Effect of grain refinement on mechanical properties and sliding wear resistance of extruded Sc-free 7042 aluminum alloy

The present study deals with an investigation on dry sliding wear behavior of grain refined Sc-free 7042 aluminum alloy by using a pin-on-disc wear test machine. Al–5Ti–1B and Al–15Zr master alloys were used as grain refining agents. The optimum amounts of added Ti and Zr in the alloy were found to be 0.03 wt.% and 0.3 wt.%, respectively. Extrusion was carried out and T6 heat treatment ware applied for all rod specimens before testing. Significant improvement in mechanical properties was obtained with the addition of grain refiners. The worn surfaces were characterized by energy dispersive X-ray spectrometry microanalysis. Results showed that the wear resistance of unrefined alloy increased with the addition of both grain refiners. Furthermore, the worn surface studies showed a mixed type of wear mechanisms; delaminating, adhesive and abrasive which took place at higher applied load.     

High temperature mechanical properties of low alloy steel foams produced by powder metallurgy

Cu–Ni–Mo and Mo based steel foams having different porosity levels for high temperature applications were produced by the space holder-water leaching technique in powder metallurgy. Steel powders were mixed with binder (polyvinylalcohol) and spacer (carbamide), and compacted. Spacer in the green compacts was removed by water leaching at room temperature and porous green compacts were sintered at 1200 °C for 60 min in hydrogen atmosphere. The successful application of foams at higher temperatures requires a good understanding of their high temperature mechanical properties. Compression tests were carried out on steel foams with different porosities at temperatures varying from room temperature to 600 °C in argon atmosphere. Effect of high temperature on compressive properties of the steel foams was investigated. It was found that the compressive strength of steel foams was greater at elevated temperatures than that at room temperature. This occurs across a range of temperatures up to 400 °C. Beyond this point the compressive strength decreased as the temperature increased. The reason for the enhancement of the compressive strength of Cu–Ni–Mo and Mo based steel foams is expected to be due to the effect of the dynamic age-hardening.     

Microstructures and mechanical properties of ultrafine-grained Ti/AZ31 magnesium matrix composite prepared by powder metallurgy

The microstructure of ultrafine grain for magnesium alloys can result in drastic enhancement in their room temperature strength, but the issue of low strength at elevated temperature becomes more serious as well due to grain boundary slide. Here ultrafine-grained Ti/AZ31 magnesium matrix composites with high strength at both room and elevated temperature were prepared by vacuum hot pressing and subsequent hot extrusion. The microstructure of the composite samples before and after consolidation processing was characterized, and the mechanical properties of the as-consolidated bulk samples were measured at room and elevated temperatures. The results indicate that after extrusion ultrafine-grained magnesium alloys were obtained and Ti particulates with particulate size of ～310?nm disperse in Mg matrix. The magnesium grain of AZ31-15at.%Ti grows from 66?nm to 800?nm. Meanwhile, the relative densities of Ti/AZ31 composites are higher than 99%. The yield strength (YS) of extruded AZ31-15at.%Ti composite at room temperature is 341?MPa, being 2.4 times higher than original AZ31 alloy. Theoretical estimation shows that remarkably enhanced room-temperature mechanical strength attributes to grain boundary strengthening with the contribution ratio of 74%. In addition, the peak stress of extruded AZ31-15at.%Ti composite at 573?K is 82?MPa and ultrafine Ti dispersions are responsible for the enhanced strength.     

Effect of Mo and Ti on the microstructure and microhardness in AlCoFeNiMoTi high entropy alloys prepared by mechanical alloying and conventional sintering

In this investigation, the synthesis of equiatomic AlCoFeNi, AlCoFeNiMo, AlCoFeNiTi, and AlCoFeNiMoTi high entropy alloys, fabricated by mechanical alloying and conventional sintering processes is presented aiming to elucidate the effect of Mo and Ti additions on the properties of the AlCoFeNi base system. X-ray diffraction studies revealed that after 15 h of milling, only BCC and FCC structures were formed. It was also found that by increasing the crystallite size after sintering, phase transformations and composition variations were observed for all the systems studied but BCC and FCC structures prevailed. Further, the addition of the different alloying elements had a significant effect on the microhardness of the HEAs and particularly, the addition of Mo and Ti to form the AlCoFeNiMoTi system presented the highest value of 894 HV0.2. Finally, it was also found that Mo- containing alloys presented considerable porosity.     

Microstructural and mechanical properties of biodegradable iron foam prepared by powder metallurgy

Research into biodegradable porous materials has been increasingly focused on iron-based materials because such materials possess suitable properties for orthopedic applications. In this study, we prepared porous iron with porosities of 32–82 vol.% by powder metallurgy using ammonium bicarbonate as a space-holder material. We studied the influence of initial powder size and compacting pressure on sample microstructure, contamination and mechanical characteristics. The experimental results were analyzed as well, using Gibson–Ashby model and this analysis showed a good agreement in theoretical and experimental data. Whereas increasing compression pressure decreased porosity, the use of finer iron powder led to an increase in porosity. Increasing the amount of space-holder material in the initial mixture increased the total porosity, improved compressibility and consequently decreased the number of pores originating from imperfect compaction. A higher compacting pressure and the use of finer powder enhanced both the flexural and compressive properties. Even the most porous samples prepared from the fine iron powder possessed mechanical properties comparable to human cancellous bone. Based on these results, we can claim that the use of fine initial iron powder is necessary to obtain highly porous iron, which appears to be suitable for orthopedic applications.     

A novel high-entropy alloy with multi-scale precipitates and excellent mechanical properties fabricated by spark plasma sintering

Body-centered-cubic (BCC) high entropy alloys (HEAs) usually exhibit high strength but poor ductility. To overcome such strength-ductility trade-off, a novel (FeCr)₄₅(AlNi)₅₀Co₅ HEA was presented in this paper, which was designed and fabricated with mechanical alloying (MA) followed by spark plasma sintering (SPS), and has a heterogeneous microstructure with multi-scale precipitates. Electron microscopy characterization revealed that the sizes of the precipitates range from nano (<300 nm), sub-micron (300～800 nm) to micron (>1 μm). The bulk HEA exhibits excellent mechanical properties, of which the compressive yield strength, fracture strength, and plasticity at room temperature can reach 1508 MPa, 3106 MPa and 30.4 %, respectively, which are much higher than that of most HEAs prepared by Powder Metallurgy reported in the literatures, suggesting that the HEA developed is highly promising for engineering applications. The excellent mechanical properties of the bulk HEA can be attributed to that the multi-scale precipitates are fully coherent with the matrix, which could reduce the misfit strain at the interface, and relieve the stress concentration during deformation.     

An investigation of the microstructural effects on the mechanical and electrochemical properties of a friction stir processed equiatomic CrMnFeCoNi high entropy alloy

The electrochemical properties of a friction stir processed (FSPed) equiatomic CrMnFeCoNi high-entropy alloy (HEA) was investigated in an aerated 0.5 M Na2SO4 electrolyte solution at room temperature.The microstructural analysis reveals a highly refined stir zone (SZ) with an average grain size that decreases from the top region of the SZ to the bottom region of the SZ (also known as shear-processed zone;SPZ).However,the region below the SPZ,(i.e.below the plunge depth) experienced an increase in average grain size and dislocation densities compared to the other regions.There is no secondary phase observed in the FSPed region,however,the microstructural evolution in the FSPed region affects the electrochemical behavior of the HEA.Cr2O3 passive layer was observed to form on the FSPed HEA,leading to excellent corrosion properties from the polarization corrosion tests.Grain refinement in the SZ enhances the rapid formation of the passive layer,thus,leading to better corrosion properties in the front surface of the FSPed HEA.The localized corrosion behavior of the FSPed HEA was predicted to be caused by the micro-galvanic nature of the HEA,which leads to an increase in polarization at the anodic sites (pits).A numerical model was established using the corrosion parameters from the experiment to simulate the localized corrosion behavior on the surface of the FSPed HEA in a neutral environment.The predicted initial pitting potential and corresponding current density agree well with the experimental results.The model is also capable of tracking the dissolution of the pits over longer periods.     

The relationship between thermo-mechanical history,microstructure and mechanical properties in additively manufactured CoCrFeMnNi high entropy alloy

Here,a single-track CoCrFeMnNi high entropy alloy(HEA)was successfully fabricated by laser melting deposition(LMD).Combining the experimental observations and numerical simulation,the microstruc-ture and mechanical properties of the as-deposited parts were systematically studied from the perspective of thermo-mechanical history experienced during the LMD process.The strengthening mech-anisms of the LMDed CoCrFeMnNi HEA parts were clarified.The frictional stress strengthening,grain boundary strengthening and dislocation strengthening contributed the whole yield strength of the parts.Dislocation strengthening dominated the strengthening mechanism.It was expected that the establish-ment of the relationship between thermo-mechanical history,microstructure and mechanical properties of the LMDed CoCrFeMnNi HEA could shed more insights into achieving HEA parts with the desired microstructure and high performance.     

Optimization of bond strength between gold alloy and porcelain through a composite interlayer obtained by powder metallurgy

The purpose of this study was to evaluate the influence of a composite interlayer (at the metal-ceramic interface) on the shear bond strength of a metal-ceramic composite when compared with a conventional porcelain fused to metal (PFM).Several metal-ceramic composites specimens were produced by hot pressing. To identify which was the best composition for the interlayer several composites, with different relations of metal/ceramic volume fraction, were bonded to metal and to ceramic substrates. The bond strength of the composites to substrates was assessed by the means of a shear test performed in a universal test machine (crosshead speed: 0.5 mm/min) until fracture. Some interfaces of fractured specimens as well as undestroyed interface specimens were examined with optical microscope and scanning electron microscope (SEM/EDS).The shear bond strength results for all composites bonded to metal and to ceramic substrates were significantly higher (>150 MPa) than those registered in the upper range of conventional porcelain fused to metal (PFM) techniques (∼80 MPa). The use of a composite interlayer proved to enhance metal/ceramic adhesion in 160%.     

Effect of ball milling time on the structural characteristics and mechanical properties of nano-sized Y2O3 particle reinforced aluminum matrix composites produced by powder metallurgy route

Mechanical milling presents an effective solution in producing a homogenous structure for composites. The present study focused on the production of 0.5 wt% yttria nanoparticle reinforced 7075 aluminum alloy composite in order to examine the effects of yttria dispersion and interfacial bonding by ball milling technique. The 7075 aluminum alloy powders and yttria were mechanically alloyed with different milling times. The milled composites powders were then consolidated with the help of hot pressing. Hardness, density, and tensile tests were carried out for characterizing the mechanical properties of the composite. The milled powder and the microstructural evolution of the composites were analyzed utilizing scanning and transmission electron microscopy. A striking enhancement of 164% and 90% in hardness and ultimate tensile strength, respectively, were found compared with the reference 7075 aluminum alloy fabricated with the same producing history. The origins of the observed increase in hardness and strength were discussed within the strengthening mechanisms' framework.     

Preparation of nitinol by non-conventional powder metallurgy techniques

The aim of the present paper was to compare the evolution of Ni–Ti intermetallics in two non-conventional production techniques for the synthesis of NiTi shape memory alloy. Short term ultrahigh energy mechanical alloying is proposed to be able to describe the early stages of the milling process, which was not described in the literature previously, and to obtain intermetallics in shorter process durations. The reactive sintering using high heating rate (>300°C?min^??1) is a process designed to suppress the formation of secondary intermetallics and to reduce the porosity of the product. The same phases' formation sequence was determined for both processes. The detrimental Ti₂Ni phase forms preferentially, and therefore, its presence cannot be avoided in any of the investigated techniques.     

Characterization and properties of CuZrAlTiNi high entropy alloy coating obtained by mechanical alloying and vacuum hot pressing sintering

CuZrAlTiNi High entropy alloy (HEA) coating was synthesized on T10 substrate using mechanical alloying (MA) and vacuum hot pressing sintering (VHPS) technique. The MA results show that the final product of as-milled powders is amorphous phase. The obtained coating sintered at 950 °C is compact and about 0.9 mm in thickness. It is composed of a couple of face-centered cubic (FCC), one body-centered cubic (BCC) solid solutions and AlNi₂Zr phase. The interface strength between coating and substrate is 355.5 MPa measured by three point bending test. Compared with T10 substrate, the corrosion resistance of CuZrAlTiNi HEA coating is enhanced greatly in the seawater solution, which is indicated by the higher corrosion potential, wider passivation region, and secondary passivation. The average microhardness of the coating reaches 943 HV_0.2, and is about 3.5 times higher than the substrate, which is mainly ascribed to the uniformly dispersed nano-size precipitates, phase boundary strengthening and solid solution strengthening. Moreover, the wear resistance of the coating is slightly improved in comparison with the substrate.     

An improved process for grain refinement of large 2219 Al alloy rings and its influence on mechanical properties

The large 2219 Al alloy rings used to connect propellant tank components of a satellite launch vehicle to each other are conventionally manufactured by radial-axial ring rolling at 460°C with 50% deformation,but often suffer from coarse elongated grain and low ductility. An improved process(hot ring rolling at460°C with 30% deformation, then air cooling to 240°C, followed by ring rolling at 240°C with 20% deformation) was tested for ring manufacturing. The corresponding microstructure evolution and mechanical properties of the produced rings were studied. The results show that the improved process can successfully be applied to manufacture the large 2219 Al alloy rings without formation of macroscopic defects,resulting in a product with fine and uniform grains after heat treatment. The fracture mechanism of both rings was mainly intergranular fracture. With the resulting grain size refinement due to the improved process, more homogeneous slip occured and the crack propagation path became more tortuous during the tensile testing process. Thus, the elongation in all three orthogonal directions was greatly improved,and the axial elongation increased from 3.5% to 10.0%.     

Additive manufacturing of high entropy alloys:A practical review

The novel idea of alloying,which is based on the utilization of multiple principal elements in high concen-trations,has created a novel class of promising materials called high entropy alloys(HEAs).So far,several HEAs with outstanding properties beyond those of conventional alloys have been discovered,and new superior high-entropy alloys are still expected to be developed in the future.However,the fabrication process of HEAs through conventional manufacturing techniques suffers from significant limitations due to the intrinsic requirements of HEAs.Additive manufacturing(AM),on the other hand,has provided new opportunities for fabricating geometrically complex HEAs with the possibility of in situ tailoring of their microstructure features.Considering the growing interest in AM of HEAs during most recent years,this review article aims at providing the state of the art in AM of HEAs.It describes the feedstock requirements for laser based AM techniques.Thereafter,a comprehensive picture of the current state of nearly all HEAs processed by laser metal deposition(LMD),selective laser melting(SLM)and selec-tive electron beam melting(SEBM)is presented.Special attention is paid to the features of AM derived microstructures along with their outstanding properties and underlying mechanisms for various mate-rial processing combinations.The AM of interstitial solute hardening HEAs,HEA matrix composites as well as non-beam based AM of HEAs will also be addressed.The post-AM treatments and the strategies to fabricate defect-free HEAs are summarized.Finally,a conclusion of current state and future prospects of additive manufacturing of HEAs will be presented.     

Microstructure and mechanical properties of CoCrNi-Mo medium entropy alloys:Experiments and first-principle calculations

The effect of Mo additions on the microstructures and mechanical properties of CoCrNi alloys was investigated,meanwhile,ab initio calculations are performed to quantitatively evaluate the lattice distortion and stacking fault energy(SFE).The yield strength,ultimate tensile strength,and elongation of(CoCrNi)₉₇Mo₃alloy are 475 MPa,983 MPa and 69%,respectively.The yield strength is increased by~30%and high ductility is maintained,in comparison with CoCrNi alloy.Besides the nano-twins and dislocations,the higher density of stacking faults is induced during the tensile deformation for(CoCrNi)₉₇Mo₃alloy.Ab initio calculation results indicate the mean square atomic displacement(MSAD)and SFE value of(CoCrNi)₉₇Mo₃alloy is 42.6 pm²and-40.4 mJ/m²at 0 K,respectively.The relationship between mechanical properties and MSAD,SFE for various multiple principal element alloys is discussed.     

Texture and mechanical properties of ultrafine-grained Mg-3Al-1Zn alloy sheets prepared by high-ratio differential speed rolling

Mechanical properties and textures of the ultrafine grained (UFG) Mg-3Al-1Zn (AZ31) alloy with a mean grain size of 1 μm produced by high-ratio differential speed rolling were investigated. The resulting material exhibited high strength and relatively high ductility at ambient temperature. The high strength was attributed to grain-size and texture strengthening, while the high ductility was attributed to suppression of inhomogeneous twinning and increased strain-rate-sensitivity. The rolling temperature and the amount of shear strain accumulated during HRDSR affected the basal texture intensity and the rotation angle of the basal poles. Bimodal grain-size distribution obtained by annealing the UFG AZ31 at 573 K for a short time period resulted in considerable improvement of uniform elongation.     

Microstructural evolution in ultrasonically processed in situ AZ91 matrix composites and their mechanical and wear behavior

AZ91 alloy matrix composites reinforced with phases formed in situ from the addition of Si particles were fabricated by solidification under ultrasonic vibrations. Application of high-intensity ultrasonic field to the melt resulted in optimized size, morphology and distribution of in situ formed Mg₂Si particles. The amount of Mg₂Si particles increased, its size was refined and the distribution became uniform. Heterogeneous nucleation from the addition of silicon particles and enhanced nucleation from rapid cooling refined the grain size of the matrix in the composites. Hardness and ultimate compressive strength of the composites increased as compared to that of the cast AZ91 alloy. Composites exhibited improved sliding wear behavior of under varying normal loads. Identified dominant wear mechanism at lower sliding velocities is abrasion. Improvement in mechanical and sliding wear properties of the composites is attributed to the refinement of both matrix and reinforcement phases and improved dispersion of the reinforcement under ultrasonic vibrations.     

Effects of La addition on the elevated temperature properties of the casting Al-Cu alloy

The tensile strength and creep resistance at elevated temperatures of the casting Al-Cu alloy with La addition have been investigated. The results show that La can remarkably improve the tensile strength and creep resistance of the alloy. With 0.2-1.0 wt.% La addition, Al₁₁La₃ phase is found in the grain boundaries and spaces among the dendrites in the modified alloy. The tensile strength and creep resistance of the alloy with 0.3 wt.% La addition are found to be optimum.     

Superior mechanical properties and strengthening mechanisms of lightweight AlxCrNbVMo refractory high-entropy alloys(x = 0, 0.5,1.0) fabricated by the powder metallurgy process

Light and strong AlxCrNbVMo(x=0,0.5,and 1.0) refractory high-entropy alloys(RHEAs) were designed and fabricated via a the powder metallurgical process.The microstructure of the AlxCrNbVMo alloys consisted of a single BCC crystalline structure with a sub-micron grain size of 2-3 μm,and small amounts(4 vol.%) of fine oxide dispersoids.This homogeneous microstructure,without chemical segregation or micropores was achieved via high-energy ball milling and spark-plasma sintering.The alloys exhibited superior mechanical properties at 25 and 1000℃ compared to those of other RHEAs.Here,CrNbVMo alloy showed a yield strength of 2743 MPa at room temperature.Surprisingly,the yield strength of the CrNbVMo alloy at 1000℃ was 1513 MPa.The specific yield strength of the CrNbVMo alloy was increased by 27 % and 87 % at 25 and 1000℃,respectively,compared to the AlMo_(0.5) NbTa_(0.5)TiZr RHEA,which exhibited so far the highest specific yield strength among the cast RHEAs.The addition of Al to CrNbVMo alloy was advantageous in reducing its reduce density to below 8.0 g/cm~3,while the elastic modulus decreased due to the much lower elastic modulus of Al compared to that of the CrNbVMo alloy.Quantitative analysis of the strengthening contributions,showed that the solid solution strengthening,arising from a large misfit effect due to the size and modulus,and the high shear modulus of matrix,was revealed to predominant strengthening mechanism,accounting for over 50 % of the yield strength of the AlxCrNbVMo RHEAs.     

Microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion

The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.