Mechanical behaviour and microstructural evolution of constrained groove pressed nickel sheets

Pure nickel sheets are subjected to severe plastic deformation by constrained groove pressing technique at room temperature thereby imparting an effective plastic strain of 3.48. The evolution of mechanical behaviour with increasing number of passes revealed intensive increase in strength properties after first pass; however marginal increase is observed subsequently. Gain in ductility which is attributed to dislocation recovery is observed after third pass along with marginal drop in strength. Microstructural evolution during groove pressing of sheets is characterized by X-ray diffraction profile analysis using Williamson–Hall method. Besides the observation of strong shear texture in constrained groove pressed sheets, improvement in strain isotropy with increased straining is revealed from Williamson–Hall plots. The sub-grain/cell size estimated by analysing the diffraction profile of deformed sheets is found to be ～1390 nm after processing up to three passes. Low grain refinement efficiency observed in this process compared to other severe plastic deformation techniques at similar strain conditions is explained by the deformation characteristics and loading behaviour experienced by the sheets during constrained groove pressing.     

Impact mechanical response and microstructural evolution of Ti alloy under various temperatures

A compressive split-Hopkinson pressure bar and transmission electron microscope (TEM) are used to investigate the mechanical behaviour and microstructural evolution of a Ti alloy (Ti–1.1Mo–5.2Zr–2.9Al–0.35Fe–0.05N–0.20 O–0.02H in wt.%) deformed at strain rates ranging from 8 × 10² s^?1 to 8 × 10³ s^?1 and temperatures between 25 °C and 900 °C. In general, the results indicate that the mechanical behaviour and microstructural evolution of the alloy are highly sensitive to both the strain rate and the temperature conditions. The flow stress curves are found to include both a work-hardening region and a work-softening region. The strain rate sensitivity parameter, β, increases with increasing strain and strain rate, but decreases with increasing temperature. The activation energy varies inversely with the flow stress, and has a low value at high deformation strain rates or low temperatures. The microstructural observations reveal that the strengthening effect evident in the deformed alloy is a result primarily of dislocations and the formation of α phase. The dislocation density increases with increasing strain rate, but decreases with increasing temperature. Additionally, the square root of the dislocation density varies linearly with the flow stress. Correlating the mechanical properties of the current Ti alloy with the TEM observations, it is concluded that the precipitation of α phase dominates the fracture strain. TEM observations reveal that the amount of α phase increases with increasing temperature below the β transus temperature. The maximum amount of α phase is formed at a temperature of 700 °C and results in the minimum fracture strain under the current loading conditions.     

Mechanical behavior and microstructural evolution of a Mg AZ31 sheet at dynamic strain rates

The mechanical behavior of an AZ31 Mg sheet has been investigated at high strain rate (10³ s^?1) and compared with that observed at low rates (10^?3 s^?1). Dynamic tests were carried out using a Hopkinson bar at temperatures between 25 and 400 °C. Tensile tests were carried out along the rolling and transverse directions and compression tests along the rolling and the normal directions in both strain rate ranges. The tension–compression yield asymmetry as well as the yield and flow stress in-plane and out-of-plane anisotropies were investigated. The microstructure of the initial and tested samples was examined by electron backscatter diffraction. The dynamic mechanical behavior is characterized by the following observations. At high temperatures the yield asymmetry and the yield anisotropies remain present and twinning is highly active. The rate of decrease in the critical resolved shear stress of non-basal systems with temperature is smaller than at quasi-static rates. Rotational recrystallization mechanisms are activated.     

原位TiB2颗粒增强7075铝基复合材料的热压缩变形行为和显微组织演变

采用等温热压缩实验研究不同变形条件下(变形温度300～450℃、应变速率0.001～1 s?1)原位TiB2颗粒增强7075铝基复合材料的热成形行为、损伤机制和显微组织演变.结果表明,复合材料在低温和高应变速率下的主要损伤机制是颗粒断裂和界面脱粘,而在高温和低应变速率下主要是基体的韧窝断裂.此外,复合材料在高温、低应变...     

铝锂合金的合金化与微观组织演化

评述了铝锂合金的发展历程和应用以及我国铝锂合金研究所取得的成果。阐述了第3代铝锂合金的成分、微观组织和性能特点,讨论了合金化元素在调控铝锂合金微观组织进而改善性能方面的作用。大量研究表明,这些合金化元素可改变铝锂合金中原有析出相的大小、形状、分布,或刺激新强化相的析出,也可以细化晶粒、控制再结晶和晶粒取向。时效析出过程的本质和动力学在很大程度上取决于合金化元素之间的交互作用和合金原子的团簇化过程。     

Phase transformation and microstructural evolution in solder joints

Soldering is the most important of joining technologies and there are numerous solder joints in modern electronic products. The phases and microstructures of solder joints are critical to their properties. Various remarkable phenomena caused by phase transformation and microstructural evolution in solder joints have been reported. The phenomena include ripening, layer detachment, liquation, cruciform pattern formation, solid-state amorphization, alternating layer formation, shift of reaction paths, and the effects of electromigration.     

The computer simulation of microstructural evolution

This paper reviews the kinetic Monte Carlo Potts model for simulating microstructural evolution. When properly implemented, that model provides a fast and flexible tool for evaluating a variety of materials systems in two and three dimensions, generating snapshots of the evolving microstructure with time. Examples of the model are provided, along with potential applications. Editor’s Note: A hypertext-enhanced version of this article can be found at www.tms.org/pubs/journals/JOM/0109/Holm-0109.html. For more information, contact E.A. Holm, Sandia National Laboratories, Materials and Process Modeling, Albuquerque, NM 87185-1411; (505) 844-7669; fax (505) 844-9781; e-mail eaholm@sandia.gov.     

Recrystallization of SRR99 single-crystal superalloy： Kinetics and microstructural evolution

As-cast SRR99 specimens were shot-peened and then annealed at 1250 and 1300°C for different times to investigate the kinetics of recrystallization.It was found that the relationship between annealing time and volume fraction of recrystallized grains could be described by the Johnson-Mehl-Avrami equation.Based on the kinetics analysis of the recrystallization process,the apparent activation energy for recrystallization was determined.In addition,the microstructural evolution during the recrystallization process at 1300°C was studied.Inwards,recrystallization first occurs in the dendritic core regions at the shot-peened surface.With the dissolution of coarse γ' particles in the interdendritic regions,the recrystallized grain boundaries move through the interdendritic regions.Finally,the fully developed grains nearly have a uniform depth.The recrystallization process at the shot-peened surface is similar to that of wrought materials,which includes nucleation of recrystallization,growth of recrystallized nuclei into the matrix,and growth of recrystallized grains by swallowing up each other.     

Mechanical properties and microstructural evolution of modified 9Cr-1Mo steel after long-term aging for 50,000 h

The mechanical properties and microstructural evolution of modified 9Cr-1Mo steel have been studied to investigate steel property changes after long-term isothermal aging at 600 °C for 50,000 h. The microhardness and strength were maintained constantly after aging but the impact energy was dramatically reduced by 62 % during the aging period. From the viewpoint of microstructural evolution after the aging process, Cr-enrichment and Fe-depletion took place within the M₂₃C₆-type precipitates in the as-aged steel and V-depletion also happened within the VX-type precipitates after aging. In addition, the precipitates of the M₂Mo-type Laves phase and the segregation of the impurity atoms would be formed during the long-term aging period. It was considered that the sharp reduction of the impact energy could be related to the formation of the Laves phases and the impurity segregation after aging at 600 °C. The phase stability was also verified by the specific heat results up to 950 °C from a DSC test. It was concluded from this study that the modified 9Cr-1Mo steel would keep its microstructural stability at 600 °C during the long-term aging period of 50,000 h, which was equivalent to the in-service life of the SFR fuel cladding.     

Crystallization and microstructural evolution of cordierite-based thick film dielectrics

Microstructural evolution of glass–ceramic dielectric thick films based on a nonstoichiometric cordierite, 2.4MgO·2Al₂O₃·5SiO₂, containing B₂O₃, P₂O₅, and PbO were investigated in conjunction with nucleation and crystal growth. The cordierite thick films were deposited by screen printing on a 96% alumina substrate, and then fired in the temperature range of 850–950°C in a N₂ atmosphere. Surface microstructure characteristics of the thick films depended on PbO content. Heterogeneous nucleation originated predominantly from the interface between the densified thick film and the alumina substrate. The added PbO segregated to the remaining glass during crystallization, and the relative concentration of Pb in the glass was proven to increase with rising temperature. Most of the remaining glass protruded from the film surface at the final temperature of 950°C. Penetration of the alumina substrate by the remaining glass was also observed. The influences of temperature and PbO on the microstructural evolution including nucleation and growth will be discussed with possible explanations for the observed results.     

Synthesis and microstructural evolution of nanocrystalline Al-Fe-V-Si-Nd alloy powder

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Flow behavior and microstructural evolution in nickel during hot deformation

The hot deformation behavior of pure nickel with coarse,columnar grains in the temperature range of 950-1150℃ at intervals of 50℃ and in the strain rate range of 0.001-10.000 s~(-1) at intervals of one order of magnitude was investigated by isothermal hot compressive testing with the compression ratio of 70%.The results reveal that the strain rate and the temperature strongly affect the flow stress during hot deformation and that flow stress increases with the increase in strain rate while decreases with temperature increasing.Moreover,the relationship among flow stress,strain rate and temperature can be represented by the Zener-Hollomon parameter with the calculated apparent activation energy of 312.403 kJ ·mol~(-1),and the variation of activation energy is sensitive to strain rate rather than temperature.In addition,the dynamic recrystallization(DRX)analysis reveals that the DRX behavior of nickel is evidently affected by both deformation temperature and strain rate and that the distinct mechanisms of nucleation are the bulging of serrated grain boundaries and the development of twinning.     

Mechanical and microstructural single-crystal Bauschinger effects: Observation of reversible plasticity in copper during bending

We study the Bauschinger effect on a bent and straightened micro-sized single-crystal copper beam (width: 8.64 μm; thickness: 7.05 μm) over three consecutive cycles. The reverse yield strengths (straightening step) are much smaller than those in forward loading (bending step). An upper bound estimate shows a load drop of 73% (1st cycle), 76% (2nd cycle) and 83% (3rd cycle) relative to the forward yield stress. Electron backscatter diffraction reveals a dramatic reduction in the bending-induced misorientation gradients upon load reversal (straightening), documenting an unexpected form of microstructure reversibility. The observed Bauschinger softening is interpreted in terms of two effects. The first consists of internal backstresses that support load reversal. They are created by polarized dislocation arrays that are accumulated during forward bending. The second effect is the reduced requirement to activate dislocation sources during reverse loading as the dislocations that were stored during bending did not participate much in cross-hardening and, hence, serve as mobile dislocations upon reverse loading. After straightening the misorientation gradients are largely removed but the non-polarized dislocations remain. We therefore introduce a revised terminology, namely the “mechanical Bauschinger effect” and the “microstructural Bauschinger effect”. The former term describes a yield stress drop and the latter one the degree of microstructure reversibility upon load path changes.     

Mechanical and microstructural properties of low-carbon steel-plate-reinforced gray cast iron

In the present experimental study, strengthening of gray cast iron by reinforcing with steel plates was investigated in the as-cast and normalized conditions. Normalizing heat treatments were applied to the specimens at temperatures of 800 °C and 850 °C. Three-point bend specimens were manufactured from gray cast iron and also from gray cast iron with reinforced steel plates. Flexural strengths of the steel-plate-reinforced cast iron were calculated for four distinct volume fractions (V_r = 0.04, 0.08, 0.12, and 0.16). The current study shows that the steel-plate-reinforced gray cast iron has higher flexural strength and flexural modulus than the cast iron without reinforcement. The flexural strength considerably increases with slight increase in normalization temperature. Optical and scanning electron microscopy analyses were used to examine flake morphology and microstructures of gray cast iron and steel-plate-reinforced gray cast iron. It is noted that carbon diffuses from the gray cast iron to the steel plates. A transition region containing partially dissolved graphite and having high hardness was observed due to the carbon diffusion.     

Al-Cu-Li合金均匀化处理参数优化和微观组织演化

摘 要：对Al-Cu-Li铸态合金进行单级和双级均匀化处理,通过光学显微镜（OM）、扫描电镜（SEM）、能谱分析（EDS）、X衍射（XRD）和差热分析（DSC）研究合金元素分布和微观组织演化,结果表明：Al-Cu-Li合金铸态组织存在严重枝晶偏析,由晶内到晶界Cu元素分布十分不均匀,Mg、Zn、Mn和Ag变化不明显。晶界处存在大量的非平衡共晶相,主要包括Al2Cu、含有少量Mg元素的Al2Cu相,以及Al2CuMg相。经双级均匀化（495℃×24h 515℃×24h）处理后,大部分非平衡共晶相和部分第二相（Al2CuMg和Al2CuLi）溶解到合金基体,但仍有部分富-Fe和富-Mn相残留在晶界不能回溶。Al2CuMg相的熔点低于Al2Cu相,两者分别在495℃和515℃先后溶解。通过均匀化动力学分析,确定Al-Cu-Li铝锂合金最佳的均匀化制度为495℃×24h 515℃×24h,该双级均匀化制度与动力学分析结果一致。     

60NiTi合金的热压缩变形与显微组织演变（英文）

采用Gleeble-3500热模拟试验机对在变形温度500～650℃和应变速率0.001～1 s^-1条件下的60NiTi合金进行热压缩变形,分析其热变形行为和显微组织,建立变形本构模型,绘制热加工图。结果表明,当压缩温度升高或应变速率降低时,峰值应力减小。合金的热变形激活能为327.89 k J/mol,热加工工艺参数为变形温度600～650℃和应变速率0.005～0.05 s^-1。当变形温度升高时,合金的再结晶程度增大;当应变速率增大时,位错密度和孪晶数量增大,Ni₃Ti相易于聚集;Ni₃Ti析出相有利于诱发合金基体的动态再结晶。动态回复、动态再结晶和孪生是60NiTi合金热变形的主要机制。     

激光熔覆Co-Cr-Ni-Mo涂层的粘塑性摩擦及组织演变行为(英文)

通过激光熔覆Co-Cr-Ni-Mo合金与WC-Co硬质合金之间的旋转摩擦变形实验,研究钴基合金的粘塑性摩擦及纳米组织形成机制。考察粘塑性摩擦过程的摩擦系数、界面温度及轴向缩短量随时间的变化关系。结果表明以上物理量首先进入快速上升阶段,然后进入稳态阶段,其中第一个阶段属于滑动摩擦,第二阶段属于粘塑性摩擦。粘塑性摩擦后,激光熔覆涂层从表面至内部可分为粘塑性变形区、热力影响区、激光原始组织3个典型区域。粘塑性变形可将原始组织中的网状M23C7相破碎为弥散分布的等轴形状纳米晶粒。粘塑性区的宽度为37～131μm,其典型组织特征为晶粒尺度小于50nm的M23C7相及α-Co相,甚至含有少量接近非晶态结构。因而,粘塑性摩擦将激光熔覆合金的硬度由HV600提高至HV997。