Variability in the yield strength of a metallic glass at micron and submicron length scales

This work presents an in-depth investigation of the dispersion in the compressive yield strength of a Zr-based bulk metallic glass as the specimen dimensions decrease from the micron to the submicron length scale. While the investigated alloy belongs to one of the so-called “ductile” glass families, the degree of variation in strength exhibited in the submicron size range approaches that of conventional brittle ceramics. Using a three-parameter Weibull analysis, however, we find that the engineering reliability of the glass is independent of specimen size. The glass exhibits a size-independent failure-free stress of 1825 MPa, below which yielding is not expected. The three-parameter Weibull modulus is also size independent. The dispersion in yield strengths above the failure-free stress is due to the change in the sampling volume for specimens 1 μm in diameter or larger, while interactions between defects and the surface actually limit the breadth of the strength distribution observed in specimens 200–330 nm in diameter relative to what may be expected for that volume. Using an instability criterion for a penny-shaped crack under shear with interfacial friction, a relationship between yield strength and the corresponding defect size is established, and the defect concentration is estimated.     

On the Thermodynamic Aspect of Interaction Between Dislocations and Highly Mobile Interstitial Solute:With Special Focus on Recent Progress in Pd–H System:A Review

It is well known that, in kinetics, the interaction between dislocations and interstitial solute normally exerts strong solute drag effect on dislocations, leading to strong solution hardening of the metals. However, due to the low mobility of interstitial solute in many metals, thermodynamic aspect of the interaction between dislocations and interstitial solute is often unobservable and omitted. It will be shown in this article by reviewing the H-induced behaviors in metal–H systems, especially the recent progress in Pd–H system that, when the interstitial solute atoms are highly mobile and able to collect in the vicinity of mobile dislocations easily, the scenario will be remarkably different. The interaction between dislocations and these highly mobile interstitial solute atoms, in thermodynamics, will reduce the line energy of dislocations and will facilitate the generation of dislocations, leading to an increase in dislocation density and an enhanced strain hardening of metals upon plastic deformation.     

Deformation and fracture behaviors of Ti-based metallic glass under multiaxial stress state

The deformation and fracture behaviors of a Ti-based metallic glass (MG) under a multiaxial stress state were investigated using a small punch test. It was found that controlling both the initiation and propagation of a shear crack can significantly stabilize the plastic deformation of Ti-based MG by forming multiple shear bands and delaying the shear crack from reaching the critical crack length. Radial, circumferential and spiral shear band patterns and corresponding fracture modes were observed. The relationship between the shear band pattern and the stress state was established. This finding implies that a MG could be stabilized and become ductile in nature under suitable stress conditions.     

Interactions between mechanically generated defects and corrosion phenomena of Zr-based bulk metallic glasses

Using the example of a Zr-Ti-Al-Cu-Ni bulk metallic glass with an ex situ approach, two aspects were studied: (i) the effects of pre-formed corrosion pits on the room temperature deformation behaviour, and (ii) the effect of mechanically generated defects on the local corrosion susceptibility. For the latter, Vickers micro-indents generated with different loads were also employed as model defects. It is revealed that pre-formed corrosion pits act as favourable surface sites to which shear bands are linked and most likely nucleate, which leads to a slight reduction in the maximum compression stress. Further, mechanically generated defects have to be considered as critical weak points at a glassy sample surface which can act as preferential sites for local corrosion initiation and propagation. Local chemical and structural changes due to the formation of shear bands as a response to mechanical load are discussed as possible reasons for this. For potential high-load applications of bulk metallic glasses this interaction must be considered as an important lifetime limiting aspect.     

Quasi phase transition model of shear bands in metallic glasses

A quasi phase transition model of shear bands in metallic glasses (MGs) is presented from the thermodynamic viewpoint. Energy changes during shear banding in a sample–machine system are analyzed following fundamental energy theorems. Three characteristic parameters, i.e. the critical initiation energy ΔG_c, the shear band stability index k₀, and the critical shear band length l_c, are derived to elucidate the initiation and propagation of shear bands. The criteria for good plasticity in MGs with predominant thermodynamic arrest of shear bands are proposed as low ΔG_c, large k₀, and small l_c. The model, combined with experimental results, is used to analyze some controversial phenomena of deformation behavior in MGs, such as the size effect, the effect of testing machine stiffness and the relationship between elastic modulus and plasticity. This study has important implications for a fundamental understanding of shear banding as well as deformation mechanisms in MGs and provides a theoretical basis for improving the ductility of MGs.     

The metallurgical and deformation behaviours of laminar metal matrix composites after ballistic impact

In this study, two types of laminar metal matrix composite (MMC) samples were manufactured. Their failure mechanisms and deformation behaviours were investigated to evaluate ballistic performance. These laminated samples consist of sub-MMC layers and un-reinforced Al layers. The sub-MMC layers were MMC layers produced from particulate-Al₂O₃-reinforced AA 2024 aluminium alloys. These layers were manufactured using both squeeze casting and hot pressing techniques. They were laminated with un-reinforced AA 2024 aluminium alloy layers as A-type and B-type using the same techniques. The ballistic tests were carried out with a 7.62 mm × 51 mm armour piercing (AP) projectile, and the responses of the targets to ballistic impact was evaluated by examining the hole surface generated by the projectiles. A-type and B-type laminated structures showed different failure mechanisms. Deep craters occurred on both sides of the hole of the A-type target, while for B-type targets, petalling and bulging occurred at the entrance and exit of the hole, respectively.     

Interpreting size effects of bulk metallic glasses based on a size-independent critical energy density

Sample-size effects on compressive deformation behavior have been carefully studied for several bulk amorphous alloy systems. The yielding strength is almost unchanged for all sized samples but the compressive plasticity is increased with the decrease of the sample sizes. It was found that there existed a critical plane energy density above which the catastrophic fracture would occur. This critical energy density is dependent on the composition of the glassy samples but independent of the sample sizes. Large samples reach the critical plane energy density within much smaller strain, thus resulting in the observed size-dependence of the macroscopic plasticity.     

Composition dependence of the microstructure and mechanical behavior of Ti–Zr–Cu–Pd–Sn–Nb bulk metallic glass composites

Ti-based bulk metallic glass composite alloys Ti₅₆Zr₆Cu_19.8Pd_8.4Sn_1.8Nb₈, Ti₆₄Zr₄Cu_13.2Pd_5.6Sn_1.2Nb₁₂ and Ti₆₈Cu_13.2Pd_5.6Sn_1.2Nb₁₂ were designed to obtain the microstructure composing of β-Ti dendrites and glassy matrix. The compressive and three-point bending properties were investigated. It was found that the actual microstructure of the Nb-added alloys consisted of primarily precipitated β-Ti dendrites, network-like glassy matrix, and extra island-like Ti₂Cu intermetallic phase with different volume fractions. Under compressive loading, all the Nb-added alloys presented higher yield strength combined with remarkably increased plasticity. Under bending condition, however, the alloys Ti₅₆Zr₆Cu_19.8Pd_8.4Sn_1.8Nb₈ and Ti₆₄Zr₄Cu_13.2Pd_5.6Sn_1.2Nb₁₂ with higher Ti₂Cu volume fractions became completely brittle. The alloy Ti₆₈Cu_13.2Pd_5.6Sn_1.2Nb₁₂ could keep its plastic deformability due to the decreased Ti₂Cu volume fraction. Compressive deforming behavior of the Nb-added alloys was determined by the ductile β-Ti phase and glassy matrix, nevertheless, bending deforming behavior of the alloys was determined by the volume fraction and distribution of the brittle intermetallics.     

金属塑性接触变形和接触强度及其测定方法(一)

以布氏硬度实验、塑性接触变形实验为主要研究手段,以金属的小接触变形为假设前提。对金属材料接触变形过程中的4个阶段：弹性接触变形阶段、屈服变形阶段、塑性接触变形阶段、弹性恢复阶段作了深入细致的研究。为弹性接触极限强度、塑性接触强度、变形力与变形量之间的数学关系模型的建立奠定了理论基础。     

传热有限元在金属热塑性成形中的应用

以热锻造冲孔和无模拉伸为例阐述了传热有限元数值析在金属热塑性成形问题中的应用方法,对正确模拟预测工艺过程,提高产品质量有积极的意义。     

Cu-Nb-C-Nb多芯复合线材的制备及表征

采用粉末套管工艺,结合集束拉拔技术制备出了石墨烯包覆铌粉末增强Cu-Nb的多芯复合线材(3#)、石墨烯未包覆铌粉末增强Cu-Nb多芯复合线材(4#)、及纯铌粉增强Cu-Nb多芯复合线材(5#) 3种结构复合线材。通过优化热处理工艺发现,线材在750℃/60h热处理后,与线材的Nb (110)衍射峰强度相比加工态样品发生了明显的增强。微观结构及EDS能谱分析说明,高温热处理有利于Cu/Nb界面之间的轻微扩散,增加了界面的结合强度,线材的塑性和韧性得到了明显改善。通过对3种线材微观结构、力学性能及电学性能的分析表明,石墨烯包覆铌粉末的Cu-Nb-C-Nb线材导电性能优于其它2种线材。最后,分析了3种不同线材的塑性变形机制及引起性能变化的微观机理。提出了进一步优化工艺,为高强高导多元结构复合线材的制备开创了一种全新的方向。     

基于绿色制造的金属塑性精成形

金属塑性成形加工在产品制造过程中具有举足轻重的地位。但传统的加工方法存在以下不足：材料利用率很低;能源浪费严重;振动、噪声对操作工人以及周边环境产生严重危害和污染。本文综合分析了传统金属塑性成形加工在资源、能源及环境等方面存在的普遍症结,提出了节约资源、能源、环境保护,确保可持续发展是金属塑性成形理论与技术发展的重大课题。实现绿色塑性成形加工,应从技术、工艺和设备三方面入手,走金属塑性精成形之路。