A group of ductile metallic glasses prepared by modifying local structure of icosahedral quasicrystals

Inspired by research into the association between icosahedral local orders and the plasticity of metallic glasses(MGs),beryllium(Be) is added to the icosahedral quasi-crystal forming alloy Zr40Ti40Ni20. In this way, bulk metallic glasses(BMGs) with favorable compressive plasticity are fabricated. Therein, the icosahedral quasi-crystalline phase is the main competing phase of amorphous phases and icosahedral local orders are the main local atomic motifs in amorphous phases.The alloys of(Zr40Ti40Ni20)76Be24and (Zr40Ti40Ni20)72Be28with their greater plastic strain capacity show similar characteristics to highly plastic amorphous systems: The serrated flow of compression curves always follows a near-exponential distribution. The primary and secondary shear bands intersect each other, bifurcate, and bend. Typical vein patterns are densely distributed on the fracture surfaces. The relaxation enthalpy of four MGs is linearly correlated with the plastic strain, that is, the greater the relaxation enthalpy, the larger the plastic strain.

基于熵和剪切带开发新型非晶合金

非晶合金是非晶材料家族中的新成员,由于缺乏原子排列长程有序特征,具有独特的性能和广泛应用前景.然而,非晶合金的形成、成分设计和形变问题在该领域长期未得到解决,制约着新型非晶合金的高效开发.传统上,多组元非晶合金的成分设计主要遵循相图研究范式,以温度调控为出发点,确定低液相线的成分为落脚点,尤其强调深共晶成分.然而,对于缺乏相图的多组元体系则力所不及,亟需新的理论突破.本文综述了非晶转变中的热力学与动力学基本规律,重点探讨了热力学与动力学性质之间的关联性,尤其是梳理了材料熔化熵与熔点粘度、熔点等多个非晶形成经典参量之间的内在关联,提出并论证了熔化熵和振动熵为非晶形成的关键热力学参量,可有效指导高性能非晶合金的设计和表征.结合实际应用中对非晶材料的工程要求,讨论了化学成分调控对非晶合金剪切带的萌生密度和扩展稳定性的影响,并总结了在单调加载和疲劳加载条件下剪切带行为对非晶合金断裂过程的作用.最后展望了振动熵和构型熵之间争议问题,提出未来工作中需要发展具有物理意义玻璃形成能力指标和可靠数据库的需求,混合加载模式下的韧性以及疲劳和韧性之间的权衡等问题.本文为进一步建立高性能非晶材料成分设计路径提供了参考与指导.

Rejuvenated metallic glass strips produced via twin-roll casting

The energy state and atomic level structure of metallic glasses(MGs)are very sensitive to their cooling rates,and a lower cooling rate generally causes a lower energy and more relaxed state of MGs.In this work,the Zr41.2 Ti13.8 Cu12.5 Ni10 Be22.5(Vit.1)ribbons with a thickness of 40μm and 110μm and the strips with a thickness of 320μm and 490μm were produced by single-roll melt spinning and twin-roll casting,respectively.The increase in thickness of either ribbons or strips results in a lower energy state with a smaller relaxation enthalpy,a lower content of free volume,and a higher hardness.Although the cooling rate of the twin-roll produced 320μm-thick strip is almost one magnitude lower than that of the single-roll produced 110μm-thick ribbon,the former,however,possesses a rejuvenated energy state as compared to the latter.Molecular dynamics simulations reveal that the squeezing force during twin-roll casting affects the evolution of connection types of clusters,and the 2-atom and 4-atom connections are prone to be retained,which results in a higher energy state of MGs.Such a rejuvenation process during twin-roll casting can overwhelm the relaxation process caused by the lower cooling rate.Therefore,twin-roll casting is not only a method being capable for producing strips with a large thickness,but also prone to obtain a high energy state of the MG strip.

Enhancing strength and plasticity by pre-introduced indent-notches in Zr36Cu64 metallic glass:A molecular dynamics simulation study

The deformation behavior in Zr36 Cu64 metallic glasses with pre-introduced indent-notches has been studied by molecular dynamics simulation at the atomic scale.The indent-notches can trigger the formation of densely-packed clusters composed of solid-like atoms in the indent-notch affected zone.These denselypacked clusters are highly resistant to the nucleation of shear bands.Hence,there is more tendency for the shear bands to nucleate outside the indent-notch affected zone,which enlarges the deformation region and enhances both the strengthening effect and the plastic deformation ability.For indent-notched MGs,when determining the initial yielding level,there is a competition process occurring between the densely-packed clusters leading to the shear band formation outside the indent-notch affected zone and the stress-concentration localizing deformation around the notch roots.When the indent-notch depth is small,the stress-concentration around the notch root plays a dominant role,leading to the shear bands initiating from the notch root,reminiscence of the cut-notches.As the indent-notch depth increases,there are many densely-packed clusters with high resistance to deformation in the indent-notch affected zone,leading to the shear band formation from the interface between the indent-notch affected zone and the matrix.Current research findings provide a feasible means for improving the strength and the plasticity of metallic glasses at room temperature.