The atomistic mechanisms occurring during the processes of aging and rejuvenation in glassy materials involve very small structural rearrangements that are extremely difficult to capture ...experimentally. Here we use in-situ X-ray diffraction to investigate the structural rearrangements during annealing from 77 K up to the crystallization temperature in Cu
Zr
Al
Hf
Co
bulk metallic glass rejuvenated by high pressure torsion performed at cryogenic temperatures and at room temperature. Using a measure of the configurational entropy calculated from the X-ray pair correlation function, the structural footprint of the deformation-induced rejuvenation in bulk metallic glass is revealed. With synchrotron radiation, temperature and time resolutions comparable to calorimetric experiments are possible. This opens hitherto unavailable experimental possibilities allowing to unambiguously correlate changes in atomic configuration and structure to calorimetrically observed signals and can attribute those to changes of the dynamic and vibrational relaxations (α-, β- and γ-transition) in glassy materials. The results suggest that the structural footprint of the β-transition is related to entropic relaxation with characteristics of a first-order transition. Dynamic mechanical analysis data shows that in the range of the β-transition, non-reversible structural rearrangements are preferentially activated. The low-temperature γ-transition is mostly triggering reversible deformations and shows a change of slope in the entropic footprint suggesting second-order characteristics.
Increased formability of aluminium alloys has been demonstrated via cryogenic deformation. In previous studies, the microstructures of samples deformed at low temperatures were analysed after ...reheating to room temperature (RT) and storage. However, after heating the dislocation structure and density of the deformed material do not reflect the cryogenic situation. In this work, we investigate the evolution of flow stress during recovery in Al-Mg and Al-Mg-Si alloys. We examine the RT recovery behaviour of samples pre-strained at 77 K to different strain levels, and evaluate the structural stability upon subsequent deformation. We also study microstructural evolution via in-situ synchrotron X-ray diffraction, starting from initial conditions at cryogenic temperatures to long-term RT-recovery. Recovery of cryogenically deformed samples at RT results in reduction of the flow stress, in dependence on RT storage. The recovery process can be divided into three distinct sections, each based on a different mechanism characterized by either the arranging or the annihilation of dislocations. Subsequent further straining at room temperature after cryogenic forming also generates plastic instabilities and premature fracture due to unfavourable hardening and recovery assisted softening interplay.
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•Room temperature recovery after cryogenic deformation can be divided into three different mechanistic phases•These can be distinguished by dislocation rearrangement, dislocation annihilation and a change in strength (stress drop)•Room temperature deformation directly after cryogenic deformation can result in immediate fracture
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•Five fully biocompatible Ti-based metallic glasses with different metalloid and soft metal content for a synergistic improvement in corrosion properties.•For Ti40Zr40 bearing alloys ...the GFA drops due to the low negative enthalpy of mixing between Ti and Zr atoms.•With increasing Ti and decreasing Zr content, the maximum of the broad diffuse XRD peak shifts to higher 2-Theta angles.•Ti60Zr20Si8Ge7B3Sn2 and Ti50Zr30Si8Ge7B3Sn2 have a very high pitting potential and wider passivation region compared with other Ti-based MG alloys.
We introduce five new biocompatible Ti-based metallic glass (MG) compositions with different metalloid and soft metal content for a synergistic improvement in corrosion properties. Without any potentially harmful elements such as Cu, Ni or Be, these novel alloys can eliminate the risk of inflammatory reaction when utilized for permanent medical implants. Excluding Cu, Ni or Be, which are essential for Ti-based bulk MG production, on the other hand, confines the glass-forming ability of novel alloys to a moderate level. In this study, toxic-element free MG alloys with significant metalloid (Si–Ge–B, 15–18 at.%) and minor soft element (Sn, 2–5 at.%) additions are produced in ribbon form using conventional single-roller melt spinning technique. Their glass-forming abilities and their structural and thermal properties are comparatively investigated using X-ray diffraction (XRD), synchrotron XRD and differential scanning calorimetry. Their corrosion resistance is ascertained in a biological solution to analyze their biocorrosion properties and compare them with other Ti-based bulk MGs along with energy dispersive X-ray. Ti60Zr20Si8Ge7B3Sn2 and Ti50Zr30Si8Ge7B3Sn2 MG ribbons present a higher pitting potential and passivation domain compared with other Ti-based MG alloys tested in similar conditions. Human mesenchymal stem cell metabolic activity and cytocompatibility tests confirm their outstanding cytocompatibility, outperforming Ti-Al6-V4.
A down‐sized high‐pressure torsion device is developed to be used in an INSTRON deformation machine available at the High Energy Materials Science beamline at PETRA III. This setup allows to obtain ...synchrotron diffraction patterns in situ during severe plastic straining. Two different materials are studied: in pure Ni, the dislocation density and coherently scattering domain size are analyzed; in NiTi shape memory alloy, amorphization and a reverse martensitic phase transformation are investigated. The in situ experiments facilitate the characterization of the microstructural evolution of these materials depending on uniaxial loading, hydrostatic pressure, and torsional strain.
In situ X‐ray diffraction during high‐pressure torsion deformation is implemented. In pure Ni, an extreme amount of lattice defects is quantified with static recovery effects upon pressure release. The NiTi alloy exhibits amorphization even under hydrostatic pressure only, which increases strongly with torsional straining. The occurrence of reverse martensite to austenite transformation induced by deformation could be observed.
The thermal stability of deformation-induced dislocations was investigated in polypropylene (PP) during annealing by means of in-situ X-ray diffraction using synchrotron radiation. The samples were ...cold rolled to high strains (ε = 1.2) in order to introduce a high number of dislocation lattice defects and immediately stored in liquid nitrogen afterward. Then, stepwise annealing was applied from −180 °C up to above the melting temperature (165 °C) while synchrotron X-ray diffraction patterns were recorded at each step. The resulting low noise, high angular resolution diffraction patterns were evaluated using multireflection X-ray profile analysis (MXPA), revealing parameters such as the dislocation density and the thickness of the crystalline lamellae as a function of the annealing temperature. Two significant decreases of the dislocation density were found at annealing temperatures of about 10 and 85 °C. These distinct changes in the dislocation density could be identified as the mechanisms of β- and α-relaxation, respectively, by performing additional dynamic mechanical thermal analysis (DMTA). This behavior could be attributed to an increased intrinsic mobility of the macromolecules at these temperatures accompanied by thermal activation of dislocations, resulting in their mutual annihilation or their movement into the adjacent amorphous phase. The reduction of the dislocation density at the glass transition (β-relaxation) occurs because the stabilizing effect of backstresses originating from the amorphous phase is lost. At the α-relaxation the reduction in the dislocation density is attributed to defect propagations within the crystalline lamellae as well as in the amorphous phase and the recrystallization of intralamellar mosaic blocks (i.e., grains).
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•New family of AgxAu85-xSi15 amorphous alloys for 20 < x < 60 is synthesized by co-sputtering.•Temperature dependence of electrical resistance indicated small amounts of ...nanocrystalline phase confirmed by HRTEM.•All alloys exhibit high hardness, 2% of elastic strain and 1–2% of tensile plasticity when tested on polymer substrates.•Complex chemistry-properties and structure-properties relationships of the alloy family are elucidated.
To expand the basis for property-oriented material design, complex relationships between chemistry, atomistic structure, and properties of sputter-deposited AgxAu85-xSi15 alloys are investigated using versatile experimental methods. According to differential scanning calorimetry and X-ray diffraction data, the alloys are amorphous within a wide compositional range of 20 ≤ x ≤ 60 at%. However, high-resolution transmission electron microscopy revealed noticeable amounts of crystallites in limiting compositions with x = 20 at% and x = 60 at%. Glass transition and crystallization temperatures grow with increasing Ag content, while room-temperature resistivity and temperature coefficient of resistance are shown to be highly sensitive to the amount of crystalline phase. Neither the existence of nanocrystallites nor the substitution of Ag by Au affected significantly the mechanical properties. All compositions exhibit similar nanoindentation hardness of about 4.3 GPa, Young’s modulus of about 55 GPa, and macroscopic elastic limit of about 2%. The unique combination of high hardness, high elastic strain but low Young’s modulus together with the expected chemical inertness inherited from noble metals makes this alloy family suitable for biomedical applications in form of coatings or in jewelry if an economic bulk production route will be developed.
The aim of this study is to investigate the effects of cryogenic and room temperature pre-deformation on subsequent artificial ageing of Al-Mg-Si alloys. Naturally aged and pre-aged samples were ...strained to 5%, 10% and 20% at RT (25 °C) and under liquid nitrogen, and artificially aged at 185 °C. Pre-deformation generally increases ageing kinetics for both the naturally aged and pre-aged alloys, which increase in proportion to the degree of pre-deformation, and which are slightly more pronounced for the cryogenic condition. The peak strength is constant, except for when a low degree of pre-deformation is used, in which case it is slightly reduced. Cryogenically deformed samples show an increased strength and hardness, compared to samples pre-deformed at RT, when subjected to an equal magnitude of strain. This difference is reduced during artificial ageing. Synchrotron measurements reveal that this behaviour can be linked to the greater dislocation density, which is not completely recovered even after prolonged ageing at 185 °C.
Abstract
Residual stress engineering is widely used in the design of new advanced lightweight materials. For metallic glasses, attention has been given to structural changes and rejuvenation ...processes. High-energy scanning X-ray diffraction strain mapping reveals large elastic fluctuations in notched metallic glasses after deformation under triaxial compression. Microindentation hardness mapping hints at a competing hardening–softening mechanism after compression and reveals the complementary effects of stress and structure modulation. Transmission electron microscopy proves that structure modulation and elastic heterogeneity distribution under room temperature deformation are related to shear band formation. Molecular dynamics simulations provide an atomistic understanding of the confined deformation mechanism in notched metallic glasses and the related fluctuations in the elastic and plastic strains. Thus, future focus should be given to stress modulation and elastic heterogeneity, which, together with structure modulation, may allow the design of metallic glasses with enhanced ductility and strain-hardening ability.
The effects of severe plastic deformation (SPD) by means of high-pressure torsion (HPT) on the structural properties of the two iron-based metallic glasses Fe
Cu
Nb
Si
B
and Fe
Co
Si
B
P
Cu
have been ...investigated and compared. While for Fe
Cu
Nb
Si
B
, HPT processing allows us to extend the known consolidation and deformation ranges, HPT processing of Fe
Co
Si
B
P
Cu
for the first time ever achieves consolidation and deformation with a minimum number of cracks. Using numerous analyses such as X-ray diffraction, dynamic mechanical analyses, and differential scanning calorimetry, as well as optical and transmission electron microscopy, clearly reveals that Fe
Co
Si
B
P
Cu
exhibits HPT-induced crystallization phenomena, while Fe
Cu
Nb
Si
B
does not crystallize even at the highest HPT-deformation degrees applied. The reasons for these findings are discussed in terms of differences in the deformation energies expended, and the number and composition of the individual crystalline phases formed. The results appear promising for obtaining improved magnetic properties of glassy alloys without additional thermal treatment.
The viscoelastic behavior of four different bulk metallic glass (BMG) systems,
i.e.
, Cu
46
Zr
46
Al
8
, Cu
44
Zr
44
Al
8
Co
4
, Cu
44
Zr
44
Al
8
Hf
4
, and Cu
44
Zr
44
Al
8
Co
2
Hf
2
, is ...investigated concerning its deformation-mode dependence via dynamic mechanical analysis (DMA) in 3-point bending (TPB), tension, and torsion modes. At temperatures below the glass transition, the considered BMGs deform primarily elastic, and the mechanical response is independent of the testing frequency, whereas, in the glass transition region, the viscoelastic component dominates. Crystallization decreases the viscoelastic contribution, whereas plastic deformation leads to an increase in atomic mobility for all three deformation modes. Compared to tension and torsion, TPB is found to be more sensitive to dynamic mechanical stress. TPB generates a complex stress state in the matrix and can thus introduce substantial variations in loss modulus. Structural analyses carried out by transmission electron microscopy and X-ray diffraction confirmed the amorphous nature of the base composition and structural changes when heated to the intermediate peak temperature observed at 743 K for the TPB mode. Compared to Cu
46
Zr
46
Al
8
, 4 at. pct Co addition in the Cu
46−x\2
Zr
46−x\2
Al
8
Co
x
amorphous alloy leads to a glass showing relatively higher thermomechanical stability around its glass transition. This study provides evidence for the enhancement of the mechanical properties of CuZr-based BMGs at elevated temperatures by microalloying.
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