Improving the strength and SCC resistance of an Al-5Mg-3Zn alloy with low-angle grain boundary structure

The strength of traditional Al-Mg alloys is relatively low because it mainly relies on solid solution strengthening.Adding a third component to form precipitation can improve their strength,but it usually leads to high-stress corrosion cracking(SCC)sensitivity due to the formation of high-density precipitates at grain boundaries(GBs).So far,it is still challenging to improve the strength of Al-Mg alloys without re-ducing SCC resistance.Herein,a nanostructured Al-5Mg-3 Zn alloy with a good yield strength of 336 MPa and good elongation was successfully produced.By dynamic plastic deformation and appropriate anneal-ing treatment,near-equiaxed nanograins were introduced in the nanostructured Al-5Mg-3 Zn alloy with a high proportion(71%)of the low-angle grain boundary.TEM statistical investigations show that the pre-cipitation of active T’phase at GBs has been greatly suppressed in the nanostructured Al-5Mg-3 Zn alloy at sensitized conditions,and the area fraction of GB precipitates is reduced from 72%to 21%,which sig-nificantly decreases the SCC susceptibility.This study provides guidance for developing advanced Al-Mg alloy with high SCC resistance.

Microstructural evolution and oxidation inα/βtitanium alloy under fretting fatigue loading

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation,thus potentially causing unanticipated catastrophic fractures.The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous,which obstructs the understanding of fretting fatigue issues,and is unable to guarantee the reliability of structures for long-term operation.Here,fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of anα/βtitanium alloy up to 108 cycles.Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone.The grain size in the topmost nanostructured layer could be refined to~40 nm.The grain refinement process involves the initial grain rotation,the formation of low angle grain boundary(LAGB;2°–5°),the in-situ increments of the misorientation angle,and the final subdivision,which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays.The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary(HAGB)and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer,thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress.Eventually,it facilitates surface degradation and the formation of catastrophic fractures.

SEM/EBSD analysis of quartz cementation and compaction microstructures during diagenesis of sandstone

Compaction and silicon cementation are the dominant processes reducing porosity and permeability in quartzose sandstones during diagenesis. Despite the wealth of information about quartz cementation, there are still unanswered questions related to mechanisms of growth of quartz cement and the diagenesis processes. In this study we present an electron backscatter diffraction （EBSD） analysis, combined with optics and cathodoluminescence （CL） information, for the quartz sandstones from the Upper Triassic Xujiahe Formation of Sichuan Basin, in order to reveal the microstructural and crystallographic features of the silica cementation and detrital grain during the compaction. The EBSD is a crucial technique to provide essential crystallographic data on the quartz grain and its cement. Quartz cement is shown to be syntaxial to its host quartz grain. EBSD data-based orientation maps show dauphin6 twinning and low angle boundary to be common in the host grains and quartz cement of the samples. The dauphin6 twins occurred in grain-grain contacts and in cement-crystal boundaries, and commonly crossed grain cement boundaries. These features indicate that there may be two types of dauphin6 twins, one inherited twins from the source area and the other developed by compaction-induced grain boundary deformation. These investigations suggest that strong mechanical compaction may occur after and/or during quartz cement growth in the later diagenesis of the Xujiahe sandstones. EBSD has a capability of revealing microstructural information and regarding mechanisms of diagenesis crystal growth in quartzose sandstones.