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Unveiling the mechanism of deformation-induced supersaturation

A Cu-6at%Ag cast alloy was deformed by means of high-pressure torsion to different applied strain levels until a steady-state regime is reached. The continuous structural refinement is attended by the successive dissolution of the Ag precipitates in the Cu matrix. The results show that the Ag regions need to fall below a phase size of ~ 5 nm to fully dissolve. Atomistic calculations indicate that the final dissolution can be explained based on the enthalpy difference between the solid solution and layered systems which are in between the coherent and semi-coherent structure. These findings are supported by detailed microstructural investigations.

Anomalous Cu phase observed at HIP bonded Fe-Cu interface

Hot isostatic pressing (HIP) processes are widely used for removing inner defects, densifying sintered components, consolidating particles and powders, and interfacial diffusion bonding. However, microscopic views of the phenomena have not been fully understood. X-ray absorption fine structure (XAFS) experiments were performed to study the interfacial region of the HIP bonded Fe-Cu sample. XAFS analyses clearly show that the bond distance around Cu is extraordinarily short compared with the bulk fcc Cu. The Cu species in the Fe-Cu HIP bonded sample takes a bcc structure even in the Cu-rich phase at room temperature. This anomalous bcc phase of Cu may derive from the HIP diffusion bonding process, which is performed below the melting points of both the elements. Cu atoms can diffuse into Fe with the bcc structure and settle in the bcc sites.

Micro-mechanisms of failure in nano-structured maraging steels characterised throughin situmechanical tests

High-pressure-torsion (HPT) processing introduces a large density of dislocations that form sub-grain boundaries within the refined nano-scale structure, leading to changes in precipitate morphology compared to hot-rolled maraging steels. The impact of such nanostructuring on the deformation and fracture micro-mechanisms is being reported for the first time usingin situcharacterization techniques along with transmission electron microscopy and atom probe tomography analysis, in this study. Digital image correlation has been used to quantify the full field strain maps in regions of severe strain localization as well as to determine the fracture toughness through critical crack tip opening displacements. It is seen that the phenomenon of planar slip leads to strain softening under uniaxial deformation and to crack branching under a triaxial stress state in hot rolled maraging steels. On the other hand, nano-structuring after HPT processing creates a large number of high angle grain boundaries as dislocation barriers, leading to strain hardening under uniaxial tension and nearly straight crack path with catastrophic fracture under triaxial stress state. Upon overaging, the hot-rolled sample shows signature of transformation induced plasticity under uniaxial tension, which is absent in the HPT processed overaged samples, owing to the finer reverted austenite grains containing higher Ni concentration in the latter. In the overaged fracture test samples of both the hot-rolled and HPT conditions, crack tips show a signature of strain induced transformation of the reverted austenite to martensite, due to the accompanying severe strain gradients. This leads to a higher fracture toughness even while achieving high strengths in the overaged conditions of the nanocrystalline HPT overaged samples. The results presented here will aid in design of suitable heat treatment or microstructure engineering of interface dominated nano-scale maraging steels with improved damage tolerance.

Analysis of the Formation Mechanism of Surface Cracks of Continuous Casting Slabs Caused by Mold Wear

Surface cracks are easily produced after friction between continuous casting billets and copper layers in mold cavity, but the formation mechanism is not clear. Based on a steel-based hot-dip copper plating experiment, this study simulated the action behavior of copper adhering to the surface of a continuous casting billet after mold wear and systematically analyzed the formation mechanism of cracks caused by copper infiltration on the surface of the continuous casting billet. It is shown that when the copper liquid adheres to the surface of the slab, in addition to the diffusion of Cu in the steel, Fe is also dissolved in the copper liquid, accelerating the solidification of the copper liquid on the surface of the slab and forming a stable fusion combination between copper and steel. At the same time, due to the enrichment of the Fe-C phase and a large number of vacancies at the grain boundary, the grain boundary becomes the dominant area of copper–steel fusion bonding. For a continuous casting process in which the temperature is kept higher than 900 ℃, Cu’s solubility is high and the diffusion coefficient is very low in Fe, which makes it very difficult for Cu accumulated in the grain boundary to diffuse into the steel matrix during the continuous casting process, resulting in a grain boundary with a greatly weakened strength becoming the origin of cracks in the bending and straightening deformation of the billet.

Structural, Mechanical, and Tribological Characterization of Magnetic Pulse Compacted Fe–Cu Bimetallic Particles Produced by Electric Explosion of Dissimilar Metal Wires

Bimetallic 73 wt.% Fe–Cu nanoparticles have been produced using electric explosion of two immiscible metal wires and then consolidated into disks using magnetic pulse compaction. The compacted disks have been characterized for phase composition, mechanical strength, and high-temperature steel ball-on-disk sliding friction. The sample possessed good flexural and compression strength. Friction and wear reduction were observed during sliding test at 400 °C, which was explained by intense tribosynthesis of cuprospinel CuFe2O4 nanoparticles, which served to reduce adhesion between the ball and disk.

Synthesis of a bulk nanostructured metastable Al alloy with extreme supersaturation of Mg

Nanostructuring of bulk metals is now well documented with the development of severe plastic deformation (SPD) for improving the physical and mechanical properties of engineering materials. Processing by high-pressure torsion (HPT), which was developed initially as a grain refinement technique, was extended recently to the mechanical bonding of dissimilar metals during nanostrcturing which generally involves significant microstructural heterogeneity. Here we introduce, for the first time, a bulk metastable Al-Mg supersaturated solid solution by the diffusion bonding of separate Al and Mg metal solids at room temperature using HPT. Exceptional hardness was achieved homogeneously throughout the metastable alloy with a record maximum supersaturated Mg content of ~38.5 at.% in the Al matrix having a grain size of ~35–40 nm. Our results demonstrate the synthesis of a bulk nanocrystalline metastable alloy with good microstructural stability at room temperature where such bulk solids are not yet reported for mechanical alloying by powder metallurgy.

High Temperature Flow Behavior of Ultra-Strong Nanoporous Au assessed by Spherical Nanoindentation

Nanoporous metals have attracted attention in various research fields in the past years since their unique microstructures make them favorable for catalytic, sensory or microelectronic applications. Moreover, the refinement of the ligaments down to the nanoscale leads to an exceptionally high strength. To guarantee a smooth implementation of nanoporous metals into modern devices their thermo-mechanical behavior must be properly understood. Within this study the mechanical flow properties of nanoporous Au were investigated at elevated temperatures up to 300 °C. In contrast to the conventional synthesis by dealloying of AuAg precursors, the present foam was fabricated via severe plastic deformation of an AuFe nanocomposite and subsequent selective etching of iron, resulting in Au ligaments consisting of nanocrystalline grains, while remaining Fe impurities excessively stabilize the microstructure. A recently developed spherical nanoindentation protocol was used to extract the stress-strain curves of nanoporous Au. A tremendous increase of yield strength due to ligament and grain refinement was observed, which is largely maintained at high temperatures. Reviewing literature will evidence that the combined nanocrystalline and nanoporous structure leads to remarkable mechanical properties. Furthermore, comparison to a previous Berkovich nanoindentation study outlines the conformity of different indentation techniques.

In situ atomic-scale observation of oxidation and decomposition processes in nanocrystalline alloys

Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu-Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu-Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu-Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions.

Mixing instabilities during shearing of metals

Severe plastic deformation of solids is relevant to many materials processing techniques as well as tribological events such as wear. It results in microstructural refinement, redistribution of phases, and ultimately even mixing. However, mostly due to inability to experimentally capture the dynamics of deformation, the underlying physical mechanisms remain elusive. Here, we introduce a strategy that reveals details of morphological evolution upon shearing up to ultrahigh strains. Our experiments on metallic multilayers find that mechanically stronger layers either fold in a quasi-regular manner and subsequently evolve into periodic vortices, or delaminate into finer layers before mixing takes place. Numerical simulations performed by treating the phases as nonlinear viscous fluids reproduce the experimental findings and reveal the origin for emergence of a wealth of morphologies in deforming solids. They show that the same instability that causes kilometer-thick rock layers to fold on geological timescales is acting here at micrometer level.

The mechanisms behind deformation of multiphase solids are elusive. Here, the authors use X-rays and simulations to show that the same mechanisms causing rocks to fold occur at the micrometer scale in dual-metal lamellas of Ag/Cu and Al/Cu under high-pressure torsion, leading to vortices formation.

Nanoscale spheroidized cementite induced ultrahigh strength-ductility combination in innovatively processed ultrafine-grained low alloy medium-carbon steel

We describe here innovative processing of low alloy medium-carbon steel with a duplex microstructure composed of nanoscale spheroidized cementite (Fe₃C) in an ultrafine-grained (UFG) ferritic steel. After multi-pass rolling and intermittent annealing at 550 °C for 300 s, the obtained UFG-1 steel showed an average ferrite grain size of ~430 nm, containing nanoscale spheroidized cementite (Fe₃C) particles with an average size of ~70 nm. On annealing at 600 °C for 300 s, the average size of ferritic grains was increased to ~680 nm and the average size of spheroidized Fe₃C particles increased to ~90 nm, referred as UFG-2 steel. Tensile tests indicated that UFG-1 steel had high yield strength (σ_y) of 1260 MPa, and ultimate tensile strength (σ_UTS) of 1400 MPa. These values are higher than that of UFG-2 steel (σ_y = 1080 MPa and σ_UTS = 1200 MPa), suggesting that the strengthening contribution is a cumulative effect of decrease in ferrite grain size and nanoscale cementite. The incoherent interfaces between nanosized particles and the matrix acted as a strong barrier to dislocation motion. The study underscores that nanosized precipitates not only provide strength but also contribute to ductility, which is very encouraging for improving the ductility of medium-carbon steels.

On nanostructured molybdenum-copper composites produced by high-pressure torsion

Nanostructured molybdenum-copper composites have been produced through severe plastic deformation of liquid-metal infiltrated Cu30Mo70 and Cu50Mo50 (wt%) starting materials. Processing was carried out using high-pressure torsion at room temperature with no subsequent sintering treatment, producing a porosity-free, ultrafine-grained composite. Extensive deformation of the Cu50Mo50 composite via two-step high-pressure torsion produced equiaxed nanoscale grains of Mo and Cu with a grain size of 10-15 nm. Identical treatment of Cu30Mo70 produced a ultrafine, lamellar structure, comprised of Cu and Mo layers with thicknesses of ∼ 5 and ∼ 10 - 20 nm , respectively, and an interlamellar spacing of 9 nm. This microstructure differs substantially from that of HPT-deformed Cu-Cr and Cu-W composites, in which the lamellar microstructure breaks down at high strains. The ultrafine-grained structure and absence of porosity resulted in composites with Vickers hardness values of 600 for Cu30Mo70 and 475 for Cu50Mo50. The ability to produce Cu30Mo70 nanocomposites with a combination of high-strength, and a fine, oriented microstructure should be of interest for thermoelectric applications.

Synthesis and Mechanical Characterisation of an Ultra-Fine Grained Ti-Mg Composite

The importance of lightweight materials such as titanium and magnesium in various technical applications, for example aerospace, medical implants and lightweight construction is well appreciated. The present study is an attempt to combine and improve the mechanical properties of these two materials by forming an ultra-fine grained composite. The material, with a composition of 75 vol% (88.4 wt%) Ti and 25 vol% (11.4 wt%) Mg , was synthesized by powder compression and subsequently deformed by high-pressure torsion. Using focused ion beam machining, miniaturised compression samples were prepared and tested in-situ in a scanning electron microscope to gain insights into local deformation behaviour and mechanical properties of the nanocomposite. Results show outstanding yield strength of around 1250 MPa, which is roughly 200 to 500 MPa higher than literature reports of similar materials. The failure mode of the samples is accounted for by cracking along the phase boundaries.

Structural evolution and strain induced mixing in Cu-Co composites studied by transmission electron microscopy and atom probe tomography

A Cu-Co composite material is chosen as a model system to study structural evolution and phase formations during severe plastic deformation. The evolving microstructures as a function of the applied strain were characterized at the micro-, nano-, and atomic scale-levels by combining scanning electron microscopy and transmission electron microscopy including energy-filtered transmission electron microscopy and electron energy-loss spectroscopy. The amount of intermixing between the two phases at different strains was examined at the atomic scale using atom probe tomography as complimentary method. It is shown that Co particles are dissolved in the Cu matrix during severe plastic deformation to a remarkable extent and their size, number, and volume fraction were quantitatively determined during the deformation process. From the results, it can be concluded that supersaturated solid solutions up to 26 at.% Co in a fcc Cu-26 at.% Co alloy are obtained during deformation. However, the distribution of Co was found to be inhomogeneous even at the highest degree of investigated strain.

On the remarkable thermal stability of nanocrystalline cobalt via alloying

Nanostructured Co materials are produced by severe plastic deformation via alloying with small amounts of C and larger amounts of Cu. The thermal stability of the different nanostructured Co materials is studied through isothermal annealing at different temperatures for various times and compared to the stability of severe plastically deformed high-purity nanocrystalline Co. The microstructural changes taking place during annealing are evaluated by scanning electron microscopy, transmission electron microscopy and microhardness measurements. In the present work it is shown that the least stable nanostructured material is the single-phase high purity Co. Alloying with C improves the thermal stability to a certain extent. A remarkable thermal stability is achieved by alloying Co with Cu resulting in stabilized nanostructures even after annealing for long times at high temperatures. The essential reason for the enhanced thermal stability is to be found in the immiscibility of both components of the alloy.

A deformation mechanism of hard metal surrounded by soft metal during roll forming

It is interesting to imagine what would happen when a mixture of soft-boiled eggs and stones is deformed together. A foil made of pure Ti is stronger than that made of Cu. When a composite Cu/Ti foil deforms, the harder Ti will penetrate into the softer Cu in the convex shapes according to previously reported results. In this paper, we describe the fabrication of multilayer Cu/Ti foils by the roll bonding technique and report our observations. The experimental results lead us to propose a new deformation mechanism for a hard metal surrounded by a soft metal during rolling of a laminated foil, particularly when the thickness of hard metal foil (Ti, 25 μm) is much less than that of the soft metal foil (Cu, 300 μm). Transmission Electron Microscope (TEM) imaging results show that the hard metal penetrates into the soft metal in the form of concave protrusions. Finite element simulations of the rolling process of a Cu/Ti/Cu composite foil are described. Finally, we focus on an analysis of the deformation mechanism of Ti foils and its effects on grain refinement, and propose a grain refinement mechanism from the inside to the outside of the laminates during rolling.

Engineering interface structures and thermal stabilities via SPD processing in bulk nanostructured metals

Nanostructured metals achieve extraordinary strength but suffer from low thermal stability, both a consequence of a high fraction of interfaces. Overcoming this tradeoff relies on making the interfaces themselves thermally stable. Here we show that the atomic structures of bi-metal interfaces in macroscale nanomaterials suitable for engineering structures can be significantly altered via changing the severe plastic deformation (SPD) processing pathway. Two types of interfaces are formed, both exhibiting a regular atomic structure and providing for excellent thermal stability, up to more than half the melting temperature of one of the constituents. Most importantly, the thermal stability of one is found to be significantly better than the other, indicating the exciting potential to control and optimize macroscale robustness via atomic-scale bimetal interface tuning. Taken together, these results demonstrate an innovative way to engineer pristine bimetal interfaces for a new class of simultaneously strong and thermally stable materials.