Acta Metallurgica Sinica-English Letters最新文献

The application of Mg alloys is always accompanied by various coating technology, but a reliable model predicting the service life of coatings on Mg alloys is lacking but urgent. In this work, a semi-mechanistic model was proposed to predict the service life of plasma electrolytic oxidation (PEO) coating/electrophoretic coatings on a VW63Z Mg alloy; the model was decomposed into three parts: a first part depicting the degradation time of organic coating (L₁) and the diffusion time of electrolyte in the inorganic coating (L₂), respectively; a second part interpreting the breakdown of coatings due to the corrosion process (L₃); a final part establishing an algorithm converting the accelerated tests into the real service environment (α); the effect of structural stress and dissimilar metal joints on the service life of coatings was also considered. Based on the ongoing accelerated experiments, the semi-mechanistic model could be able to predict the service life of both PEO coatings and composite coatings on VW63Z Mg alloy with a satisfiable precision.

The high Si-bearing 15Cr–9Ni–Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment (SHT) at 850 °C for 3 h. The effects of 550 °C aging on the α′-martensitic transformation of the as-welded and the SHT weld metals were investigated. The results showed that the weld metal had poor thermal stability of austenite. The precipitation of NbC during the 850 °C SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α′-martensite. The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α′-martensite at the early stage of 550 °C aging. The homogenized diffusion of C and Cr in the matrix during 550 °C aging led to the restoration of austenitic thermal stability and the decrease of α′-martensite content. The C-depleted α′-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix, while the Cr-depleted α′-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.

The Ti-45Nb (mass%) alloy’s corrosive and biocompatible response in simulated physiological conditions was investigated before and after its additional high-pressure torsion (HPT) and laser irradiation processing. The grain size reduction from 2.76 µm to ~ 200 nm and the appearance of laser-induced morphologically altered and highly oxidized surface led to the significant improvement of alloy corrosion resistance and cell–implant interaction. Moreover, an additional increase of the laser pulse energy from 5 to 15 mJ during the alloy irradiation in the air led to an increase in the surface oxygen content from 13.64 to 23.89% accompanied by an increase of excellent cell viability from 127.18 to 134.42%. As a result of the controlled alloy microstructural and surface modifications, the formation of protective bi-modal mixed Ti- and Nb-oxide external scale was enabled. The presence of this surface oxide scale enhanced the alloy’s resistance to corrosion deterioration and simultaneously boosted cell viability and proliferation.

Alternating shear stress is a critical factor in the accumulation of damage during rolling contact fatigue, severely limiting the service life of bearings. However, the specific mechanisms responsible for the cyclic shear fatigue damage in bearing steel have not been fully understood. Here the mechanical response and microstructural evolution of a model GGr15 bearing steel under cyclic shear loading are investigated through the implementation of molecular dynamics simulations. The samples undergo 30 cycles under three different loading conditions with strains of 6.2%, 9.2%, and 12.2%, respectively. The findings indicate that severe cyclic shear deformation results in early cyclic softening and significant accumulation of plastic damage in the bearing steel. Besides, samples subjected to higher strain-controlled loading exhibit higher plastic strain energy and shorter fatigue life. Additionally, strain localization is identified as the predominant damage mechanism in cyclic shear fatigue of the bearing steel, which accumulates and ultimately results in fatigue failure. Furthermore, simulation results also revealed the microstructural reasons for the strain localization (e.g., BCC phase transformation into FCC and HCP phase), which well explained the formation of white etching areas. This study provides fresh atomic-scale insights into the mechanisms of cyclic shear fatigue damage in bearing steels.

Previously, the in-plane mechanical anisotropy of Zr hot-rolled plates is ascribed mainly to the different activities of the deformation modes activated when loading along different directions. In this work, a quantitative study on the deformation behavior of a pure Zr hot-rolled plate under tension along the rolling direction (RD) and transverse direction (TD) reveals that both the activities of deformation modes and the anisotropy of grain boundary strengthening account for a tensile yield strength anisotropy along the TD and RD. Crystal plasticity simulations using viso-plastic self-consistent model show that prismatic slip is the predominant deformation mode for tension along the RD (RD-tension), while prismatic slip and basal slip are co-dominant deformation modes under tension along the TD (TD-tension). A low fraction of (left{10bar{1}2right}) twinning is also activated under TD-tension, while hardly activated under RD-tension. The activation of basal slip with a much higher critical resolve shear stress under TD-tension contributes to a higher yield strength along the TD than along the RD. The grain boundary strengthening effect under tension along the TD and RD were compared by calculating the activation stress difference ((Delta {text{Stress}})) and the geometric compatibility factor (({m}^{prime})) between neighboring grains. The results indicate a higher grain boundary strengthening for TD-tension than that for RD-tension, which will lead to a higher yield strength along the TD. That is, the anisotropy of grain boundary strengthening between TD-tension and RD-tension also plays an important role in the in-plane anisotropy along the RD and TD. Afterward, the reasons for why there is a grain-boundary-strengthening anisotropy along the TD and RD were discussed.

AZ31B magnesium alloys are commonly used in lightweight structures of automobile and aerospace industry. In this work, the ultrasonic-induced transient liquid phase bonding technique is deployed to joint AZ31B magnesium alloys at 490 °C with Ag interlayer in the atmosphere. The effect of ultrasonic vibration on microstructure and mechanical properties of welding seam is investigated. As vibration duration increases, the width of the welding seam increases firstly and then decreases. When the ultrasound vibration sustained for one second, the eutectic reaction occurred in the contacting interface between Ag interlayer and the base metal AZ31B, resulting in formation of liquid phase and metallurgical bonding. When the ultrasound vibration sustained for seven second, the width of the welding seam increased to a maximum of about 303.1 μm and the average shear bond strength of the welding joint attained the peak value of about 66.87 MPa. When the ultrasound vibration sustained over seven seconds, the liquid phase is gradually squeezed out and the width of the welding seam decreases. α-Mg and AgMg₃ can be observed in the welding seam. The analysis shows that α-Mg has a certain strengthening effect. The whole process of appearance, growth and integration of α-Mg is analyzed.

The new developed γ/γʹ Co–Al–Nb-base alloys show great potentials as high-temperature materials. However, finding appropriate compositions to improve performance of alloys still poses a great challenge to the development of Co–Al–Nb-base alloys. Motivated by the lack of alloying effects on fundamental properties of critical γʹ phase, we systematically performed a theoretical investigation on the effect of alloying elements TM (TM: Ti, V, Cr, Zr, Mo, Ta, W, Re, and Ru) on phase stabilities and mechanical properties of L1₂-type γʹ (Co, Ni)₃(Al, Nb). By analyzing the stability of γʹ phase with respect to its competitive B2 and D0₁₉ phases, the results shown that Ti, V, and Cr enhance the L1₂ stability and widen the L1₂–D0₁₉ energy barrier, in which V yields the maximum influence. The analysis of electronic structure indicated that the alternation of valence electrons at fermi level would be the atomic origin for doping TM in γʹ phase. The calculated results of mechanical properties shown that V and Cr are expected to be optimal dopant for enhancing the strength and the ductility of γʹ phase. The addition of Ta is also beneficial for enhancing the strength at the slight expense of ductility of γʹ phase. By drawing the mechanical maps, the preferred composition range for the phases with desired properties is roughly demarcated in theory for the multi-addition of V/Cr and V/Ta in γʹ phase. The findings would be useful for optimizing the performance of novel γ/γʹ Co–Al–Nb-base superalloys.

In this work, the microstructure and mechanical properties of large cross-sectioned Mg–9Gd–3Y–1.2Zn–0.5Zr (VWZ931) samples produced by the small extrusion ratio has been investigated. The as-extruded VWZ931 sample with diameter of ~ 30 mm can exhibit the high yield strength (YS) of 339 MPa, ultimate tensile strength (UTS) of 387 MPa and elongation of 8.2%, respectively. After peak-aged, the YS and UTS of the Mg samples were significantly increased to 435 MPa and 467 MPa. The small extrusion ratio leads to the low fraction of dynamic recrystallized (DRX) grains in VWZ931 sample, and the texture hardening effect can be fully utilized to achieve high strength. The combined effect of precipitation strengthening due to the long-period stacking ordered phases and the (beta)′ phase, grain boundary strengthening due to the fine DRX grains, heterogeneous deformation-induced strengthening caused by bimodal microstructure, can together contribute to the high strength of present Mg alloy. The findings can shed light on designing other large-sized Mg wrought alloys with high mechanical performance.

Through carrying out the high-temperature tensile experiments on an as-extruded Mg–11wt%Y alloy at 350 °C, 400 °C, 450 °C, 500 °C and 550 °C, the mechanical behavior and fracture mechanisms at elevated temperatures are investigated and compared. Tensile results show that with the increase of temperature, the yield strength and ultimate tensile strength of the alloy increase at first and then decrease, while that the elongation ratio decreases firstly and then increases. For the sample being tested at 350 °C, the values of yield strength, ultimate tensile strength and the elongation ratio are 188 MPa, 266 MPa and 11%, respectively. At 400 °C, the yield strength and ultimate tensile strength reach the maximum values of, respectively, 198 MPa and 277 MPa, but the elongation ratio is the lowest and its value is only 8%. When the applied temperature is increased to 550 °C, the values of yield strength and ultimate tensile strength, respectively, decrease to 140 MPa and 192 MPa and the elongation ratio increases to 38%. Failure analysis demonstrates that the fracture surfaces of different samples are mainly composed of plastic dimples and exhibit the typical characteristic of ductile fracture. The observation to the fracture side surfaces indicates that at the temperatures of 350 °C and 400 °C, microcracks mainly initiate in the interior of Mg₂₄Y₅ particles. When the temperatures are 450 °C, 500 °C and 550 °C, the cracks preferentially initiate at the Mg₂₄Y₅/α-Mg interfaces.

Mg alloy seamless tubes (MASTs) were prepared through three-high rotary piercing process, effect of billet temperature, feed angle and plug advance on microstructure, texture and mechanical properties of tubes were investigated. The effect on the deformation mechanism and improving mechanical properties mechanism of this process for MASTs were studied. The results show that the grain size could be refined to 11.3–31.1% of the initial grain size and the microstructure was more uniform due to the accumulation of strain. The formation of high strain gradient at the grain boundary activated the non-basal slip. This piercing process could change the grain orientation of as-extruded billet and eliminate the initial basal texture to produce new favorable texture. And the process could accelerate the continuous dynamic recrystallization process. After piercing, yield strength of pierced tubes decreased by 6.7%, ultimate tensile strength (UTS) and elongation increased by 32.4 and 45%, respectively, at optimal parameters. The plate-shaped β₁-Mg₁₇Al₁₂ orientation transformed from basal plates to prismatic plates, facilitating the increase in UTS and ductility. The decrease size of nanoscale precipitates could reduce the cracking possibility. The critical resolved shear stress ratios of pyramidal (10−11) slip and (11−22) slip to basal slip for the sample including prismatic plates both decreased compared to that including basal plates. This could enhance the ductility of tube sample. Moreover, grain boundary sliding could contribute to a better ductility via coordinating deformation and reducing stress concentration during piercing process.