Segregation-induced Grain Boundary Research Articles

Effect of solutes on texture evolution during grain growth in ZK60 alloy by phase field simulation

A three-dimensional (3D) multiscale phase field model was proposed to investigate the effect of Ca, Y and Al solutes on the texture evolution of ZK60 alloy during grain growth at 573 K under an applied compression stress of 270 MPa, in which the characteristics of the elastic anisotropy and grain boundary (GB) segregation caused by the solutes were taken into account. The results show that either Ca or Y addition can weaken the basal texture of ZK60, while Al addition enhances it. The deviation of the grain size distribution from the Hillert model becomes greater with the addition of Al, while Ca or Y addition reduces the deviation. The solute segregation-induced GB energy change has a very limited effect on the texture evolution compared with elastic anisotropy. The revealed mechanism is of great significance for the microstructure design of the ZK60 alloy by an appropriate alloying strategy.

The effect of solute segregation on stability and strength of Cu symmetrical tilt grain boundaries from the first-principles study

Grain boundary (GB) stability and strength are two essential points for nanocrystalline materials. In this study, the effects and underlying mechanisms of segregation-induced GB stabilization and GB strengthening in various Cu GBs for selected 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) were investigated through first-principles total energy calculations. The segregation energy results show that all other elements excluding Ni can segregate to GB thus improving the GB stability. The segregation driving force is associated with the volume of Voronoi cell of solute, in the trend that increases along with the volume of Voronoi cell growth. The stabilizing mechanism should attribute to the decrease in the number of antibonding electrons. As to GB strength, the embrittling potency results show that Mg, Cr, Zr and Ag are enhancers in Σ5 [001](310) GB, while for Σ5 [001](210) and Σ11 [110](113) GBs, only Cr and Zr are enhancers. The embrittling potency is concerned with segregation energy and the volume of Voronoi cell of solute, in a trend that increases with the segregation energy decline and the volume of Voronoi cell growth. The enhancing mechanism lies in the formation of covalent-like bonds between solute and matrix atoms. Furthermore, by comparing the stabilizing and strengthening effectiveness of alloying elements in various Cu GBs. The trend that the worse the stability and strength of GBs, the better the stabilizing and strengthening effects of alloying elements were revealed.

Relation of embrittlement to phosphorus grain-boundary segregation for an advanced Ni–Cr–Mo RPV steel

Embrittlement is an essential issue for reactor pressure vessel (RPV) steels. Grain-boundary segregation (GBS) of impurity phosphorous is one of the major reasons for non-hardening embrittlement of an RPV steel. The main aim of the present work is to establish a relation of embrittlement to phosphorous GBS for an advanced SA508-4N Ni–Cr–Mo RPV steel so that one can forecast its embrittling tendency. To obtain this type of relation, the steel samples doped with phosphorous are quenched from 890 to 1100 °C to obtain different prior austenite grain sizes (PAGSs) and then tempered at 650 °C, followed by ageing at different lower temperatures so as to achieve different phosphorous segregation levels at prior austenite grain boundaries. The characterization work is done with the use of mini-Charpy impact tests (specimen: 2.5 mm × 2.5 mm in cross section) together with Auger electron spectroscopy, scanning electron microscopy, and optical microscopy. The results indicate that there is a linear relationship between ductile-to-brittle transition temperature (DBTT/oC) and phosphorous GBS concentration (Cp/at.%) for both cases (quenching from 890 °C: PAGS = 34 μm; quenching from 1100 °C: PAGS = 112 μm). The relations can be expressed as DBTT=13.13Cp−335.70 (PAGS = 34 μm) and DBTT=6.69Cp−223.87 (PAGS = 112 μm). The rate of material embrittlement, characterized by the slope of the line, is associated with both the phosphorous segregation-induced grain-boundary (GB) embrittlement and the total GB area per unit volume. Interestingly, there is a critical phosphorous GBS concentration, below which the DBTT of the coarse-grained specimen is higher due to its higher intrinsic brittleness and above which that of the fine-grained specimen is higher at the same phosphorous GBS level due to the considerable phosphorous segregation-induced GB embrittlement along with a much larger total GB area per unit volume.

Grain boundary segregation behavior in Fe-rich Sm-Co-Fe-Cu-Zr magnets

Grain boundary (GB) deficiencies, including magnetically softer phase and precipitate-free-zones (PFZs), have been recognized as important microstructural origins for the lower-than-ideal energy product of precipitate-hardening Sm-Co-Fe-Cu-Zr magnets with high Fe content. However, the underlying co-aggregations or co-depletions of the interactive compositional variables and their effect on the heterogeneous precipitation near GBs are still unclear, being one of the key issues to optimize the microstructure towards high magnetic performance. Herein, the GB metallurgical segregation behavior during decomposition process of a commercially sintered magnet Sm25Co42.9Fe23.5Cu5.6Zr3.0 (wt.%) was systematically investigated. The concurrently-happened co-aggregations of Sm/Zr/Cu and co-depletions of Co/Fe near GBs lead to further growth and new formation of Zr-stabilized Smn+1Co5n-1-type (n = 2 for 1:3R, n = 3 for 2:7R, n = 4 for 5:19R) sheet-like precipitates, which in turn strongly affect the precipitation behavior surrounding them. The further growth of primary Smn+1Co5n-1 sheet-like precipitates leads to misaligned Sm-rich PFZs (without 1:5H and 1:3R precipitates that are normally formed within grain interiors), while the formation of secondary Smn+1Co5n-1 sheet-like precipitates leads to Sm-lean PFZs, characterized by misaligned regions without 1:5H and 1:3R precipitates or coherent band-like regions without 1:5H precipitates. Further magnetic domain studies and magnetic measurements revealed that the GB segregation-induced Smn+1Co5n-1 soft phases and misaligned PFZs not only reduce the efficient domain wall pinning sites but also weaken the [001] grain texture, being detrimental to both coercivity and remanence. To achieve high magnetic performance, the processing condition should be carefully controlled to balance the competitive contributions between the segregation-induced GB deficiencies and the normal cellular nanostructure within grain interiors.