High-strength Nanocrystalline Research Articles

Structure and corrosion properties of multilayer metal/nitride/oxide ceramic coatings formed on austenitic-martensitic steel by magnetron deposition

The design of protective coatings preventing the degradation and failure of austenitic steels in humid climate is a relevant issue. In this work, a conventional magnetron sputtering method was utilized in order to fabricate a four-layer Ni/Al-Si-N/Ni/Al-Si-O coating onto austenitic-martensitic steel. At the first stage of deposition, a three-layer coating was formed in an Ar+N2 atmosphere using a mosaic target (Ni or Al-Si), while the second stage included the deposition of the outer ceramic Al-Si-O layer in an Ar+O2 gas mixture. The studies of the structure, hardness, adhesion strength and electrochemical behavior (in 3.5 wt.% NaCl) of the «coating/steel» system were performed by means of X-ray Diffraction, electron microscopy, mechanical (indentation and scratch) tests and electrochemical impedance spectroscopy. The ceramic Al-Si-O layer exhibits an amorphous structure. The adjacent nickel layers have submicrocrystalline columnar grains. The ceramic Al-Si-N layer possesses a nanocrystalline columnar structure based on AlN + Si3N4 phases. Hardness behavior of the multilayer coating varies nonmonotonically due to a contribution of the soft amorphous phase in the upper surface layer and high-strength nanocrystalline phases in the underlying sublayers. It has been revealed that as-deposited multilayer coating decreases a corrosion rate of the substrate. The layered structure of the coating hinders the transport and diffusion of chloride ions to the substrate and decreases the corrosion rate of the substrate. However, a transport of chloride ions into the inner layers of the coating may occur through the microstructural defects preserved in the amorphous Al-Si-O layer. The scheme describing an initiation of corrosion damage and formation of corrosion products under critical corrosion conditions was proposed.

Pulsed electrodeposition of homogenous and heterogeneous solid solution layered structure in high strength nanocrystalline Co[sbnd]Cu alloys

A supersaturated solid solution nanostructure of an immiscible CoCu system is among the promising nanocrystalline (NC) alloys offering enhanced thermal and mechanical stability. In this paper, experiments were conducted to study the phase transformation and tailoring microstructure from a homogeneous/single-phase to a heterogeneous/dual-phase solid solution layered CoCu nanostructure via pulsed electrochemical deposition (PED). The increasing concentration of sodium dodecyl sulfate (SDS) in electrolyte played a crucial role in stabilizing a homogeneous solid solution CoCu nanostructure. On the other hand, the opposite approach led to the formation of heterogeneous solid solution CoCu alloys in the form of multilayered nanostructure consisting of the primary and secondary solid solution CoCu differing in 10–15 at.% Cu. The separation of primary and secondary phase is more prominent with the extended pulsed periods. The mechanical properties of selected single- and dual-phase alloys were investigated through micro-pillar compression, revealing that a combination of a high yield strength and an elevated strain hardening can be achieved in NC materials through a combined solid solution strengthening and hardening through nano-twin (NT).

Intermetallic particle heterogeneity controls shear localization in high-strength nanostructured Al alloys

The mechanical behavior of two nanocrystalline Al alloys, Al-Mg-Y and Al-Fe-Y, is investigated with in-situ micropillar compression testing. Both alloys were strengthened by a hierarchical microstructure including grain boundary segregation, nanometer-thick amorphous complexions, carbide nanorod precipitates with sizes of a few nanometers, and submicron-scale intermetallic particles. The maximum yield strength of the Al-Mg-Y system was measured to be 950 MPa, exceeding that of the Al-Fe-Y system (680 MPa), primarily due to a combination of more carbide nanorods and more amorphous complexions. Both alloys exhibited yield strengths much higher than those of commercial Al alloys, and therefore have great potential for structural applications. However, some micropillar specimens were observed to plastically soften through shear banding. Post-mortem investigation revealed that intermetallic-free deformation pathways of a few micrometers in length were responsible for this failure. Further characterization showed significant grain growth within the shear band. The coarsened grains maintained the same orientation with each other, pointing to grain boundary mechanisms for plastic flow, specifically grain rotation and/or grain boundary migration. The presence of intermetallic particles made it difficult for both matrix and intermetallic grains to rotate into the same orientation due to the different lattice parameters and slip systems. Therefore, we are able to conclude that a uniform distribution of intermetallic particles with an average spacing less than the percolation length of shear localization can effectively prevent the maturation of shear bands, offering a design strategy for high-strength nanocrystalline Al alloys with both high strength and stable plastic flow.

Crush-origin pseudotachylyte in granitic mylonites of continental exhumed Ailaoshan-Red River shear zone southeastern Asia

Tectonic pseudotachylytes have provided valuable insights into the evolution of seismic activity during crustal deformation. However, the origin and formation processes of pseudotachylytes are still debated. Thin pseudotachylyte veins were first recognized within the granitic mylonites of the continental exhumed Ailaoshan-Red River strike-slip shear zone, southeastern Asia, which record the deformation characteristics from ductile (mylonite) to brittle deformation (ultracataclasite). Mylonitization resulted in the recrystallization of quartz aggregates/ribbons, fine-grained plagioclase, and the concentration of biotite into biotite-rich foliation planes. The pseudotachylyte veins have various angles oblique to the host rock foliation, which record superimposition of the latter brittle deformation onto former ductile fabrics. Observations of the microstructure and quartz clast size distribution (obey power-law distribution) suggest that crushing was the dominant process in the formation of pseudotachylyte in the samples. Plagioclase within the pseudotachylyte vein presents a complex texture of high-strength nanocrystalline to partly amorphous material. Detailed microstructure observation revealed that amorphous materials form by a solid-state amorphization process and is different from friction melts. The changes in composition between plagioclase clasts and amorphous materials (enriched in Ca, but depleted in Na) provide evidence of mechanochemical effect that accompanies fault motion and comminution process. When a sufficient amount of amorphous material is formed, it has great significance in fault weakening through the coseismic slip of the strain partition.

Achieving high-strength nanocrystalline WE43 Mg alloy by a combination of cold rotary swaging and aging treatment

In this work, a bulk nanocrystalline WE43 alloy prepared through rotary swaging was subjected to the aging treatment at different temperatures. The mechanical property and microstructure variations during aging were systematically studied. Aging at 175 °C enhanced the alloy to be maximumly with 516 MPa in ultimate tensile strength (UTS), 477 MPa in yield strength (YS) and 8.5% in elongation to failure. The significant strengthening effect resulted from rare-earth elements segregation at grain boundary. During aging at 225 °C, tremendous enhancement was obtained in the initial stage of aging, while the formation of massive precipitates along grain boundary deteriorated the strength as aging time was prolonged.

High-strength nanocrystalline intermetallics with room temperature deformability enabled by nanometer thick grain boundaries.

Although intermetallics are attractive for their high strength, many of them are often brittle at room temperature, thereby severely limiting their potential as structural materials. Here, we report on a previously unidentified deformable nanocrystalline CoAl intermetallics with Co-rich thick grain boundaries (GBs). In situ micropillar compression studies show that nanocrystalline CoAl with thick GBs exhibits ultrahigh yield strength, exceeding 4.5 gigapascals. Unexpectedly, nanocrystalline CoAl intermetallics also show prominent work hardening to a flow stress of 5.7 gigapascals up to 20% compressive strain. Transmission electron microscopy studies show that deformation induces abundant dislocations inside CoAl grains with thick GBs, which accommodate plastic deformation. Molecular dynamics simulations reveal that the Co-rich thick GBs play a vital role in promoting nucleation of dislocations at the Co/CoAl interfaces, thereby enhancing the plasticity of the intermetallics. This study provides a perspective to promoting the plasticity of intermetallics via the introduction of thick GBs.

Mechanism of cementite decomposition in 100Cr6 bearing steels during high pressure torsion

Severe plastic deformation leads to cementite decomposition in pearlitic and martensitic alloys, resulting in high-strength nanocrystalline ferrite. This effect can be employed to strengthen pearlitic wires but it can also be associated with material failure by white etching cracks (WECs) that are primarily known to concern bearings or rails. We investigate the decomposition of spheroidal cementite in the martensitic bearing steel 100Cr6 with 62 Rockwell hardness during high pressure torsion at 9.5 GPa applied pressure. The hard martensitic matrix and the even harder spheroidal cementite precipitates behave plastically very differently. The enforced macroscopic plastic deformation is almost entirely carried by the matrix. Plastic material flow of the matrix around the spheroidal cementite leads to wear of the spheroidal cementite as indicated by continuously increasing levels of chromium in the matrix. Plastic deformation of spheroidal cementite via dislocation gliding supposedly accelerates this process as slip steps generated thereby are preferential sites of wear at the matrix/cementite interface. Larger spheroidal cementite precipitates are more prone to plastic deformation and to decomposition than smaller ones. The mechanism likely holds true in general for multiphase materials with large strain difference between phases subjected to high pressure torsion. Although the formation of WECs in 100Cr6 bearings might be slowed down by reducing the size of the spheroidal cementite precipitates, it is unlikely that this could entirely prevent this failure mechanism.

Fabrication and characterization of nanostructured immiscible Cu–Ta alloys processed by high-pressure torsion

Nanostructured Cu–Ta alloys show great potential as high strength nanocrystalline materials due to their excellent mechanical properties and limited grain growth at high temperatures. This report describes the fabrication of nanostructured immiscible Cu–Ta alloys in bulk by high-pressure torsion (HPT) using a stack of Cu/Ta/Cu discs at room temperature. A microstructural study after HPT processing showed that the internal Ta layer breaks into small individual flakes which distribute uniformly over the Cu matrix through increases in the numbers of HPT turns. There is solid-state diffusion between the Cu and Ta when the HPT processing increases to 100 turns due to microstructural refinement and increasing crystalline defects. After processing through 150 turns, a composite microstructure of two phases is formed including supersaturated Cu–Ta solid solutions (Cu 81 Ta 19 and Ta 78 Cu 22 alloys) with a crystallite size of ∼35–45 nm. This fine microstructure produces exceptional mechanical properties including a high hardness of over 350 Hv corresponding to ∼3.43 GPa, a tensile strength of ∼1300 MPa and a tensile elongation of about 40%. • Immiscible Cu–Ta alloys successfully fabricated from Cu/Ta/Cu stacked discs by HPT. • The microstructures contain Cu-rich and Ta-rich layers, and Ta-rich clusters. • The HPT-processed Cu–Ta alloy has excellent tensile strength and elongation.

High Strength Nanocrystalline Ordered Cu3Pd Alloy Produced by Severe Cold-Drawing and Annealing

The paper explores the structure, mechanical properties, electrical resistance and ordering of the Cu3Pd alloy after severe, cold plastic deformation by multipass drawing and annealing. The alloy structure after severe deformation and subsequent annealing is studied by scanning and transmission electron microscopy. The proposed modes of the mechanical-thermal treatment allow producing the ordered Cu3Pd alloy wire with high strength and plastic properties combined with low electrical and high corrosion resistances.

Структурно-фазовые превращения и свойства атомно-упорядочивающегося сплава Cu3Pd, подвергнутого мегапластической деформации и отжигу

Структурно-фазовые превращения и свойства атомно-упорядочивающегося сплава Cu3Pd, подвергнутого мегапластической деформации и отжигу

Regulating the mechanical properties of nanocrystalline nickel via molybdenum segregation: an atomistic study

The effects of segregation of impurity molybdenum (Mo) atoms on the tensile mechanical properties of nanocrystalline nickel (Ni) are investigated with molecular dynamics simulation. The results show that the segregation of Mo atoms induces an obvious increase in the elastic modulus and strength of nanocrystalline Ni, and the strengthening effect is more significant with smaller grain size. When the grain size decreases below a critical value, at which the softening occurs in non-segregated Ni-Mo alloy, no evident softening phenomenon is observed in Mo-segregated systems. Furthermore, based on a bicrystal configuration, it is found that Mo atoms segregating to the grain boundary reduce the energy and mobility of the grain boundary, increasing the grain boundary stability and thus accommodating the strengthening. The present findings will shed light on the fabrication of high strength nanocrystalline materials by controlling the segregation of atoms.

Tribological Properties and Structure of Aluminum—Lithium Alloys

This paper presents the results of a study of structural transformations and the main tribological properties (coefficient of friction and wear rate) of Al–2.2 Li and Аl–3.1 Сu–2.0 Li–0.1 Zr (wt %) alloys. Optical metallography and transmission electron microscopy have been used to examine the alloy structure after heating and friction. Tribological tests of the alloys in pair with steel are carried out by sliding friction using a pin-on-plate scheme during reciprocating motion of a sample. The sliding velocity is 0.07 m/s and the load is 294 N. Friction is carried out in air and in a nitrogen gas at room temperature. Alloying of the Al–Li alloy with copper (3.1 wt %) and zirconium (0.1 wt %) is shown to significantly increase the wear resistance, enhance the frictional hardening, and reduce the friction coefficient of the alloys. The positive effect of this alloying on the tribological properties of the Al–Li alloy is caused by friction-induced severe strain hardening. A high-strength nanocrystalline structure consisting of a mixture of matrix crystals and a metastable δ' (Al3Li) phase is formed in a surface layer to 10 μm thick. Planar dislocation slip is observed in both alloys. This sliding mechanism is assumed to be related to the low (f ~ 0.25) coefficient of friction of both alloys. Artificial aging carried out under different four regimes increases the hardness of alloys, but significantly reduces their resistance to wear. This paper offers an explanation for the effects obtained.

Microstructure and mechanical properties of magnesium silicide prepared via spark plasma assisted combustion synthesis

The current study reports the swift synthesis of high strength nanocrystalline magnesium silicide (Mg2Si) doped with bismuth (Bi) of 0–0.025 at.% via spark plasma assisted combustion synthesis. The proposed synthesis approach is rapid (∼54 min) against the conventional liquid and solid state synthesis routes which warrants prolong duration. XRD and TEM studies reveal the in-situ evolution of Mg2Si phase and substantial doping of Bi. The synthesized Mg2Si compound exhibits higher density (>95). Mg2Si0.975-Bi0.025 demonstrates the benchmarked Vickers hardness of 484.2 ± 15.44 Hv1 and un-doped Mg2Si shows the excellent fracture toughness of 2.09 ± 0.03 MPa√m.

The influence of nanocrystalline CoNiFeAl0.4Ti0.6Cr0.5 high-entropy alloy particles addition on microstructure and mechanical properties of SiCp/7075Al composites

The influence of nanocrystalline CoNiFeAl0.4Ti0.6Cr0.5 high-entropy alloy (HEA) particles addition on the microstructure and mechanical properties of SiCp/7075Al composites was investigated systematically. The CoNiFeAl0.4Ti0.6Cr0.5 high-entropy alloy was prepared by mechanical alloying, and the (5 vol% HEAp + 40 vol% SiCp)/7075Al (HEA-7075Al) and 45 vol% SiCp/7075Al (SiC-7075Al) composites were subsequently fabricated by squeeze casting. The HEA particles were well retained in the Al matrix and remained nanocrystalline following the squeeze casting and heat treatment process, which indicated that the CoNiFeAl0.4Ti0.6Cr0.5 high-entropy alloy has the advantage of sluggish diffusion behavior and thermal stability. The time for the HEA-7075Al composite reaching the aged-peak period (19.5 h) was slightly longer than that of the SiC-7075Al composite (15 h), indicating that the HEA particles addition has postponed the aging process, which can be ascribed to the decrease of dislocation density in the Al matrix, evaluated from XRD. Additionally, in contrast to the SiC-7075Al composite, the HEA-7075Al composite exhibits higher strength, better plasticity and higher modulus with the average values of 712 MPa, 0.82% and 171 GPa, respectively, which is the results of the good interface between HEA particles and Al matrix, high strength nanocrystalline HEA particles and proper dislocation density present in Al matrix.

Yield stress of ultrafine-grained or nanocrystalline materials with a bimodal grain size distribution

An attractive processing route for enhancing the yield strength of high-strength nanocrystalline metals and alloys while maintaining high ductility is to develop a bimodal grain size distribution (GSD), in which, supposedly, the finer grains provide strength, and the coarser grains maintain or even enhance ductility. We present a theoretical model predicting the strength of such a system, and show, analytically, how the yield stress is related to the various parameters of the bimodal GSD, such as volume fraction of the two components of the bimodal distribution and their standard deviations.

Tensile nanomechanics and the Hall-Petch effect in nanocrystalline aluminium

This work carries out comprehensive classical molecular dynamics simulations of uniaxial tensile deformation for nanocrystalline Al samples with a broad range of different mean grain sizes. The largest nanocrystalline Al sample has a mean grain size of about 30nm and contains over 100 millions atoms in the modeling system. The grain size dependence of yield stress and the atomic fraction of dislocations are quantified and the size dependence tensile nanomechanics are also revealed. Deformation twinning and grain boundary migration are observed as main mechanisms of tensile deformation in the nanocrystalline Al. In particular, the complete Hall-Petch relationship for the nanocrystalline Al bulk is determined for the first time in this study, from which three distinct regions are identified including the normal, inverse, and extended regions in the Hall-Petch relationship. We expect these findings will provide useful insights in the nanomechanics of nanocrystalline Al. The quantitation analyses on dislocations and mechanical properties may facilitate the design and development of high strength nanocrystalline Al and Al based alloys as well as future testing procedures for promising structural and transportation applications.

The local strength of individual alumina particles

We implement the C-shaped sample test method and micro-cantilever beam testing to measure the local strength of microscopic, low-aspect-ratio ceramic particles, namely high-purity vapor grown α-alumina Sumicorundum® particles 15–30 µm in diameter, known to be attractive reinforcing particles for aluminum. Individual particles are shaped by focused ion beam micromachining so as to probe in tension a portion of the particle surface that is left unaffected by ion-milling. Mechanical testing of C-shaped specimens is done ex-situ using a nanoindentation apparatus, and in the SEM using an in-situ nanomechanical testing system for micro-cantilever beams. The strength is evaluated for each individual specimen using bespoke finite element simulation. Results show that, provided the particle surface is free of readily observable defects such as pores, twins or grain boundaries and their associated grooves, the particles can achieve local strength values that approach those of high-perfection single-crystal alumina whiskers, on the order of 10 GPa, outperforming high-strength nanocrystalline alumina fibers and nano-thick alumina platelets used in bio-inspired composites. It is also shown that by far the most harmful defects are grain boundaries, leading to the general conclusion that alumina particles must be single-crystalline or alternatively nanocrystalline to fully develop their potential as a strong reinforcing phase in composite materials.

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.

Corrosion properties of high-strength nanocrystalline Al 84 Ni 7 Gd 6 Co 3 alloy produced by hot pressing of metallic glass

Abstract The corrosion properties of the nanostructured Al 84 Ni 7 Gd 6 Co 3 alloy produced by hot pressing of the metallic glassy powder, have been investigated. The results reveal that the nanostructure Al 84 Ni 7 Gd 6 Co 3 alloy shows different corrosion behavior compared to the pure aluminum, because of the presence of high volume fraction of Al-based intermetallic compounds. The corrosion resistance of Al 84 Ni 7 Gd 6 Co 3 alloy is significantly influenced by the amount of aluminum and porosity in the inter-particle area. The main corrosion mechanisms observed in the Al 84 Ni 7 Gd 6 Co 3 alloy are found to be the pitting corrosion (corrosion of aluminum in the consolidated particles) and crevice corrosion (corrosion of aluminum in the inter-particle area).

Hybrid nanostructured aluminum alloy with super-high strength

Methods to strengthen aluminum alloys have been employed since the discovery of the age-hardening phenomenon in 1901. The upper strength limit of bulk Al alloys is ~0.7 GPa by conventional precipitation strengthening and increases to >1 GPa through grain refinement and amorphization. Here we report a bulk hybrid nanostructured Al alloy with high strength at both room temperature and elevated temperatures. In addition, based on high-resolution transmission electron microscopic observations and theoretical analysis, we attribute the strengthening mechanism to the composite effect of the high-strength nanocrystalline fcc-Al and nano-sized intermetallics as well as to the confinement effect between these nano phases. We also report the plastic deformation of nano-sized intermetallics and the occurrence of a high density of stacking faults and twins in fcc-Al after low-strain-rate deformation at room and high temperatures. Our findings may be beneficial for designing high-strength materials for advanced structural applications. An international team of scientists create an aluminum alloy that is twice as strong as those made by conventional techniques. Zhi Wang from IFW Dresden in Germany and Tohoku University in Japan and co-workers from China, Germany, Japan and Austria have developed a technique that allows the microstructure of aluminum alloys to be carefully controlled, enabling them to create hybrid structures consisting of nanoscale intermetallic compounds in an aluminum matrix. Previous attempts to create strong alloys included making an aluminum-based material that is non-crystalline, like glass, or one that is made up of crystalline nanograins. But these approaches lead to materials that are unsuitable for use at high temperatures. In contrast, the aluminum alloy created by the researchers exhibits high strength at both ambient and elevated temperatures. We report a bulk hybrid nanostructured Al alloy with super-high strength at both room and elevated temperatures. The strengthening mechanisms were clearly elucidated, which are mainly attributed to the composite structure and confinement effect between the nano phases. The confining effect can effectively suppress the premature brittle fracture of the nano-intermetallic phases. The microstructural strategy and strengthening mechanisms in this work may be beneficial for the scientific community in understanding and designing high-strength materials.