室温微成形尺寸效应对材料强韧性影响的研究现状

重点梳理了近年来微成形尺寸效应理论的研究进展,归纳总结了现有文献中关于微成形中变形行为的各种尺寸效应与材料屈服强度、塑性、抗拉强度、断裂韧性的关系,包括:晶粒尺寸、厚度尺寸以及两者之间的比值对屈服强度有重要影响;伸长率随厚度的增加而增加;抗拉强度与厚度尺寸和晶粒尺寸之间存在复杂关系;材料的断裂韧性与各种尺寸有密不可分的关系。并分析了这些研究结论存在差异的原因,同时,指出现有微成形尺寸效应机理目前存在有争议和亟待解决的关键问题。     

黄铜箔拉伸屈服强度的尺寸效应

为了研究金属箔的塑性变形性能与尺寸的相关性,在常温下对不同厚度和晶粒尺寸的黄铜箔试样进行了单向拉伸实验.结果表明:随着厚度或晶粒尺寸的减小,箔的屈服强度都会升高,晶粒尺寸对屈服强度的影响满足Hall-Petch细晶强化关系,厚度减小使屈服强度升高也可以主要归结于晶粒尺寸的减小.此外,当箔的厚度小于100 靘时,厚度/晶粒尺寸比不能表征屈服强度的尺寸效应.     

微细C5210磷青铜板料的尺寸效应研究

为研究常温下晶粒尺寸和厚度对C5210磷青铜力学性能的影响,引入尺寸效应影响因子φ=厚度/晶粒尺寸。结果表明,厚度不同时,随着φ从14.7减小到6.3,屈服强度减小了48%,延伸率由25.5%减小到18.2%;晶粒尺寸不同时,随着φ的减小,屈服强度先快速下降,当φ减小到3.5后便缓慢下降,延伸率则先增大再减小,φ为3.5时延伸率达到最大值29.2%。通过扫描电镜观察拉伸试样断口,均为韧性断裂。厚度不同时,随着φ的增大,韧窝数量增多且尺寸增大,材料塑性较好;晶粒尺寸不同时,φ从10.6减小至3.5,断面收缩率增加,韧窝尺寸增大,材料塑性较好。而当φ值继续减小到0.8时,韧窝数量减少,且尺寸变小,材料的塑性变差。     

纳米Pt膜的晶粒尺寸及其对热导率的影响

采用电子束-物理气相沉积法（EB-PVD）制备了6个厚度为15-62nm的铂薄膜,研究了纳米薄膜的晶粒尺寸及其对热导率的影响规律．当薄膜厚度小于30nm时,晶粒平均尺寸接近于薄膜的厚度;晶粒尺寸随着薄膜厚度的增加而增大并趋于定值;当薄膜厚度大于30nm时,晶粒尺寸约为20nm．受薄膜的表面和内部晶界的综合影响,铂纳米薄膜的热导率大大低于体材料的值,并且纳米薄膜的热导率随着薄膜厚度的增加而增大并趋于一个低于体材料热导率的值．     

固溶时效对Ti6242合金组织性能的影响

对轧制态Ti6242合金棒材进行固溶时效热处理,分析了固溶温度对材料组织与性能的影响;并通过图像处理得到了不同固溶温度下的组织参数,对组织与性能的关系作定量分析。结果表明,相变点(差热法测得相变点为991℃)以下,固溶温度在910~985℃时的抗拉强度和屈服强度随温度的升高呈下降趋势,塑性变化较小;随固溶温度的升高,等轴α尺寸及片层厚度的增加,克服了条状α增加对强度的贡献作用,导致抗拉强度及屈服强度降低,对塑性影响较小。在相变点之上,在1000~1030℃固溶时,抗拉强度和屈服强度变化较小,而塑性迅速下降;这是因为1000℃固溶时,等轴α含量降低显著(5%~6%),组织的变形协调能力下降,塑性降低,条状α尺寸的增加引起了抗拉强度和屈服强度降低,1015~1030℃固溶时,其显微组织为魏氏体,表现为强度和塑性的急剧减小,1030℃固溶时片层α的长度减小,宽度增大,材料性能有所回升。     

应变梯度塑性理论模拟晶粒尺寸对铝多晶体强度的影响

采用经典塑性理论与基于微观机制的应变梯度（MSG）塑性理论对不同晶粒尺寸铝多晶的应力-应变关系进行了模拟分析,结果表明：基于微观机制的应变梯度本构方程所得的应力-应变曲线中,随着晶粒尺寸的减小,塑性应变功明显增加．晶粒直径为20μm的应力-应变曲线稍高于经典塑性理论得到的曲线,这进一步说明随着晶粒尺寸的增大,应变梯度的贡献逐渐减小,同时计算所得的屈服强度与晶粒尺度关系拟合的直线与铝晶体的Hall-Petch直线比较吻合。     

C5210磷青铜薄板成形行为的尺度效应（英文）

为研究厚度、晶粒尺寸对C5210磷青铜薄板力学性能和成形性能的影响,通过不同温度退火热处理得到不同晶粒尺寸的试样,然后在常温下对具有不同厚度和晶粒尺寸的试样进行单向拉伸试验。结果表明:当厚度在50~800μm范围内,材料的屈服强度随着厚度的减小而增大,而加工硬化指数和伸长率随着厚度的减小而减小,提出了描述屈服强度随厚度减小而增大关系的修正模型;材料的屈服强度随着晶粒尺寸的增大而减小,但加工硬化指数随着晶粒尺寸的增大而增大,伸长率则随着晶粒尺寸的增大先增大到一个峰值后再减小。通过扫描电镜观察拉伸试样的断口形貌发现所有试样断裂方式均为韧性断裂,并且随着厚度的增大断口韧窝的密集度增大,而晶粒尺寸越大的试样断口韧窝密集度越小。     

微细黄铜板料尺寸效应

为研究微细尺度下材料的晶粒尺寸d、厚度t对其力学性能的影响,引入尺寸效应影响因子φ=t/d,在常温下对具有不同晶粒尺寸和板厚的H62黄铜薄板试样进行单向拉伸实验。结果表明,随着φ的减小,流动应力减小,延伸率下降;板料硬化指数n值,随板料厚度的增加而增大,随晶粒尺寸的增大而增大。拉伸试样均为韧窝微孔聚合型断裂,0.5mm和1.0mm厚度试样断口处韧窝多而浅,且为抛物线形,材料韧性好,塑性强;板厚0.1mm和0.2mm的试样断口韧窝深而大,硬化指数n值大,但塑性差。     

亚微米厚铜薄膜的微观结构及疲劳损伤行为

在具有高弹性和力学稳定性的柔性基底上,用磁控溅射系统制备了亚微米厚铜薄膜,利用透射电镜（TEM）、扫描电镜（SEM）电子背散射成像及X射线衍射（XRD）对铜薄膜进行了微观结构表征．采用恒载荷幅控制研究了亚微米厚度铜薄膜的疲劳损伤行为．结果表明：退火后的铜薄膜呈现强烈的（111）织构,薄膜中存在大量的微米、纳米尺度孪晶．在恒载荷幅作用下,亚微米厚的薄膜不易产生疲劳挤出和微裂纹,疲劳裂纹容易在界面处萌生,孪晶附近的位错塞积及界面附近变形的不协调性导致了疲劳裂纹的产生．而亚微米厚铜薄膜疲劳强度的提高来源于薄膜厚度、晶粒尺寸和孪晶尺寸三个微尺度的约束．     

热处理对QT900连续油管晶粒尺寸和性能的影响规律

采用电阻炉对QT900连续油管进行不同温度下的加热,通过空冷和炉冷两种不同的方式对其进行冷却。采用MIAPS显微图像分析处理系统以及力学性能测试手段,研究了加热温度和冷却方式对QT900连续油管晶粒尺寸及力学性能的影响规律。结果表明:随着加热温度的升高,QT900连续油管的晶粒尺寸不断长大,在空冷条件下获得的晶粒尺寸比炉冷的晶粒尺寸小,加热温度超过850℃时,加速冷却可显著细化晶粒尺寸;QT900的强度和硬度均随着加热温度的升高,呈下降趋势,加热至850℃时QT900的强度和硬度都出现大幅下降;塑性随加热温度的升高有所增大。QT900连续油管650℃加热并炉冷能在保证强度的同时提高塑性,降低硬度。     

High Strength-Ductility Nb-microalloyed Low Martensitic Carbon Steel:Novel Process and Mechanism

A low-carbon Nb-microalloyed Fe–Mn–Si-based steel was treated by a novel quenching–partitioning–tempering(Q–P–T) process as a modified quenching and partitioning(QP) process. After processed by Q–P–T treatment,this steel exhibits excellent mechanical properties such as high product of strength and elongation. The addition of Nb markedly raises both the yield strength and tensile strength of Q–P–T martensitic steel, especially the yield strength, which can be attributed to the strong grain refinement strengthening and precipitation strengthening of Nb. The Nb addition can also lead to a little increase in ductility. The Nb-microalloyed steel treated by Q–P–T process displays much higher ductility than that treated by traditional quenching and tempering(QT) process. The mechanisms of Q–P–T process on ductility enhancement were fully analyzed and can be attributed to high quenching temperature and considerable amount of retained austenite.     

Shear band spacing under bending of Zr-based metallic glass plates

Metallic glasses often exhibit marked ductility when subjected to compressive or bending loads as a result of multiple shear band formation. This observed ductility depends upon sample geometry; thin plates show ductility in bending while thicker plates of the same composition fracture under similar loading. The thickness dependence of yielding and fracture of metallic glass plates subjected to bending is considered in terms of the shear band processes responsible for these properties. Experimental results show that shear band spacing and length scale with the thickness of the plate at a ratio of 1:10. Both shear band offset and shear band spacing increase with increasing curvature; shear band offset as the square of the plate thickness. As bending is increased beyond yield, shear band spacing continues to increase until the strain is accommodated by a few long shear bands. Continued bending leads to crack formation and failure.     

铜基非晶合金板弯曲塑性及剪切带间距的研究

制备了一种压缩断裂塑性应变接近2％的铜基块状非晶板材,并利用三点弯曲实验进行了其弯曲塑性、剪切带间距与试样厚度关系的研究．结果表明：非晶合金的弯曲断裂塑性应变明显依赖于试样厚度,即弯曲断裂塑性应变随试样厚度增加呈指数衰减关系减少;弯曲时剪切带间距随试样厚度增加而线性增加;剪切带间距与试样厚度的比值对于同一非晶合金为恒定值,但随非晶合金种类的不同而变化,与非晶合金的塑性变形能力有关．     

Temperature dependent mechanical properties of ultra-fine grained FeCo–2V

The tensile properties of ultra-fine grained ordered FeCo–2V have been investigated as a function of testing temperature. Samples with grain sizes of 100, 150 and 290 nm have been tested at temperatures ranging from 25 to 500 °C. Extremely high yield strengths (up to 2.1 GPa) were measured at room temperature with appreciable ductility of between 3 and 13%. These strengths were found to decline only gradually as the testing temperature was increased to 400 °C, while ductility was generally enhanced, up to 22%. The high strengths are attributed to grain boundary strengthening that is particularly effective due to ordering. Measured ductility was dependent on the relative values of yield strength, fracture strength and work hardening rate. Discontinuous yielding and appreciable Lüders strain (3–6%) were observed and were dependent on the initial structure and on the testing temperature.     

Effect of Grain Size on the Superplastic Behavior of a 7475 Aluminum Alloy

Specimens of a super plastic 7475 aluminum alloy with grain sizes ranging between 9 and 35 |xm were tensile tested at a strain rate of 1 × 10 ^{- 4}/s at 457 and 517 °C. At 517 °C, the ductility was found to decrease with an increase in grain size. At 457 °C, on the other hand, the ductility was found to increase initially and then decrease for grain sizes larger than 14 μm. The latter decrease in ductility is attributed to the lowered ability for grain- boundary sliding with decreasing grain- boundary area. In the as- received material (grain size of 9 μm), the observed low ductility is attributed to an inhomogeneous microstructure.     

Micro–macro interactions and effect of section thickness of hpdc AZ91 Mg alloy

This work is aimed to conclude the effect of section thickness of a high pressure die cast (hpdc) Mg alloy on the tensile properties as ambiguous conclusions are presented in the literature. Tensile tests were performed on as-cast hpdc AZ91 alloy and the effect of section thickness on the tensile properties such as yield strength (YS), ultimate tensile strength (UTS), ductility and fracture strain (FS) are explained. Additionally, explanation on the microfailure mode of the material is presented to explain the influence of different microfeatures on the failure process. The average size, area fraction and clustering tendency of pores and Mg₁₇Al₁₂ (β) particles as well as average grain size are quantified and their effects on section thickness are obtained. The results confirm that the UTS, YS, ductility and FS are mainly influenced by the area fraction, size distribution and spatial arrangement of pores and phases.     

Twin-Induced Plasticity of an ECAP-Processed TWIP Steel

The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.     

Effect of microstructure on plastic deformation of Cu at low homologous temperatures

The mechanical properties of polycrystalline Cu (purity 99.95%) prepared by severe plastic deformation were studied at low homologous temperatures from 0.5 K to room temperature. Material with three different microstructures was prepared by annealing of ultrafine-grained Cu. At cryogenic temperatures (0.5 and 4.2 K) the material exhibited an inverse temperature dependence of the yield stress and unstable plastic deformation accompanied by serrations on the stress–strain curves. These low-temperature anomalies were accentuated with grain refinement. At cryogenic temperatures, enhanced ductility was observed and the Hall–Petch relation was found to hold. Microhardness and yield stress were much more temperature dependent in fine-grained than in coarse-grained material, and there is a correlation between the flow stress at a fixed strain and the microhardness. This study has demonstrated that, apart from enhanced discontinuous plastic flow, severe plastic deformation improves the strength of copper at cryogenic temperatures without sacrificing ductility.     

Superior fatigue crack growth resistance, irreversibility, and fatigue crack growth–microstructure relationship of nanocrystalline alloys

While previous studies have reported that nanocrystalline materials exhibit poor resistance to fatigue crack growth (FCG), the electro-deposited nanocrystalline Ni–Co alloys tested in this paper show superior resistance to FCG. The high damage tolerance of our alloy is attributed to the following: alloying with Co, low internal stresses resulting in stability of the microstructure, and a combination of high strength and ductility. The high density of grain boundaries interact with the dislocations emitted from the crack tip, which impedes FCG, as predicted by the present model and measured experimentally by digital image correlation. Further, the addition of Co increases the strength of the material by refining the grain size, reducing the fraction of low angle grain boundaries, and reducing the stacking fault energy of the material, thereby increasing the prevalence of twinning. The microstructure is stabilized by minimizing the internal stress during a stress relief heat treatment following the electro-deposition process. As a result grain growth does not occur during deformation, leaving dislocation-mediated plasticity as the primary deformation mechanism. The low internal stresses and nanoscale twins preserve the ductility of the material, thereby reaching a balance between strength and ductility, which results in a superior resistance to FCG.