Microstructure and mechanical properties of as-cast Mg-4Zn-0.5Zr-0.2Cu-0.2Ce alloy

In this study, an approach is proposed to improve the microstructure and mechanical properties of Mg-4Zn-0.5Zr alloy by combining trace Cu and rare earth Ce addition. The results showed that Cu and Ce additions led to obvious grain refinement and the formation of Mg-Zn-Cu and Mg-Zn-Ce phases. The Mg-Zn-Ce phase was identified to have an orthorhombic structure. The length of the [0001]_α rods in the Cu-containing alloys remarkably decreased. The yield strength increased slightly after Cu and Ce co-addition, which was attributed to grain refinement and precipitation strengthening. The coarse Mg-Zn-Ce phase distributed at the grain boundaries would reduce the ductility by promoting crack propagation during tensile processes.     

Precipitates,zones and transitions during aging of Al-Zn-Mg-Zr 7000 series alloy

Abstract

Age hardening of an industrial 7000 series alloy in the temperature range 70-150° C has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate η′ and the stable phase η-MgZn₂, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450°C is emphasized.     

Microstructure,precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment

IN718 alloy was fabricated by laser powder bed fusion (PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment (SHT) at 980 °C or 1065 °C for 1 h. SHT at 980 °C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065 °C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980 °C and 1065 °C were two-step aged, which consisted of annealing at 720 °C for 8 h followed by annealing at 620 °C for 8 h. Transmission electron microscopy revealed the precipitation of γ' and γ” in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980 °C and 1065 °C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065 °C and two-step aging due to precipitation with greater amount and uniform distribution.     

挤压态Mg-2Sn-2Bi-0.5Ca-0.2Mn合金的显微组织和力学性能

低合金化的Mg-Sn-Bi基合金具有较高的拉伸延展性和挤压成形性,是开发高强韧镁合金的理想材料。为了弥补其强度不足的缺点,本文通过微合金化设计了一种新型的低合金化Mg-2Sn-2Bi-0.5Ca-0.2Mn镁合金,该合金在挤压温度为523 K、挤压比为25∶1的条件下被成功挤压成形。采用电子背散射衍射仪(EBSD)、X射线衍射分析仪(XRD)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)等技术表征挤压态合金的组织特征和相组成,并利用拉伸试验机测试了挤压态合金的室温拉伸性能,此外,还对合金的强韧化机制和加工硬化行为进行了详细的讨论。结果表明：挤压态合金主要由α-Mg、Mg₃Bi₂以及Mg₂Bi₂Ca相组成,且表现出几乎完全的动态再结晶组织和典型的挤压镁合金织构;合金的拉伸屈服强度为287.2 MPa,抗拉强度为353.0 MPa,伸长率为20.0%,具有良好的强韧性匹配度。合金展现出的高屈服强度是晶界强化、第二相强化和织构强化共同作用的结果;合金的断口形貌表现出典型的韧性断裂特征,然而粗大Mg<...     

Microstructure and mechanical properties of an indirect-extruded Mg-8Sn-1Al-1Zn alloy

The microstructure and mechanical properties of an indirect-extruded Mg-8Sn-1Al-1Zn (TAZ811) alloy were investigated and compared with those of a commercial Mg-3Al-1Zn (AZ31) alloy. In the extruded condition, the TAZ811 alloy shows a much smaller grain size but a stronger basal texture than the AZ31 alloy. In addition, the TAZ811 alloy contains fine Mg₂Sn particles in the microstructure, whereas the AZ31 alloy reveals relatively coarse and sparse Al-Mn particles. The TAZ811 alloy showed tensile and compressive strengthening as well as a reduction in yield asymmetry between tension and compression, which is mainly due to grain refinement and the presence of fine Mg₂Sn particles.     

MB15在半固态等温处理中的组织和成分演变

为考查等温处理技术制备半固态成形用MB15镁合金非枝晶锭料的可能性,借助于液淬技术、光学和电子显微分析,研究了MB15镁合金的微观组织和成分演变.结果表明:MB15镁合金在半固态等温热处理过程中能形成晶粒细小、晶粒粗化速度缓慢的球状组织,但其圆整程度低于AZ91D合金.随着温度的升高和保温时间的延长,Zn向晶界扩散速度加快,晶界处的Zn含量逐渐增加,晶内Zr偏析逐渐扩散.冷却时,α-Mg与Zr在晶内富Zn处发生包晶反应,晶界处发生共晶反应和共析反应.     

Microstructure, local and global mechanical properties of friction stir welds in aluminium alloy 6005A-T6

The effect of the welding speed on the microstructure, local and overall mechanical properties of friction stir welded joints has been investigated in the aluminium alloy 6005A-T6. The fine hardening precipitation within the heat-affected zone has been characterized by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Post-welding heat treatments have been applied to obtain indications on the level of solid solution supersaturation in the as welded state. The local mechanical behaviour was determined using thin specimens extracted from various regions of the weld. The overall properties were measured on samples cut perpendicular to the weld. Specific attention was devoted to the relationship between the local microstructure and local hardening properties in the weakest region, which govern the overall strength and ductility of the welds.     

Microstructure evolution and mechanical properties of 316H weld metal during aging

ABSTRACT

A standard 316H welding wire was used to obtain 6% δ-ferrite in the weld metal by the automatic tungsten inert gas welding. The effect of aging time at 750°C on microstructure and mechanical properties of the weld metal were analysed. The results showed that as the aging time increased, the δ-ferrite fraction decreased, and finally, δ-ferrite was no longer seen, the M₂₃C₆ fraction first increased and then decreased, and the σ phase fraction increased. Finally, the fractions of M₂₃C₆ and σ phase kept stable, but the size increased. Accordingly, as the aging time increased, the yield and tensile strengths first decreased significantly, then increased obviously and finally decreased slightly again, while the elongation and impact energy decreased all the time.     

Effects of heat treatment on the microstructure and mechanical properties of AA2618 DC cast alloy

Direct chill (DC) cast ingot plates of AA2618 alloy have been increasingly used for large-mold applications in the plastics and automotive industries. The effects of different heat treatments on the microstructure and mechanical properties of AA2618 DC cast alloy were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and hardness and tensile testing. The as-cast microstructure contained a considerable amount of coarse intermetallic phases, including Al₂CuMg, Al₂Cu, Al₇Cu₄Ni, Al₇Cu₂(Fe,Ni) and Al₉FeNi, resulting in poor mechanical properties. Solution treatment at 530 °C for 5 h dissolved the first three phases into the solid solution and consequently improved the mechanical properties of the alloy. By utilizing the appropriate aging temperature and time, different combinations of strength and ductility could be obtained to fulfill the design requirements of large-mold applications. The strengthening of AA2618 DC cast alloy under the aging conditions studied was caused by GPB zones and S′ precipitates. The evolution of both precipitates in terms of their size and density was observed to have a significant effect on the mechanical properties of the alloy.     

等温时效对新型Sn-Ag基复合钎料显微组织和力学性能的影响

研究了等温时效对Sn-3.5Ag共晶钎料及其复合钎料的力学性能和显微组织变化的影响。为了弥补传统复合钎料制备和服役中强化颗粒容易粗化的问题, 制备了不同种类最佳配比的具有纳米结构的有机无机笼型硅氧烷齐聚物(POSS)颗粒增强的Sn-Ag基复合钎料。对钎焊接头在不同温度（125、150、175℃）下进行时效,通过SEM和EDAX分析了钎料与基板间金属间化合物层(IMC)的生长情况。结果表明, 经过不同温度时效,复合钎料钎焊接头界面处金属间化合物的生长速率比Sn3.5Ag共晶钎料慢, 复合钎料的IMC生长的激活能分别为80、97和77kJ/mol,均高于Sn3.5Ag共晶钎料。经过150℃时效1000h后,复合钎料钎焊接头的剪切强度分别下降了22%、13%和18%,下降幅度相当或明显小于Sn-3.5Ag钎料钎焊接头。      

Effect of aging treatment on the microstructures and mechanical properties evolution of 25Cr-20Ni austenitic stainless steel weldments with different Nb contents

The microstructure evolutions and the mechanical properties of the 25Cr-20Ni austenitic stainless steel weld metals with different Nb contents were investigated during the long term aging treatment at 700~?C.M_(23)C_6,Nb(C,N),α-Cr phase and Nb-nitride phase(Z phase)were observed in the microstructures of the aged weld metals.The results showed that theα-Cr phase precipitated in the interdendritic regions of the weld metals after being exposed to~ 700?C for 500 h and the element Nb accelerated the precipitation of theα-Cr phase significantly.The density of theα-Cr phase decreased with the increase of the distance away from the primary Nb(C,N).Additionally,theα-Cr phase showed a crystallographic relationship with the austenitic matrix,■.It was observed that the Z phase precipitated in the periphery of the Nb(C,N)and may replace the Nb(C,N)after long term exposure to high temperature.The transformation of the Nb(C,N)into Z phase suggested that the Z phase had a higher stability than the Nb(C,N)par~ticles at 700?C for long term aging.The tensile strength of the Nb-bearing weld metal showed a continuous decrease at the initial stage of the aging treatment and then went up slightly with the prolonged aging time.However,the elongations and the impact energies of the weld metals decreased monotonously with the increase of the aging time.     

Surface dissolution and amorphization of electrocatalysts during oxygen evolution reaction: Atomistic features and viewpoints

Electrochemical electrolysis with an aqueous electrolyte is a key method to obtain green hydrogen energy, convert carbon dioxide into high value-added carbon-based chemicals, and produce ammonia by nitrogen reduction at ambient conditions. For better efficiency of these electrolysis processes, lowering the comparatively large activation barrier of the sluggish oxygen evolution reaction (OER) at the anode side is necessary. Therefore, suitable electrocatalysts should be adopted to facilitate the OER. As a highly oxidizing environment along with high overpotential is unavoidable at the anode during electrolysis, degradation of both the activity and durability of OER catalysts inevitably takes place. In this Review, we classify four significant origins directly affecting the stability of oxide-based OER catalysts: (1) Alkali and alkaline-earth metal dissolution; (2) transition-metal leaching; (3) lattice oxygen evolution; and (4) rapid dissolution of phosphorus and chalcogens. In particular, because these origins usually induce amorphization or reconstruction at the surface, we systematically summarize atomic-level evidence largely based on transmission electron microscopy in each section. Providing integrated viewpoints for a better understanding of catalyst degradation, we believe that this Review offers valuable insight toward designing new OER catalysts with enhanced durability and activity.     

全Cu_(3)Sn焊点在高温时效下的组织及力学性能EI北大核心CSCD

对全Cu_(3)Sn焊点进行620℃下不同持续时间的时效处理,研究时效过程中接头微观组织演变,并利用纳米压痕实验及剪切实验表征时效后焊点的力学性能变化。结果表明:在时效过程中,Cu/Cu_(3)Sn界面以平面状析出Cu_(20)Sn_(6)并持续生长,直至Cu_(3)Sn被完全消耗。随后Cu_(20)Sn_(6)向Cu_(20)Sn_(6)和Cu_(13.7)Sn组成的两相层转变,Cu_(13.7)Sn通过消耗两相层在Cu/两相层的界面处以波浪状析出并继续生长,直至占据整个界面区,该过程中伴随着焊缝中间位置孔洞数量和尺寸的生长,最终聚合成微裂纹。Cu_(20)Sn_(6),Cu_(3)Sn,Cu_(13.7)Sn相的硬度分别为9.62,7.15,4.67 GPa,弹性模量分别为146.5,134.0,133.2 GPa。随时效时间的增加,焊点的抗剪强度呈先增大后减小的趋势,在120 min内保持大于20.1 MPa;其断口形貌和断裂路径也随之发生变化。     

Influence of Zr on structure, mechanical and thermal properties of Ti-Al-N

Multinary Ti-Al-N thin films are used for various applications where hard, wear and oxidation resistant materials are needed. Here, we study the effect of Zr addition on structure, mechanical and thermal properties of Ti_1-xAl_xN based coatings under the guidance of ab initio calculations. The preparation of Ti_1-x-zAl_xZr_zN by magnetron sputtering verifies the suggested cubic (NaCl-type) structure for x below 0.6-0.7 and z ≤ 0.4. Increasing the Zr content from z = 0 to 0.17, while keeping x at ~ 0.5, results in a hardness increase from ~ 33 to 37 GPa, and a lattice parameter increase from 4.18 to 4.29 Å. The latter are in excellent agreement with ab initio data. Alloying with Zr also promotes the formation of cubic domains but retards the formation of stable wurtzite AlN during thermal annealing. This leads to high hardness values of ~ 40 GPa over a broad temperature range of 700-1100 °C for Ti_0.40Al_0.55Zr_0.05N. Furthermore, Zr assists the formation of a dense oxide scale. After 20 h exposure in air at 950 °C, where Ti_0.48Al_0.52N is already completely oxidized, only a ~ 1 μm thin oxide scale is formed on top of the otherwise still intact ~ 2.5 μm thin film Ti_0.40Al_0.55Zr_0.05N.     

Structural investigations of Y2O3 dispersoids during mechanical milling and high-temperature annealing of Fe-15Y2O3-xTi (x = 0–15) model ODS alloys

A model Oxide Dispersion Strengthened (ODS) alloy powder of composition Fe – 15 wt. % Y₂O₃ – x wt. % Ti (x = 0, 2, 5, 10 and 15) were synthesized by high energy mechanical milling in Ar atmosphere for a prolonged duration of 60 h. Synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations suggested the amorphisation of Y₂O₃ nano-crystallites, irrespective of Ti content, which is further studied by Raman spectroscopy. The Raman spectroscopy analysis confirms the presence of YO bonding in the milled powder and ruled out the possibility of elemental dissociation of Y₂O₃ and dissolution into the Fe matrix. Annealing of the milled powders containing different amounts of Ti led to formation of different types of oxide complexes which were also studied using synchrotron XRD and TEM studies. The role of Ti in refining the dispersoids through formation of Y_1.6Ti_1.8Fe_0.4O_6.6 is established through these studies.     

Microstructure, mechanical and tribological properties of Ti(C, N)/a-C gradient composite films

Ti(C, N)/a-C composite films with compositional gradient from Ti-TiN-Ti(C, N) to Ti-containing a-C layers have been prepared by closed-field unbalanced magnetron sputtering. Within the composite films, the carbon contents gradually increase and achieve maximum in the a-C layer by increasing the power applied to the graphite targets, the nitrogen contents gradually decrease to zero from Ti(C, N) layer of the interface to a-C layer of the films. In order to achieve a good combination of the mechanical and tribological properties in the composite films, a designed experimental parameter basing on various substrate rotation speeds is also selected. Results show that the compositional gradient result in the microstructure change of composite films where the Ti(C, N) layers consist of fine nanocolumnar Ti(C, N) grains and the a-C layers consist of 2-7 nm TiC nanocrystallites embedded in an amorphous C matrix. The Ti(C, N) layers also exhibit clear multilayer structure where the period thickness gradually decreases as substrate rotation speed increases. Under higher rotation speed, disappearance of the multilayer structure is accompanied with simultaneous increase in the crystallinity of Ti(C, N) layer and also the Ti(C, N) grain size. In the a-C layer, the TiC nanocrystallites embedded in the a-C matrix is produced by the high rotation speeds. The Ti(C, N)/a-C gradient composite films exhibit high microhardness values (~5000 H_V) and low friction coefficient (~ 0.15), which is related to the hard Ti(C, N) layer and self-lubricate a-C layer, respectively. The combination of the Ti(C, N) layer with a-C layer increases the load and the wear resistance capacity of the composite films, which gives satisfactory friction performance in the pin-on-disk tests with a wear rate of 3.7 × 10^− 17 m³/mN.     

Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment

In order to examine the combined effect of plastic deformation and aging process, the Al 7075 alloy was subjected to equal channel angular pressing (ECAP) deformation by route B_C in various ECAP and aging conditions: pre-ECAP aging, post-ECAP aging and dynamic aging during ECAP at 393 K and 423 K. Followed by ECAP and aging treatment, Vickers microhardness and tensile test were performed and microstructural observations were undertaken using transmission electron microscopy (TEM) and X-ray diffractometer (XRD). TEM investigation showed that ultrafine-grained (UFG) materials with grain size less than 500 nm could be obtained after three or four passes of ECAP. Precipitates characterization revealed that maximum mechanical properties are achieved when the microstructure mainly consists of fine dispersion of small η^′ precipitates and minor quantities of GP zones. Dynamic aged specimens at 393 K and 423 K represented maximum and minimum mechanical properties, respectively, due to formation of fine η^′ precipitates plus GP zones and η^′ plus η precipitates, respectively. Dynamic aging during ECAP at 393 K appeared preferable to other procedures for attaining maximum mechanical properties as well as saving time and energy.     

Microstructure evolution,mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6Zn-2Gd-0.5Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primary α-Mg matrix, α-Mg + W (Mg₃Zn₃Gd₂) eutectic along grain boundaries, and icosahedral quasicrystalline I (Mg₃Zn₆Gd) phase within α-Mg matrix. During homogenization process, α-Mg + W (Mg₃Zn₃Gd₂) eutectic and I phase gradually dissolved into α-Mg matrix, while some rod-like rare earth hydrides (GdH₂) formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505 °C for 16 h according to the microstructure evolution. Furthermore, the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.     

Microstructure evolution and mechanical properties of GH984G alloy with different Ti/Al ratios during long-term thermal exposure

GH984G alloy is a low cost Ni–Fe based wrought superalloy designed for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the microstructure evolution and tensile properties of GH984G alloy with different Ti/Al ratios during thermal exposure at different high temperatures are investigated. Detailed microstructure analysis reveals that the Microstructure of alloys with different Ti/Al ratios are similar after standard heat treatment, and the primary precipitates are γ′, MC, M₂₃C₆ and M₂B. However, η phase precipitates at grain boundary in the alloy with high Ti/Al ratio after thermal exposure at 750 °C for 570 h. By contrast, the microstructure stability of the alloy with lower Ti/Al ratio is excellent. There is no detrimental phase even if after thermal exposure at 750 °C for 5000 h in the alloy with lower Ti/Al ratio. γ′ coarsening plays a great role on the tensile strength, and the critical size range of γ′ could be defined as approximately 27–40 nm. The influence of η phase on tensile strength has close relationship with its volume fraction, the high volume fraction results in the decrease of tensile strength. The tensile strength of the alloy with lower Ti/Al ratio is obviously higher than the alloy with higher Ti/Al ratio and the yield strength has no obvious decrease during long-term thermal exposure at 700 °C. It is demonstrated that the thermal stability of microstructure and mechanical properties of GH984G alloy can be improved by moderately decreasing Ti/Al ratio to satisfy the requirement of A-USC plants.