Solute Clusters/Enrichment at the Early Stage of Ageing in Mg-Zn-Gd Alloys Studied by Atom Probe Tomography

Three-dimensional distribution of solute elements in an Mg–Zn–Gd alloy during ageing process is quantitatively characterized by three-dimensional atom probe(3DAP) tomography. Based on the radius distribution function, it is found that Zn–Gd solute pairs in Mg matrix appear mainly at two peaks at early stage of ageing, and the separation distance between Zn and Gd atoms could be well rationalized by the first-principle calculation. Moreover, the fraction of Zn–Gd solute pairs increases first and then decreases due to the precipitation of long-period stacking ordered(LPSO) structures. Both the composition of the structural unit in LPSO structure and the solute enrichment around it are quantified. It is found that Zn and Gd elements are synchronized in the LPSO structure, and solute segregation of pure Zn or Gd is not observed at the transformation front of the LPSO structure in this alloy. In addition, the crystallography of transformation front is further determined by 3DAP data.

Faceted interfaces: a key feature to quantitative understanding of transformation morphology

Faceted interfaces are a typical key feature of the morphology of many microstructures generated from solid-state phase transformations.Interpretation,prediction and simulation of this faceted morphology remain a challenge,especially for systems where irrational orientation relationships(ORs)between two phases and irrational interface orientations(IOs)are preferred.In terms of structural singularities,this work suggests an integrated framework,which possibly encompasses all candidates of faceted interfaces.The structural singularities are identified from a matching pattern,a dislocation structure and/or a ledge structure.The resultant singular interfaces have discrete IOs,described with low-index g’s(rational orientations)and/orΔg’s(either rational or irrational orientations).Various existing models are grouped according to their determined results regarding the OR and IO,and the links between the models are clarified in the integrated framework.Elimination of defect types as far as possible in a dominant singular interface often exerts a central restriction on the OR.An irrational IO is usually due to the elimination of dislocations in one direction,i.e.,an O-line interface.Analytical methods using both three-dimensional and two-dimensional models for quantitative determinations of O-line interfaces are reviewed,and a detailed example showing the calculation for an irrational interface is given.The association between structural singularities and local energy minima is verified by atomistic calculations of interfacial energies in fcc/bcc alloys where it is found that the calculated equilibrium cross-sections are in a good agreement with observations from selected alloys.

Surface effect induced phase transformation by Mn removal during annealing and its textures in cold-rolled high manganese transformation-induced plasticity steel

The surface effect induced transformation texture during vacuum annealing of cold-rolled high manganese transformation-induced plasticity(TRIP)steels was studied.Due to Mn removal occurring at the surface layer,γ→δdiffusional phase transformation leads to the formation of hard pancake-shaped ferrite grains due to solution strengthening at the surface and the centre layer remains as austenite+martensite after annealing.In the case of slow heating,{112}/{111}<110>textures for the surface ferrite were strengthened with the increase in temperature and holding time,indicating an inheritance of rolling textures.By increasing the heating rate of annealing,the rotated cube texture was developed in surface ferrite.This kind of multiphase sandwich structure with hard ferrite surface layer and tough austenite dominant centre can increase tensile strength and should also improve deep drawing properties,therefore providing new possibility of controlling properties for the application of high manganese TRIP steel.