Degradation research of protective coating on AZ91D Mg alloy components via simulated contamination

Accelerated environmental(hygrothermal)exposure experiments are performed on organic paints coated on commercial die-cast AZ91D Mg alloys to investigate the effects of contamination on blistering.Specifically,artificial human perspiration spray is used to contaminate the substrate surface.Blistering occurred only for paints that are spread on surfaces with the perspiration present.More blisters gradually form at longer test times,and the volume of blistering increases.Scanning electron microscopy indicates that blistering is initiated by contamination and/or substrate corrosion at the interface of the organic paints and the substrate.Blistering is characterized for two samples exposed to the hygrothermal environments for various times,and is found to be initially empty in the early stages.Hydrophilic chloride contaminants from the perspiration lead to in situ adhesion loss.Simultaneously,the paints volume expands,and the associated compressive stress causes it to bulge.After long-term test exposure,chloride anions corrode the substrate under the films,and MgO,Mg(OH)_(2),and Mg_(2)(OH)_(3)Cl corrosion products fill the blisters.Finally,a model of blistering evolution is discussed.

Dynamic recrystallization mechanism of as-cast nickel base superalloy N10276 during primary hot working

The dynamic recrystallization(DRX)mechanism of as-cast nickel base superalloy N10276 during primary hot working was investigated by compression tests at temperatures of 1000–1200℃ and strain rates between 0.01 and 10 s^(-1).Optical microscopy,scanning electron microscopy,electron backscattered diffraction technique and transmission electron microscopy were used to characterize the evolution of microstructure.At higher deformation temperature or lower strain rate,the true stress–true strain curves exhibit the characteristic of a peak stress followed by a steady state flow stress under large strains,confirming the occurrence of DRX.The degree of DRX increases with elevating deformation temperature.With the progress of DRX,low angle grain boundaries gradually decrease,while high angle grain boundaries increase continuously.Microstructure studies have shown that discontinuous dynamic recrystallization is the main recrystallization mechanism.Since there are few original grain boundaries and twin boundaries,and lack of second phase particles for particle stimulated nucleation,geometrically necessary boundaries are formed as supplementary nucleation sites through sub-grain boundary rotation and deformation twin boundaries.The annealing twins dominated by Σ3 grain boundaries are generated in large quantities during the growth of recrystallized grains.

Microstructure and Mechanical Properties of TWIP Steel Joints

As a new type of high manganese steel,the twinning induced plasticity(TWIP)steels have attracted a growing interest in the automotive industry due to their good performance.Thin plates of TWIP steel were welded by laser beam welding(LBW)and gas tungsten arc welding(GTAW).The microstructure result shows that GTAW joint has obvious heat-affected zone(HAZ),while the HAZ of LBW joint is almost invisible.The X-ray diffraction result shows that the phase compositions of both joints are austenitic and no phase transition occurs.Energy dispersive spectrometry result shows that there is violent evaporation of Mn element in LBW joint,while the proportion of Mn element in GTAW joint is almost unchanged.Tensile tests and micro-hardness measurements were performed to take into account the mechanical properties of joints manufactured by the two different processes.The micro-hardness profiles of both joints present a typical saddle distribution,and the hardness of GTAW seam is lower than that of LBW seam.The failure positions of LBW joints are all located in base metal while the GTAW joints are all at the weld toe due to the softening of HAZ.By means of scanning electron microscopy,a typical ductile fracture is observed in LBW joint,while a brittle fracture with quasi-cleavage fracture characteristic is observed in GTAW joint.

Strengthening Mechanisms for Ti-and Nb-Ti-micro-alloyed High-strength Steels

The strengthening mechanisms of hot-rolled steels micro-alloyed with Ti(ST-TQ500)and Nb-Ti(NTTQ500)were investigated by examining the microstructures of steels using optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM).The results revealed almost no differences in the solute solution strengthening and fine-grained strengthening of the two steels,whereas the contributions of precipitation strengthening and dislocation strengthening were different for ST-TQ500 and NT-TQ500.The measured precipitation strengthening effect of ST-TQ500 was 88 MPa higher than that of NT-TQ500;this difference was primarily attributed to the stronger precipitation effect of the Ti-containing nanoscale particles.The dislocation strengthening effect of ST-TQ500 was approximately 80 MPa lower than that of NT-TQ500.This is thought to be related to differences in deformation behavior during the finishing rolling stage;the inhibition of dynamic recrystallization from Nb in NT-TQ500(Nb-Ti)may lead to higher density of dislocations in the microstructure.

Evaluation on Fatigue Performance and Fracture Mechanism of Laser Welded TWIP Steel Joint Based on Evolution of Microstructure and Micromechanical Properties

The fatigue performance and fracture mechanism of laser welded twinning induced plasticity(TWIP)steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.The optical microscopy was used to analyze the evolution of microstructure.The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers.The micromechanical behaviors of the various zones were characterized using nanoindentation.The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal.The microstructures,tensile properties and fatigue strength of base metal as well as welded metal were analyzed.The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy,in order to identify fatigue crack initiation sites and propagation mechanisms.Moreover,the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.

Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in NaCl solution

The corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy was investigated in air and 3.5 wt%NaCl solution. Epoxy coating as a new method was used to improve the corrosion fatigue property of the material.Results show that the fatigue limit(FL) of the coated specimens is higher than that of the uncoated specimens in3.5 wt% NaCl solution because of the strengthening and blocking functions of the epoxy coating. The FL of the coated specimens in 3.5 wt% NaCl solution is as high as that in air. It implies that the coated specimens are not as sensitive to the environment as the magnesium alloy. The low tensile strength and the short elongation of the pure epoxy coating lead to that the fatigue crack of the coated specimen is always initiated from the epoxy-coating film Pores and pinholes accelerate the fatigue crack initiation process. Pinholes are caused by the corrosion reactions between the epoxy coating and the NaCl solution.

Microstructure and mechanical properties of 20Si2CrNi3MoV steel treated by HDQP process

A combined process of hot-deformation plus two-step quenching and partitioning(HDQP)treatment was applied to a low-carbon 20 Si2CrNi3 MoV steel,and transmission electron microscopy(TEM),scanning electron microscopy(SEM),X-ray diffraction(XRD),Vickers hardness and tension test were used to characterize the microstructure and mechanical properties.More stable retained austenite due to fine microstructures and typical curved micromorphology is obtained,and the newly-treated steel obtains more retained austenite because of the effect of hot deformation.The retained austenite fraction increases and then decreases with the increasing quenching temperature from 200 to 350°C.The maximum retained austenite fraction(18.3%)and elongation(15%)are obtained to enhance the ductility.