Heterogeneous structural changes correlated to local atomic order in thermal rejuvenation process of Cu-Zr metallic glass.

In this study, we investigated the atomistic mechanism of structural excitation in a thermal process (thermal rejuvenation) of metallic glass. In a molecular dynamics framework, Cu-Zr metallic glass was rejuvenated by a thermal process composed of an isothermal heat treatment at a temperature above the glass transition temperature T g , followed by fast cooling. Atomistic analyses of the local rearrangement, potential energy, and geometrical structure revealed structural changes correlating to the local atomic order in the rejuvenation process. In the early stage of the heat treatment for thermal rejuvenation, the structural excitation exhibited spatial heterogeneity at the nanometer scale. More-excited regions (i.e., regions with large atomic non-affine and affine transformations) exhibited low-ordered structures and vice versa, implying that the local structural excitation is significantly correlated with the local atomic order. The structural excitation transitioned from partial to whole as the isothermal process proceeded above T g . Although rejuvenation decreased the ordered structure, the calculation results suggested the formation of newly ordered local structures and newly disordered local structures correlated to local structural excitations and atomic dynamics in the thermal process. These results indicate that the heterogeneous structure evolution of the rejuvenation process induces a redistribution of the local atomic order in the microstructure of metallic glasses.

Thermal rejuvenation in metallic glasses.

Structural rejuvenation in metallic glasses by a thermal process (i.e. through recovery annealing) was investigated experimentally and theoretically for various alloy compositions. An increase in the potential energy, a decrease in the density, and a change in the local structure as well as mechanical softening were observed after thermal rejuvenation. Two parameters, one related to the annealing temperature, Ta/Tg, and the other related to the cooling rate during the recovery annealing process, Vc/Vi, were proposed to evaluate the rejuvenation phenomena. A rejuvenation map was constructed using these two parameters. Since the thermal history of metallic glasses is reset above 1.2Tg, accompanied by a change in the local structure, it is essential that the condition of Ta/Tg ≥ 1.2 is satisfied during annealing. The glassy structure transforms into a more disordered state with the decomposition of icosahedral short-range order within this temperature range. Therefore, a new glassy structure (rejuvenation) depending on the subsequent quenching rate is generated. Partial rejuvenation also occurs in a Zr55Al10Ni5Cu30 bulk metallic glass when annealing is performed at a low temperature (Ta/Tg ~ 1.07) followed by rapid cooling. This behavior probably originates from disordering in the weakly bonded (loosely packed) region. This study provides a novel approach to improving the mechanical properties of metallic glasses by controlling their glassy structure.

Synthesis and mechanical properties of highly structure-controlled Zr-based metallic glasses by thermal rejuvenation technique.

A novel thermal rejuvenation treatment facility for Zr-based bulk metallic glass (BMG) was developed, consisting of a rapid heating and indirect liquid nitrogen quenching process. The re-introduction of free volume into thermally rejuvenated BMG results in more disordered state. The rejuvenation improves ductility, implying that the re-introduced free volume aids in the recovery of the shear transformation zone (STZ) site and volume. Actually, it is confirmed that relaxation significantly reduces STZ volume; however, it is recovered by thermal rejuvenation. Molecular dynamics simulations also indicate that rejuvenation enhances homogeneous deformation. The current findings indicate that the thermal rejuvenation method is extremely effective for recovering or improving the ductility of metallic glass that has been lost due to relaxation.

Breakdown of One-to-One Correspondence in Energy and Volume in a High-Pressure Heat-Treated Zr-Based Metallic Glass During Annealing.

The glassy state of a high-pressure heat-treated Zr50Cu40Al10 metallic glass was investigated from energetic and volumetric perspectives. The specific heat (Cp) data of initial heating and subsequent cooling from the supercooled liquid region indicates that the sample behaviour cannot be explained simply by the derivative of the well-known enthalpy (H)-temperature (T) curve. Unlike the Cp data, the thermal-expansion coefficient (α) value increased monotonically during the first heating step, which suggests the collapse of a one-to-one correspondence between the energy and volume during the process. The α data of the cooling process follow almost the same path as those of the as-cast process, whereas the corresponding Cp curves do not. This result implies that the volume appears to be more sensitive to obeying external heat compared with energy, which highlights the different time scale for achieving an equilibrium state in energy and volume. The second heating data of the Cp and α exhibit an inverse relationship against the corresponding first heating set, which confirms the breakdown of a one-to-one correspondence during annealing. The newly constructed energy-density diagram shows that the treated sample is rejuvenated volumetrically but is relaxed energetically during aging, which has never been observed experimentally previously.

Various Rejuvenation Behaviors of Zr-Based Metallic Glass by Cryogenic Cycling Treatment with Different Casting Temperatures.

The rejuvenation behavior of an Zr50Cu40Al10 (at.%) metallic glass upon cryogenic cycling treatment has been investigated. At a high casting temperature, the microstructure of the glass is quite homogenous and thus, internal stress cannot be generated during cycling. Therefore, the glass cannot be rejuvenated by cryogenic cycling treatment. In the contrary, by lowering the casting temperature, nano-sized heterogeneity can be induced and subsequently generates the internal stress and rejuvenates the glass. Once the glass is rejuvenated, the more induced free volume can plasticize the glass with a higher plastic strain. These findings point out that the synthesis conditions can tailor the heterogeneity of the glass and subsequently affect the following rejuvenation behavior upon thermal treatment. It can also help understand the mechanisms of rejuvenation of metallic glass upon cryogenic cycling treatment.

Controlled rejuvenation of amorphous metals with thermal processing.

Rejuvenation is the configurational excitation of amorphous materials and is one of the more promising approaches for improving the deformability of amorphous metals that usually exhibit macroscopic brittle fracture modes. Here, we propose a method to control the level of rejuvenation through systematic thermal processing and clarify the crucial feasibility conditions by means of molecular dynamics simulations of annealing and quenching. We also experimentally demonstrate rejuvenation level control in Zr(55)Al(10)Ni(5)Cu(30) bulk metallic glass. Our local heat-treatment recipe (rising temperature above 1.1T(g), followed by a temperature quench rate exceeding the previous) opens avenue to modifying the glass properties after it has been cast and processed into near component shape, where a higher local cooling rate may be afforded by for example transient laser heating, adding spatial control and great flexibility to the processing.

Change in local environment upon quasicrystallization of Zr-Cu glassy alloys by addition of Pd and Pt.

The effects of Pd and Pt, which are known quasicrystal (QC)-forming elements, on the local atomic structure in Zr(70)Cu(30) glassy alloys are investigated. A QC phase precipitates from a glassy phase above a certain temperature by a cooperative-like motion of icosahedral clusters. Quasicrystallization is accompanied by a significant change in the local environment around the Zr atoms and a slight change around the noble metal. However, the local environment around the Cu atoms remains almost the same during QC formation. It is suggested that two types of icosahedral polyhedra exist in the glassy state: one has a relatively perfect icosahedral structure formed around the Zr atoms. The other is in a distorted state around the Cu atoms. We speculate that the medium-range order (i.e. QC nucleus) has a Zr-centered icosahedral cluster as its core, and the QC grows via aggregation of possible clusters in the initial stage. Pd or Pt atoms stabilize and/or connect individual Zr-centered icosahedral clusters, facilitating the formation of the nucleus and growth of the QC phase.

Evaluation of the local environment for nanoscale quasicrystal formation in Zr(80)Pt(20) glassy alloy using Voronoi analysis.

The local atomic structure of nano-quasicrystal-forming Zr(80)Pt(20) binary glassy alloy was investigated by reverse Monte Carlo modeling based on the results of x-ray diffraction. A prepeak at Q∼17 nm(-1) originating from the unique bonding between the Pt-Pt pair is observed in the structure factor. Voronoi analysis indicates that an icosahedral-like polyhedron is formed around Pt. It is also found that icosahedral-like polyhedra exist around Zr; however, the fraction of perfect icosahedra is considerably lower than that in the nano-quasicrystalforming Zr(70)Pd(30) glassy alloy. A difference in the local environment between the two binary quasicrystal-forming glassy alloys is suggested.