Zidong Wang

Thermodynamic model for precipitation of carbonitrides in microalloyed steels and its application in Ti–V–C–N system

Based on mass balance and solubility product equations, a thermodynamic model enabling the calculation of equilibrium carbonitride composition and relative amounts as a function of steel composition and temperature was developed, which provides a method to estimate the carbonitride complete dissolution temperature for different steel compositions. Actual carbonitride precipitation behavior was further verified in Ti–V–C–N microalloyed steel system. The model suggests that for higher [V] and [Ti] dissolved in steels, it is available to decrease the addition of C and N during alloy composition design. The resultant longer fatigue life of the modified steel could be attributed to the more [V] and [Ti] dissolved in the matrix, inducing finer dispersion of carbonitrides. Therefore, this model is proved to be effective in determining better chemical composition for high-performance steels, leading to possible reductions in the cost of production and improvements in the combined mechanical properties of the steels.

Fretting Wear Behavior and Mechanism of Inconel 690 Alloy Related to the Displacement Amplitude

ABSTRACT Fretting wear tests on Inconel 690 alloy were carried out at different displacement amplitudes. The results indicated that with an increase in displacement amplitude, the ratio of tangential force to normal load and wear volume increased. The fretting mode gradually transformed from mostly stick, mixed stick–slip, to full sliding, showing the competition of fretting-induced fatigue cracking and fretting-induced wear. There was a gradient plastic strain created by fretting, which resulted in the formation of a nanocrystalline tribologically transformed structure (TTS) and plastic deformation layers. The plastic strain in the plastic deformation layer gradually increased with an increase in displacement amplitude.

Formation and capturing of nanoparticles in Cu-1wt. %Fe alloy melt during directional solidification process

A single crystal Cu-1wt. %Fe alloy with finely dispersed iron-rich nanoparticles which keep coherent interface with the copper matrix was prepared under directional solidification. Formation of nanoparticles in the alloy melt was investigated by performing differential scanning calorimeter tests and designed water quenching experiment at a certain temperature. Results show that iron-rich nanoparticles are formed in the Cu-1wt. %Fe alloy melt before primary α-Cu forms, which is not consistent with equilibrium phase diagram. Mechanism that iron-rich nanoparticles are uniformly captured in the matrix was described, which is that numerous nanoparticles follow Brownian motions and are engulfed in the solidified matrix which makes it possible to form uniformly distributed nanoparticles reinforced single crystal Cu-1wt. %Fe alloy.

Effect of anisotropy on deep cellular crystal growth in directional solidification

The effect of anisotropic surface tension and anisotropic interface kinetics on deep cellular crystal growth is studied. An asymptotic solution of deep cellular crystal growth in directional solidification is obtained by using the matched asymptotic expansion method and the multiple variable expansion method. The results show that as the anisotropic parameters increase, the total length of deep cellular crystal increases and the root depth increases, whereas the curvature of the interface near the root increases or the curvature radius decreases.

Thermodynamic Analysis of Ti3O5Nanoparticles Formed in Melt and Their Effects on Ferritic Steel Microstructure

Based on the Wagner’s formalism combined with mass conservation, a thermodynamic analysis method has been developed previously. This method enables the calculation of the equilibrium matrix composition, precipitate composition and precipitate total molar fraction for TixOy(s) in molten metal, which can be determined at any appropriate temperature. In this present study, the Ti3O5 phase precipitation and the quantitative relationship between the addition of Ti, O and Ti3O5 in the molten steel were studied using the thermodynamic model. Using the combined multipoint dispersion supply method, electromagnetic stirring and well-dispersed 5-nm Ti3O5 nanoparticles were fabricated in the ferrite matrix of the as-cast high-strength steel with 0.05 wt % Ti—0.002 wt % O. The as-cast microstructure was improved by the homogeneously dispersed Ti3O5 nanoparticles through heterogeneous nucleation and grain refinement.

Hydrogen redistribution under stress-induced diffusion and corresponding fracture behaviour of a structural steel

ABSTRACT Hydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960 MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10−6 to 10−4 s−1. In the pre-charged sample with a certain hydrogen content of 0.62 ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10−4 to 5 × 10−6 s−1, while it stabilised at the strain rate slower than 5 × 10−6 s−1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material. This paper is part of a thematic issue on Hydrogen in Metallic Alloys