Naqiong Zhu

Modeling of nucleation and growth of M23C6 carbide in multi-component Fe-based alloy

Three-dimensional atom probe (3DAP) technique has been used to study the nucleation and growth of M23C6 carbide in a supersaturated multi-component Fe-based alloy aged at 800°C. 3D images indicate that the radius of M23C6 carbide after ageing for 10 min is about 9 nm. Concentration profiles of alloy elements in the carbide are also obtained. Combined with PANDAT and Thermo-Calc software, attempts to model the early stages of precipitation are present. The calculated particle size and composition of M23C6 carbide is in good agreement with 3DAP data.

Experimental and Computational Study of Diffusion Mobilities for fcc Ni-Cr-Mo Alloys

Based on the available thermodynamic parameters and experimental diffusivities in the literature as well as measured in the present work, the diffusion mobilities of Ni, Cr, and Mo for fcc Ni-Cr-Mo alloys have been assessed by means of the CALPHAD method. Comprehensive comparisons between the calculated and experimental data show that an excellent agreement is obtained for the diffusion data of ternary diffusion couples, such as the interdiffusion coefficients, concentration profiles, and the diffusion path.

In-situ analysis of retained austenite transformation in high-performance micro-alloyed TRIP steel

Microstructures and mechanical properties of Ti-V micro-alloyed TRIP (transformation-induced plasticity) steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). The results indicated that the steel exhibited high ultimate tensile strength (1079 MPa), sufficient ductility (28%) and the highest product of strength and ductility (30212 MPa · %) heat treated after intercritical annealing at 800 °C for 3 min and bainitic annealing at 430 °C for 5 min. In addition, the change of volume fraction of retained austenite (VF-RA) versus tensile strain was measured using in-situ analysis by X-ray stress apparatus and micro-electronic universal testing machine. It was concluded that a-value could be used to evaluate the stability of retained austenite (S-RA) in the investigated Ti-V micro-alloyed TRIP steel. The smaller a-value indicated the higher stability of retained austenite (S-RA) and the higher mechanical properties of Ti-V micro-alloyed TRIP steel.

Microstructures and mechanical properties of ferrite-based lightweight steel with different compositions

The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). It was shown that the ferrite-based lightweight steels with 5 wt. % or 8 wt. % Al were basically composed of ferrite, austenite and κ-carbide. As the annealing temperature increased, the content of the austenite in the steel gradually increased, while the κ-carbide gradually decomposed and finally disappeared. The mechanical properties of the steel with 5 wt. % Al and 2 wt. % Cr, composed of ferrite and Cr7 C3 carbide at different annealing temperatures, were significantly inferior to those of others. The steel containing 5 wt. % Al, annealed at 820 °C for 50 s then rapidly cooled to 400 °C and held for 180 s, can obtain the best product of strength and elongation (PSE) of 31242 MPa · %. The austenite stability of the steel is better, and its PSE is higher. In addition, the steel with higher PSE has a more stable instantaneous strain hardening exponent (n value), which is mainly caused by the effect of transformation induced plasticity (TRIP). When the κ-carbide or Cr7C3 carbide existed in the microstructure of the steel, there was an obvious yield plateau in the tensile curve, while its PSE decreased significantly.