内生非晶复合材料组织与力学性能调控研究进展

非晶合金因其独特的短程有序、长程无序原子结构特征, 使其具有了一系列优异的力学、物理、化学等性能, 在先进金属结构材料领域具有巨大的潜在应用价值。但非晶合金在室温承载变形时, 原子团簇发生剪切转变形成的大量自由体积会演化为高度局域化剪切带, 局域化剪切带由于缺乏介质的阻碍会发生失稳扩展, 导致非晶合金极易发生室温脆断, 特别单轴拉伸时基本无塑性。为克服这个缺憾, 研究者们提出将微米级尺寸的晶体相引入非晶来抑制剪切带的失稳扩展, 使得内生第二相增韧非晶复合材料具有了明显的拉伸塑性能力, 因此倍受材料学界的关注。近年来, 研究者们陆续通过成分设计、制备技术、热处理工艺等方法来实现非晶复合材料的塑性变形能力的提升, 使得非晶复合材料有望走向实际的工程应用。本文围绕内生第二相增韧非晶复合材料的微观组织调控这一关键科学问题, 从影响非晶复合材料微观组织结构的因素(合金成分设计、制备工艺参数、微观结构构筑等)到微观组织对其室温力学性能的影响机制两方面的研究成果进行了系统总结, 重点阐述了近10年来内生第二相增韧非晶复合材料领域组织调控及其室温力学性能关联性方面的研究进展, 并且对内生非晶复合材料研究领域目前的存在的问题和挑战进行了展望, 以期为高强高韧内生第二相增韧非晶复合材料的设计与制备提供理论参考。     

非晶合金的相变韧塑化

块体非晶合金因其独特的原子结构而具有许多优异的力学性能,成为近年来材料领域的研究热点之一,但是由于其在变形过程中的室温脆性和应变软化等关键问题,一直制约其大规模工程应用。为解决此问题,块体非晶合金领域的研究者们提出了多种方案,其中利用"相变诱导塑性"概念制备块体非晶合金复合材料来韧塑化非晶合金成为卓有成效的方案之一,通过此方法成功地制备出同时具有拉伸塑性和加工硬化能力的非晶合金复合材料。然而,该类块体非晶合金复合材料要求的形成条件更严格,同时具有更复杂的多相协调变形过程和更独特的性能优化方案。从该类块体非晶合金复合材料的形成、性能特点、韧塑化机理及性能优化等方面进行综述,并对其未来发展进行了简要展望。     

非晶合金中锯齿流变动力学的研究

非晶合金在外力场的作用下会出现间接性的锯齿流,锯齿具有空间和时间的无序分布性,能够反映塑性变形过程中剪切带的演化过程。借助于混沌理论、自组织临界理论、统计分析、分形和平均场理论等数学方法进行了锯齿动力学研究。发现非晶合金的塑性流变行为与材料的本征结构、试样尺寸、加载试验机的刚度、温度和应变速率等密切相关,揭示了非晶合金的塑性变形过程中剪切带滑移不稳定性的演化特点。试样尺寸小、低温或高应变速率下加载的韧性非晶试样的塑性流变动力学呈现类自组织临界状态,锯齿的幅值分布具有无标度性特点,剪切带之间的交互作用强,剪切带过程相对稳定。低温下大的分形维数说明剪切带分叉速率快,触发了剪切带之间的交互作用。简单的平均场理论证实了非晶合金的塑性可受应变速率调控。这些结论为进一步探索非晶合金的塑性提供了新的思路。     

Zr-Al-Cu-Ni-Ag非晶复合材料的显微硬度和剪切带形貌

使用差氏扫描量热仪、维氏显微硬度计和扫描电子显微镜研究了过冷液相区内不同温度等温退火处理对Zr-Al-CuNi-Ag块体非晶合金的显微硬度和剪切带形核、扩展的影响。结果表明,等温退火处理后非晶合金的初晶相为二十面体准晶相(I-相),其体积分数随着退火温度的升高呈现上升的趋势;复合材料的显微硬度随着晶化体积分数的升高呈现增大的趋势;界面压痕下方剪切带密度随着晶化体积分数的升高则呈现下降的趋势。这是等温退火导致非晶合金发生结构弛豫和第二相析出综合作用的结果。     

铝锂合金的外强化、外韧化效应及机理

一、前言 近十多年,国际上在改善铝锂合金塑性、断裂韧性、疲劳性能等方面取得了长足的进展,同时发现铝锂合金的强化韧化特性不同于普通铝合金。例如:它的未再结晶结构的强塑性配合、断裂韧性、疲劳性能等均明显优于再结晶结构;一定条件下,峰值时效的断裂韧性最高;其低温强度、塑性、断裂韧性、疲劳性能等均优于室温;虽其疲劳裂纹萌生寿命短和短裂纹扩展抗力低,但仍有较高水平的疲劳极限或疲劳寿命。用经典的强化韧化理论不能解释这些现象。根据铝锂合金强化韧化研究的最新进展,结合作者的研究结果,本文论述了该合金的外强化、外韧化机理。 二、拉伸性能与薄带强化、分层强化 早在70年代就发现,在铝锂合金中加入少量锆,固溶时形成Al_3Zr相而阻碍晶界迁移,最终热处理后仍保留未再结晶结构,因之可明显改善合金的强塑性配合。目前,几乎所有工业铝锂合金都含有锆,并在未再结晶状态应用。80年代初至今,许多人赞同这样一个观点:未再结晶晶粒间的僮向差小,位错易穿越之进入相邻晶     

非晶合金中剪切温升的研究进展

区别于传统的晶态金属材料,非晶合金(BMGs)不具备长程有序结构,其塑性变形载体为剪切带。剪切带一旦形成,便很快发展成为裂纹,引发材料的灾难性断裂。剪切不稳定性的研究有助于非晶合金塑性变形机理的理解,并可为非晶合金塑性变形能力的提高提供设计思路。近年来,基于非晶合金的结构特点,科研工作者努力探究非晶合金的剪切不稳定性,主要提出了结构软化诱导的剪切不稳定性和热软化引发的剪切不稳定性两种机制。本文重点总结了非晶合金中剪切温升的研究进展,介绍了测试应变速率、外部约束、试验机刚度和测试温度对剪切温升的影响,指明非晶合金中剪切引入的热远低于玻璃转变温度,暗示热软化对剪切不稳定性的影响是微弱的。本文最后对非晶合金中剪切不稳定性机制的研究方向进行了展望。     

不同微观结构Zr_(55)Al_(10)Ni_5Cu_(30)块体非晶合金的晶化过程

用差示扫描量热仪分别对具有相似晶体体积分数和晶化激活能的Zr55Al10Ni5Cu30块体非晶合金铸态、轧制态试样进行等温和连续升温实验,研究了不同微观结构块体非晶合金的晶化过程。结果表明,在晶化初期(小于30 min),两个试样具有相似的晶化速率;晶化后期(大于30 min),轧制态试样表现出较快的晶化速率。这在一定程度上表明,用JMA公式和晶化开始温度Tx及峰值温度Tp计算出的晶化激活能不能全面反映非晶合金的热稳定性。另外,剪切带中原子之间相互联接的减弱以及短程有序的强化,使轧制态试样热稳定性降低和晶化过程变快。     

块体非晶合金压缩变形研究进展

在约化压缩变形温度t r=T/T g为77 K/T g≤t rt rc1、t rc1≤t rt rc2和t rc2≤t rT x/T g时,一般块体非晶合金的压缩变形行为分别对应不均匀脆性断裂、非牛顿流变和牛顿流变;当应变速率约为10-4s-1时,临界约化压缩变形温度t rc1在0.839~0.920之间,t rc2=1.011;随着应变速率的降低,块体非晶合金由不均匀的脆性断裂转变为非牛顿流变的t rc1、由非牛顿流变转变为牛顿流变的t rc2均降低。低t r压缩变形试样表面的剪切带特征明显,但高t r压缩变形由于均匀变形和热影响作用使剪切带不明显,且块体非晶合金的塑性随剪切带数量的增加而提高;随着t r的升高,块体非晶合金的断口显微组织演化过程分别为复杂不均匀的脉状纹络、较均匀的脉状纹络、熔滴状和岩浆流状组织。另外在极低约化压缩温度t r下Ni60Pd20P17B3和Ti40Zr25Ni3Cu12Be20块体非晶合金表现出非牛顿流变行为。非晶在一定温度的压缩变形会导致部分或所有非晶纳米晶化。     

温度对核电用GH690合金力学行为的影响

以国产蒸汽发生器传热管用GH690合金为研究对象,通过评价其断裂韧性及拉伸特性,结合光学显微镜、扫描电镜和透射电镜分析,研究了合金由室温-623K的力学性能.研究结果表明,室温下GH690合金低的层错能,易生成形变孪晶,使得合金在孪生的协调下塑性变形能力提高,同时孪晶促进裂纹扩展转向,使合金在断裂过程中吸收更多的能量,维持合金高的断裂韧性.随着温度的升高,合金的层错能增加,导致形变孪晶生成困难,合金应力集中程度加剧,裂纹从而平直扩展,合金的断裂韧性降低.由于合金的室温层错能较低,合金在拉伸时能够通过孪生协调变形,同时生成的孪晶阻碍了位错的滑移而提高了合金的强度和塑性.随着形变温度的升高,合金通过孪生协调变形的能力降低,导至合金的变形机制由孪生转变为滑移,滑移产生的加工硬化效应小于孪生,故合金的强度和延伸率随之降低.     

Fe43Cr16Mo16C18B5Y2大块非晶合金的晶化行为及力学性能研究

采用铜模吸铸法制备出Fe43Cr16Mo16C18B5Y2块体非晶合金,并用XRD、SEM、DSC、硬度和压痕实验分别研究了该合金的结构、压缩断口形貌、晶化特征、硬度和断裂韧度.由热分析曲线得到玻璃转变温度(Tg)、晶化起始温度(Tx)和晶化峰值温度(Tp),这些特征温度具有明显的动力学效应.用Kissinger方法计算出不同升温速率下该Fe基块体非晶合金的玻璃转变激活能Eg、晶化激活能Ex、激活能Ep,结果表明该合金具有较高的热稳定性.力学实验结果表明,该块体非晶合金的硬度高达1178kg/mm2,断裂韧度为7.614MPa·m1/2,呈典型的脆性断裂,通过压缩断口形貌的观察发现该块体非晶合金的断裂呈现剪切断裂模式.     

Effects of crystalline particles on mechanical properties of strip-cast Zr-base bulk amorphous alloy

Effects of crystalline phase particles formed in a strip-cast Zr-base bulk amorphous alloy on strength, ductility, and fracture toughness were investigated by directly observing microfracture processes using an in situ loading stage installed inside a scanning electron microscope chamber. The compressive and fracture toughness test results indicated that strength, ductility, and fracture toughness of the strip-cast amorphous alloy were higher than those of the as-cast monolithic amorphous alloy, although the strip-cast alloy contained a considerable amount (4.5 vol.%) of hard, brittle crystalline particles. According to the in situ microfracture observation, crystalline particles were easily cracked under low stress levels, acted as blocking sites of shear band or crack propagation, and provided initiation sites of multiple shear bands. Thus, the improvement of mechanical properties in the strip-cast alloy could be explained by mechanisms of (1) blocking of crack propagation, (2) formation of multiple shear bands, and (3) crack deflection by crystalline particles.     

Revealing the relationships between alloy structure,composition and plastic deformation in a ternary alloy system by a combinatorial approach

A high-throughput approach based on magnetron co-sputtering of alloy libraries is employed to inves-tigate mechanical properties of crystalline and amorphous alloys in a ternary palladium(Pd)-tungsten(W)-silicon(Si)system with the aim to reveal the difference in plastic deformation response and extract the relevant structure-property relationships of the alloys in the system.It was found that in contrast to crystalline alloys,the amorphous ones,i.e.,metallic glasses,exhibited a much smaller fluctuation range in the plasticity parameters(Er2/H and Wp/Wt),indicating a significant difference in the plastic deformation mechanism controlling the mechanical properties for the respective alloys.We propose that the inho-mogeneous deformation of amorphous alloys localized in thin shear bands is responsible for the weaker compositional dependence of both plasticity parameters,while dislocation gliding in crystalline materials is significantly more dependent on the exact structure,thus resulting in a larger scattering range.Based on the representative efficient cluster packing model,a set of composition-dependent atomic structural models is proposed to figure out the structure-property relationships of amorphous alloys in Pd-W-Si alloy system.     

Oxygen addition for improving the strength and plasticity of TiZr-based amorphous alloy composites

The addition of a small amount of oxygen improves the mechanical properties,especially plasticity,of Ti45.7Zr33Ni3Cu5.8Be12.5 amorphous alloy composites (AACs) at room temperature (298 K).Compared to the plasticity of AACs without added O (5％),the plasticity of the composites with 0.73 at.％ O (nominal composition) was much higher (11％).Even at O content higher than 0.73 at.％,the AACs exhibited good plasticity.The highest plasticity of ～12.3 ％ was observed with 2.87 at.％ O.Two distinct mechanisms are proposed to explain the enhanced plasticity of the AACs.At low O content,although deformation-induced phase transformation was suppressed,a substantial amount of α'martensite was formed.The microstructural features of α'martensite,such as thinner laths and homogeneous distribution,induced the formation of multiple shear bands in the amorphous matrix.At high O content,deformation-induced phase transformation was seriously suppressed.A dispersed nano ω phase was formed during rapid solidification in AACs with O content higher than 1.45 at.％.This resulted in a weakening in the anisotropy of β dendrites and led to their homogenous deformation.Furthermore,multiple shear bands were formed in the amorphous matrix.Apart from plasticity,the strength of the AACs also increased with an increase in the O content.This phenomenon was explained in terms of three mechanisms,viz.the solid-solution-strengthening effect of O,fine-grain strengthening of β dendrites,and secondary phase strengthening by the nano ω phase.     

Vickers indentation tests in a Zr62.55Cu17.55Ni9.9Al10 bulk amorphous alloy

Vickers indentation tests were conducted on a Zr_62.55Cu_17.55Ni_9.9Al₁₀ bulk amorphous alloy to investigate the evolution of shear bands and its plastic deformation dimension via a bonded interface technique. Under all indentation loads, the plastic deformation is accommodated through semi-circular and radial shear bands. The plastic deformation dimension increases with increasing the indentation loads. A simplified λ = C(P)^0.5 model was put forward to predict and estimate the plastic deformation dimension characterized by shear bands in the subsurface. For the Zr_62.55Cu_17.55Ni_9.9Al₁₀ amorphous alloy, C is about 15.314 µm/N^0.5. The normalized shear band zone is independent to the indentation load.     

Tensile ductility and necking of metallic glass

Metallic glasses have a very high strength, hardness and elastic limit. However, they rarely show tensile ductility at room temperature and are considered quasi-brittle materials. Although these amorphous metals are capable of shear flow, severe plastic instability sets in at the onset of plastic deformation, which seems to be exclusively localized in extremely narrow shear bands approximately 10 nm in thickness. Using in situ tensile tests in a transmission electron microscope, we demonstrate radically different deformation behaviour for monolithic metallic-glass samples with dimensions of the order of 100 nm. Large tensile ductility in the range of 23-45% was observed, including significant uniform elongation and extensive necking or stable growth of the shear offset. This large plasticity in small-volume metallic-glass samples did not result from the branching/deflection of shear bands or nanocrystallization. These observations suggest that metallic glasses can plastically deform in a manner similar to their crystalline counterparts, via homogeneous and inhomogeneous flow without catastrophic failure. The sample-size effect discovered has implications for the application of metallic glasses in thin films and micro-devices, as well as for understanding the fundamental mechanical response of amorphous metals.     

Mechanical Properties, Damage and Fracture Mechanisms of Bulk Metallic Glass Materials

The deformation, damage, fracture, plasticity and melting phenomenon induced by shear fracture were investigated and summarized for Zr-, Cu-, Ti- and Mg-based bulk metallic glasses （BMGs） and their composites. The shear fracture angles of these BMG materials often display obvious differences under compression and tension, and follow either the Mohr-Coulomb criterion or the unified tensile fracture criterion. The compressive plasticity of the composites is always higher than the tensile plasticity, leading to a significant inconsistency. The enhanced plasticity of BMG composites containing ductile dendrites compared to monolithic glasses strongly depends on the details of the microstructure of the composites. A deformation and damage mechanism of pseudo-plasticity, related to local cracking, is proposed to explain the inconsistency of plastic deformation under tension and compression. Besides, significant melting on the shear fracture surfaces was observed. It is suggested that melting is a common phenomenon in these materials with high strength and high elastic energy, as it is typical for BMGs and their composites failing under shear fracture. The melting mechanism can be explained by a combined effect of a significant temperature rise in the shear bands and the instantaneous release of the large amount of elastic energy stored in the material.     

Effects of particulate reinforcement and strain‐rates on deformation and fracture behavior of Aluminum 6061‐T6 under high velocity impact

Aluminum matrix composites (AMC) exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of AMC is the driving force for the need to adequately understand their deformation and failure mechanisms under various types of loading conditions. In this study, plastic deformation of alumina particle reinforced Aluminum 6061‐T6 matrix composite is investigated and compared to that of an un‐reinforced Aluminum 6061‐T6 alloy at high strain‐rates under compressive loading. Dynamic stress‐strain curves are obtained using direct impact Split Hopkinson Pressure Bar (SHPB). Particulate reinforcement increases the deformation resistance of the aluminum alloy at high strain‐rates. Strain localization along narrow adiabatic shear bands is observed in both the reinforced and un‐reinforced alloy. Whereas the microstructure of shear bands in un‐reinforced alloy showed finer grain size compared to that of the bulk material, the shear bands observed in the AMCs are darker than the bulk material and the reinforcing particles are observed to be more closely spaced along the shear bands.     

W丝增强块状非晶基复合材料单轴压缩形变的研究

利用扫描电子显微镜和透射电子显微镜，研究了室温单轴压缩下W丝增强块状非晶基复合材料的形变特征，结果表明：非晶基体内产生了大量的剪切带，剪切带分布特征与W丝密切相关；非晶基体的微观结构发生了改变，局域内自由体积显著增加。     

Mechanical properties for bulk metallic glass with crystallites precipitation and second ductile phase addition

Monolithic phase bulk metallic glasses (BMGs) produced by a copper mold casting method and BMG composites containing in-situ brittle crystallites and out-situ tungsten fiber produced by a water quenching method were obtained. Mechanical properties including cyclic deformation and fracture toughness were investigated. Under symmetrically cyclic stress control, the life of tungsten fiber reinforced amorphous alloy is much longer than that of the monolithic amorphous alloy. The composite containing tungsten fibers that retard the crack propagation exhibits cyclic softening while the partially crystallized amorphous alloy exhibits stable cycling. The regions of crack initiation, stable propagation and final fracture were observed on the fracture surface. Crystalline brittle phases do not retard the crack propagation but become sites of crack initiation. Tungsten fiber reinforced BMG has the largest fracture toughness while BMG with quenched-in crystallites the smallest. Tungsten fibers stabilize crack growth in the matrix and extend the strain to failure of the composite, while brittle crystallites speed up the crack propagation even though they act as obstacles when shear bands reach them in some cases.