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.     

Microstructural evolution of bulk nanocrystalline Ni during creep

Experiments were conducted on electrodeposited (ED) nanocrystalline (nc) Ni with an average initial grain size of about 20 nm at 393 K to study the shape of the creep curves. In addition, microstructure was examined by means of transmission electron microscopy (TEM). The results show that the creep curves are characterized by the presence of a well-defined steady-state stage. An examination of the microstructure indicates that while grain growth occurs during deformation, the grain size attains a constant value once steady state creep is approached. A comparison between grain size measurements obtained by the TEM technique and those obtained via the X-ray diffraction method shows that the use of the latter method may lead to an underestimation of the value of the average grain size.     

Effect of copper on the mechanical properties of alloys formed by powder metallurgy

Alloys formed by powder metallurgy are typically porous, which reduces their strength. In this study, we attempt to improve the mechanical properties of an alloy composed of 0.6 wt% C, 1.0 wt% Ni, 0.3 wt% Mo, 0.7 wt% Mn and the balance Fe by addition of 8 wt% Cu. To form the alloys, powders are blended and compacted in a dual-action hydraulic press and then sintered in a furnace at 1150 °C. Alloys with and without Cu are used in specific parts designed for impact testing. Stress analysis is performed using ANSYS, which validates the operation of the parts. The strength of the body geometry according to its design is determined by considering the manufactured material and the loads that it is subjected to during operation. SEM images revealed that the alloy without Cu contains martensite and bainite phases with large, irregular pores. In contrast, the alloy with Cu has a considerably lower pore concentration. During sintering, Cu forms a liquid phase that can fill the spaces between the particles of the alloying powders. The result is an alloy with increased density and toughness; the density of the alloy increases from 7.2 to 7.8 g/cm³ upon addition of Cu, and its toughness increases from 22 to 34 J.     

Microstructural characteristics and mechanical properties of magnetron sputtered nanocrystalline TiN films on glass substrate

Nanocrystalline TiN thin films were deposited on glass substrate by d.c. magnetron sputtering. The microstructural characteristics of the thin films were characterized by XRD, FE-SEM and AFM. XRD analysis of the thin films, with increasing thickness, showed the (200) preferred orientation up to 1·26 μm thickness and then it transformed into (220) and (200) peaks with further increase in thickness up to 2·83 μm. The variation in preferred orientation was due to the competition between surface energy and strain energy during film growth. The deposited films were found to be very dense nanocrystalline film with less porosity as evident from their FE-SEM and AFM images. The surface roughness of the TiN films has increased slightly with the film thickness as observed from its AFM images. The mechanical properties of TiN films such as hardness and modulus of elasticity (E) were investigated by nanoindentation technique. The hardness of TiN thin film was found to be thickness dependent. The highest hardness value (24 GPa) was observed for the TiN thin films with less positive micro strain.     

Microstructural characteristics of gas tungsten arc synthesised Fe-Cr-Si-C coating

Abstract

The gas tungsten arc (GTA) method was used to synthesise Fe-Cr-Si-C alloy coatings, and processing effects on the coating were investigated experimentally. Coatings were developed on an AISI type 1040 steel substrate. Four different regions were obtained in the surface coating; and in these regions either a hypoeutectic or a hypereutectic microstructure was found. The hypoeutectic microstructure consisted of primary dendrites of austenite (γ) phase and eutectic M₇C₃ (M=Cr,Fe) carbides. On the other hand, the hypereutectic microstructure consisted of M₇C₃ primary carbides and eutectic. A hypoeutectic or hypereutectic microstructure was determined by the combination of particularly carbon concentration, solidification rate, and extent of substrate melting. The higher hardness of the hypereutectic microstructure is attributed especially to the formation of M₇C₃ primary carbides. The lower hardness of the hypoeutectic microstructure is related to three effective parameters: first, the presence of γ phase in the primary dendrites; second, excessive dilution from the base material; and third, relatively low concentrations of chromium and carbon.     

Improvement in physical and mechanical properties of aluminum/zircon composites fabricated by powder metallurgy method

Metal–matrix composites (MMCs) are known as the most useful and high-tech composites in our world as well as aluminum (Al) as the best metal for producing these composites. Combining aluminum and zircon (ZrSiO₄) will yield a material with the best corrosive resistance and mechanical properties like strength at high temperatures. Also, the abrasive wear behavior of these composites will be improved. In the present investigation, a study on aluminum/zircon composites has been carried out. Micro-structures of these composites in powder metallurgy conditions show different size distribution of zircon with different proportions in the composite. Also, there is a case-study about density and compressive strength and hardness of aluminum/zircon composites. The green specimens prepared by isostatic pressing of prepared powders with different zircon percentages, were sintered at two temperatures. These specimens were then investigated by different physical and mechanical testing methods to observe in which conditions the best properties would be obtained. The most improved compression strength was obtained with the specimen including 5% of zircon sintered at 650 °C.     

Consolidation and mechanical behaviour of nanocrystalline iron powder

Abstract

Iron nanopowders, with an average diameter of <100 nm, were consolidated using plasma pressure compaction (P²C). The consolidated samples produced were of the order of 15 mm in diameter and 4·1 mm thick. The time and pressure of compaction were varied in order to study the effects of processing variables on the microstructure and the mechanical behaviour in compression of the materials. It was found that the compliance of the loading curves was a function of the mesoscale porosity in the specimens. The strain to failure was found to be a function of the porosity on the microscale. The optimal strength and ductility were achieved in specimens in which a fully dense (on the microscale), platelet structure had developed.     

Corrosion behavior of bulk nanocrystalline copper in ammonia solution

Corrosion behavior of copper bulk with grain size of 48 nm prepared by inert gas condensation and in situ warm compress (IGCWC) technique was investigated in 0.3 wt.% ammonia solution. The nanocrystalline (NC) Cu sample displayed an active-passive-transpassive behavior with the formation of duplex passive films but without stable passive regions in potentiodynamic polarization process. NC Cu exhibited slightly inferior corrosion resistance when compared with coarse-grained (CG) Cu. It could be explained by both the higher grain boundary density that could accelerate corrosion reactions and the loose passive film formed on the surface of NC Cu that couldn't provide effective protection.     

Microstructural features, texture and strengthening mechanisms of nanostructured AA6063 alloy processed by powder metallurgy

Nanostructured AA6063 (NS-Al) powder with an average grain size of ∼100 nm was synthesized by high-energy attrition milling of gas-atomized AA6063 powder followed by hot extrusion. The microstructural features of the consolidated specimen were studied by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) techniques and compared with those of coarse-grained AA6063 (CG-Al) produced by hot powder extrusion of gas-atomized powder (without using mechanical milling). The consolidated NS-Al alloy consisted of elongated ultrafine grains (aspect ratio of ∼2.9) and equiaxed nanostructured grains. A high fraction (∼78%) of high-angle grain boundaries with average misorientation angle of 33° was noticed. Microtexture evaluation by plotting pole-figures and orientation distribution function (ODF) analysis showed Copper and P texture components for both the consolidated Al alloys. Tensile test at room temperature and microhardness measurement revealed that a significant improvement in the strength of AA6063 alloy is obtained through refinement of the grain structure. The strengthening mechanisms are discussed based on the dislocation-based models. The role of high-angle and low-angle grain boundaries on the strengthening mechanisms is discussed.     

Analysis of ductility of nanostructured copper prepared by powder metallurgy

The strain rate sensitivity (m) was determined as a function of the strain rate using mechanical jump tests, on a nanostructured copper (grain size of 90 nm) prepared by powder metallurgy. The largest value is m = 0.050 ± 0.005, measured at 1 × 10⁻⁵ s⁻¹. Apparent activation volume was derived giving an insight of the micromechanisms involved in the plastic deformation. Nanostructured face centred cubic metals exhibit elongation due to their strain rate sensitivity and the related delay in the localisation of the deformation. The present analysis brings elements of the micromechanism involved in the plasticity, providing with the guide to design a relevant ultra-fine grained architecture with improved ductility.     

Strengthening effects in precipitation and dispersion hardened powder metallurgy copper alloys

The hardening of copper and copper alloy matrix using powder metallurgy (PM) techniques and different ways for dispersoids formation, as well as analysis of their single and combined effects on the strength of obtained material at room and elevated temperatures, have been presented and discussed. Gas atomized Cu–3.8 wt.%Ti and Cu–0.6 wt.%Ti–2.5 wt.%TiB₂ (Cu–Ti–TiB₂) powders and mechanically alloyed powder Cu–4 wt.%TiB₂ were used as starting materials. The powders were consolidated by hot isostatic pressing (HIP) and hot pressing (HP). Optical, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDAX), as well as transmission electron microscope (TEM) were used for microstructure characterization of the compacts. High strengthening of the Cu–Ti compacts was achieved by thermal treatment (aging) as a consequence of the development of modular structure and precipitation of metastable Cu₄Ti_(m). Hardening in the Cu–Ti–TiB₂ compacts is due to simultaneous influence of the following factors: the development of modular structure, precipitation of metastable Cu₄Ti_(m), and the presence of TiB₂ dispersoid nanoparticles. In case of Cu–TiB₂ compacts, high starting values of hardness and hardness on the elevated temperatures result from the presence of finely distributed TiB₂ particles in copper matrix obtained by mechanical alloying. Cu–Ti–TiB₂ composite yields much higher hardness values compared with the binary Cu–Ti alloys, owing to primary TiB₂ dispersions formed during atomization. Separation of metastable Cu₄Ti precipitate and the presence of significantly finer TiB₂ particles in the copper matrix are the reason for higher hardness values at peak temperatures (400–500 °C) in multiple-hardened copper alloy compared to the dispersion-hardened.     

Development of TiB and nanocrystalline Ti-reinforced novel hybrid Ti nanocomposite produced by powder metallurgy

Journal of Materials Science - Titanium monoboride (TiB) whisker-reinforced titanium (Ti) matrix composites were produced by powder metallurgy, through vacuum sintering. TiB is formed by thermal...     

Microstructural characterization of nanocrystalline nickel produced by surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), nanocrystalline surface layers are produced in pure Ni plates. The average crystallite size, root mean square (r.m.s.) microstrain, dislocation density, and stored elastic energy are determined by X-ray diffraction (XRD) line profile analysis. The average crystallite size obtained by XRD is compared with the grain size observed from transmission electron microscopy (TEM) image. The high-resolution TEM (HRTEM) micrograph confirms the presence of high density of dislocations obtained by XRD, and reveals that most of dislocations distribute at the subgrain boundaries with few inside the subgrains.     

Synthesis and characterization of bulk amorphous/amorphous composite alloys through powder metallurgy route

In the present study, amorphous Ni₆₀Nb₂₀Zr₂₀ and Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized separately using a mechanical alloying (MA) technique. The dual-amorphous-phased (Ti₅₀Cu₂₈Ni₁₅Sn₇)_100−x (Ni₆₀Nb₂₀Zr₂₀) _x (x = 0, 10, 20, and 30 vol%) powders were prepared by mixing the corresponding amorphous powders. The dual-amorphous-phased powders were then consolidated into bulk amorphous/amorphous composite (BA/AC) alloy discs. The amorphization status of as-prepared powders and bulk BA/AC composite discs was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of the BA/AC discs showed that the Ni₆₀Nb₂₀Zr₂₀ phase is distributed homogeneously within the Ti₅₀Cu₂₈Ni₁₅Sn₇ matrix. The (Ti₅₀Cu₂₈Ni₁₅Sn₇)₇₀(Ni₆₀Nb₂₀Zr₂₀)₃₀ BA/AC disc exhibited a relative density of 96.6% and its Vickers microhardness was 726 kg/mm².     

The ferromagnetic transition in MnAs synthesised by mechanical alloying and powder blending techniques

A study of the structure and properties of MnAs synthesized by mechanical alloying and powder blending techniques has been carried out. Mechanical alloying resulted in the formation of MnAs during milling. Following heat treatment samples prepared using both techniques exhibited a magnetostructural transformation from the paramagnetic B31 to the ferromagnetic B8₁ phase with decreasing temperature or increasing magnetic field. The transformation temperatures and fields were found to depend on milling procedure and heat treatment conditions. Powder blended and heat treated samples exhibited excellent stability during transformation cycling.     

Microstructural characterisation of vacuum sintered T42 powder metallurgy high-speed steel after heat treatments

High-speed steel powders (T42 grade) have been uniaxially cold-pressed and vacuum sintered to full density. Subsequently, the material was heat treated following an austenitising + quenching + multitempering route or alternatively austenitising + isothermal annealing. The isothermal annealing route was designed in order to attain a hardness value of ～50 Rockwell C (HRC) (adequate for structural applications) while the multitempering parameters were selected to obtain this value and also the maximum hardening of the material (～66 HRC). Microstructural characterisation has been carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microstructure consists of a ferrous (martensitic or ferritic) matrix with a distribution of second phase particles corresponding to nanometric and submicrometric secondary carbides precipitated during heat treatment together with primary carbides. The identification of those secondary precipitates (mainly M₃C, M₆C and M₂₃C₆ carbides) has allowed understanding the microstructural evolution of T42 high-speed steel under different processing conditions.     

Effect of temperature-related factors on densification,microstructure and mechanical properties of powder metallurgy TiAl-based alloys

In the present work, the influence of temperature-related factors, including sintering temperature, heating step and temperature-control mode, on the densification, microstructure and mechanical properties of Ti-46.5Al-2.15Cr-1.90Nb-(B, Y, Mo) alloys prepared by SPS has been investigated and discussed in detail. The results obviously indicate that the sintering temperature plays a key role on densification and phase transition, when compared with the heating step and temperature-control mode. Based on the experimental results and theoretical analysis, the densification process and microstructural evolution of TiAl-based alloys during sintering are studied. Moreover, the mechanical properties of the sintered alloys are determined by the combined effects of the densification and microstructure. The obtained results will help to optimize the microstructure and properties for this kind of intermetallic alloys through controlling sintering parameters during powder metallurgy process.     

Freeform fabrication of functional aluminium prototypes using powder metallurgy

Freeform fabrication methods allow the direct formation of parts built layer by layer, under the control of a CAD drawing. Most of these methods form parts in thermoplastic or thermoset polymers, but there would be many applications for freeform fabrication of fully functional metal or ceramic parts. We describe here the freeforming of sinterable aluminium alloys. In addition, the building approach allows different materials to be positioned within a monolithic part for an optimal combination of properties. This is illustrated here with the formation of an aluminium gear with a metal-matrix composite wear surface.     

Microstructure and wear behavior of a porous nanocrystalline nickel-free austenitic stainless steel developed by powder metallurgy

This paper investigates the microstructure and dry sliding wear characteristics of a porous Cr–Mn–N austenitic stainless steel prepared by powder metallurgy. The densification of the mechanically alloyed 18Cr–8Mn–0.9N stainless steel powder is performed by sintering at 1100 °C for 20 h and subsequently water-quenching. This procedure gives rise to the development of a nanostructured austenitic stainless steel with a relative density of 85%. The porous biocompatible stainless steel exhibits an outstanding wear resistance compared with AISI 316L stainless steel samples. This is attributed to its considerable intrinsic hardness and its specific configuration of pores.