Thermodynamics and kinetics of solution nitriding

In order to clarify the thermodynamics and kinetics of solution nitriding, the non-uniform nitrogen distribution formed by solution nitriding was investigated in austenitic (18Cr–10Ni) and ferritic (25Cr) stainless steels, and then it was simulated with DICTRA. The main issue for austenitic stainless steel is the surface reaction determined by the mass transfer from gas to solid, while that for ferritic stainless steel is the ferrite to austenite transformation and the austenite growth kinetics induced by nitrogen absorption. DICTRA simulations gave close agreement with the experimental results and demonstrated that solution nitriding is obviously controlled by mass transfer of nitrogen from gas to solid as well as diffusion in the solid, and austenite growth is controlled by nitrogen diffusion in austenite. In addition, the analysis suggests that nitrogen should have a stronger interaction with chromium than that treated in the thermodynamic database and austenite nucleation induced by nitrogen absorption takes place without any change in Cr composition.     

Computer Simulation of Fe-Al-Si System Diffusion Couples

Diffusion couples, Fe-6.8 wt% Al-1.0 wt% Si/Fe and Fe-6.3 wt% Al-0.9 wt% Si/Fe were constructed and separately annealed at 1050°C for 3 h and at 1000°C for 64 h. The concentration profiles of Fe, Al and Si atoms in these couples were measured by electron probe micro-analysis (EPMA), while the diffusion behavior was also simulated by coupling thermodynamic and kinetic properties of Fe-Al-Si system. The simulation results were in good agreement with the measured concentration profiles showing the validity of dynamic parameters of Fe-Al-Si system. Calculation was made for Fe-7 wt% Al-1 wt% Si/Fe diffusion couples at 1000?C with different diffusion time. Silicon uphill was found under the influence of aluminum.     

Ferromagnetic ordering and mobility end-members for impurity diffusion in bcc Fe

Based on the experimental data available in the literature, the mobility end-members for impurity diffusion of twelve elements, i.e. Ag, Al, As, Au, Cu, Mo, Nb, Sb, Sn, Ti, W and Zn, in bcc Fe have been critically studied. The effect of ferromagnetic ordering is taken into consideration, which allows the dramatic change of kinetic behaviors around the Curie temperature to be considered. Comprehensive comparisons between the calculated and experimentally measured impurity diffusion coefficients are made, where the good agreement confirms not only the parameters obtained in this work but also the model developed by Jönsson. This work contributes to the establishment of a general mobility database to computationally study microstructure evolution upon heat treatment and performance prediction during service.     

Atomic mobilities, uphill diffusion and proeutectic ferrite growth in Fe–Mn–C alloys

Following the treatment in CALPHAD, experimental data on diffusivities in Fe–Mn and Fe–C binary systems are critically evaluated with the DICTRA software to derive atomic mobilities. The effect of magnetic ordering on diffusion in bcc phase is taken into account, and the obtained atomic mobilities are expressed as functions of temperature and compositions with the Redlick–Kister polynomials. Based on the mobility parameters obtained in this work for the end-members and the interaction terms, comprehensive comparisons between the calculated and experimentally measured quantities are made. Due to the lack of experimental diffusivities for the ternary system, extrapolation based on binary information is performed, the results of which are used to study uphill diffusion of C in fcc Fe–Mn–C alloys. Such C diffusion against its own concentration gradient is a common occurrence for ternary systems containing one interstitial element, provided that the initial alloy compositions of diffusion couples are well chosen. In addition, the operating tie line evolution for proeutectic ferrite growth is also investigated, where C diffusion-controlled fast and Mn diffusion-controlled slow growths are discussed.     

Diffusional mobility for fcc phase of Al–Mg–Zn system and its applications

Diffusional mobility for fcc phase of the Al–Mg and Al–Mg–Zn systems was critically assessed by using the DICTRA software (Diffusion Controlled Transformation). Good agreement was obtained from comprehensive comparisons between the calculated and experimental diffusion coefficients. The developed mobility database enables reasonable prediction of diffusion and solidification behaviours resulting from interdiffusion, such as concentration profile of diffusion couples and solidification curve of the Al–Mg alloys.     

Assessment of atomic mobilities for fcc Co–Ti–V alloys

Based on the experimental interdiffusion coefficients from our previous work and the critically reviewed experimental diffusivities available in the literature, the atomic mobilities of Co, Ti and V in fcc Co–Ti–V alloys were assessed by means of DICTRA (DIffusion Controlled TRAnsformation) software. Askill's empirical relations were utilized to assess the self-diffusion coefficients of fcc Ti and fcc V. Comprehensive comparisons between the DICTRA-calculated diffusivities and the experimental data indicate that the presently obtained atomic mobilities can reproduce most of the diffusivities in binary and ternary systems. In addition, a further verification on the obtained atomic mobilities was carried out through comparing the DICTRA-simulated concentration profiles/diffusion paths of the diffusion couples with the corresponding experimental ones. This work contributes to the establishment of a kinetic database for multi-component cemented carbide.     

Interdiffusion and atomic mobilities in fcc Ag–Mg and Ag–Mn alloys

On the basis of Ag/Ag–Mg and Ag/Ag–Mn semi-infinite diffusion couples together with electron probe microanalysis technique, the interdiffusion coefficients in fcc phases of Ag–Mg and Ag–Mn alloys were measured between 873 and 1173 K via the Sauer-Freise method. A statistic method was applied to evaluate the errors of the identified interdiffusivities in terms of the propagation of errors. Based on available thermodynamic information and the experimental interdiffusion coefficients, the atomic mobilities for the fcc Ag–Mg and Ag–Mn systems are obtained by using the DICTRA software package. The quality of the assessed atomic mobilities was confirmed by the comprehensive comparisons between calculated concentration profiles and the experimental data.     

Experimental Study and Kinetic Calculation on Migration of γ/α Interface Boundary During High Temperature Aging for 2205 Duplex Stainless Steel

Aging treatments in the temperature range of 1000 to 1200 ℃ for different time intervals(10,30,and 60 min)in a solution-treated 2205 duplex stainless steel(DSS)are carried out respectively.The migration of γ/α interface boundary after different aging treatments is measured by optical metallograph and calculated by DICTRA software.The experimental results are in agreement with calculated ones.The migration of γ/α interface boundary with increased keeping time depends on the chemical potential of chromium in each phase.At aging temperature of 1100 ℃,the back diffusion effect of chromium results in the opposite direction migration of γ/α interface.Therefore,the ferrite volume fraction is lower at first and then it is higher after 1800 s diffusion.