Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2008"

The origin of “rare earth” texture development in extruded Mg-based alloys and its effect on tensile ductility

Abstract: The extrusion behaviour, texture and tensile ductility of five binary Mg-based alloys have been examined and compared to pure Mg. The five alloying additions examined were Al, Sn, Ca, La and Gd. When these alloys are compared at equivalent grain size, the La- and Gd-containing alloys show the best ductilities. This has been attributed to a weaker extrusion texture. These two alloying additions, La and Gd, were found to also produce a new texture peak with 〈 1 1 2 ¯ 1 〉 parallel to the extrusion direction. This “rare earth texture” component was found to be suppressed at high extrusion temperatures. It is proposed that the 〈 1 1 2 ¯ 1 〉 texture component arises from oriented nucleation at shear bands.

Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe–Mn–C steel

Abstract: We present a thermochemical model of the stacking-fault energy (SFE) in the Fe–Mn–C system with few percent of Cu, Cr, Al and Si in addition. Aluminium strongly increases the SFE, contrary to chromium, while the effect of silicon is more complex. Copper also increases the SFE, but strongly decreases the Neel temperature. The SFE is the relevant parameter that controls mechanical twinning, which is known to be at the origin of the excellent mechanical properties of these steels. Using this model, copper containing Fe–Mn–C grades were elaborated with SFE below 18 mJ/m 2 , in the range where ɛ-martensite platelets form instead of microtwins during plastic deformation. This substitution of deformation modes, confirmed by X-ray diffraction, does not significantly damage the mechanical properties, as long as the SFE is greater than 12 mJ/m 2 , which avoids the formation of α′-martensite.

Contribution of Orowan strengthening effect in particulate-reinforced metal matrix nanocomposites

Abstract: The relative contribution of three strengthening factors in particulate-reinforced metal matrix nanocomposites (MMNCs), namely, load-bearing effect, enhanced dislocation density strengthening effect and Orowan strengthening effect, is evaluated. Orowan strengthening mechanism is found to play a significant role in MMNCs. The relative contribution of Orowan strengthening effect increases with decreasing size of nanoparticles. However, there exists a critical particle size below which the Orowan strengthening effect drops abruptly. The critical particle size, independent of the volume fraction of nanoparticles, is found to be about 5.44 times the magnitude of the Burgers vector of dislocations in the matrix.

Dynamic recrystallization during high temperature deformation of magnesium

Abstract: As a consequence of the high critical stresses required for the activation of non-basal slip systems, dynamic recrystallization plays a vital role in the deformation of magnesium, particularly at a deformation temperature of 200 °C, where a transition from brittle to ductile behavior is observed. Uniaxial compression tests were performed on an extruded commercial magnesium alloy AZ31 at different temperatures and strain rates to examine the influence of deformation conditions on the dynamic recrystallization (DRX) behavior and texture evolution. Furthermore, the role of the starting texture in the development of the final DRX grain size was investigated. The recrystallized grain size, measured at large strains (ɛ ∼ −1.4) seemed to be more dependent on the deformation conditions than on the starting texture. In contrast to pure magnesium, AZ31 does not undergo grain growth at elevated deformation temperatures, i.e. 400 °C, even at a low strain rate of 10−4 s−1. Certain deformation conditions gave rise to a desired fully recrystallized microstructure with an average grain size of ∼18 μm and an almost random crystallographic texture. For samples deformed at 200 °C/10−2 s−1, optical microscopy revealed DRX inside of deformation twins, which was further investigated by EBSD.

The role of friction stir welding tool on material flow and weld formation

Abstract: In this investigation an attempt has been made to understand the mechanism of friction stir weld formation and the role of friction stir welding tool in it. This has been done by understanding the material flow pattern in the weld produced in a special experiment, where the interaction of the friction stir welding tool with the base material is continuously increased. The results show that there are two different modes of material flow regimes involved in the friction stir weld formation; namely “pin-driven flow” and “shoulder-driven flow”. These material flow regimes merge together to form a defect-free weld. The etching contrast in these regimes gives rise to onion ring pattern in friction stir welds. In addition to that based on the material flow characteristics a mechanism of weld formation is proposed.

Microstructure and compressive properties of AlCrFeCoNi high entropy alloy

Abstract: An AlCrFeCoNi high entropy alloy was prepared by vacuum arc melting. Only diffraction peak corresponding to a BCC crystal structure is observed for this AlCrFeCoNi high entropy alloy. The microstructure of this AlCrFeCoNi alloy is polygonal grains with intragranular dendritic segregation. Dendritic segregation area is found to be Al, Ni rich and Cr, Fe deplete, while interdendritic segregation area is Cr, Fe rich and Al, Ni deplete. The distribution of Co is essentially identical. The fine microstructure of dendritic segregation area and of interdendritic segregation area is found to be nanoscale spherical precipitates morphology and basket-weave morphology, respectively. Results of EDS attached on high resolution scanning electron microscope (SEM) revealed that these morphological characteristics are also resulted from elements segregation. This AlCrFeCoNi high entropy alloy exhibits excellent compressive properties. The yield stress, compressive strength and plastic strain of the alloy reaches 1250.96, 2004.23 MPa, and 32.7%, respectively. The fracture mechanism of this AlCrFeCoNi high entropy alloy is observed as cleavage fracture and slip separation.

Mechanical properties of polypropylene hybrid fiber-reinforced concrete

Abstract: This paper investigates the mechanical properties of polypropylene hybrid fiber-reinforced concrete. There are two forms of polypropylene fibers including coarse monofilament, and staple fibers. The content of the former is at 3 kg/m3, 6 kg/m3, and 9 kg/m3, and the content of the latter is at 0.6 kg/m3. The experimental results show that the compressive strength, splitting tensile strength, and flexural properties of the polypropylene hybrid fiber-reinforced concrete are better than the properties of single fiber-reinforced concrete. These two forms of fibers work complementarily. The staple fibers have good fineness and dispersion so they can restrain the cracks in primary stage. The monofilament fibers have high elastic modulus and stiffness. When the monofilament fiber content is high enough, it is similar to the function of steel fiber. Therefore, they can take more stress during destruction. In addition, hybrid fibers disperse throughout concrete, and they are bond with mixture well, so the polypropylene hybrid fiber-reinforced concrete can effectively decrease drying shrinkage strain.

Microband-induced plasticity in a high Mn–Al–C light steel

Abstract: Room temperature tensile behavior of a high Mn–Al–C steel in the solid solution state was correlated to the microstructures developed during plastic deformation in order to clarify the dominant deformation mechanisms. The steel was fully austenitic with a fairly high stacking fault energy of ∼85 mJ/m 2 . The tensile behavior of the steel was manifested by an excellent combination of strength and ductility over 80,000 MPa% in association with continuous strain hardening to the high strain. In addition, the austenite phase was very stable during deformation. The high stacking fault energy and firm stability of austenite were attributed to the high Al content. In spite of the high stacking fault energy, deformed microstructures exhibited the planar glide characteristics, seemingly due to the glide plane softening effect. In the process of straining, the formation of crystallographic microbands and their intersections dominantly occurred. Microbands consisting of geometrically necessary dislocations led to the high total dislocation density state during deformation, resulting in continuous strain hardening. This microband-induced plasticity is to be the origin of the enhanced mechanical properties of the steel.

Size effect on strength and strain hardening of small-scale [111] nickel compression pillars

Abstract: This study investigates uniaxial compression behavior of focused ion beam (FIB) manufactured [1 1 1] nickel (Ni) small-scale pillars, ranging in diameter from approximately 25 μm to below 200 nm, in order to examine the effect of crystallographic orientation on the mechanical properties. This study is unique from other micro-pillar studies in that the [1 1 1] orientation has a considerably lower Schmid factor, and has multiple slip systems available. The [1 1 1] Ni pillars show a strong increase in yield stress and work hardening with decreasing diameter. The relationship between yield stress and diameter (σy ∝ d−0.69) matches well with previous small-scale pillar studies. Strain hardening, which has been inconsistently observed in other micro-pillar studies, is found to be a function of both diameter and orientation. Although the precise mechanism for hardening is unknown, transmission electron microscopy reveals dislocations throughout the pillar and into the base material suggesting that dislocation interactions and deformation below the pillar play a role in the observed strain hardening. Furthermore, a slight crystallographic rotation of the pillar is observed likely contributing to the observed mechanical properties. By exploring the role of crystallography on the plastic deformation behavior, this study provides additional insight into the nature of the size effect.

Grain size effects on the tensile properties and deformation mechanisms of a magnesium alloy, AZ31B, sheet

Abstract: The grain size dependence of the tensile properties and the deformation mechanisms responsible for those properties are examined for Mg alloy, AZ31B, sheet. Specifically, the Hall–Petch effect and strain anisotropy ( r -value) are characterized experimentally, and interpreted using polycrystal plasticity modeling. {1 0 . 2} extension twins, {1 0 . 1} contraction twins, and so-called “double-twins” are observed via microscopy and diffraction-based techniques, and the amount of twinning is found to increase with increasing grain size. For the sheet texture and tensile loading condition examined, {1 0 . 2} extension twinning is not expected, yet the polycrystal plasticity model predicts the observed behavior, including this ‘anomalous’ tensile twinning. The analysis shows that the Hall–Petch strength dependence, of the polycrystal as a whole, is primarily determined by the grain size dependence of the strength of the prismatic slip systems.

Interfacial design of Cu-based composites prepared by powder metallurgy for heat sink applications

Abstract: Thermal aspects are becoming increasingly important for the reliability of the electronic components due to the continuous progress of the electronic industries. Therefore, the effective thermal management is a key issue for packaging of high performance semiconductors. The ideal material working as heat sink and heat spreader should have a CTE of (4–8) × 10−6 K−1 and a high thermal conductivity. Metal matrix composites offer the possibility to tailor the properties of a metal by adding an appropriate reinforcement phase and to meet the demands in thermal management. Copper/SiC and copper/diamond composites have been produced by powder metallurgy. The major challenge in development of Cu/SiC is the control of the interfacial interactions. Silicon carbide is not stable in copper at the temperature needed for the fabrication of Cu/SiC. It is known that the bonding between diamond and copper is very weak in the Cu/diamond composite. Improvements in bonding strength and thermo-physical properties of the composites have been achieved by • a vapour deposited molybdenum coating on SiC powders to control interface reactions, • using atomized Cu(X) alloys with minor additions of carbide formers, e.g. X = Cr, B, to improve the interfacial bonding in Cu-diamond composites.

Study on microstructure and mechanical properties of laser rapid forming Inconel 718

Abstract: Inconel™ 718 (IN718) has been deposited using laser rapid forming (LRF) from the gas atomized (GA) and plasma rotation electrode preparation (PREP) powders. The mechanical properties of LRF IN718 were evaluated and compared in between as-deposited and heat-treated state. The results show that the existence of the porosities in as-deposited samples, caused by the hollow particles in the GA powders, results in the low ductility and stress rupture properties for LRF GA IN718, since it will promote the occurrence of the micro-porous coalescence failure in the tensile samples. However, the ultimate tensile strength for heat-treated LRF GA IN718 is comparable to that of the wrought IN718, which is 1.5 times of that of the as-deposited samples. It is found that there exists a continuous thin film of Nb-rich MC carbides along the grain boundaries on the fracture surface of the stress rupture samples, which makes cracks initiate and propagate along this path easily, which also results in the poor stress rupture life for LRF GA IN718. The porosities and microcracks in LRF sample were successfully eliminated by using PREP powders, which leads to a substantial improvement in both tensile and stress rupture properties of LRF IN718.

Microstructure and tensile properties of friction stir welded AZ31B magnesium alloy

Abstract: The microstructural change in AZ31B-H24 magnesium (Mg) alloy after friction stir welding (FSW) was examined. The effects of tool rotational speed and welding speed on the microstructure and tensile properties were evaluated. The grain size was observed to increase after FSW, resulting in a drop of microhardness across the welded region from about 70 HV in the base metal to about 50 HV at the center of the stir zone. The obtained Hall–Petch type relationship showed a strong grain size dependence of the hardness. The aspect ratio and fractal dimension of the grains decreased towards the center of the stir zone. The welding speed had a significant effect on the microstructure, with larger grains at a lower welding speed. The yield strength and ultimate tensile strength increased with increasing welding speed due to a lower heat input. A lower rotational speed of 500 rpm led to higher yield strength than a higher rotational speed of 1000 rpm. The friction stir welded joints were observed to fail mostly at the boundary between the weld nugget and thermomechanically affected zone at the advancing side. Fracture surfaces showed a mixture of cleavage-like and dimple-like characteristics.

Structure-property correlations in Al 7050 and Al 7055 high-strength aluminum alloys

Abstract: The 7XXX series age-hardenable high-strength aluminum alloys find useful applications in the field of aerospace engineering. Constant efforts are being made to tailor the mechanical and corrosion properties of these alloys as per requirements for a particular application. These properties are a function of factors like microstructure, chemical composition and processing parameters. An effort has been made to collate the information available from different studies conducted on alloys Al 7050 and Al 7055. Databases were created to consolidate the information about microstructure, mechanical properties and corrosion behavior for the two alloys. Existing models were utilized to predict strength and fracture toughness for these alloys and these models were validated using experimental values and a qualitative evaluation was made for the corrosion behavior of these alloys. Available data were utilized to prepare maps that are intended to serve as guides to design aluminum alloys with desired combination of properties.

Microstructure, mechanical properties and bio-corrosion properties of Mg–Zn–Mn–Ca alloy for biomedical application

Abstract: Microstructure, mechanical properties and bio-corrosion properties of as-cast Mg–Zn–Mn–Ca alloys were investigated for biomedical application in detail by optical microscopy, scanning electronic microscopy (SEM), mechanical properties testing and electrochemical measurement. SEM and optical microscopy observation indicated that the grain size of the as-cast alloys significantly decreased with the increasing of Ca content up to 0.5 wt.%. Further increasing of Ca content did not refine the grain more. The phase constitute was mainly controlled by the atomic ratio of Zn to Ca. When the ratio was more than 1.0–1.2, the alloy was mainly composed of primary Mg and lamellar eutectic (α-Mg + Ca 2 Mg 6 Zn 3 ), while the alloy was composed of primary Mg and divorced eutectic (α-Mg + Mg 2 Ca + Ca 2 Mg 6 Zn 3 ) when the atomic ratio was less than 1.0–1.2. The yield strength of the as-cast alloy increased but the elongation and the tensile strength increased first and then decreased with the increasing of Ca content. It was thought that Mg 2 Ca phase deteriorated the tensile strength and ductility. Electrochemical measurements indicated that Mg 2 Ca phase improved the corrosion resistance of the as-cast alloy.

Fabrication and characterization of carbon/epoxy composites mixed with multi-walled carbon nanotubes

Abstract: In this study, a high-intensity ultrasonic liquid processor was used to obtain a homogeneous mixture of epoxy resin and multi-walled carbon nanotubes (CNTs). The CNTs were infused into epon 862 epoxy resin through sonic cavitation and then mixed with W curing agent using a high-speed mechanical agitator. The trapped air and reaction volatiles were removed from the mixture using a high vacuum. Flexural tests and fracture toughness tests were performed on unfilled and CNT-filled epoxy to identify the effect of adding CNTs on the mechanical properties of epoxy. The highest improvement in strength and fracture toughness was obtained with 0.3 wt% CNT loading. The nanophased matrix filled with 0.3 wt% CNT was then used with weave carbon fabric in a vacuum-assisted resin transfer molding (VARTM) set up to fabricate composite panels. Flexural tests, thermogravimetric analysis (TGA), and dynamic mechanical analysis (DMA) were performed to evaluate the effectiveness of adding CNTs on the mechanical and thermal properties of the composite. The glass transition temperature, decomposition temperature, and flexural strengths were improved by infusing CNTs. Based on the experimental result, a linear damage model has been combined with the Weibull distribution function to establish a constitutive equation for neat and nanophased carbon/epoxy. Simulated result show that that infusing CNTs increases Weiubll scale parameter, but decrease Weibull shape parameter.

Enhancement in thermal and mechanical properties of polyurethane foam infused with nanoparticles

Abstract: Nanotechnology is presently seen as one of the most promising approaches in the field of materials science towards the development of advanced materials for future engineering applications. Recent and ongoing researches on polymer/inorganic nanocomposite have shown dramatic enhancements in thermal, mechanical and chemical properties over the neat polymer without compromising density, toughness, and processibility. This study investigates the effects of different types of nanoparticles on thermal and mechanical performance of rigid polyurethane (PUR) foam. Three different types of nanoparticles, namely spherical TiO 2 , platelet nanoclay, and rod-shaped carbon nanofibers (CNFs) are considered. A sonication technique is used to disperse the nanoparticles into part-A of the PUR foam precursor followed by mechanical mixing into part-B and then the whole mixture is poured into a preheated aluminum mold and cured inside the oven. In all cases only 1 wt.% nanoparticles are used, and have found significant thermal and mechanical properties enhancement of the nanophased foam. Among nanoparticles, CNFs infused PUR foam shows maximum enhancement, whereas TiO 2 infused PUR foam shows the minimum.

Room temperature formability of a magnesium AZ31 alloy: Examining the role of texture on the deformation mechanisms

Abstract: The deformation behavior, texture and microstructure evolution of six sample types of a commercial magnesium alloy AZ31 with different processing histories were investigated during plane strain compression at room temperature using a channel-die device. Although all the samples were deformed under the same conditions, i.e. temperature and strain rate, the initial state of the samples prior to deformation was responsible for the final texture and microstructure. Stress–strain curves showed a maximum ductility of 28% for the sample with a hot rolling history. EBSD analysis was carried out to give a better insight into the operating deformation mechanisms. Besides the expected { 1 0 1 ¯ 2 } -tensile twinning, { 1 0 1 ¯ 1 } -compression twinning and { 1 0 1 ¯ 1 } − { 1 0 1 ¯ 2 } -double twinning were also observed in some specimens and were correlated to microcrack formation, which caused an early shear failure.

Microstructural characteristics and mechanical properties of Ti-6Al-4V friction stir welds

Abstract: Friction stir welding (FSW) was applied to 3 mm-thick Ti–6Al–4V plates under different rotational speeds. Defect-free welds were successfully produced at rotational speeds of 400 and 500 rpm. The base material (BM) had a deformed α/β lamellar microstructure. FSW produced a full lamellar structure with refined prior β grains in the SZ, while the HAZ contained a bimodal microstructure consisting of the equiaxed primary α and α/β lamellar structure within the prior β structure. An increase in rotational speed increased the sizes of α colonies and prior β grains. The SZ exhibited higher hardness than the BM, with the lowest hardness found in the HAZ. Results of the transverse tensile test showed that all welds fractured in the HAZ and that they exhibited lower strength and elongation than the BM. The tensile test for only the SZ showed it to be characterized by higher strength and elongation than the BM.

Secondary ageing in an aluminium alloy 7050

Abstract: Secondary precipitation takes place in alloy 7050 at 65 °C after underageing at 130 °C and quenching (T6I4-65 temper) and results in a significantly increased number density of the η ′ platelets, the precipitates also formed in the T6 temper. The modified microstructure results in tensile properties comparable to that of the T6 temper, but with significantly improved fracture toughness. A combined transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) study has shown that secondary ageing at 65 °C results in evolution of the GPI zones formed during underageing into the η ′ phase. Ageing at 65 °C alone results in the formation of GPII zones, which provide lesser strengthening than the η ′ platelets. The DSC study revealed six exothermic reactions corresponding to the formation of six different types of precipitate during the DSC scan.

Mechanisms of cracking and delamination within thick thermal barrier systems in aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration

Abstract: An analysis has been conducted that characterizes the susceptibility to delamination of thermal barrier coated (TBC) hot-section aero-turbine components when penetrated by calcium-magnesium-alumino-silicate (CMAS). The assessment has been conducted on stationary components (especially shrouds) with relatively thick TBCs after removal from aero-engines. In those segments that experience the highest temperatures, the CMAS melts, penetrates to a depth about half the coating thickness, and infiltrates all open areas. Therein the TBC develops channel cracks and sub-surface delaminations, as well as spalls. Estimates of the residual stress gradients made on cross-sections (by using the Raman peak shift) indicate tension at the surface, becoming compressive below. By invoking mechanics relevant to the thermo-elastic stresses upon cooling, as well as the propagation of channel cracks and delaminations, a scenario has been presented that rationalizes these experimental findings. Self-consistent estimates of the stress and temperature gradients are presented as well as predictions of channel cracking and delamination upon cooling.

Influence of bainite/martensite-content on the tensile properties of low carbon dual-phase steels

Abstract: This investigation aims to examine structure–property relations of ferrite–bainite dual-phase (FBDP) steels and to compare these against that of ferrite–martensite dual-phase (FMDP) steels. For this purpose, a series of FBDP and FMDP steels containing wide variation (20–90%) of the harder constituents have been prepared from a low carbon Nb-micro-alloyed base material by suitable heat treatments. Hardness and tensile properties of the developed steels have been examined against the volume fraction of bainite or martensite. The nature of variation of the estimated mechanical properties such as hardness, yield and tensile strength, percentage elongation and strain-hardening exponent with the amount of the harder constituents of the FBDP and FMDP steels exhibits subtle to significant differences. These differences have been explained using the influence of the nature of the microstructural constituents and their mutual interactions. Low carbon FBDP steel with 60–70% bainite appears to possess excellent potential for structural applications.

Properties of Ti–6Al–4V non-stochastic lattice structures fabricated via electron beam melting

Abstract: This paper addresses foams which are known as non-stochastic foams, lattice structures, or repeating open cell structure foams. The paper reports on preliminary research involving the design and fabrication of non-stochastic Ti–6Al–4V alloy structures using the electron beam melting (EBM) process. Non-stochastic structures of different cell sizes and densities were investigated. The structures were tested in compression and bending, and the results were compared to results from finite element analysis simulations. It was shown that the build angle and the build orientation affect the properties of the lattice structures. The average compressive strength of the lattice structures with a 10% relative density was 10 MPa, the flexural modulus was 200 MPa and the strength to density ration was 17. All the specimens were fabricated on the EBM A2 machine using a melt speed of 180 mm/s and a beam current of 2 mA. Future applications and FEA modeling were discussed in the paper.

Enhancement of tensile ductility and stretch formability of magnesium by addition of 0.2 wt%(0.035 at%)Ce

Abstract: Mg–0.2 wt%(0.035 at.%)Ce alloy was hot-rolled and its mechanical properties were investigated by conducting tensile and Erichsen tests at room temperature and 433 K. The rolled Mg–Ce alloy exhibited greater elongation to failure and higher stretch formability than the rolled pure Mg. This was attributed to a reduction in basal texture intensity and the splitting of the basal plane by the addition of a small amount of Ce (0.2 wt%). Also, the small amount of Ce strongly affected the recrystallization behavior during hot rolling. Microstructural observation revealed that the prismatic slip was activated in the Mg–Ce alloy. The enhancement of the non-basal slip by the addition of Ce was not attributed to a reduction in the c/a ratio. An increase in stacking fault energy due to the addition of Ce is suggested to play a vital role in the activation of the non-basal slip.

Processing and properties of ultra-high temperature ceramics for space applications

Abstract: The processing and the properties of two ultra-high temperature ceramics (UHTCs) designed for the manufacturing of aerospace sharp-shaped hot-structures are presented, along with the results obtained in the electrical discharge machining (EDM) of these UHTCs into sharp hot-structure components. The powder mixtures in the (ZrB2–SiC)-based systems were brought to full density by hot-pressing. The hot-pressed bodies were characterized by fine and uniform microstructures (typical grain size Beyond basic mechanical properties, the machinability of UHTC blocks into a more complex shape by means of the EDM was also assessed. Sharp-shaped hot-structures in the form of a nose-cone were produced from 12 cm × 10 cm hot-pressed cylindrical blocks. The machined surfaces showed limited roughness Ra

Gamma double prime precipitation kinetic in Alloy 718

Abstract: Gamma double prime (γ′′), precipitation was studied in Alloy 718 using isothermal and isochronal aging heat treatments applied between 943 and 1003 K. It is shown, that the coarsening behavior of γ′′ precipitates follows the coarsening kinetic predictions of the Lifshitz–Slyozov–Wagner (LSW) theory. The activation energy for γ′′ growth has been determined as equal to 272 kJ mol−1 and seems to be controlled by volume diffusion of niobium in the matrix. The energy of the γ′′/matrix interface, Γ, has been found to be 95 ± 17 mJ m−2 by assuming that the γ′′ precipitates adopt a disk shape which minimizes the total energy. This energy includes a volume distortion term calculated from the Eshelby inclusion theory and a surface component which is assumed to be isotropic. This interfacial energy is discussed and compared with the energy of γ′/matrix and γ′′/matrix interfaces in other superalloys. The constant K′′ of the LSW law time dependence has been calculated using the value of interfacial energy and the activation energy of γ′′ precipitates coarsening and is found to be in good agreement with our experimental values.

Fabrication and characterization of TiO2–epoxy nanocomposite

Abstract: A systematic study has been conducted to investigate the matrix properties by introducing nanosize TiO2 (5–40 nm, 0.5–2% by weight) fillers into an epoxy resin. Ultrasonic mixing process, via sonic cavitations, was employed to disperse the particles into the resin system. The thermal, mechanical, morphology and the viscoelastic properties of the nanocomposite and the neat resin were measured with TGA, DMA, TEM and Instron. The nano-particles are dispersed evenly throughout the entire volume of the resin. The nanofiller infusion improves the thermal, mechanical and viscoelastic properties of the epoxy resin. The nanocomposite shows increase in storage modulus, glass transition temperature, tensile modulus, flexural modulus and short beam shear strength from neat epoxy resin. The mechanical performance and thermal stability of the epoxy nanocomposites are depending on with the dispersion state of the TiO2 in the epoxy matrix and are correlated with loading (0.0015–0.006% by volume). In addition, the nanocomposite shows enhanced flexural strength. Several reasons to explain these effects in terms of reinforcing mechanisms were discussed.

Cryomilled nanostructured materials: Processing and properties

Abstract: Nanostructured (i.e., 1–200 nm grain size) and ultrafine-grained (i.e., 200–500 nm grain size) metals are of interest, not only as a result of their unusual combinations of physical and mechanical properties, but also because they can be readily synthesized using well-developed synthesis techniques. Cryomilling, i.e., mechanical alloying in liquid nitrogen, is representative of a class of synthesis techniques that attain the nanostructured state via severe plastic deformation. In this overview, published data related to cryomilled materials are reviewed and discussed with particular emphasis on cryomilling mechanisms; microstructure and thermal stability of cryomilled powders; primary consolidation and secondary processing methods; microstructural evolution during consolidation; and mechanical response of consolidated materials. The deformation behavior and the underlying mechanisms that govern cryomilled materials are discussed and compared with those of nanostructured materials processed via other methods, in an effort to shed light into the fundamental behavior of ultrafine-grained and nanostructured materials.

Flow behavior and microstructures of superalloy 718 during high temperature deformation

Abstract: Flow behavior and microstructures of superalloy 718 were investigated by hot compression tests performed at temperatures ranging from 950 to 1100 °C with strain rates of 10−3 to 1 s−1. The dependence of the peak stress on deformation temperature and strain rate can be expressed by a hyperbolic-sine type equation. The activation energy for superalloy 718 is determined to be 443.2 kJ mol−1. A power exponent relationship between the peak strain and the Z parameter is obtained. Microstructure analysis shows that the dynamically recrystallized grain size is inversely proportional to the Z parameter. The nucleation mechanisms of DRX are closely related to the value of Z parameter. Under low Z conditions, DRX nucleation and development are mainly assisted by the formation of twins near the original grain boundaries.

The influence of cooling rate and manganese content on the β-Al5FeSi phase formation and mechanical properties of Al–Si-based alloys

Abstract: The present study investigates the influence of manganese level and cooling rate on the formation of iron compounds and mechanical properties of Al–9%Si alloys containing 0.3%Fe. It has been established that high cooling rates and Mn additions are not able to totally nullify the formation of β-Al5FeSi-needles onto α-Al15(Fe,Mn)3Si2-Chinese scripts even at Mn:Fe ratio of 2:1, but produces improvement in tensile strength unlike the ductility which suffers a loss with the increment of Mn concentrations.