查看更多>>摘要：CoCrFeNi high entropy alloys (HEAs) with yttrium (Y) additions (1 and 4 at. %) were nanostructured by mechanical alloying process and annealed at various temperatures between 500 degrees C and 1100 degrees C. The structure, grain growth, and hardness were studied as a function of solute addition and annealing temperature using X-ray diffraction (XRD), focused ion beam (FIB), and scanning transmission electron microscope (S/TEM) techniques, and hardness test. The thermo-physical calculations were utilized to discuss the phase evolution after mechanical alloying and annealing with respect to added solutes. The results showed that Y additions did not affect the main crystal structure of the base CoCrFeNi HEA as the solid solution with a single face-centered cubic (fcc) crystal structure was maintained even after 1 h annealing at 1100 degrees C. The as-milled nanocrystalline grain size of CoCrFeNi HEA yielded extensive grain growth with the temperature exposures reaching 291 nm and 1.4 mu m after annealing at 900 degrees C and 1100 degrees C, respectively. However, Y additions retarded the grain growth and decreased the average grain size upon annealing as compared to the base HEA. That is, 1 and 4 at. % Y additions stabilized the grain size around 88 nm and 95 nm (both determined by TEM) after annealing at 900 degrees C and 1100 degrees C, respectively. Accordingly, the as-milled hardness of CoCrFeNi HEA dropped from 475 HV to 220 HV after annealing at 1100 degrees C, while the reduction in hardness was relatively gradual with Y additions and retained around 435 HV with 4 at. % Y addition even after annealing at 1100 degrees C. Such thermal stability may facilitate the use of HEAs at high temperatures and enable the consolidation routes of powders into dense nanocrystalline compact HEAs.
查看更多>>摘要：Intermetallic TiAl-based alloys show a high susceptibility to the consistency of their surface region expressed by, e.g., a tendency to environmental embrittlement. In the present work, the prospects of near surface grain refinement of the beta-stabilised TNM alloy and the first results of mechanical tests are presented and discussed. Grain refinement was done by remelting the surface region by a laser-based process. Adjusted parameters result in a refined surface area to a depth of approx. 225 mu m and at the same time a smooth surface. Subsequent heat treatment at 900 degrees C for 6 h leads to a fine-grained duplex microstructure in the remelted area, with an average grain size of about 500 nm compared to 7.3 mu m of the initial state. In the heat-affected zone below the remelted region, lamellar grains disappear by dissolving gamma-lamellae during remelting. These lamellae are re-formed during heat treatment, but their spacing is significantly refined. Tensile tests reveal an increase of the tensile strength of remelted and heat-treated specimens by about 16% if compared to those with the initial microstructure. Failure in the remelted and heat-treated tensile specimens originates from the refined lamellar grains of the heat-affected zone.
查看更多>>摘要：Cooling rate is the key factor affecting soft magnetic properties of amorphous ribbons. So, is the faster the cooling rate conducive to better soft magnetic performance as theoretical hypotheses? The uncertainty of the debate on this issue in practical application motivates the present work. (Fe0.9Co0.1)86Ni1B13 amorphous ribbons with different thicknesses were prepared by deliberately varying the cooling rate. It was found that the amorphous ribbon prepared at a lower cooling rate exhibits better soft magnetic properties than the one prepared at a higher cooling rate. The reasons behind the improvement of the soft magnetic properties of amorphous ribbons were investigated in this work. The results prove that cooling rate strongly affect the surface morphology and residual stress of amorphous ribbons and thus the magnetic structure and properties of amorphous ribbons. On the premise of ensuring the formation of amorphous structure, high surface quality and low defects density of the ribbon prepared at low cooling rate promise candidates for the potential application in electric devices rather than the ribbons prepared at a high cooling rate.
查看更多>>摘要：In the rapidly developing modern industrial fields, the materials used are often required to have simultaneously high strength and high electrical conductivity. However, these two properties often conflict and cannot be met at the same time. In the present study, we developed the Cu/Cu-Zr-Al bulk metallic glass (BMG) composites which exhibited simultaneously high strength and high electrical conductivity, as well as large plasticity. The BMG composites were fabricated by using spark plasma sintering crystalline Cu-coated Cu-Zr-Al metallic glass composite powders. The effect of particle size of the Cu-Zr-Al metallic glass powders on mechanical properties and electrical properties of the BMG composites was investigated systematically. Microstructure of contact in-terfaces between crystalline Cu and Cu-Zr-Al metallic glass was also characterized and discussed. This study is expected to provide an effective technical approach for the development of new high strength and high con-ductivity materials.
Sanyal, SouriddhaBhuyan, PallabiMandal, Tapas K. Bandyopadhyay Sumantra
17页
查看更多>>摘要：In this study, a Mg-0.9Al-0.6Mn-0.2Si-0.1Ca alloy (in as-cast form) has been subjected to homogenization treatment followed by different thermomechanical processing, namely hard plate hot forging (HPHF) and hot rolling (HR), at a fixed working temperature (723 K). The HPHF specimen has developed a bimodal grain size distribution comprising of fine recrystallized grains and coarse deformed (non-recrystallized) grains due to incomplete dynamic recrystallization. Most of these grains have basal orientations along the forging plane, leading to a strong basal fiber texture with low Schmid factor and low Taylor factor. The HR specimen, on the contrary, has developed a uniform grain size distribution consisting of mostly fine grains, with twins in some of the relatively coarser grains. Although a large number of grains in the HR specimen have basal orientations, some of the very fine newly recrystallized grains and the twinned regions attain non-basal orientations, leading to an overall weaker basal fiber texture. The Mg17Al12 phase from the as cast specimen has dissolved during both the thermomechanical treatments owing to its low melting point. Contrarily, the thermally stable Al8Mn5 precipitates are present in all the specimens. The finer grain size and higher dislocation density, along with the finer nano-sized Al8Mn5 precipitates, have resulted in greater strengthening in the HR specimen, as compared to the HPHF specimen. Furthermore, the weaker basal fiber texture, higher Schmid factor and higher Taylor factor have led to much better ductility as well as work hardening response in the HR specimen, than in the HPHF condition.
查看更多>>摘要：In order to improve the mechanical properties of TiAl alloys, micro-nano Ti2AlC-reinforced TiAl composites were successfully fabricated by means of spark plasma sintering (SPS) using the ball-milled multilayer graphene oxide and Ti-48Al-2Nb-2Cr pre-alloyed powders. The micro-nano Ti2AlC phase precipitated at the interface between alpha(2)-Ti3Al and gamma-TiAl phases utilizing the reaction of TiAl and high-activity graphene. Fully lamellar structure was obtained after sintering above 1300 degrees C, and Ti2AlC phase characteristic were depended on sintering temperature and graphene content. The compressive strength and fracture strain of TiAl-0.5G alloy were improved at room temperature and elevated temperature, which was improved by 23.61% and 5.03% compared to the TiAl-0G alloy at room temperature. The tribological properties of TiAl composites were significantly improved by micro-nano Ti2AlC at the room temperature, and the average friction coefficient of TiAl-0.5G alloy is 0.217 compared with TiAl-0G alloy is 0.312, and the wear mechanism is ploughing wear. The micro-nano Ti2AlC improves the strength and oxidation resistance of TiAl composites at 650 degrees C, which is detrimental to the wear resistance due to the lower ductility and the third wear by the loose oxide particles at elevated temperature.
查看更多>>摘要：Diffusion bonding of high niobium beta-gamma TiAl (HNBG) to Nb-Si alloy was carried out using Ti foil as interlayer. The interfacial microstructural evolution, growth kinetics and mechanical properties of HNBG/Ti/Nb-Si joint were investigated in detail. The microstructure observations indicate that the interfacial microstructure of the joint was composed of beta-(Nb, Ti)5Si3, (Nb,Ti)ss//alpha-Ti, beta-Ti//alpha 2//beta/B2 from zone I to zone IV. The beta-Ti phase and alpha-Ti phase in zone II obey the orientation relationship, {110}beta || {0002}alpha and <111>beta|| <11-20>alpha. The gamma phase in HNBG alloy and beta/B2 phase in zone IV obey the orientation relationship, {111}gamma || {110}beta/B2 and <110>gamma || <111>beta/B2. The growth activation energies of beta-(Nb, Ti)5Si3 and beta/B2 phases were 166 kJ/mol and 507 kJ/mol respectively. The content of beta phase in zone II and the average grain size of alpha 2 phase in zone III increased with bonding temperature or bonding time. The shear test results showed that the ultimate shear strength was 385 MPa at 1000 degrees C-10 min-10 MPa. Fracture mainly occurred beween the interface of zone III and IV, and between the interface of zone IV and HNBG alloy. The fracture mode was mainly characterized by brittleness rupture along the phase boundaries of alpha 2 and beta/B2, beta/B2 and HNBG alloy with slight amount of cleavage fracture in zone IV.
Moghaddam, Ahmad OstovariAbdollahzadeh, AminSamodurova, MarinaShaburova, Nataliya...
7页
查看更多>>摘要：(NiCoFeCuMn)Zn-3 and (NbTaVNiTiFe)Al-3 high entropy intermetallic compound (HEIC) powders are fabricated by induction melting followed by ball milling. The fabricated (NiCoFeCuMn)Zn-3 HEIC powder possesses a single gamma-brass D8(2)-type structure, while (NbTaVNiTiFe)Al-3 exhibits D0(22) HEIC phase along with Ni4Ti3 and FeNi intermetallic inclusions. The microstructure of (NiCoFeCuMn)Zn-3 HEIC powder consists of a homogenous distribution of constituent elements. In the microstructure of (NbTaVNiTiFe)Al3, (TaNb)-rich and (FeNi)-rich regions are formed. A modified electronegativity parameter is proposed as a new indicator to design single high-entropy phases in ordered (A1A2A3 ... An)B-m systems. The application of the prepared HEICs as precursor powders for detonation spraying (DS) is further examined. The results indicate that both powders can exhibit sufficient thermal stability during DS. The coatings exhibit sound bonding with substrate, and consist of lamellar microstructure with low porosities. It is predicted that HEICs, as a promising class of high entropy materials, are interesting candidates for a wide range of applications from coatings to catalysts.
查看更多>>摘要：Abnormal grain growth (AGG) in nanocrystalline (CoCrFeNi)(100-x)Zr-x (x = 1 and 4 at. %) HEAs, prepared through high energy mechanical alloying, was comprehensively investigated upon annealing. Transmission electron microscopy (TEM), including high angle annular dark field imaging (HAADF) and energy dispersive spectroscopy (EDS) mapping, focused ion beam microscopy (FIB), and X-ray diffraction experiments (XRD) were utilized to investigate the microstructures as a function of added Zr content and temperature exposures. The results showed that nanocrystalline grains of the as-milled HEAs did not increase significantly upon annealing up to 700 degrees C as the nanocrystalline grain sizes were retained. However, grain growth was observed in (CoCrFeNi)(99)Zr-1 after annealing at 900 degrees C, which turned into AGG after annealing at a higher temperature of 1100 degrees C, disrupting the equiaxed grain structures observed at 900 degrees C. Although the increased amount of Zr doping reduced the average grain size in (CoCrFeNi)(96)Zr-4, bimodal grain structure existed in the microstructure composed of a matrix with 255 nm grain size and abnormally grown grains up to 3 mu m. The observed AGG was attributed to the pinning effect of in-situ formed secondary oxide phases. The microstructural evolution as a function of Zr doping and annealing temperatures was also correlated with the microhardness test results. The AGG and bimodal grain structure reported for the Zr doped CoCrFeNi HEA may open a new avenue to produce HEAs with the enhanced strength-ductility combination due to the incorporation of larger grains and in-situ formed oxide phases in a fine-grained matrix.
查看更多>>摘要：For metallic materials, it is challenged to enhance the strain hardening rate under uniaxial tension when high flow stress is achieved. Here, we present a strategy enabling to obtain superior strain hardening ability via fabricating an incompletely recrystallized structure in a precipitation-strengthened Co-rich CoCrNiAlTi mediumentropy alloy. It involves to introduce a low-temperature ageing process in the CoCrNiAlTi alloy experienced high-strain cold rolling process. Accompanied with the formation of the partially recrystallized microstructure, high-density band-like L12-type ordered phase has formed via discontinuous precipitation. During the plastic straining process, deformation twins have been triggered by the high-stress field owing to its metastable structure. As expected, the alloy demonstrates synergistically substantial strengthening and toughening effects, thus, a superb mechanical property combination with ultimate tensile stress of 2.20 GPa and ductility of over 5.0% has been achieved at ambient temperature.
NSTL
Elsevier