Nucleation in Systems with Elastic Forces

Systems with long-range interactions when quenced into a metastable state near the pseudo-spinodal exhibit nucleation processes that are quite different from the classical nucleation seen near the coexistence curve. In systems with long-range elastic forces the description of the nucleation process can be quite subtle due to the presence of bulk/interface elastic compatibility constraints. We analyze the nucleation process in a simple 2d model with elastic forces and show that the nucleation process generates critical droplets with a different structure than the stable phase. This has implications for nucleation in many crystal-crystal transitions and the structure of the final state

The Driving Force for Lath and Plate Martensite and the Activation Energy for Isothermal Martensite in Ferrous Alloys

From information on Ms for lath and lenticular martensite the driving force for the start of a formation of the two types of martensite was calculated in a number of Fe-X systems. By plotting the calculated driving force against temperature the results indicate that the driving force for formation of martensite may not be much affected by solution hardening but mainly be a function of temperature. From the kinetics of isothermal αmartensite in ferrous alloys one can clearly distinguish between two groups of alloys, high alloy steels and carbon containing steels. High alloy steels with low Ms temperature have a temperature dependence corresponding to a very low activation energy, possibly 7 kJ/mol. It can hardly depend on any diffusion process. Carbon containing steels have a temperature dependence corresponding to an activation energy of about 80 kJ/mol. Its rate of formation can be explained by assuming that it is triggered by submicroscopic plates of bainite formed with a rate of carbon diffusion

Microstructure development in a high-nickel austenitic stainless steel using EBSD during in situ tensile deformation

Plastic deformation of surface grains has been observed by electron backscatter diffraction technique during in situ tensile testing of a high-nickel austenitic stainless steel. The evolution of low- and high-angle boundaries as well as the orientation changes within individual grains has been studied. The number of low-angle boundaries and their respective misorientation increases with increasing strain and some of them also evolve into high-angle boundaries leading to grain fragmentation. The annealing twin boundaries successively lose their integrity with increasing strain. The changes in individual grains are characterized by an increasing spread of orientations and by grains moving towards more stable orientations with 〈111〉 or 〈001〉 parallel to the tensile direction. No deformation twins were observed and deformation was assumed to be caused by dislocation slip only.Open Access APC beslut 13/2018</p

EBSD analysis of surface and bulk microstructure evolution during interrupted tensile testing of a Fe-19Cr-12Ni alloy

Abstract The microstructure evolution in both surface and bulk grains in a pure Fe-19Cr-12Ni alloy has been analyzed using electron backscatter diffraction after tensile testing interrupted at different strains. Surface grains were studied during in situ tensile testing performed in a scanning electron microscope, whereas bulk grains were studied after conventional tensile testing. The evolution of the deformation structure in surface and bulk grains displays a strong resemblance but the strain needed to obtain a similar deformation structure is lower in the case of surface grains. Both slip and twinning are observed to be important deformation mechanisms, whereas deformation-induced martensite formation is of minor importance. Since the stacking fault energy (SFE) is low, 17mJ/m2, dynamic recovery by cross slip of un-dissociated dislocations is unfavorable. This reduces the annihilation of dislocations which in turn leads to a significant increase of low angle boundaries with increasing strain. The low SFE also favors formation of deformation twins which reduces the slip distance, leading to a hardening similar to the Hall-Petch relation. The combination of a low ability for cross-slip and a reduced slip distance caused by twinning is concluded to be the main reason for maintaining a high strain-hardening rate up to strains close to necking.Open Access APC beslut 12/2018</p