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.     

Strengthening the mechanical properties and wear resistance of CoCrFeMnNi high entropy alloy fabricated by powder metallurgy

In this study, an equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by a rapid solidified gas atomization process. Subsequently, the high-energy mechanical milling was carried out to further refine the microstructure of pre-alloyed powder to improve the sintering ability and strengthening of HEAs. The microscopic results show that the powder morphology significantly changed from spherical to flatten, flake, irregular, and partially spherical shape with increasing milling time. The XRD results exhibited HEA bulks consisting of major FCC and minor Cr₇C₃ phases. The hardness of HEA bulks increased from 270±10 Hv to 450±10 Hv with increasing milling time, while the compressive yield strength increased from 370 MPa to 1050 MPa due to grain boundary strengthening and dislocation strengthening. Meanwhile, the lowest coefficient of friction ～0.283 and specific wear rate ～1.03×10^-5 mm³/Nm were obtained for the 60 min milled HEA due to increased surface hardness and oxidation behavior. The developed powder metallurgy approach could be considered as a promising way to improve the strength and wear resistance when compared to the conventional processed CoCrFeMnNi HEAs.     

Investigation of characteristics of Cu based,Co-CrC reinforced composites produced by powder metallurgy method

This study was carried out to assess the contribution of Co and CrC reinforcement particles to Cu main matrix powders by powder metallurgy (P/M) method as well as to mechanical properties, abrasion resistance and microstructure properties. For this purpose, the matrix material Cu powders, Co and CrC reinforcement particles, of which the weight percents are as follows: 5% (2% Co + 3% CrC), 10% (4% Co + 6% CrC) and 15% (6% Co + 9% CrC), were added. The samples were produced by cold pressing at room temperature and 450 MPa pressure, using the P/M method. Microstructure characterization of composite samples was performed by SEM-EDS and XRD analysis. Mechanical characterization of composite samples was carried out by analyzing the data of density determinations, hardness measurements, abrasion resistance tests and tensile tests. As a result of this study, it was observed that as the reinforcement ratio increased, the relative density of the samples decreased while the hardness of the samples increased. As a result of the findings obtained from the analysis of the tensile tests and other tests performed, it was clearly seen that the reinforcement particle ratio of wt. 10% was the optimum reinforcement ratio for this study.     

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.     

Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials using powder metallurgy: A review

The bio-active and biodegradable properties of hydroxyapatite (HA) make this material a preferred candidate for implants such as bone replacement in replacing natural tissues damaged by diseases and accidents. However, the low mechanical strength of HA hinders its application. Combining HA with a biocompatible material with a higher mechanical strength, such as a titanium (Ti) alloy, to form a composite has been of interest to researchers. A HA/Ti composite would possess characteristics essential to modern implant materials, such as bio-inertness, a low Young’s modulus, and high biocompatibility. However, there are issues in the material processing, such as the rheological behavior, stress-shielding, diffusion mechanism and compatibility between the two phases. This paper reviews the HA and Ti alloy interactions under various conditions, in vitro and in vivo tests for HA/Ti composites, and common powder metallurgy processes for HA/Ti composites (e.g., pressing and sintering, isostatic pressing, plasma spraying, and metal injection molding).     

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.     

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.     

Whiskers of Al2O3 as reinforcement of a powder metallurgical 6061 aluminium matrix composite

An Al-Mg-Si alloy matrix composite reinforced with 10 vol.% of alumina whiskers (Al₂O₃w) has been processed by powder metallurgy and investigated. The Al₂O₃w were produced as single crystal c-axis alpha-alumina fibres at pre-pilot scale via vapour-liquid-solid (VLS) deposition in a cold-wall air-tight furnace with alumina linings. As far as we know, this is the first report of the utilization of whiskers of Al₂O₃ as reinforcing elements for Al alloys. Tensile tests have been performed on the composite at room and high temperatures. Results show that the AA6061 alloy reinforced with the as-produced Al₂O₃ whiskers has remarkably high mechanical properties at room temperature. This is attributed to the high quality of the Al₂O₃ single crystals and to the strong bonding attained between them and the 6061 alloy matrix.     

Application of neural network and genetic algorithm to powder metallurgy of pure iron

In the present paper, soft computing techniques are applied to optimize the powder metallurgy processing of pure iron. An artificial neural network is trained to predict the stress resulting from a given trend in strain and sintering temperature. To prepare an appropriate model, pure iron powders are compacted and sintered at various temperatures. Subsequently, compression test is conducted at room temperature on the bulked samples. The sintering temperatures and the corresponding stress–strain records are used as sets of data for the training process. The performance of the network is verified by putting aside one set of data and testing the network against it. Eventually, by using a genetic algorithm, an optimization tool is created to predict the optimum sintering temperature for a desired stress–strain behavior. Comparison of the predicted and experimental data confirms the accuracy of the model.     

Surface modifications induced by shot peening and their effect on the plane bending fatigue strength of a Cr-Mo steel produced by powder metallurgy

The effect of shot peening on the plane bending fatigue strength of a 7.1 g/cm³ sintered Cr-Mo steel was investigated. Shot peening provides surface densification, strain hardening, compressive residual stresses up to −700 MPa, without impairing the dimensional and geometrical precision of specimens. Plane bending fatigue strength increases of 30%, irrespective to the different residual stress profiles obtained by changing the shot peening parameters. The improvement is mainly due to the surface densification and strain hardening.     

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.     

Effect of rotation speed and milling time to attain Nb-10Hf-1Ti alloy powders on morphology and microstructure of Nb-10Hf-1Ti powder

The mechanical alloying process was employed to produce C103 alloy with Nb-10% Hf-1% Ti (wt.%) composition using Nb, Hf and Ti powders. The mechanical alloying process was performed in an argon atmosphere in the chamber and bullets of tungsten carbide with a ball-to-powder weight ratio (BPR) of 20:1 at rotation speed of 200, 300 and 400 rpm for 2, 5 and 8 h. At rotation speeds of 200 and 300 rpm particle size decreased and became more spherical during MA. While increasing milling time at 400 rpm caused agglomeration of particles. XRD results showed that increasing milling time at a constant rotation speed has no considerable effect on reduction of crystallite size, but the lattice strain is strongly affected by it and increased obviously with further rotation speed. The results showed that the optimum milling time and rotation speed to attain Nb-10Hf-1Ti alloy powders with the least amount of contamination and appropriate morphology are 5 h and 300 rpm, respectively.     

Preliminary study on the fabrication of 14Cr-ODS FeCrAl alloy by powder forging

A simple powder forging process was presented herein to fabricate an Fe-14Cr-4.5Al-2W-0.4Ti-0.5Y2O3 ODS FeCrAI alloy.The forged alloy exhibits a high density that exceeds 97％of the theoretical density.The ODS alloy was investigated in terms of the residual porosity,morphology and phase structure of oxide nanoparticles,impact toughness and tensile properties.It was found that refined grains were obtained during powder forging.A residual porosity less than 1.1％has no impact on the precipitation of oxide nanoparticles.The average diameter of the oxide particles is 7.99 nm,with a number density of 2.75 x 1022 m-3.Almost all of the oxides are identified as orthorhombic YAlO3 particles.The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700℃,and enable the ductile-to-brittle transition temperature to be close to room temperature.However,a slight decrease in strength at 1000℃and the Charpy upper shelf energy has been suggested to be due to the residual porosity.These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.     

A comparison of composite metal foam's properties and other comparable metal foams

New closed cell composite metal foams are processed using casting and powder metallurgy (PM) techniques. The foam is comprised of steel hollow spheres packed into a random loose arrangement, with the interstitial spaces between spheres occupied with a solid metallic matrix. The characterization of composite metal foams was carried out using monotonic compression, compression-compression fatigue, loading-unloading compression, micro-hardness and nano-hardness testing. The microstructure of the composite metal foams was studied using optical, scanning electron microscopy imaging and electron dispersive spectroscopy. The composite metal foams displayed superior (5-20 times higher) compressive strengths, reported as 105 MPa for cast foams and 127 MPa for PM foams, and much higher energy absorbing capability as compared to other metal foams being produced with similar materials through other technologies.     

Characterization of the prior particle boundaries in a powder metallurgy Ti_2AlNb alloy

Ti_2 AlNb-based alloy powder metallurgy(PM) compacts were prepared via hot isostatic pressing(HIP)under relatively low temperature(920 and 980℃) and at certain pressure(130 MPa).The microstructure,composition and orientation of B2,α_2 and O phases in the compacts were characterized and analyzed with an aim to investigate the effect of unsuitable HIPping parameters on the appearance of prior particle boundary(PPB),which seriously affects the mechanical properties of the alloy.The results show that more α_2 phase is the characteristics of the PPB in Ti_2AlNb-based alloy when HIPped at relatively low temperature.Increasing HIPping temperature to the upper part of the two-phase region can effectively inhibit the formation of PPB.Electron backscatter diffraction measurements show the specific orientation relationship between phases,which helps us understand the origin of a2 and O phase and the corresponding transformation path.The HIPping at a higher temperature can weaken the micro-texture intensity of the α_2 and O phase due to the increase of misorientation in B2 phase.The α_2 phase at cell wall keeps the Burgers orientation relationship(BOR) with the grain on one side,and does not satisfy the BOR with the other.It is found that some O phase variants inside the cell HIPped at 980℃ can only maintain α_2-O OR with α_2 owing to the α_2→O phase transformation forming the O phase,while these O variants deviate from B2-O OR with B2 phase.     

Shear strength prediction of Ni–Ti alloys manufactured by powder metallurgy using fuzzy rule-based model

Powder metallurgy is an important manufacturing method and developing models that can predict the characteristics of the products is remarkable for researchers. There are many models discussed in the literature for prediction of the product properties however, nonlinear modeling methods including artificial neural networks (ANNs) and fuzzy models have shown better performance.     

Fabrication of Ti–Nb–Ag alloy via powder metallurgy for biomedical applications

Ti and some of its alloys are widely used as orthopedic implants. In the present study, Ti–26Nb–5Ag alloys were prepared by mechanical alloying followed by vacuum furnace sintering or spark plasma sintering (SPS). The microstructure and mechanical properties of the Ti–Nb–Ag alloys were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), compressive and micro-hardness tests. The effect of different sintering methods on the microstructure and properties of Ti–Nb–Ag alloy was discussed. The results showed that the titanium alloy sintered by vacuum furnace exhibited a microstructure consisting of α, β and a small amount of α″ martensite phase; whilst the SPS sintered alloy exhibited a microstructure consisting of α, β and a small amount of α″ martensite phase, as well as a nanostructured Ag homogeneously distributed at the boundaries of the β phases. The Ti–Nb–Ag alloy sintered by SPS possessed fracture strength nearly 3 times of the alloy sintered by vacuum furnace.     

Structure and properties of ductile CuAlMn shape memory alloy synthesized by mechanical alloying and powder metallurgy

A ductile Cu–Al–Mn–Ti–B shape memory alloy with high fatigue strength has been prepared via mechanical alloying and powder metallurgy. With increasing milling time, the size of the crystallite grains decreases. Cu diffraction pattern appeared only after milling at a speed of 300 rpm for 25 h. The single phase CuAlMnTiB solid solution powder after 35 h milling was hot-pressed and extruded to form the final alloy. The quenched alloy had a single β phase at room temperature and its yield strength, maximum strength and strain were measured to be 390 MPa, 1015 MPa and 14.4%, respectively. The aged alloy showed a martensite structure at room temperature and had a shape memory recovery of 92% after 120 cycles.     

Microstructural and thermal processing effects on adding 1 and 3 w/o Ti to a powder metallurgy processed quaternary Ni-Cr-Fe-Al alloy

The effect of the addition of 1 and 3 w/o Ti to a quaternary (Ni-Cr-Fe-Al) alloy on the phase transformations that might occur in the material on sintering were simulated using a thermodynamic modelling tool. These predictions were subsequently compared with experimental results obtained by X-ray diffraction and metallography. As well, the onset of melting and the transformation temperature of the Ti modified alloys were corroborated by Differential Scanning Calorimetry (DSC). From SEM and point count analyses, the microstructure, including the % porosity and volume fraction of gamma prime precipitates, remained relatively unchanged from the quaternary without Ti. This may have been due to the presence of sub-micron precipitates not detected in the Ti-containing samples. However, an increase in lattice parameters on adding both 1 and 3 w/o Ti to the quaternary was determined from X-ray diffraction measurements. Finally, the software modelling provided a reasonable prediction for both microstructure and thermal processing thereby offering a means to simulate both design and characterisation of the experimental material, both during sintering and on cooling.     

Powder metallurgy of the porous Ti-13Nb-13Zr alloy of different powder grain size

The objective of the present project was to determine the effects of powder granulation (fraction of grain size) for the Ti-13Nb-13Zr alloy, produced by powder metallurgy, on its porosity, grain cohesion, compressive strength, and Young`s modulus. Two powder fractions, 45–105 µm, and 106–250 µm were applied. The 50 mass pct of NH<sub>4</sub>HCO<sub>3</sub> was added as a space holder. The specimens were in compaction stage uniaxially pressed at pressure 625 MPa for 120 s. The brown bodies were sintered at a temperature 1150°C for 3.5 h. The well-joined grains were observed for both powder granulations. The increase in powder granulation resulted in an increase of porosity from 51% to 59%, and it was only 30% with no space holder used. The compressive strength increased with decreased porosity from 57 to 236 MPa. Young`s modulus was measured as 4.8 GPa for finer powder and 0.9 GPa for coarser powder. It is evident from the results obtained that the applied process parameters, the space holder and its fraction, and the use powder granulation between 45 and 105 µm bring out the porous material fulfilling mechanical and biological requirements specific of load-bearing titanium implants.