Showing papers in "Metals and Materials International in 2013"

Zinc phosphate conversion coatings on magnesium alloys: A review

Abstract: Phosphating is one of the most widely used surface treatments of steels and aluminum due to its low-cost, easy mass production, good corrosion resistance and good adhesion with paint. Many researchers have tried to expand applications of the phosphating process, especially to magnesium alloys for automobiles and aerospace applications. Recently, the coatings on magnesium alloys by zinc phosphate conversion coatings (Zn3(PO4)2·4H2O) have been intensively studied. This paper reviews the state-of-the-art of phosphate conversion coatings developed for magnesium alloys, in terms of coating properties, phosphate conversion coatings processes and compositions of phosphating bath.

Thermal desorption spectroscopy study of the interaction of hydrogen with TiC precipitates

Abstract: Thermal desorption spectroscopy (TDS) was used to study hydrogen-trap interactions for an experimental steel (0.025 wt%C-0.09%Ti). After lab processing, the microstructure consisted of small (∼20 μm) ferrite grains containing nanometer TiC precipitates. After hot and cold rolling, the material contained some hydrogen (originated from the hot rolling) in irreversible traps, the TiC precipitates. After annealing in hydrogen, the TDS spectra consisted of a high temperature peak, attributed to irreversible trapping by TiC precipitates. Annealing slightly increased the TiC precipitate size. Both the peak temperature and peak area increased with increasing annealing temperature. The increase in peak area occurred together with the increase in TiC precipitate size. The TDS spectra for samples annealed at 800 °C, and electrochemically charged, contained (i) a low temperature peak which decreased in height with increasing desorption time, and (ii) a high temperature peak that did not change significantly with desorption time, and was similar to those after gaseous charging. The low temperature peak was attributed to reversible traps such as grain boundaries, whereas the high temperature peak was attributed to irreversible trapping by TiC precipitates. The high temperature TDS peak was composed of constituent peaks with essentially the same activation energy of 145 kJ/mol.

Corrosion and wear behaviors of boronized AISI 316L stainless steel

Abstract: In this study, the effects of a boronizing treatment on the corrosion and wear behaviors of AISI 316L austenitic stainless steel (AISI 316L) were examined. The corrosion behavior of the boronized samples was studied via electrochemical methods in a simulation body fluid (SBF) and the wear behavior was examined using the ball-on-disk wear method. It was observed that the boride layer that formed on the AISI 316L surface had a flat and smooth morphology. Furthermore, X-ray diffraction analyses show that the boride layer contained FeB, Fe2B, CrB, Cr2B, NiB, and Ni2B phases. Boride layer thickness increased with an increasing boronizing temperature and time. The boronizing treatment also increased the surface hardness of the AISI 316L. Although there was no positive effect of the coating on the corrosion resistance in the SBF medium. Furthermore, a decrease in the friction coefficient was recorded for the boronized AISI 316L. As the boronizing temperature increased, the wear rate decreased in both dry and wet mediums. As a result, the boronizing treatment contributed positively to the wear resistance by increasing the surface hardness and by decreasing the friction coefficient of the AISI 316L.

Effect of solidification parameters on mechanical properties of directionally solidified Al-Rich Al-Cu alloys

Abstract: Al(100−x)-Cux alloys (x=3 wt%, 6 wt%, 15 wt%, 24 wt% and 33 wt%) were prepared using metals of 99.99% high purity in vacuum atmosphere. These alloys were directionally solidified under steady-state conditions by using a Bridgman-type directional solidification furnace. Solidification parameters (G, V and ), microstructure parameters (λ1, λ2 and λE) and mechanical properties (HV, σ) of the Al-Cu alloys were measured. Microstructure parameters were expressed as functions of solidification parameters by using a linear regression analysis. The dependency of HV, σ on the cooling rate, microstructure parameters and composition were determined. According to experimental results, the microhardness and ultimate tensile strength of the solidified samples was increased by increasing the cooling rate and Cu content, but decreased with increasing microstructure parameters. The microscopic fracture surfaces of the different samples were observed using scanning electron microscopy. Fractographic analysis of the tensile fracture surfaces showed that the type of fracture significantly changed from ductile to brittle depending on the composition.

Role of Ca treatment in hydrogen induced cracking of hot rolled API pipeline steel in acid sour media

Abstract: The effect of Ca treatment on hydrogen-induced cracking (HIC) resistance of hot rolled pipeline steel was evaluated. HIC testing was carried out in acidic condition according to NACE standard; results clearly prove that HIC resistance is very sensitive to Ca/S ratio. When Ca/S ratio is below the stoichiometric ratio, HIC occurred at mid-thickness of the steel regardless of the S content. This is closely related to the formation of spherical CaS inclusion with Ca treatment instead of MnS inclusion, which acts on crack initiation sites.

Influence of growth rate on microstructure, microhardness, and electrical resistivity of directionally solidified Al-7 wt% Ni hypo-eutectic alloy

Abstract: Al-7 wt% Ni alloy was directionally solidified upwards with different growth rates, V (8.3–489.5 μm/s) at constant temperature gradient, G (4.2 K/mm) using a Bridgman-type growth apparatus. The dependence of the dendritic microstructures such as primary dendrite arm spacing (λ1) and secondary dendrite arm spacing (λ2) on the growth rate were determined using a linear regression analysis. The present experimental results were also compared with similar previous experimental results. Measurements of microhardness (HV) and electrical resistivity (ρ) of the directionally solidified samples were carried out. The dependence of the microhardness and electrical resistivity on the growth rate (V) was also analyzed. According to these results, it has been found that, for increasing values of V, the values of HV and ρ increase. However, the values of HV and ρ decrease with increasing values of λ1 and λ2.

Optical and electrical properties of CuO thin films deposited at several growth temperatures by reactive RF magnetron sputtering

Abstract: Copper oxide thin films were deposited on glass substrates at various growth temperatures by the reactive radio-frequency magnetron sputtering method. The band gap energy, carrier concentration and figure of merit of the CuO thin films were found to depend significantly on the growth temperature. All of the CuO films, irrespective of growth temperature, showed a monoclinic structure with the main CuO $$\left( {\bar 111} \right)$$ orientation, and the crystallite size, determined by using Scherrer’s formula, was about 50 nm for the thin film deposited at 25 °C. The highest figure of merit occurred for the film grown at 300 °C with an optical transmittance of 62.9% in the wavelength range of 800–1100 nm. The results suggest that the optimum growth temperature for growing high-quality CuO thin films is 300 °C.

Effect of silicate structure on thermodynamic properties of calcium silicate melts: Quantitative analysis of Raman spectra

Abstract: The distribution of silicate anionic species (Qn units, n=0, 1, 2, 3) and the chemical speciation of oxygen in CaO-SiO2-MO (M=Mn and Mg) slags were investigated by micro-Raman spectroscopic analysis. Furthermore, the thermochemical properties were evaluated using a concentration of free oxygen and a degree of polymerization. A good linear relationship was obtained between sulfide capacity and concentration of free oxygen in the CaO-SiO2 (-MnO) melts at 1500 to 1600 °C. However, even though there was more abundant free oxygen in the CaO-SiO2-MgO system than in the CaO-SiO2 system, the sulfide capacity of the former was lower than the latter, indicating that the sulfur dissolution behavior in the silicate melts cannot be simply explained by the content of free oxygen, because the composition dependency of the stability ratio of oxygen and sulfide ions should be taken into account. The excess free energy of CaO, MgO and MnO linearly decreased as the ln (Q3/Q2) increased. The effect of the degree of polymerization on the excess free energy of mixing of MgO-containing slag was larger than that of MnO-containing slag, which was explained by the difference of the ionization potential between Mn2+ and Mg2+ ions.

Effect of weight percentage and particle size of B4C reinforcement on physical and mechanical properties of powder metallurgy Al2024-B4C composites

Abstract: In this study, Al2024-B4C composites containing 0, 5, 10 and 20 wt% of B4C particles with two different particle sizes (d50=49 μm and d50=5 μm) as reinforcement material were produced by a mechanical alloying method. Two new particle distribution models based on the size of reinforcement materials was developed. The microstructure of the Al2024-B4C composites was investigated using a scanning electron microscope. The effects of reinforcement particle size and weight percentage (wt%) on the physical and mechanical properties of the Al2024-B4C composites were determined by measuring the density, hardness and tensile strength values. The results showed that more homogenous dispersion of B4C powders was obtained in the Al2024 matrix using the mechanical alloying technique according to the conventional powder metallurgy method. Measurement of the density and hardness properties of the composites showed that density values decreased and hardness values increased with an increase in the weight fraction of reinforcement. Moreover, it was found that the effect of reinforcement size and reinforcement content (wt%) on the homogeneous distribution of B4C particles is as important as the effect of milling time.

$$\left\{ {10\bar 12} \right\}$$ twinning behavior in magnesium single crystal

Abstract: In order to investigate $$\left\{ {10\bar 12} \right\}$$ tensile twinning behavior, the magnesium single crystal was deformed by compressing along the $$\left[ {2\bar 1 \bar 10} \right]$$ direction at room temperature, as $$\left\{ {10\bar 12} \right\}$$ tensile twinning easily takes place when the compression direction is perpendicular to the c-axis. Numerous $$\left\{ {10\bar 12} \right\}$$ primary tensile twins were activated during deformation, and the Schmid factor (SF) criterion was applied to the six $$\left\{ {10\bar 12} \right\}$$ twin variants. The analysis shows that the majority of the $$\left\{ {10\bar 12} \right\}$$ primary twins belong to high SF variants, and high SF twin boundaries provided nucleation sites for low SF variants. The $$\left\{ {10\bar 12} \right\}$$ secondary tensile twins were formed inside the high SF of wide $$\left\{ {10\bar 12} \right\}$$ primary twin bands, and the basal plane of the $$\left\{ {10\bar 12} \right\}$$ secondary twin was tilted about 60° with respect to the original parent matrix. In the case of the $$\left\{ {10\bar 12} \right\}$$ secondary tensile twin, relatively low SF variants were activated while counterparts with higher SF variants were absent.

Characterization of deposited layer fabricated by direct laser melting process

Abstract: Deposition dimensions are important in the final applications of products made by direct laser melting (DLM). This investigation used a 200 W fiber laser to produce single-line beads from stainless steel 316L powder using a variety of different energy distributions. To investigate the deposited layer, deposition width, height, penetration depth, and side surface roughness were measured. In order to validate the effectiveness of the two main process parameters (laser power and scan rate), multi-layered beads were fabricated by the sequential layering of single lines. It was found that with an increase in linear energy density, the wetting angle was reduced, and the average roughness was also increased with linear energy density. An equation that predicts the deposition height for a multi-layered bead is proposed and experimentally validated in this study. For deposited layer applications, the material properties of the deposited layer, such as contact angle, interfacial contact resistance, and flexural strength are estimated. The rougher deposited layers show higher contact angle and interfacial contact resistance. The flexural strength of the DLM fabricated specimen is above 250 MPa.

Grain refinement and tensile strength of carbon nanotube-reinforced Cu matrix nanocomposites processed by high-pressure torsion

Abstract: In recent years, the processing of metallic materials via severe plastic deformation has been widely applied to manufacture bulk specimens of ultrafine grained/nanocrystalline structures. In this study, bulk nanocomposites of carbon nanotube-reinforced Cu were manufactured by consolidation of mixtures of coarse grained Cu powders and CNTs of two volume fractions (5 vol% and 10 vol%) using high-pressure torsion, a typical SPD method. The effects of CNT reinforcements on the microstructural evolution of the Cu matrix were investigated using electron backscatter diffraction and scanning/transmission electron microscopy; the results showed that the Cu matrix grain size was reduced to ∼114 nm, and the CNTs were well dispersed in the matrix. Due to the effect of the UFG Cu and CNTs, the tensile strength (350 MPa) of the nanocomposite was higher than that (190 MPa) of Cu processed by the powder HPT process without CNTs. However, the Cu-CNT 10 vol% indicated a decreased tensile strength due to an increased interface area between the matrix and CNTs at high volume fractions of CNTs.

Prediction of effect of volume fraction, compact pressure and milling time on properties of Al-Al2O3 MMCs using neural networks

Abstract: An artificial neural network (ANN) model was developed to predict the effect of volume fraction, compact pressure and milling time on green density, sintered density and hardness of Al-Al2O3 metal matrix composites (MMCs). Al-Al2O3 powder mixtures with various reinforcement volume fractions of 5, 10, 15% Al2O3 and milling times (0 h to 7 h) were prepared by mechanical milling process and composite powders were compacted at various pressure (300, 500 and 700 MPa). The three input parameters in the proposed ANN were the volume fraction, compact pressure and duration of the milling process. Green density, sintered density and hardness of the composites were the outputs obtained from the proposed ANN. As a result of this study the ANN was found to be successful for predicting the green density, sintered density and hardness of Al-Al2O3 MMCs. The mean absolute percentage error for the predicted values didn’t exceed 5.53%. This model can be used for predicting Al-Al2O3 MMCs properties produced with different reinforcement volume fractions, compact pressures and milling times.

Effect of roll speed ratio on deformation characteristics of IF steel subjected to differential speed rolling

Abstract: The paper investigates the effect of the roll speed ratio on the deformation characteristics of interstitial free (IF) steel processed by the differential speed rolling (DSR) technique. An intense plastic strain induced by a single DSR with various roll speed ratios ranging from 1:1 to 1:4 for the lower and upper rolls was successfully imparted to the samples, resulting in the formation of fine elongated ferrite grains as the roll speed ratio increased. Observations of the preferred orientation using electron back-scattered diffraction and transmission electron microscopy revealed that the {110} slip was readily activated as the dominant deformation mode in order to accommodate the intense plastic strain imposed by the DSR. In addition, the microhardness values of the IF steel sample deformed via DSR under a roll speed ratio of 1:4 increased, and this is discussed on basis of the frictional force between the roll and the sample during DSR deformation.

Predicting the flow stress behavior of Ni-42.5Ti-3Cu during hot deformation using constitutive equations

Abstract: The hot deformation behavior of ternary Ni-42.5Ti-3Cu alloy was modeled. Hot compression tests were carried out at the temperatures from 800 °C to 1000°C and at the strain rates of 0.001 s−1 to 1 s−1. The experimental results were then used to determine the constants for developing constitutive equations. There was an unacceptable fitting between the predicted and experimental results using Zener-Hollomon parameter in a hyperbolic sinusoidal equation form. The mismatches among the experimental and predicted results were observed almost for all tested conditions. By modifying the Zener-Hollomon parameter for the compensation of strain rate, a very good agreement was achieved between the predicted values and experimental ones. Both predicted and experimental stress-strain curves illustrate the occurrence of dynamic recrystallization. Also, in both cases, the peak and steady state stresses raised with decrease of temperature and increase of strain rate. The very good agreement between the measured and predicted results indicates the high accuracy of developed model and constitutive equations which can be used for predicting and analyzing the hot deformation behavior of Ni-42.5Ti-3Cu.

Deformation analysis for cold rolling of Al-Cu double layered sheet by physical modeling and finite element method

Abstract: The deformation characteristics of Al-Cu double layered sheet during rolling with various process parameters were studied by both a physical modeling technique and the finite element method. Physical modeling and the finite element method are complementary, due to their different advantages and limitations. Physical modeling simulates metal forming operations by using a model workpiece under conditions similar to those in actual production. The deformation characteristics of double layered sheet during rolling were also simulated using a commercial finite element code, FORGE™. The effects of process parameters, such as total reduction ratio, initial thickness ratio and differential speed ratio on the rolling characteristics were the primary focus of the investigation. In addition, an analytical model for double layered sheet rolling is also proposed with the use of a force-thickness diagram. From the results, the effect of the process parameters on the rolling of the Al-Cu double layered sheet has been determined.

Effect of V Addition on Hardness and Electrical Conductivity in Cu-Ni-Si Alloys

Abstract: The effect of vanadium (V) addition on the microstructure, the hardness and the electrical conductivity of Cu-2.8Ni-0.7Si alloys was investigated. The V-free, the 0.1 wt% V-added, the 0.2 wt% V-added Cu base alloys were exposed to the same experimental conditions. After the cold rolling of the studied alloys, the matrix was recrystallized during the solution heat treatment at 950 °C for 2 h. However, small amounts of vanadium substantially suppressed the recrystallization and retarded the grain growth of the Cu base alloys. The added vanadium accelerated the precipitation of Ni2Si intermetallic compounds during aging and therefore it contributed positively to the resultant hardness and electrical conductivity. It was found that the hardness and the electrical conductivity increased simultaneously with increasing aging temperature and time with accelerated precipitation kinetics by the addition of vanadium. In the present study, the Cu-2.8Ni-0.7Si alloy with 0.1 wt%V was found to have an excellent combination of the hardness and the electrical conductivity when it was aged at 500 °C.

Microstructure and fatigue resistance of high strength dual phase steel welded with gas metal arc welding and plasma arc welding processes

Abstract: This study presents the microstructure and high cycle fatigue performance of lap shear joints of dual phase steel (DP590) welded using gas metal arc welding (GMAW) and plasma arc welding (PAW) processes High cycle fatigue tests were conducted on single and double lap joints under a load ratio of 01 and a frequency of 20 Hz In order to establish a basis for comparison, both weldments were fabricated to have the same weld depth in the plate thickness The PAW specimens exhibited a higher fatigue life, a gentle S-N slope, and a higher fatigue limit than the GMAW specimens The improvement in the fatigue life of the PAW specimens was primarily attributed to the geometry effect that exhibited lower and wider beads resulting in a lower stress concentration at the weld toe where cracks initiate and propagate Furthermore, the microstructural constituents in the heat-affected zone (HAZ) of the PAW specimens contributed to the improvement The higher volume fraction of acicular ferrite in the HAZ beneath the weld toe enhanced the PAW specimen’s resistance to fatigue crack growth The double lap joints displayed a higher fatigue life than the single lap joints without changing the S-N slope

Effects of Ti addition and heat treatments on mechanical and electrical properties of Cu-Ni-Si alloys

Abstract: The mechanical and electrical properties of Cu-5.98Ni-1.43Si and Cu-5.98Ni-1.29Si-0.24Ti alloys under heat treatment at 400 and 500 °C after hot- and cold-rolling were investigated, and a microstructural analysis using transmission electron microscopy was performed. Cu-5.98Ni-1.29Si-0.24Ti alloy displayed the combined Vickers hardness/electrical conductivity value of 315.9 Hv/57.1%IACS. This was attributed to a decrease of the solution solubility of Ni and Si in the Cu matrix by the formation of smaller and denser δ-Ni2Si precipitates. Meanwhile, the alloyed Ti was detected in the coarse Ni-Si-Ti phase particles, along with other large Ni-Si phase particles, in Cu-5.98Ni-1.29Si-0.24Ti.

Constitutive analysis and processing map for hot working of a Ni-Cu alloy

Abstract: The hot deformation behavior of a Ni-Cu alloy was studied using hot compression testing in the temperature range of 950 °C–1150 °C and at strain rates of 0.001 s−1-1 s−1. Flow curves at low strain rates, up to 0.01 s−1, were typical of DRX characterized by a single peak, while at higher strain rates, the typical form of a DRX flow curve was not observed. The power-law constitutive equation was used to correlate flow stress to strain rate and temperature, and the apparent activation energy of hot deformation was determined to be about 462.4 kJ/mol. The peak strain and stress were related to the Zener-Hollomon parameter and the modeling formula was proposed. The dependence of flow stress to the Z changed at ln Z=38.5, which was considered to be a critical condition for the change in the mechanism of dynamic recrystallization. The efficiency of power dissipation was determined to be between 10–35 percent at different deformation conditions. According to the dynamic material model, stable flow was predicted for the studied temperature and strain rate ranges. Highly serrated grain boundaries at low strain rates were considered to be a reason for the occurrence of continuous dynamic recrystallization. On the contrary, at high strain rates, equiaxed grain structure was attributed to the typical discontinuous dynamic recrystallization.

Joining of dissimilar AZ31B magnesium alloy and SS400 mild steel by hybrid gas tungsten arc friction stir welding

Abstract: The joining of dissimilar materials, magnesium alloy (AZ31B) and mild steel (SS400), was performed using a hybrid gas tungsten arc-friction stir welding (HGTAFSW) method that applied a preceding gas tungsten arc welding (GTAW) preheating heat source to a mild steel plate surface during friction stir welding (FSW). The mechanical and microstructural characteristics of the HGTAFS welds were evaluated and compared to those of FS welds to confirm the effect of the additional GTAW preheating heat source. The tensile strength of the HGTAFS welds was approximately 91% of that of the magnesium alloy base metal but higher than that of the FS welds. This was attributed to the enhanced material plastic flow and partial annealing effect in the magnesium alloy and mild steel materials by GTAW reheating of the mild steel side, which induced a significant increase in the elongation of the welds. The concentration profiles indicated that no intermetallic FeAl and FeAl3 compounds had formed according to the phase diagram, which led to a decrease in joint strength. Overall, the use of HGTAFSW by applying a GTAW preheating heat source to a mild steelplate surface resulted in a mechanically sounder and metallurgically defect-free welds compared to FSW.

Effects of antimony trisulfide (Sb2S3) on sliding friction of automotive brake friction materials

Abstract: The effect of antimony trisulfide (Sb2S3) on the tribological properties of automotive brake friction materials was investigated using a Krauss type tribometer and a 1/5 scale dynamometer with a rigid caliper. Results showed that Sb2S3 improved fade resistance by developing transfer films on the disc surface at elevated temperatures. On the other hand, the rubbing surfaces of the friction material exhibited contact plateaus with a broader height distribution when it contained Sb2S3, indicating fewer contact junctions compared to the friction material with graphite. The friction material with Sb2S3 also exhibited a lower stick-slip propensity than the friction material with graphite. The improved fade resistance with Sb2S3 is attributed to its lubricating capability sustained at high temperatures, while the lower stick-slip propensity of the friction material with Sb2S3 is associated with the slight difference between its static and kinetic coefficients of friction and high normal stiffness.

Nickel-based layered double hydroxide from guest vanadium oxide anions

Abstract: The layered double hydroxides (LDHs) are well known for ionic exchange properties to intercalate anionic compound in interlayer region. The Ni/V LDH composite was explored by co-precipitation method. The maximum capacitance of Ni/V LDH composite was 2612 F g−1 at the scan rate 2 mV s−1. As-synthesized Ni/V LDH composite provides a three-dimensional conducting network frame for manifesting electrochemical capacitance. This is because the insertion of vanadium oxide anions (VOx −) into nickel interlayer space between nickel layers is compensated for hydroxyl vacancies during synthesis of lower-pH condition. The network frame makes it possible to promote fast electron transfer for Ni/V LDH composite electrode material and consequently allows the improvement of the electronic conductivity for Ni/V LDH composite electrode material. Thus, the Ni/V LDH composite exhibits high capacitance than β-Ni(OH)2 due to its unique properties, with vanadium oxide anions embedded in the turbostratic structure and shorter diffusion pathway.

Nanocrystalline Co 3 O 4 fabricated via the combustion method

Abstract: A facile and rapid combustion method has been used to prepare nano-crystalline Co3O4 spinel employing urea as a combustion fuel. The fabrication was carried out by refluxing a mixture of cobalt nitrate and urea followed by calcination, for 3 h in static air atmosphere, at 400 °C. The thermal genesis of the Co3O4 was explored by means of thermogravimetric and differential thermal analyses in air atmosphere in the temperature range 25–1000 °C. X-ray diffraction, Fourier transform infrared spectra, and scanning electron microscopy were used to characterize the structure and morphology of the Co3O4. The obtained results conrmed that the resulting oxides were comprised of pure single-crystalline Co3O4 nanoparticles. Moreover, various comparison experiments showed that several experimental parameters, such as the reflux time and the urea/cobalt nitrate molar ratio, play important roles in the crystallite size as well as the morphological control of Co3O4 powders. Consequently, the minimum crystallite size can be obtained at 12 h reflux and a urea/cobalt nitrate molar ratio of 5.

Fabrication of Co-doped ZnO nanorods for spintronic devices

Abstract: Herein, single-crystalline Zn1−xCoxO (0.0≤x≤0.10) nanorods were prepared using a facile microwave irradiation method. Structural analyses by X-ray diffraction and transmission electron microscopy revealed the incorporation of Co2+ in the lattice position of Zn2+ ions into the ZnO matrix. Field emission scanning electron microscopy and TEM micrographs revealed that the length and diameter of the undoped ZnO nanorods were about ∼2 μm and ∼200 nm, respectively. For Co-doped ZnO, the length and diameter were found to increase with an increase of Co doping. The selected area electron diffraction pattern indicated that the Zn1−xCoxO (0.0≤x≤0.10) nanorods had a single phase nature with the preferential growth direction along the [0 0 1] plane. Raman scattering spectra confirmed the shift of the E 2 high mode toward a lower wave number, suggested successful doping of Co ions at Zn site into the ZnO. Magnetic studies showed that Co doped ZnO nanorods exhibited room temperature ferromagnetism and the magnetization value increased with an increase in Co doping. The synthesis method presented here is a simple approach to prepare ZnO based diluted magnetic semiconductors nanostructures for practical application to spintronic devices.

Strain-dependent constitutive analysis of hot deformation and hot workability of T4-treated ZK60 magnesium alloy

Abstract: The hot deformation behavior of T4-treated ZK60 magnesium alloy was investigated in a compression test conducted with a thermo-mechanical simulator at a temperature range of 523 K to 673 K and a strain rate of 0.001 s−1 to 1 s−1. The results show that the flow stress increases as the deformation temperature decreases and the strain rate increases. Strain-dependent constitutive relationships were developed using regression method and artificial neural network, and good agreements between the experimentally measured values and the predicted ones were achieved. The work hardening analysis and onset of dynamic recrystallization (DRX) were investigated. The processing map reveals a domain of DRX at the temperature range of 620–673 K and strain rate range of 0.001–0.01 s−1, with its peak efficiency of 32% at 623 K and 0.001 s−1, which are the optimum values of the parameters for hot working of the T4-treated ZK60 alloy. The strain level has a great effect on the processing maps and lower temperatures and higher strain rates should be avoided during hot working processes. DRX model indicates that DRX of ZK60 alloy is controlled by the rate of nucleation, which is slower than the rate of migration.

Strain hardening model of pure titanium considering effects of deformation twinning

Abstract: This paper is concerned with the effect of deformation twinning on the strain hardening behavior of commercially pure titanium during the compressive loading. In accordance with many studies on titanium, the strain hardening behavior of titanium during compression has different characteristics from those of general metallic materials. It has been reported that the strain hardening rate of titanium during compression can be divided into three stages. In the first stage, the strain hardening rate decreases as the strain increases due to dynamic recovery. Following the first stage, however, a sudden increase in the strain hardening rate is observed in the second stage. It is well known that the occurrence of the second stage is due to the generation of deformation twinning. After the second stage, the strain hardening rate decreases again as the strain increases in the third stage. In this paper, a strain hardening model that can represent the three stages of strain hardening is proposed based on the investigated effect of deformation twinning on the strain hardening behavior of titanium. The electron backscatter diffraction (EBSD) analyses are conducted to quantify the twin volume fraction with increase of compressive plastic strain, which provide fundamental frame of the hardening model for titanium.

Effect of vibration on microstructures and mechanical properties of 304 stainless steel GTA welds

Abstract: This study investigates the microstructures and mechanical properties of 304 stainless steel at various vibration frequencies during simultaneous vibration welding. The experimental results demonstrated that simultaneous vibration welding could accelerate the nucleation and grain refinement of the microstructures. The effect of the grain refinement was more evident at the resonant frequency (375 Hz) and a minimum content of residual δ-ferrite (4.0%). The γ phase grew in the preferential orientation of the (111) direction with and without vibration. The full width at half maximum of the diffraction peak widened after the vibration, which was attributed to the grain refinement. The residual stress could be efficiently removed through simultaneous vibration welding when the amplitude of the vibration was increased. Furthermore, the lowest residual stress (139 MPa) was found when the vibration frequency was 375 Hz. The hardness and Young’s modulus exhibited slight increases with low and medium frequencies. The hardness values were increased by 7.6% and Young’s modulus was increased by 15% when the vibration frequency was resonant (375 Hz).

Effects of microstructure and pre-strain on Bauschinger effect in API X70 and X80 linepipe steels

Abstract: In this study, effects of microstructure and pre-strain on the Bauschinger effect were investigated in two API X70 and two API X80 linepipe steel sheets fabricated by controlling the cooling condition, and their yield strength and Bauschinger parameters were measured by the tension-compression test with varying tension pre-strain. The fast-cooled steels had the higher fraction of acicular ferrite, granular bainite and martensite-austenite (MA) constituents and smaller grain sizes. The reduction in yield stress (ΔYS) of the steels having a higher fraction of MA and smaller grain sizes was higher than that of the steels having a lower fraction of MA and larger grain sizes. The ΔYS was smallest at the pre-strain of 1%, reached the maximum at the pre-strain of 2%, and then decreased with increasing pre-strain. This result could be explained by the amounts of mobile dislocations and back stress, which affected the Bauschinger effect and strain hardening effect simultaneously. Since these two effects affected the yield strength on a competing basis, the Bauschinger stress and hardening parameter were used to separately analyze these effects. It could be confirmed that the Bauschinger effect and strain hardening effect were activated at pre-strains of 1–2% and 3–4%, respectively.

Effect of a grain refiner cum modifier on mechanical properties of Al-7Si and Al-11Si alloys

Abstract: This study evaluates the influence of grain refiners/modifiers on the mechanical properties of the Al-7Si and Al-11Si alloys with an experiment of quantitative and qualitative correlations with the microstructure. Modification of Al-Si alloys with strontium additions and grain refinement with Al-Ti, Al-B and Al-T-B master alloy additions are demonstrated to be efficient on Al-Si alloys. A single master alloy with combined additions of Sr and Ti and/or B was prepared and the microstructure and mechanical properties were studied. The results show that boron rich (Al-3B-Sr and Al-1Ti-3B-Sr) master alloys are more efficient than Ti rich (Al-3Ti-Sr and Al-5Ti-1B-Sr) master alloys considering their combined grain refinement and modification effect on Al-7Si and Al-11Si alloys. However, the presence of Sr does not influence the grain refinement. Similarly, presence of grain refiner does not influence the modification of eutectic Si.