• Germain, Lisa

Abstract

For many years, the understanding of the coupling between a mechanical (applied or residual) stress field and a chemical field in a material diffusion process has been an important area of study. Long-range interstitial diffusion, for example in the case of gas nitriding, causes strong stress gradients associated with phase transformations (short-range diffusion). The phenomena of diffusion-precipitation are known and simulated in the laboratory. However, the stress-diffusion coupling (short or long range) remains an important problem. Two significant phenomena on long-range diffusion have been highlighted: the effect of a homogeneous stress (Fickian term) and the stress gradient as a driving force (Nersntien term). In addition, mechanical pretreatment is frequently used with thermochemical treatments, resulting in rarely decoupled phenomena of incompatible plastic strains and residual stresses. In this study numerical approaches limited to long-range diffusion and experimental approaches were carried out to investigate these issues. The stress-diffusion coupling is the subject of numerical simulations in 1D (2D axisymmetric) and 2D using the thermodynamics of irreversible processes. The effects of a homogeneous stress and a stress gradient were demonstrated by numerical simulations that were applied to a binary system (Fe-N). The results revealed that the Fickian coupling factor (σ_kk β) must be about 100 times larger than the diffusion coefficient D to have a significant impact on the kinetics.In order to compare these theoretical results with reality, nitriding experiments on the steel grade 33CrMoV12-9 were carried out. In particular, the influence on the diffusion kinetics of: a residual stress gradient (i.e. elastic strain gradient ∇ε^e); an applied stress gradient (i.e. elastic strain gradient ∇ε^e); a homogeneous plastic strain ε^P, a plastic strain gradient ∇ε^P. The results showed small differences on the diffusion kinetics as observed in the numerical simulations.

Topics

• impedance spectroscopy
• polymer
• phase
• experiment
• simulation
• steel
• precipitation
• interstitial