Thermal rejuvenation of an aged Au-based metallic glass by fast scanning calorimetry

A metallic glass (MG) annealed above its glass-transition temperature Tg, and cooled, may show an enthalpy increase ΔH, and other property changes. The extent of this thermal rejuvenation depends on the state of the MG (represented by effective cooling rate Φi) and the post-anneal cooling rate Φc. Previous studies examined effects of (Φc/Φi) up to 102. With a Au-based MG aged for up to 10 years at room temperature, and using fast calorimetry to anneal and then cool at up to 5000 K s−1, we extend (Φc/Φi) to 107. The rejuvenation is limited by anneal temperature or by Φc, when, for all MGs, ΔH/Tg shows a universal approximate scaling with log(Φc/Φi). We detect decoupling of vitrification from α relaxation, and highlight limitations in the use of fictive temperature to characterize glassy states. Rejuvenation of the Au-based MG decreases its elastic modulus and hardness, extending trends reported for other MGs

Reciprocating sliding wear behavior of high-strength nanocrystalline Al84Ni7Gd6Co3 alloys

Nanocrystalline Al-Ni-Gd-Co alloys with exceptionally high hardness have been recently developed from amorphous precursors. In the present work, the reciprocating sliding wear in the gross slip regime of these novel nanocrystalline Al-based alloys has been investigated under small amplitude oscillatory sliding motion using a martensitic chrome steel as the counter material. When compared to pure Al or Al-12Si alloy, these nanocrystalline alloys exhibit excellent wear resistance and a lower coefficient of friction when sliding against steel. The enhanced wear resistance of the novel nanocrystaline Al alloys is related to their ultra-high hardness and the hybrid nanostructure that mainly consists of nanometric intermetallic phases embedded in a nanocrystalline fcc-Al matrix. Three body abrasive conditions were created at the initial stages of the wear tests due to the formation of micro- and nano-particulate debris from the worn surface of the Al alloys; the debris was compacted under the subsequent sliding cycles forming layers that are protective to the extensive wear of the Al alloys

Advanced ultra-light multifunctional metallic-glass wave springs

We show that, using thermo -elastic processing, metallic -glass foils can be shaped, without being embrittled, into linear and annular wave springs. These springs exhibit an undulatory behaviour, unique to metallic -glass foils, in which under compression the number of arcs in the spring increases, increasing the load -bearing capacity and the spring constant. We evaluate the performance limits of the metallic -glass wave springs, and consider how the undulatory behaviour can be exploited. The metallic -glass springs can operate over the same load -ranges as commercially available crystalline wave springs, but have material volumes (and therefore weights) that are one to two orders of magnitude less. Their energy storage per unit material volume is as high as 2600 kJ m – 3 . We suggest that the undulatory behaviour is important in rendering the springs fail -safe in case of overload. We discuss the range of applicability of thermo -elastic processing, the likely working limit of metallic -glass wave springs, and the potential for application of metallic -glass springs in MEMS devices

Investigation on the mechanically-induced nanocrystallization in metallic glasses

Shear-induced nanocrystallization in bent ribbons of Pd40Cu30Ni10P20 metallic glass has been quantitatively investigated via synchrotron radiation. The formed nanocrystals volume fraction during deformation has been directly estimated from X-ray diffraction spectra using peaks area integration. The nanocrystallization process during deformation was found to be strongly linked with the microstructure configuration of shear bands in amorphous alloys. A constitutive model based on free volume approach has been introduced to describe the kinetic of mechanically induced nanocrystallization. The solution of the coupled constitutive equations of the model, fitted to experimental data, permits to determine the physical and mechanical parameters governing the phenomena of shear-induced crystallization in metallic glasses

シンサイ ニツイテ タイワスル コドモ ノ テツガク ノ カノウセイ

International audienc

An atomistic study of the structural changes in a Zr–Cu–Ni–Al glass-forming liquid on vitrification monitored in-situ by X-ray diffraction and molecular dynamics simulation

Structural changes in the Zr55Cu30Ni5Al10 liquid alloy on cooling from above the equilibrium liquidus temperature are studied by synchrotron radiation X-ray diffraction and compared with the results of first-principles molecular dynamics simulation. In-situ vitrification of the studied alloy is achieved using a containerless levitation technique. Subsequent analysis of the atomic and electronic structure of the alloy in liquid and glassy states reveals formation of medium-range order on cooling and its relationship with liquid fragility. The structural changes in this alloy are smaller in comparison with a more fragile one

Crystallization of Ti33Cu67 metallic glass under high-current density electrical pulses

We have studied the phase and structure evolution of the Ti33Cu67 amorphous alloy subjected to electrical pulses of high current density. By varying the pulse parameters, different stages of crystallization could be observed in the samples. Partial polymorphic nanocrystallization resulting in the formation of 5- to 8-nm crystallites of the TiCu2 intermetallic in the residual amorphous matrix occurred when the maximum current density reached 9.7·108 A m-2 and the pulse duration was 140 μs, though the calculated temperature increase due to Joule heating was not enough to reach the crystallization temperature of the alloy. Samples subjected to higher current densities and higher values of the evolved Joule heat per unit mass fully crystallized and contained the Ti2Cu3 and TiCu3 phases. A common feature of the crystallized ribbons was their non-uniform microstructure with regions that experienced local melting and rapid solidification

A novel operando approach to analyze the structural evolution of metallic materials during friction with application of synchrotron radiation

In this study, we describe an experimental setup and a new approach for operando investigation of structural evolution of materials during wear and friction. The setup is particularly suited for testing various friction pairs, including those in which both rubbing bodies are made of metals. The developed device allows circumventing the problems related to significant scattering of X-rays produced by metals and makes it possible using “real samples” in synchrotron beamlines operating in reflection mode. To demonstrate the capabilities of the device and the proposed new approach, an iron-based massive sample was subjected to thousands of friction cycles using a cemented carbide pin. The material was probed with synchrotron X-ray radiation within a few milliseconds after leaving the friction zone. The results of the microstructural and structural analysis, as well as results obtained from diverse mathematical models, allowed us to evaluate several features, including gradual accumulation of defects, microstructural refinement, dislocation density changes, surface layer oxidation, as well as several other phenomena caused by the dry sliding friction process. Mainly, it was possible to conclude that the process of wear occurred due to the cooperative action of oxidation and plastic deformation, which began during the first cycle of frictional interaction and was manifested in increasing the dislocation density, whose type was changed gradually during testing. The number of defects quickly reached a threshold value and subsequently fluctuated around it due to periodically repeated processes of defect accumulation and stress relaxation resulting from material wear. It was also observed that friction led to the quick formation of a mechanically mixed layer, consisting of the sample material and a mixture of two types of iron oxide – hematite and magnetite. The delamination of this layer was probably the primary wear mechanism