Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2003"

Single layer and multilayer wear resistant coatings of (Ti,Al)N: a review

Abstract: We review the status of (Ti,Al)N based coatings obtained by various physical vapor deposition (PVD) techniques and compare their properties. PVD techniques based on sputtering and cathodic arc methods are widely used to deposit wear resistant (Ti,Al)N coatings. These techniques were further modified to improve the metal ionization rate and to eliminate macrodroplets from plasma streams. We summarize manufacture of target/cathode, substrate materials for deposition of coatings, deposition parameters, and the effect of deposition parameters on the physical and mechanical properties of (Ti,Al)N coatings. It is shown that (Ti,Al)N coatings by PVD enhance the wear, thermal, and oxidation resistance of a wide variety of tool materials. We discuss the wear resistant properties of (Ti,Al)N for various machining applications as compared with coatings such as TiN, Ti(C,N) and (Ti,Zr)N. High hardness (∼28–32 GPa), relatively low residual stress (∼5 GPa), superior oxidation resistance, high hot hardness, and low thermal conductivity make (Ti,Al)N coatings most desirable in dry machining and machining of abrasive alloys at high speeds. Multicomponent coatings based on different metallic and nonmetallic elements combine the benefit of individual components leading to a further refinement of coating properties. Alloying additions such as Cr and Y drastically improve the oxidation resistance, Zr and V improve the wear resistance, whereas, Si increases the hardness and resistance to chemical reactivity of the film. Addition of boron improves the abrasive wear behavior of Ti–Al based coatings due to the formation of TiB2 and BN phases depending on the deposition conditions. Hafnium based nitrides and carbides have potential for resistance to flank and crater wear. The presence of a large number of interfaces between individual layers of a multilayered structure results in a drastic increase in hardness and strength. (Ti,Al)N multilayer super lattice coatings with lattice periodicity of 5–10 nm allow creation of coatings with different properties than PVD deposited single layered thick coatings with columnar grain structure. A range of (Ti,Al)N based multilayers containing layers of (Ti,Al)CN, (Ti,Nb)N, TiN, AlN/TiN, CrN, Mo and WC are also reviewed. It is now possible to design new wear resistant or functional coatings based on a multilayer or a multicomponent system to meet the demanding applications of advanced materials.

Friction stir processing: a novel technique for fabrication of surface composite

Abstract: A novel surface modifying technique, friction stir processing (FSP), has been developed for fabrication of surface composite. Al–SiC surface composites with different volume fractions of particles were successfully fabricated. The thickness of the surface composite layer ranged from 50 to 200 μm. The SiC particles were uniformly distributed in the aluminum matrix. The surface composites have excellent bonding with the aluminum alloy substrate. The microhardness of the surface composite reinforced with 27vol.%SiC of 0.7 μm average particle size was ∼173 HV, almost double of the 5083Al alloy substrate (85 HV). The solid-state processing and very fine microstructure that results are also desirable for high performance surface composites.

Processing techniques for functionally graded materials

Abstract: An overview of the achievements of the German priority program “Functionally Graded Materials (FGM)” in the field of processing techniques is given. Established powder processes and techniques involving metal melts are described, and recent developments in the field of graded polymer processing are considered. The importance of modeling of gradient formation, sintering and drying for the production of defect-free parts with predictable gradients in microstructure is discussed, and examples of a successful application of numerical simulations to the processing of functionally graded materials are given.

Development of interconnect materials for solid oxide fuel cells

Abstract: The state of the art development of interconnect materials for solid oxide fuel cells (SOFCs) is reviewed. The specific interconnect requirements from a materials design point of view are described in detail. Doped-LaCrO 3 ceramics considered as interconnects for electrolyte-supported planar SOFC suffer from their extreme sensitivity to oxygen partial pressure and high manufacturing cost. Advent of anode-supported planar SOFC allows lower operating temperatures and great efforts have been directed towards the development of metallic interconnects. Chromia formed on the surface of chromium-based alloys are intrinsically too volatile to be viable as interconnects for SOFCs operating above 800 °C. Some intermediate phases occurring between iron-based interconnect and cathode significantly increase the contact resistance. Metallic materials that are capable of developing stable oxide scales with acceptable growth rate and reasonably high electrical conductivity over the expected SOFC lifetime need to be developed.

Diffusion coefficients of some solutes in fcc and liquid Al: critical evaluation and correlation

Abstract: The diffusion coefficients of several transition elements (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and a few non-transition elements (Mg, Si, Ga, and Ge) in fcc and liquid Al are critically reviewed and assessed by means of the least-squares method and semi-empirical correlations. Inconsistent experimental data are identified and ruled out. In the case of the elements, for which plentiful experimental data are available in the literature, the least-squares analysis gives rise to the activation energies and pre-exponential factors in an Arrhenius equation. For the elements with limited experimental data or no data at all, the diffusion parameters are estimated from two semi-empirical correlations. In one correlation, the logarithmic pre-exponential factors are plotted against the activation energies for various elements in Al. In the other correlation, the activation energies are shown as a function of valences relative to Al. The diffusion coefficients calculated by using the evaluated diffusion parameters agree reasonably with the reliable experimental data. The proposed semi-empirical correlations are used to predict the diffusion coefficients of a few elements in liquid Al. A satisfactory agreement between the predicted and measured diffusion coefficients is obtained.

A review on the status of anode materials for solid oxide fuel cells

Abstract: Present review is aimed at providing a state-of-art development of anode for solid oxide fuel cell (SOFC) with principal emphasis on the materials aspect. The criteria for the anode of SOFC are first presented. The prospects and problems of the currently developed anode materials are elucidated. In particular, the electrochemical properties of the Ni/YSZ cermet anode that is the most commonly employed in the establishment of SOFC stack is described along with various approaches attempted for their improvements. The advantages and disadvantages of other anode materials are compared to offer some insights for the research and development of new generation of anode materials for SOFC.

Functionally graded materials for biomedical applications

Abstract: Functional gradation is one characteristic feature of living tissue. Bio-inspired materials open new approaches for manufacturing implants for bone replacement. Different routes for new implant materials are presented using the principle of functional gradation. An artificial biomaterial for knee joint replacement has been developed by building a graded structure consisting of ultra-high molecular weight polyethylene (UHMWPE) fibre reinforced high-density polyethylene combined with a surface of UHMWPE. The ingrowth behaviour of titanium implants into hard tissue can be improved by depositing a graded biopolymer coating of fibronectin, collagen types I and III with a gradation, derived from the mechanisms occurring during healing in vivo. Functionally graded porous hydroxyapatite (HAP) ceramics can be produced using alternative routes, e.g. sintering of laminated structures of HAP tapes filled with polymer spheres or combining biodegradable polyesters such as polylactide, polylactide-co-glycolide and polyglycolide, with carbonated nanocrystalline hydroxyapatite. HAP–collagen I scaffolds are an appropriate material for in vitro growth of bone. The scaffold has to be functionally graded in order to create an optimised mechanical behaviour as well as the intended improvement of the cell ingrowth.

On the influence of mechanical surface treatments—deep rolling and laser shock peening—on the fatigue behavior of Ti–6Al–4V at ambient and elevated temperatures

Abstract: It is well known that mechanical surface treatments, such as deep rolling, shot peening and laser shock peening, can significantly improve the fatigue behavior of highly-stressed metallic components. Deep rolling (DR) is particularly attractive since it is possible to generate, near the surface, deep compressive residual stresses and work hardened layers while retaining a relatively smooth surface finish. In the present investigation, the effect of DR on the low-cycle fatigue (LCF) and high-cycle fatigue (HCF) behavior of a Ti–6Al–4V alloy is examined, with particular emphasis on the thermal and mechanical stability of the residual stress states and the near-surface microstructures. Preliminary results on laser shock peened Ti–6Al–4V are also presented for comparison. Particular emphasis is devoted to the question of whether such surface treatments are effective for improving the fatigue properties at elevated temperatures up to ∼450 °C, i.e. at a homologous temperature of ∼0.4 T/T m (where T m is the melting temperature). Based on cyclic deformation and stress/life ( S / N ) fatigue behavior, together with the X-ray diffraction and in situ transmission electron microscopy (TEM) observations of the microstructure, it was found that deep rolling can be quite effective in retarding the initiation and initial propagation of fatigue cracks in Ti–6Al–4V at such higher temperatures, despite the almost complete relaxation of the near-surface residual stresses. In the absence of such stresses, it is shown that the near-surface microstructures, which in Ti–6Al–4V consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment.

Effect of mechanical alloying on the morphology, microstructure and properties of aluminium matrix composite powders

Abstract: A composite powder with a fine homogeneous distribution of the reinforcement phase in the whole particle can be obtained by mechanical alloying. Aluminium PM6061 unreinforced, and matrix composite reinforced with Si 3 N 4 and AlN powder, are milled in a high-energy attritor mill and the powder properties are compared with those of the same composite composition mixed in a horizontal low-energy ball mill. The correlation observed between the apparent densities and the milling time, explained by the morphological and microstructural evolution of the powder particles during the high-energy milling process, is used to determine the steady state of the process. At short milling times, the apparent density decreases as the milling time is extended, due to the deformation dominant at this stage; at longer milling times, it starts to increase with increasing milling time due to the piling up of the flattened particles and fracture of the welded particles. When mechanical alloying reaches the steady state, the apparent density is stabilized. A simple model is proposed to illustrate the mechanical alloying of a ductile–brittle component system. The particle size distribution and the microhardness of the mechanically alloyed particles are determined.

Effect of weld metal chemistry and heat input on the structure and properties of duplex stainless steel welds

Abstract: The excellent combination of strength and corrosion resistance in duplex stainless steels (DSS) is due to their strict composition control and microstructural balance. The ferrite–austenite ratio is often upset in DSS weld metals owing to the rapid cooling rates associated with welding. To achieve the desired ferrite–austenite balance and hence properties, either the weld metal composition and/or the heat input is controlled. In the current work, a low heat input process viz., EBW and another commonly employed process, gas tungsten-arc welding have been employed for welding of DSS with and without nickel enhancement. Results show that (i) chemical composition has got a greater influence on the ferrite–austenite ratio than the cooling rate, (ii) and even EBW which is considered an immature process in welding of DSS, can be employed provided means of filler addition could be devised.

Dynamic continuous recrystallization characteristics in two stage deformation of Mg-3Al-1Zn alloy sheet

Abstract: Dynamic recrystallization (DRX) characteristics of a Mg/3Al/1Zn (AZ31) alloy sheet were investigated at temperatures ranging from 200� /450 8C and constant strain rates of 1/10 4 � /2/10 4 s 1 . The average grain size of the as-received alloy was 12 mm and can be refined to 6 mm via deformation at 250 8C, 1/10 4 s 1 to a strain level of 60%. Grain refinement was less effectiv ea t higher temperatures due to rapid grain growth. The grain refinement was attributed to dynamic continuous recrystallization which involves progressive increase in grain boundary misorientation and conversion of low angle boundaries into high angle boundaries. During DRX, subgrains were developed through the conversion of dislocation cell walls into subgrain boundaries. The presence of precipitates was not essential for dynamic recrystallization in the magnesium alloy being investigated because of its limited slip systems, low stacking fault energy and high grain boundary diffusion rate. # 2003 Elsevier Science B.V. All rights reserved.

Advances in microstructure and mechanical properties of zirconium diboride based ceramics

Abstract: The use of silicon nitride as a sintering aid (5 vol.%) greatly improves the powder sinterability of zirconium diboride, in comparison to additive free ZrB2. Nearly full dense monolithic material is obtained by hot pressing at 1700 °C. The microstructure consists of fine regular ZrB2 grains and of various secondary grain boundary phases (e.g. BN, t-ZrO2, BN-rich glassy phase), mainly located at triple points. The addition of 20 vol.% of silicon carbide as a reinforcing particulate phase to the ZrB2+5vol.%Si3N4 powder mixture slows down the densification rate of ZrB2, therefore a higher hot pressing temperature (i.e. 1870 °C) is necessary to achieve nearly full density. Further addition of oxide additives (1vol.%Al2O3+0.5vol.%Y2O3) to the ZrB2–20vol.%SiC–5vol.%Si3N4 system enables the production of near fully dense composites at lower hot pressing temperature (1760 °C). The presence of SiC particles in both the ZrB2-based composites effectively improves strength, hardness and toughness, compared to monolithic zirconium diboride. Some mechanical properties are very interesting: flexural strength up to 700 and 600 MPa are measured at room temperature and 1000 °C, respectively. The properties are discussed in terms of the microstructural features.

Functionally graded materials for sensor and energy applications

Abstract: Principles, preparation, characterisation, and application of functional materials containing a gradient of their functional properties are surveyed, with main emphasis on thermoelectric (TE) materials for application in sensors and thermogenerators. Further examples of the implementation of functionally graded materials (FGM) presented are dielectric thin-film stacks for capacitors with low temperature coefficient, microwave-processed structural gradients in fuel cell electrodes, and zone-melted graded (Bi1 − xSbx)2Te3 materials for Peltier coolers. Preparation and properties of compositional gradients in TE solid solutions (FeSi2 doped by alloying, (Bi1 − xSbx)2Te3, Mg2(Si,Ge,Sn), PbTe) are analysed, as well as composites joining thermoelectrics of dissimilar chemistry and joints to metallic contacts and interlayers. Thermal spraying of doping-graded FeSi2 was developed as a preparation technique of TE silicide-based FGM. Design, preparation and test of a layered heat-flux sensor based on FeSi2 are described. A calibration test evidenced the feasibility of linearisation of thermal sensor characteristics. A theoretical design tool for functionally graded and segmented thermoelectric structures was based on a local selection criterion to identify the optimal spatial compositional distribution.

Grain refinement and properties of pure Ti processed by warm ECAP and cold rolling

Abstract: This work explored a two-step severe plastic deformation process to produce ultrafine-grained (UFG) Ti with significantly enhanced strength. Warm equal channel angular pressing (ECAP) was first used to refine the grain size of Ti billets to about 350 nm. The Ti billets were further processed by repetitive cold rolling (CR). This two-step process produced UFG Ti with strengths higher than those of common titanium alloys such as Ti–6Al–4V. This paper reports the microstructures, tensile properties, and thermal stability of these Ti billets processed by a combination of warm ECAP and CR.

Effects of laser sintering processing parameters on the microstructure and densification of iron powder

Abstract: The densification behavior and the attendant microstructural features of iron powder processed by direct laser sintering were investigated. The effects of processing parameters such as laser power, scan rate, scan line spacing, thickness of layer, scanning geometry and sintering atmosphere were studied. A specific energy input (ψ) was defined using the “energy conservation” rule to explore the effects of the processing condition on the density and the attendant microstructure of laser sintered iron. It was found that the sintered density increased sharply with increasing the specific energy input until a critical energy input had been reached (ψ∼0.2 kJ mm−3). The microstructure consists of large pores (>0.5 mm) and elongated ferrite grains parallel to the building direction. The increase in the sintered density was followed with further increasing the specific energy, but at slower rate. Intensifying the energy input over 0.8 kJ mm−3 leads to the formation of horizontally elongated pores while the sintered density remains almost constant. The inter-agglomerates are fully dense and consist of elongated ferrite grains which are oriented parallel to the building direction. The iron powder was used as a model material so the outcomes are generic and can be applied to other material systems with congruent melting point or systems which melting/solidification approach is the mechanism feasible for the rapid bonding of metal powders in direct laser sintering.

Hall–Petch relationship in friction stir welds of equal channel angular-pressed aluminium alloys

Abstract: The effect of grain size on hardness in the stir zones of friction stir (FS) welds of equal channel angular (ECA)-pressed Al alloys 1050 and 5083 was examined. The hardness was found to be essentially related to grain size through the Hall–Petch relationship in the stir zone of Al alloy 1050. The kH slope of the Hall–Petch equation for the stir zone of Al alloy 1050 was different from the previously reported ones, which was attributed to dynamic recrystallisation during friction stir welding (FSW). On the other hand, the relationship between hardness and grain size in the stir zone of Al alloy 5083 was expressed by the Hall–Petch equation with a change in slope. The change in slope was attributed to the homogeneous distribution of many fine particles.

Failure modes in plasma-sprayed thermal barrier coatings

Abstract: Commercial plasma-sprayed thermal barrier coatings (TBCs) were investigated in an effort to elucidate the failure modes during thermal-cycling. Residual stresses in the thermally grown oxide (TGO) was measured using the Cr3+ photoluminescence piezo-spectroscopy (PLPS) method and the microstructures of the TBCs were characterized as a function of thermal cycles. The average residual stress in the TGO was found to be of the order of 1 GPa. The average thermal-cyclic life of the TBCs was found to be ∼350 cycles. Microstructural observations revealed that as the TGO thickened, cracking occurred at the bond-coat/TGO interface, and in some instances cracking also occurred at the TGO/top-coat interface, but primarily at crests of bond-coat undulations. The bond-coat-TGO separation resulted in ‘layering’ of the TGO at crests due to enhanced TGO thickening in those regions. In the troughs of bond-coat undulations, cracking occurred within the top-coat when the TGO was thick. Thus, the primary failure modes in these TBCs were: (i) cracking of the bond-coat/TGO interface; (ii) cracking within the top-coat; and (iii) linking of these microcracks by fracture of the TGO. A semi-quantitative failure model has been used to rationalize some of the observed cracking modes. Based on this analysis some suggestions are made for improving TBC durability.

Rheology and colloidal structure of aqueous TiO2 nanoparticle suspensions

Abstract: Rheological behavior and suspension structure of anatase titanium dioxide (TiO2) nanoparticles dispersed in pure water have been investigated over a range of volumetric solids concentrations (φ=0.05–0.12) and shear rates (γ=101–103 s−1). The nanoparticle suspensions generally exhibited a pseudoplastic flow behavior, indicating an existence of particle aggregations in the liquid medium. The suspensions became apparently thixotropic as φ was increased above 0.1. Relative viscosity (ηr) of the suspensions followed an exponential form with φ, i.e., ηr=13.47e35.98φ, revealing a pronounced increase in the degree of particle interactions as φ increased. Fractal dimension (Df) was estimated from the suspension yield-stress (τy) and φ dependence, and was determined as Df∼1.46–1.78 for the flocculated nanoparticle suspensions. This suggested that the suspension structure was probably dominated by the diffusion-limited cluster–cluster aggregation, due mostly to the strong attractions involved in the interparticle potentials. Maximum solids loading (φm) of the suspensions was determined as φm=0.146. This relatively low value of φm (compared with the random close packing of monosized particles, φm∼0.64) partially vindicated the existence of a porous, three-dimensional aggregate network of interconnected nanoparticles in the carrier liquid.

Grain size dependence of the plastic deformation kinetics in Cu

Abstract: Data on the effect of grain size d in the range of nm to mm on the plastic deformation kinetics of Cu at 77–373 K are analyzed to determine the influence of grain size on the strain rate-controlling mechanism. Three grain size regimes were identified: Regimes I (d≈10−6–10−3 m), II (d≈10−8–10−6 m) and III (d<∼10−8 m). A dislocation cell structure characterizes Regime II, which no longer occurs in Regime II. The absence of all intragranular dislocation activity characterizes Regime III. The following mechanisms were concluded to be rate-controlling for : (a) Regime I, intersection of dislocations; (b) Regime I, grain boundary shear promoted by dislocation pile-ups; and (c) Regime III, grain boundary shear. The major effect of grain size on the intersection mechanism in Regime I is on the mobile and forest dislocation densities; the effect in Regime II is on the number of dislocations and on the number of grain boundary atom sites; the effect in Regime III is on the number of grain boundary atom sites. The transition grain size from one regime to another depends on the strain rate and temperature. Crystallographic texture is also important.

Kinetic interactions between solid iron and molten aluminium

Abstract: In order to improve the metallurgical bond between ferrous inserts and aluminium matrix in castings, the interaction between molten aluminium and solid iron is studied by immersion tests. Morphology evolution and growth kinetics of the intermetallic layers Fe2Al5 and FeAl3 were established as a function of time and temperature. For medium interaction time (

Effect of surface nanocrystallization on friction and wear properties in low carbon steel

Abstract: A nanocrystalline (nc) surface layer of about 10 μm thick was fabricated on a low carbon steel plate by means of surface mechanical attrition treatment. The grain size is about 15 nm in the top surface layer, and it increases with an increase of depth from the treated surface. Nanoindentation measurements indicated that hardness is enhanced in the nc surface layer relative to the matrix. Experiments show that the friction coefficient decreases and the wear resistance increases with the nc surface layer. The improvement in friction and wear properties may be attributed to the harder nc surface layer which reduces the degree of plowing and micro-cutting under the lower load and the degree of plastic removal and surface fatigue fracture under the higher load, respectively.

Effect of laser surface melting on corrosion and wear resistance of a commercial magnesium alloy

Abstract: Among the light metals, Mg and its alloys occupy a prominent position due to its low density, excellent machinability, and high specific strength. However, a relatively poor resistance to corrosion and wear are serious impediments against wider application of Mg alloys. In the present study, an attempt was made to enhance pitting corrosion resistance and microhardness of a commercial Mg alloy, MEZ (Zn 0.5%, Mn 0.1%, Zr 0.1%, rare earth elements 2%, Mg remaining percentage) by laser surface melting. The study included a detailed characterization of laser surface melted zone in terms of microstructure, phase analysis and its correlation with process parameters to optimize the laser processing routine. Mechanical properties like microhardness, wear, and electrochemical properties like pitting corrosion resistance of the surface melted layer were studied in detail. Microhardness of the laser surface melted layer was improved to 85–100 VHN as compared to 35 VHN of the as-received MEZ. Pitting corrosion resistance of the laser surface melted MEZ, significantly improved in a 3.56 wt.% NaCl solution because of grain refinement and redistribution of the intermetallic phases following rapid quenching associated with the process. The wear resistance of laser surface melted layer was also improved as compared to as-received MEZ.

Microstructure of cryogenic treated M2 tool steel

Abstract: Cryogenic treatment has been claimed to improve wear resistance of certain steels and has been implemented in cutting tools, autos, barrels etc. Although it has been confirmed that cryogenic treatment can improve the service life of tools, the underling mechanism remains unclear. In this paper, we studied the microstructure changes of M2 tool steel before and after cryogenic treatment. We found that cryogenic treatment can facilitate the formation of carbon clustering and increase the carbide density in the subsequent heat treatment, thus improving the wear resistance of steels.

A study of active deformation systems in titanium alloys: dependence on alloy composition and correlation with deformation texture

Abstract: Active glide and twinning systems have been studied by transmission electron microscopy (TEM) in samples of three Ti-alloys, T40 (Ti+1000 ppm O), T60 (Ti+2000 ppm O) and TiAl6V4 deformed up to 5% by uniaxial or biaxial tension. The aim of the work was to understand more clearly which glide systems are activated during deformation of polycrystalline material and how they are related to the formation of the cold rolling textures. In order to estimate the stresses necessary for the activation of the observed glide systems, the resolved shear stresses for the different deformation systems were calculated from measured crystal orientations. The main results are: in TiAl6V4 〈a〉, basal slip has a lower critical resolved shear stress, τc, than prismatic slip. 〈c+a〉 pyramidal glide shows a very low τc, which is up to two times larger than that for prismatic slip. Nevertheless, 〈c+a〉 glide systems were only rarely activated and twinning systems were never activated. Therefore, deformation with c-components may be accommodated by β-phase deformation or grain boundary sliding. The observed c-type texture is due to the strong basal glide. In T40, τc for 〈c+a〉 glide is up to 13 times higher than that for prismatic glide. However, 〈c+a〉 glide and twinning were strongly activated, leading to the observed t-type texture. In T60, the high oxygen content completely suppressed twinning and strongly reduced 〈c+a〉 glide. The less developed t-type texture is due to the combination of 〈c+a〉 and basal glide.

Effect of standard heat treatment on the microstructure and mechanical properties of hot isostatically pressed superalloy inconel 718

Abstract: Ni–Fe base superalloy, Inconel 718, was processed through powder metallurgy (P/M) hot isostatic pressing (HIP) route. In order to balance the strength and ductility, the HIPed material was given the standard heat treatment, viz. solution treatment at 980 °C for 1 h/water quenched (WQ) to room temperature and a two-step ageing treatment consisting of 720 °C for 8 h/furnace cooling (FC) at 55 °C h−1 to 620 °C and holding at 620 °C for 8 h before air cooling (AC) to room temperature. Optical microscopy and scanning electron microscopy (SEM) studies on the heat treated alloy have shown a homogeneous microstructure with fine grain size (25 μm) along with the presence of prior particle boundary (PPB) networks. Transmission electron microscopy (TEM) on the heat treated material has revealed the presence of oxides, MC carbides and δ-precipitates at the grain boundaries and a uniform precipitation of fine γ″ and γ′ strengthening phases in the matrix. Tensile and stress rupture tests were performed on the heat treated material. While the yield strength (YS) and ultimate tensile strength (UTS) of the HIPed and heat treated alloy at room temperature and 650 °C were comparable to those of conventionally processed wrought IN 718, its ductility was lower. The stress rupture life of the HIPed alloy improved marginally due to heat treatment and met the minimum specification requirement of life hours but the rupture ductility was found to be inferior to that of the wrought material. The fractography of the failed samples has revealed the transgranular ductile mode of fracture in the as-solution treated alloy, while intergranular mode of failure with the decohesion of PPBs occurred more predominantly in the aged condition. This change of fracture mode with ageing treatment shows the ductility dependence on the relative strength of the matrix and PPBs. The TEM studies on the deformed alloy have revealed that the brittle oxides and carbides at the prior particle boundaries coupled with the fine γ″ and γ′-precipitates in the matrix are responsible for low ductility at 650 °C. The investigations of the present study have led to better understanding of the structure–property relationships in HIP+heat treated alloy 718 and suggest that the standard heat treatment recommended for wrought IN 718 is not suitable for HIPed alloy and has to be modified to realise optimum properties.

Texture evolution during large-strain hot rolling of the Mg AZ61 alloy

Abstract: Grain refinement in a Mg-based AZ61 alloy of initially coarse, recrystallized microstructure was successfully achieved by thermomechanical processing (TMP) consisting of two to three hot-rolling steps with large reductions per pass. Reductions as large as 85% (equivalent to a true strain of ≈1) were achieved without surface cracking. The underlying microscopic mechanisms operative during the TMP that allowed this hcp material to accommodate such large strains per pass were investigated by macro- and microtexture analysis. A significant decrease in the intensity of the initial basal texture was observed after the first pass. This was attributed to rotational dynamic recrystallization, a mechanism by which new recrystallized grains develop, with orientations favourable for basal slip. Upon subsequent passes, basal slip becomes the main deformation mechanism. Simultaneously, grain refinement takes place by continuous dynamic recrystallization. The fine-grained microstructure thus developed showed improved superplastic behaviour in comparison with that of similar alloys processed by more elaborate methods.

Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing

Abstract: The formation of homogeneous nanocrystalline structure by nanocrystallization of amorphous NiTi subjected to high pressure torsion is demonstrated. Structural evolution during annealing was investigated and homogeneous nanocrystalline structures with different grain sizes have been obtained by controlled annealing. Nanocrystallization results in the record value of room temperature strength for this material equal to 2650 MPa with an elongation to failure of about 5%. At elevated temperatures of (0.4…0.5)Tm nanocrystalline nitinol showed a high ultimate strength with sufficient elongation (up to 200%). The observation that the shape and the size of grains after deformation remain close to that of the initial state suggests that in nanocrystalline NiTi such mechanism as grain boundary sliding and grain rotation are active and the generation and motion of dislocations play the role of accommodation of stress concentration.

On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy

Abstract: The effect of process parameters such as quench rate and precipitation heat treatment on the compromise between the toughness and the yield strength of AA7050 aluminum alloy (AlZnMgCu) are investigated, as well as the anisotropy of this compromise in the rolling plane. Fracture toughness is experimentally approached by the Kahn tear test. The microstructure is studied quantitatively in detail by a combination of scanning electron microscopy, transmission electron microscopy and small-angle X-ray scattering, and the relative fractions of the various fracture modes as a function of microstructural state are quantitatively determined on scanning electron microscopy images. Toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. A lower quench rate is shown to result in an overall reduction of toughness, and in a reduced evolution of this toughness during the aging heat treatment. The overall toughness is also lowered when the main crack propagation direction is parallel to the preferential elongation direction of the coarse constituent particles (rolling direction). The competition between intergranular and transgranular fracture is explained in terms of the modifications of the work hardening rate, and of grain boundary precipitation. The evolution of fracture toughness is qualitatively explained in terms of evolution of yield stress, strain hardening rate, grain boundary precipitation and intragranular quench-induced precipitates.

Microstructural evolution during accumulative roll-bonding of commercial purity aluminum

Abstract: The microstructure in commercial purity aluminum deformed from medium to high strain (evM=1.6–6.4) by accumulative roll-bonding (ARB) at 473 K was quantitatively examined by transmission electron microscopy. It was found that a sub-micrometer lamellar structure characterizes the microstructure at high strains (evM>1.6), and that the lamellar boundary spacing decreases and the misorientation across the lamellar boundaries increases with increasing rolling strain. This characteristic evolution has also been observed during conventional cold-rolling of commercial purity aluminum. However, a comparison between the two processes shows a significant difference in the evolution of the microstructural parameters. These differences are discussed based on the different processing conditions characterizing ARB and conventional rolling, respectively.

Reduction of porosity content generated during Nd:YAG laser welding of A356 and AA5083 aluminium alloys

Abstract: Porosity formation is greatly influenced in aluminium alloys by the low vaporisation point element (Mg, Zn) content, or by process instability such as key-hole closures that tend to entrap occluded gases during welding. Another important contribution comes from the hydrogen content, because of a very high solubility in molten aluminium that favours microporosity generation. In this paper, cw YAG laser welds on two aluminium alloys were carried out: a AA5083-O wrought alloy with a high Mg content (4.5%) and a A356 cast alloy with 7% Si and a cast oxide layer. The porosity content in laser beads was extensively studied, with the use of different experimental method (X-ray radiography+image analysis, tomography), in order to check the influence of mechanical surface preparation as well as process parameters (single or dual spot, different welding speeds). It was concluded that surface preparation as well as dual beam welding are adequate methods for reducing porosity formation tendency in laser assemblies.