tensile property of abnormal microstructure formed in hot rolled plates and sheets of α+β titanium alloys, experiment was carried out on Ti-5Al-2Cr-1Fe, Ti-4Al-3Mo-1V, Ti-6Al-4V, and Ti-8Mn alloys. Hot rolling was carried out under various conditions so as to investigate the formation process of the abnormal microstructure observed by optical microscopy, and hardness of that microstructure formed was measured by the microhardness tester.Concentration of alloying elements at the abnormal microstructure was also measured by means of electron probe microanalyzer. Tensile test was carried out on hot rolled plates and sheets with and without the abnormal microstructure to obtain the effect of the said microstructure. The results are summarized as follows:(1) The abnormal microstructure in hot rolled plates and sheets is formed, with an elongated shape, by rolling from the abnormal microstructure already existing in slab.(2) Formation of the abnormal microstructure depends on hot rolling temperature; that is, the temperature around and over β transus makes severe one.(3) The abnormal microstructure has high hardness and contains more β stabilizing elements compared to its surroundings.(4) On plates and sheets with the abnormal microstructure, the elongation, especially in the tensile property, decreases singnificantly and also severe anisotropy is detected.
Elongation
Titanium alloy
Tensile testing
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Effect of Heat Treatment on the Microstructure of Two-step Forging for High Nb Containing TiAl Alloy
Effect of heat treatment processings on the microstructure of two-step forging for high Nb containing TiAl alloy was investigated. Different heat treatment processes were employed to obtain different microstructures of Ti-45Al-9(Nb,W,B,Y) alloy. The results show that, after the heat treatment of 1310/20min+1250/2h/AC, the β phase can be efficiently eliminated and the uniform fine duplex microstructures can be gained. Followed by the heat treatment at α single phase region directly, striped FL microstructure of coarse colony and fine colony is obtained. The holding at (α+γ) phase region can result in the homogenous fine FL microstructure relatively. The FL microstructure produced by the heat treatment of HT6 is the most homogenous and finest.
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Based on analysis of the surface layer microstructure of samples obtained from a plate die casting, three characteristics of surface layer microstructure including thick and fine chilled microstructure, a small amount of chilled microstructure and no chilled microstructure, were observed. The formation reasons of microstructures with three different characteristics were discussed by simulating filling and solidification process for the plate die casting. The results indicate that the mode of collision between filling liquid and die surface as well as the solidification sequence have important effects on the formation of surface layer microstructure.
Surface layer
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Cleavage (geology)
Strain energy
Energy distribution
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Ingot
Dendrite (mathematics)
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The microstructure and hardness of ZG3Cr13 heat resistant steel after 2500h aging between 550℃～600℃ were studied.The results showed that the hardness decreased obviously during 1000h aging,then the hardness tended to be stable.The change of microstructure is related to the microstructure homogeneity after normal heat treatment.The more uniform distribution of the original microstructure,the higher stability of microstructure and higher heat resistance after aging.
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The effects of three rolling temperature on the microstructures and mechanical properties of IMI550 alloy 22 mm bars were studied. Experimental raw material having the same amount of rolling deformation,but rolling temperatures were Tβ- 60℃,Tβ- 25 ℃ and Tβ+ 15 ℃. The results showed that the microstructure of IMI550 titanium bars rolling by Tβ- 60 ℃,Tβ-25 ℃ and Tβ+ 15 ℃ were equiaxed microstructure,duplex microstructure and basketweave microstructure respectively. The room temperature tensile strength and 400 ℃ high temperature tensile strength with equiaxed microstructure,duplex microstructure and basketweave microstructure did not show significant difference,and plastic of equiaxed microstructure and duplex microstructure were also similar. The plastic of basketweave microstructure is relatively poor,but the creep properties is the best,duplex microstructure is the second,equiaxed microstructure is the worst.
Equiaxed crystals
Titanium alloy
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Equiaxed crystals
Titanium alloy
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