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.

Microstructure Characteristics and Mechanical Properties of X80 Pipeline Steels

The relation between microstructure characteristics and mechanical properties of X80 pipeline steels was investigated using optical microscopy, scanning electron microscopy, etc. It is shown that the structure consists of polygonal ferrite （PF）, quasi-polygonal ferrite （QPF）, acicular ferrite （AF）, and granular bainitic ferrite （GF）. With increasing volume fraction of M-A islands （below 3%）, the yield strength increases. With increasing content of higher angle grain boundaries（HAGBs）, the yield strength, elongation, and DWTT properties at -15 ℃ increase, and the volume fraction of M-A islands reaches its highest point in the steel containing the most volume fraction of GF.

Effect of hot rolling on microstructure and tribology behaviors of Ti-50.8Ni alloy

We link different microstructures to tribological behaviors of Ti-50.8 Ni(mole fraction, %) in reciprocating mode at room temperature(20 ℃). Hot-rolled alloys with B2 phase exhibit lower coefficient of friction and wear rate compared to the ones with B19?. Stress-induced martensitic transformation occurs during sliding. However, multi-pass hot rolling weakens the wear resistance. In this study, microstructures were characterized through electron backscatter diffraction and transmission electron microscopy(EBSD/TEM). From the concept of energy conservation, the effects of weak intensity of hot-rolled textures on the wear resistance are minimal. Based on the result that the alloy with a higher portion of coincidence site lattice boundaries shows lower martensitic start transformation temperature in the DSC curves than that with higher KAM values, the delay on B2-B19? transformation from {112}B2 twins outweighs dislocations. Moreover, widely distributed small-angle grain boundaries owing to dynamic recovery improve the wear resistance effectively compared to those that are well-recrystallized.

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.

Relationship between martensite microstructure and ductility of H13 steel from aspect of crystallography

Based on the parent austenite orientation reconstruction method,it is aimed to reveal the origination of high angle grain boundaries(HAGBs)and its relationship with ductility of H13 steel.The orientation relationship between martensite and parent austenite of quenched H13 samples was(123.5°,9.3°,192.5°),which agreed with the Kurdumov–Sachs relationship.The variant distribution of quenched samples was dominated by close-packed plane group,and its high length fraction of V1/V2 inter-variant boundaries of calculated 62.6%was mainly contributed to HAGBs(>45°).When the quenched H13 samples underwent the pre-tempering treatment,their density of HAGBs(>45°)notably increased from 1.33 to 2.39μm^(−1),which improved its total elongation from 8.3%to 11.5%.Compared with the quenched H13 samples,the length fraction of V1/V2 inter-variant boundaries of H13 samples with pre-tempering for 5,10 and 60 min was reduced by 6.7%,7.0%and 7.5%,respectively.During pre-tempering treatment,V1/V3&V5 variant pairs,etc.,merged V1/V2 variant pair by strain-induced grain boundary migration,which decreased the length fraction of V1/V2 inter-variant boundaries by 7.0%.The pre-tempering treatment significantly increased HAGBs(>45°)of H13 samples by sub-grains coarsening and strain-induced grain boundary migration mechanism.