Fine-grained Regions Research Articles

FABRICATION AND TRIBOLOGICAL PROPERTIES OF GRADIENT FINE-GRAINED OXYGEN-BOOSTING LAYER ON ECAP-TREATED PURE TITANIUM SURFACE

The gradient fine-grained oxygen-boosting layer was prepared on equal channel angular processing (ECAP)-treated titanium with thermal oxidation and oxygen boost diffusion process, and tribological properties were systematically characterized. Results show that the as-prepared boosting layer consists of surface coarse-grained region, and inner fine-grained region. The corresponding thickness and mechanical properties further increase compared to those of virgin titanium. The oxygen-boosting layer reveals excellent anti-wear properties, the dominant wear mechanism of which is abrasive.

Effect of heterogeneous microstructure on magnetization reversal mechanism of hot-deformed Nd-Fe-B magnets

The hot-deformed (HD) Nd-Fe-B magnets show heterogeneous microstructure composed of coarse and fine grain regions. It is significant to fully understand the influence of this complex microstructure on the magnetization reversal process which can give the guidance for the enhancement of the magnetic properties. In this paper, the heterogeneous microstructure of the (HD) Nd-Fe-B magnets were characterized from the morphology, size, macro-texture and micro-structure. In addition, the magnetization reversal process of the HD Nd-Fe-B magnets was systematically analyzed by magnetic measurement, in-situ domain evolution observation and micromagnetic simulation. The results indicate that the HD Nd-Fe-B magnets mainly consist of fine grain regions (FGRs) and coarse grain regions (CGRs). The FGRs show plate-like grains with fine grain size and strong c-axis texture, while the CGRs show equiaxial grains with large grain size and weak c-axis texture. In particular, it is worth noting that the texture in homogeneity exists not only between FGRs and CGRs, but also inside both the FGRs and CGRs. The dominant coercivity mechanism of the HD Nd-Fe-B magnets is domain wall pinning. Also, the experimental analysis shows that the reverse domain is formed and expanded in the CGRs at low reverse applied field, while the reverse domain occurs in the FGRs at higher reverse applied field. The micromagnetic simulation results also confirm the above magnetization reversal process. In addition, micromagnetic simulation results also show that the orientation of the grains also affects the pinning strength, besides the grain size.

Unique deformation behavior and microstructure evolution in high temperature processing of HfNbTaTiZr refractory high entropy alloy

Hot-deformation behaviors of an equiatomic HfNbTaTiZr refractory high entropy alloy having BCC structure were investigated. Uniaxial compression was carried out at different temperatures ranging from 1000 °C to 1200 °C and various strain rates from 10−4 s−1 to 10−2 s−1 where BCC single phase was stable. Stress-strain curves indicated distinctive sharp drops at yielding followed by continuous decreases of flow stress. Interrupted compression tests indicated that the sharp drop of the stress was explained from a viewpoint of dislocation unlocking possibly by solute atom(s) atmosphere or short-range ordering. Flow stress analysis was carried out by using Arrhenius type power law relationship. The apparent activation energy (Q) for hot-deformation estimated from the flow stress data was 258–232 kJ mol−1 over the entire range of strain. The flow stress analysis indicated high strain rate sensitivity (m) of the flow stress (m ∼ 0.33). The relatively high m value and dynamically recrystallized (DRX) microstructures composed of coarse unrecrystallized regions and fine DRX grains with necklace morphologies indicated a possibility of grain boundary sliding (GBS) in the fine-grained DRX regions. The fine DRX grain sizes indicated inhibited grain growth after DRX, which might be due to sluggish diffusion in the HfNbTaTiZr alloy. Preferred orientations of <001> and <111> parallel to the compression axis in the unrecrystallized coarse regions indicated normal dislocation slips in the BCC crystal, while weak and near-random texture observed in the fine-grained DRX regions supported the occurrence of GBS.

Grain size effect on twinning and annealing behaviors of rolled magnesium alloy with bimodal structure

Many wrought Mg alloys, which are manufactured by hot forming processes, have an inhomogeneous grain structure with locally different grain sizes. In the present study, variations in twinning and annealing behaviors caused by the difference in grain size within a material were investigated by subjecting a rolled Mg alloy with a bimodal grain structure to precompression and subsequent annealing. Activation of {10-12} twinning during precompression was found to be more pronounced in the coarse-grain region (CGR) than in the fine-grain region (FGR) owing to a lower twinning stress in the former. As a result, the twin structure formed in the FGR was distinctly different from that in the CGR; the CGR had a larger twinned area, stronger twin texture, fewer twin boundaries, and weaker grain refinement effect than the FGR. During the subsequent annealing, the microstructure of the FGR was greatly altered through grain growth driven by strain-induced boundary migration on account of its high internal strain energy, whereas that of the CGR remained nearly unchanged owing to its relatively lower internal strain energy. Consequently, the microstructural differences between the FGR and CGR induced by the precompression almost disappeared due to the subsequent annealing treatment, which resulted in a homogeneous microstructure.

Analysis of creep behavior of welded joints of P91 steel at 600 °C

The martensitic P91 steel (9Cr-1Mo-V-Nb) has been developed for ultra-supercritical pressure application in steam power plants. The creep rupture strength of the welded joint of this steel is limited by the fine-grained region of its heat affected zone (HAZ). The factors which result in the reduction of the creep rupture strength of the fine-grained region of HAZ were discussed. It was found that the most effective factor reducing the creep rupture strength of the fine-grained region of the HAZ of the welded joint of P91 steel, in comparison with other regions of the welded joint, is the finer prior austenite grain size. These fine prior austenite grains of the fine-grained region of the HAZ accelerate the rate of growth of martensite lath subgrains which results in a softer martensite matrix.The minimum creep rate dependence on applied stress for base metal, welded joints and the samples simulating the fine-grained region of the welded joints was described by means of the conventional power-law equation. The experimental results, both in the form of strain versus time creep curves and of minimum creep rate versus time creep curves were used to study the creep behavior of the welded joints of P91 steel at 600 °C.

Passive Vector Sensing for Non-Cooperative Emitter Localization in Indoor Environments

Indoor emitter localization is a topic of continued interest for improving wireless security as wireless technologies continue to become more advanced. Conventional methods have focused on the localization of devices relative to multi-sensor systems owing to ease of implementation with pre-existing infrastructures. This work, however, focuses on enhancing wireless security via non-cooperative emitter localization in scenarios where only a single receiver can be employed. A vector sensor is simulated and experimentally developed that extracts three-dimensional signal characteristics for room-based emitter localization and is compared to conventional methodologies such as Received Signal Strength (RSS), Time of Arrival (ToA), and Direction of Arrival (DoA). The proposed method generates time-frequency fingerprints and extracts features through dimensionality reduction. A second stage extracts spatial parameters consisting of Channel State Information (CSI) and DoAs that are analyzed using a Gaussian Mixture Model (GMM) to segregate fine-grained regions of interest within each room where the non-cooperative emitter resides. Blind channel equalization cascaded with a least squares channel estimate is used for acquiring the CSI, whereas the DoAs are obtained by unique trigonometric properties of the vector sensing antenna. The results demonstrate that a vector sensor can improve non-cooperative emitter localization and enhance wireless security in indoor environments.

Unique Deformation Behavior and Microstructure Evolution in High Temperature Processing of HfNbTaTiZr Refractory High Entropy Alloy

Hot-deformation behaviors of an equiatomic HfNbTaTiZr refractory high entropy alloy having BCC structure were investigated. Uniaxial compression was carried out at different temperatures ranging from 1000°C to 1200°C and various strain rates from 10-4 s-1 to 10-2 s-1. Stress-strain curves indicated distinctive sharp drops at yielding followed by continuous decreases of flow stress. The sharp drop of the stress was explained from a viewpoint of dislocation pinning possibly by any solute atom(s). Flow stress analysis was carried out by using Arrhenius type power law relationship. The apparent activation energy (Q) for hot-deformation estimated from the flow stress data was 124 ~ 113 kJmol-1 over the entire range of strain. The flow stress analysis indicated high strain rate sensitivity (m) of the flow stress (m ~ 0.35). The relatively high m value and dynamically recrystallized (DRX) microstructures composed of coarse unrecrystallized regions and fine DRX grains with necklace morphologies indicated a possibility of grain boundary sliding (GBS) in the fine-grained DRX regions. The fine DRX grain sizes indicated inhibited grain growth after DRX, possibly because of the sluggish diffusion in the HfNbTaTiZr alloy. Preferred orientations of and parallel to the compression axis in the unrecrystallized coarse regions indicated normal dislocation slips in the BCC crystal, while weak and near-random texture observed in the fine-grained DRX regions supported the occurrence of GBS.

Fiber Optic Sensing for Geomechanical Monitoring: (1)-Distributed Strain Measurements of Two Sandstones under Hydrostatic Confining and Pore Pressure Conditions

In this study distributed optic fiber has been used to measure both the Rayleigh and Brillouin frequency shift of two different sandstone core samples under controlled hydrostatic confining and pore pressure in the laboratory. The Berea sandstone core is relatively homogeneous, whereas the Tako sandstone core is visibly heterogeneous with a coarse-grain and a fine-grain region. Rayleigh frequency has been found to have a superior performance over Brillouin frequency in terms of better consistency (less scattering) in the tests carried out. The strain gauge readings reveal considerable anisotropy in the stiffness of the Berea core between perpendicular (vertical) and parallel to the bedding (hoop) directions. The strains converted from Rayleigh frequency shift measurements agree reasonably well with readings by one of the four hoop strain gauge channels under increasing confining/pore pressure. For the Tako sandstone core, the contrast in the grain-size, and thus rock elastic properties, is clearly reflected in the hoop strain measurement by both strain gauges and distributed optic fiber. The outcomes of the test have demonstrated successfully the use of a single optic fiber for measuring rock strain response at different regions of a heterogeneous core sample along a continuous trajectory.

Design of non-equiatomic high entropy alloys with heterogeneous lamella structure towards strength-ductility synergy

A non-equiatomic FeNiCoAlCrB (NCACB) high entropy alloy (HEA) with a heterogeneous lamella (HL) structure is fabricated through conventional thermomechanical processing. In the HL microstructure, fine-grain regions result from inhibited grain growth due to Zener pinning of boundaries by NiAl (B2) precipitates, while coarse-grained regions originate from grain growth within large deformation bands in the absence of precipitates. A back-stress strengthening mechanism, unique to deformation of heterogeneous microstructures, is verified through mechanical behavior and electron backscatter diffraction enabled geometrically necessary dislocation density analysis. This mechanism gives rise to the combination of both high strength and high ductility in HL-NCACB-HEA.

A Fine-Grained Spatial-Temporal Attention Model for Video Captioning

Attention mechanism has been extensively used in video captioning tasks, which enables further development of deeper visual understanding. However, most existing video captioning methods apply the attention mechanism on the frame level, which only model the temporal structure and generated words, but ignore the region-level spatial information that provides accurate visual features corresponding to the semantic content. In this paper, we propose a fine-grained spatial-temporal attention model (FSTA), and the spatial information of objects appearing in the video will be our main concern. In the proposed FSTA, we achieve the spatial-hard attention at a fine-grained region level of objects through the mask pooling module and compute the temporal soft attention by using a two-layer LSTM network with attention mechanism to generate sentences. We test the proposed model on two benchmark datasets, namely, MSVD and MSR-VTT. The results indicate that our proposed FSTA model can achieve competitive performance against the state of the arts on both datasets.

Influence of SLM scan-speed on microstructure, precipitation of Al3Sc particles and mechanical properties in Sc- and Zr-modified Al-Mg alloys

Additive processing via Selective Laser Melting (SLM) of Sc- and Zr-modified Al-Mg alloys (commercially known as Scalmalloy®) provides significant advantages over traditional 4xxx casting alloys. This is due to the high strength and ductility at very low mechanical anisotropy, which is a result of the alloys' very fine-grained microstructure combined with weak texture in the build-up direction. Next to their advantageous mechanical properties, the reliability of alloys processed by additive manufacturing is of great importance. Variations in microstructural features due to the use of different in processing parameters (e.g. laser scan speed) are well known for traditional alloys. This study analyses the influence of varying laser scan speeds on the static mechanical properties of SLM-processed Scalmalloy®, and discusses the evolving microstructure and precipitation of nm-sized Al3Sc particles at the correspondingly different energy inputs. It is found that by about doubling the laser scan speed the peak grain sizes in the fine-grained regions decrease from 1.1μm to 600nm, whereas in the coarse grained region almost no influence is observed. Al3Sc particles are only precipitated during processing at low scan speeds due to the corresponding high laser energy input, or by the intrinsic heat treatment from subsequent layers being built.

Oxide Dissolution and Oxygen Diffusion in Solid-State Recycled Ti-6Al-4V: Numerical Modeling, Verification by Nanoindentation, and Effects on Grain Growth and Recrystallization

The oxide dissolution and oxygen diffusion during annealing of Ti-6Al-4V solid-state recycled from machining chips by equal-channel angular pressing (ECAP) have been investigated using nanoindentation and numerical modeling. The hardness profile from nanoindentation was converted into the oxygen concentration distribution using the Fleisher and Friedel model. An iterative fitting method was then employed to revise the ideal model proposed previously, leading to correct predictions of the oxide dissolution times and oxygen concentration profiles and verifying nanoindentation as an effective method to measure local oxygen concentrations. Recrystallization started at the prior oxide boundaries where local strains were high from the severe plastic deformation incurred in the ECAP recycling process, forming a band of ultrafine grains whose growth was retarded by solute dragging thanks to high oxygen concentrations. The recrystallized fine-grained region would advance with time to eventually replace the lamellar structure formed during ECAP.

Influence of increasing Al concentration on phase, microstructure and mechanical behaviors of Ni1.5CoFeCu1−xAlxV0.5 high entropy alloys

The present work systematically investigated the influence of increasing Al concentration on the phase composition, microstructure evolution and mechanical behaviors of Ni1.5CoFeCu1−xAlxV0.5 (x = 0.1, 0.2, 0.3mol) high entropy alloys (HEAs) synthesized by mechanical alloying (MA) followed by spark plasma sintering (SPS). In the MA powders, FCC phase reduced while BCC phase increased with increase in Al content. In the alloys consolidated by SPS, BCC phase disappeared with the formation of vanadium carbides as well as aluminum oxide due to carbon and oxygen contaminations. Microstructure characterization suggests that three bulk HEAs all displayed the bimodal grain microstructure, consisting of coarse-grained (CG) regions as well as fine-grained (FG) regions with uniform dispersion of aluminum oxide nanoparticles. Furthermore, with the increase of Al concentration in the alloy, the volume fraction of FG region increased gradually and the average grain sizes of CG and FG regions reduced dramatically. Since the microstructure refinement resulted in the enhancement of grain boundary strengthening, the yield strength of alloy was improved significantly with increasing Al. In addition, the increase of Al concentration remarkably changed the strain-hardening behavior of bulk HEA by influencing the strain-hardening stages during compression.

Microstructure, plastic flow and fracture behavior of ferrite-austenite duplex low density medium Mn steel

This study addresses the microstructure, plastic flow and fracture behavior of a laboratory Al-containing low density Fe-0.3C-6.7Mn-5.9Al medium Mn steel, intercritically annealed at either 850°C or 900°C, after cold rolling. Both resulting materials present an unordinary bimodal duplex microstructure made of coarse δ-ferrite grains and fine-grained (FG) {α-ferrite + retained austenite} regions. Tensile tests at room temperature and at −50°C and Charpy impact tests showed that for both materials, each microstructural region presents its own fracture mechanisms and a specific ductile-to-brittle transition. A transition from brittle cleavage to ductile fracture is observed for coarse δ-ferrite grains, while FG regions present a transition from interfacial fracture to ductile fracture. The overall impact behavior seems to be driven by that of δ-ferrite grains.

Microstructure and Microhardness of Fiber Laser Welded Dual-Phase Steels with High-Strength Low-Alloy Steels

The fiber laser welding of dual-phase DP980 steels with high-strength low-alloy LA340 steels was studied with a concentration on microstructure, microhardness, and tensile properties of butt joints. The microstructure in the fusion zone of joints was predominantly built of lower bainite and martensite. Mainly martensite was found in the coarse-grained region of HAZ near DP980 steel, but equiaxed ferrite and tempered martensite were observed in fine-grained region of this HAZ. The microstructure consisted of acicular ferrite, upper bainite and equiaxed ferrite in the high tempered coarse-grained region of HAZ near LA340 steel. The fine-grained equiaxed ferritic microstructure was found in the low tempered region of HAZ near LA340 steel. The microhardness increased in the fusion zone and in both heat-affected zones compared to the base metals, 335 HV0.1 for DP980 steel and 186 HV0.1 for LA340 steel. The microhardness reached values from 380 to 441 HV0.1 in the fusion zone, from 314 to 464 HV0.1 in HAZ near DP980 steel, and from 181 to 380 HV0.1 in HAZ near LAD340 steel. The tensile strength of the fiber laser welded joints was 460 MPa and it reached the strength of the LA340 steel base material with failure occurring in this steel.

Thickness effects on fatigue crack propagation in submicrometer-thick freestanding copper films

To investigate the thickness effects on fatigue crack propagation properties and mechanisms of as-deposited submicrometer-thick freestanding metallic films, fatigue crack propagation experiments in freestanding copper films with a thickness B of approximately 100nm were conducted at a stress ratio R of 0.1, and compared to the films with B≈500nm. Electron-beam-evaporated copper films with B≈100nm had smaller grains compared to the films with B≈500nm. In addition, fine-grained regions with grain sizes of a few tens of nanometers were observed dispersedly. In films with B≈100nm, fatigue crack initiated and stably propagated from a notch root by applying a cyclic load. However, the arrest or retardation of fatigue crack propagation often occurred even under constant maximum stress. The fatigue crack propagation rate (da/dN) of films with B≈100nm is higher than that of films with B≈500nm, except for the crack arrest cases; films with B≈100nm showed lower fatigue fracture toughness. Moreover, fatigue crack in films with B≈100nm propagated even below the fatigue threshold of films with B≈500nm. Intermittent field emission scanning electron microscope (FESEM) observations of fatigue crack propagation behavior and fracture surface observations as well as microstructural analyses, using electron backscatter diffraction (EBSD), revealed that the fatigue cracks propagated by intergranular cracking and transgranular cracking with intrusion/extrusion formation. Compared to the films with B≈500nm, the fatigue damage along the crack path became narrower and the frequency of intrusion/extrusion formation decreased. EBSD analysis also revealed that fatigue crack stably propagated in the coarse-grained region, whereas the arrest or retardation of the fatigue crack propagation occurred when the fatigue crack tip reached the fine-grained region, with grain sizes of a few tens of nanometers, indicating that the fine-grained regions have higher resistance to fatigue damage formation than do the coarse-grained regions.

Investigation of the Origin of Coarse-Grained Bainite in X70 Pipeline Steels by EBSD Technique

Structural inhomogeneities that occurred in pipeline steels during controlled rolling and were identified as elongated regions of coarse-grained bainite were found to promote a tendency for brittle fracture of these steels. In particular, it was shown by tint etching, with subsequent analysis in polarized light with the light-optical microscope, that their volume fraction exhibited a perfect correspondence with the failure behavior. The aim of the present investigation was to comprehensively characterize the microstructure and the local texture of these regions using the EBSD technique and thereby clarify their origin. It was shown that, whereas the local texture of fine-grained regions corresponded well to the macroscopic texture of the steel band, the local texture of the structural inhomogeneities corresponds to certain minor components of the macroscopic texture. The grains of the parent austenite, from which these inhomogeneities have been produced, had orientations concentrated close to the brass component of austenite rolling texture. Hot deformation of the austenite leads to a strong variant selection (predominant realization of certain lattice orientations among plausible orientations of α-phase) during phase transformation of these grains, that result in a significant increase of the bainite packet size. Formation of bainite in the upper temperature range of bainite transformation, in its turn, leads to such a variant pairing that low-angle boundaries are formed predominantly both inside and between the packets. This further enlarges the effective grain size and correspondingly enhances the deteriorative effect of the structural inhomogeneity.

The READEX formalism for automatic tuning for energy efficiency

Energy efficiency is an important aspect of future exascale systems, mainly due to rising energy cost. Although High performance computing (HPC) applications are compute centric, they still exhibit varying computational characteristics in different regions of the program, such as compute-, memory-, and I/O-bound code regions. Some of today’s clusters already offer mechanisms to adjust the system to the resource requirements of an application, e.g., by controlling the CPU frequency. However, manually tuning for improved energy efficiency is a tedious and painstaking task that is often neglected by application developers. The European Union’s Horizon 2020 project READEX (Runtime Exploitation of Application Dynamism for Energy-efficient eXascale computing) aims at developing a tools-aided approach for improved energy efficiency of current and future HPC applications. To reach this goal, the READEX project combines technologies from two ends of the compute spectrum, embedded systems and HPC, constituting a split design-time/runtime methodology. From the HPC domain, the Periscope Tuning Framework (PTF) is extended to perform dynamic auto-tuning of fine-grained application regions using the systems scenario methodology, which was originally developed for improving the energy efficiency in embedded systems. This paper introduces the concepts of the READEX project, its envisioned implementation, and preliminary results that demonstrate the feasibility of this approach.

Magnetization reversal processes in hot-extruded τ-MnAl-C

The magnetic domain structure of hot-extruded bulk τ-Mn53Al45C2 was studied by Kerr microscopy under application of a magnetic field in-situ. The microstructure consists of recrystallized, fine-grained regions and large non-recrystallized grains which contain a high density of twins. Within these large polytwinned grains, a clear pinning interaction of magnetic domain walls at twin boundaries was observed but with a rather small pinning force. The smaller, recrystallized grains show a higher resistance to magnetization reversal. The critical single domain particle size of this material was estimated at 773nm and the fine, recrystallized grains are in the range of this size. Demagnetizing the sample following saturation using a 3T field pulse revealed that individual fine grains reverse independently from their neighbours.

Ultra-high strength Mg-9Gd-4Y-0.5Zr alloy with bi-modal structure processed by traditional extrusion

It is usual to observe that multi-scale structures can lead to combined strength and ductility both in aluminum alloys and steels, but related research has been seldom reported yet in magnesium alloys. In this study, applying traditional one step extrusion, we have successfully obtained a bimodal (Mg-9Gd-4Y-0.5Zr) alloy capable of ultra-high strength. The characterized sample reveal a bi-modal microstructure with two constitutions, i.e. stretched coarse-grain region with strong basal fiber texture and recrystallization fine-grain region. The bi-modal structured sample exhibit excellent mechanical properties with an ultimate strength 508 MPa and elongation 8% via 400 °C extrusion and subsequently 200 °C-60 h peak aging process. Ultra-high strength can be attributed to its strong extrusion texture in stretched coarse grains and dispersed nano-scale precipitates. This unique bimodal structure could be produced easily by one step extrusion, which is quite reliable and low costs in industrial applications of magnesium alloys with ultra-high strength as well as ideal ductility.