Showing papers in "Materials Characterization in 2017"

Effect of surface roughness on corrosion fatigue performance of AlSi10Mg alloy produced by Selective Laser Melting (SLM)

Abstract: The main interest in Additive Manufacturing technology relates to its ability to produce complex components with relatively reduced weight that are difficult to produce or cannot be produced by other conventional technologies. Although aluminum alloys are considered attractive material for this technology due to their high specific strength and favorable casting characteristics, their surface roughness may have a detrimental effect on their corrosion resistance and corrosion fatigue behavior in particular. The present study aims at evaluating the effect of surface roughness of AlSi10Mg produced by Selective Laser Melting (SLM) followed by stress-relief heat treatment, on the corrosion resistance and corrosion fatigue performance. This was achieved by comparing the corrosion resistance and low cycle corrosion fatigue behavior of SLM samples with their counterpart samples that were polished. The obtained results indicate that the corrosion resistance and corrosion fatigue life span of the SLM samples after polishing were improved compared to that of the unpolished SLM samples. The relatively reduced corrosion resistance and corrosion fatigue endurance of the unpolished SLM samples was related to their increased surface roughness in the form of large amounts of cavities and other surface defects that are inherently produced by the SLM process.

Microstructural evolution and mechanical properties of maraging steel produced by wire + arc additive manufacture process

Abstract: Wire + arc additive manufacture is developed for producing large-scale metallic components. In this paper, maraging steel parts were produced, and the microstructure and mechanical properties were investigated. The microhardness and tensile strength of the as deposited alloy reduced from the bottom to the top due to the transient thermal cycling, which resulted in partial aging and non-uniform formation of intermetallic compounds along the building direction. Solutionizing, followed by 3 h aging, significantly reduced the microstructural heterogeneity and increased the mechanical properties by 24.7% through the formation of large amounts of finely distributed precipitates. The as deposited alloy possessed superior strength to the wrought alloy in solutionized condition but inferior to the later in aged condition, which was attributed to the less pronounced aging response of the low-angle columnar grains characterized microstructure and the presence of retained and reverted austenite.

Structure and optical properties of Fe3O4 nanoparticles synthesized by co-precipitation method with different organic modifiers

Abstract: Synthesis and modification of magnetite nanoparticles were carried out by co-precipitation method. The influence of different organic modifiers on structure and optical properties of Fe 3 O 4 nanoparticles has been studied in detail. The X-ray diffraction method, transmission electron microscopy, Fourier transform infrared spectroscopy and UV–visible spectroscopy were used to determine crystallite size, structure, morphology and optical band-gap energy. The magnetite nanoparticles with different crystallite size at range of 2.9–12.2 nm were obtained by the modified controlled chemical co-precipitation method. The results showed that the Fe 3 O 4 nanoparticles synthesized in solution containing tartaric acid were the smallest and had the highest value of optical band-gap energy (3.01 eV). The use of dextrin allowed obtaining nanocomposite, in which magnetite nanoparticles were dispersed in polysaccharide matrix. It was confirmed that with the decrease in the crystallite size the value of the optical band-gap energy increases, which is related to quantum phenomena. Additionally, shift of characteristic peaks from Fe O bond in Fourier transform infrared spectra were observed, what is also associated with change of the nanoparticles size.

Effect of crystallographic orientation on mechanical anisotropy of selective laser melted Ti-6Al-4V alloy

Abstract: The crystallographic texture of Ti-6Al-4V produced by selective laser melting (SLM) under various laser energy densities was characterized by electron backscatter diffraction technique to explore its effect on the anisotropy in tensile properties. Results show that crystallographic orientation depending on laser energy density acts a significant role in determining the mechanical anisotropy of SLMed Ti-6Al-4V samples. The microstructure of the SLMed Ti-6Al-4V samples consists of fully martensites. As for the martensites, the fraction of basal orientations decreases, while the content of prismatic orientations increases with laser energy density increasing from 101 to 269 J/mm3. And the order of the dominated crystallographic orientation of martensites with the laser energy density is (12 3 − 0)[2 1 − 1 − 3] → (11 2 ¯ 4)[ 1 − 3 − 41] → (11 2 − 0)[1 1 − 01] → (11 2 − 0)[2 2 − 03]. There is anisotropy in tensile properties between horizontally and vertically built samples, which is more obvious with laser energy density. The formation of such anisotropy is ascribed to the higher Schmid factor values of the grains in the vertically built tensile samples than those in horizontally built ones.

Optimization of SEM-EDS to determine the C-A-S-H composition in matured cement paste samples

Abstract: Microanalysis of characteristic X-rays in the SEM is a powerful method to assess the chemical composition of phases in cement pastes, in particular the calcium silicate hydrate containing aluminium (C–A–S–H). Nevertheless, many variables may influence the results obtained, due mainly to the intimate mixing of C–A–S–H with other hydrate phases and the susceptibility of this phase to damage by the electron beam. In this study the effect of various acquisition parameters was examined, along with methods to determine an “average” C–A–S–H composition. The results acquired in the SEM were compared with the analysis of the same samples in the TEM, where phases can be analyzed without intermixing. A simple method was used to obtain compositions from SEM based analysis that are very close to those which can be obtained in the TEM.

Fabrication of a new Al-Mg/graphene nanocomposite by multi-pass friction-stir processing: Dispersion, microstructure, stability, and strengthening

Abstract: In the present research, multi-pass friction stir processing (FSP) was employed for the first time to disperse graphene in the form of graphene nano-platelets (GNPs) into an AA5052 aluminum‑magnesium alloy to fabricate a new Al-Mg/3 vol% GNPs nanocomposite. After five cumulative FSP passes, the GNPs were distributed within the metal-matrix. Field emission-scanning (FE-SEM) and transmission electron microscopy (TEM) analyses were used to examine the dispersion of GNPs, and suggested negligible deterioration of the graphene planar structure following FSP. Some clusters of graphene originating from the initial powder remained due to the high surface energy of these GNPs, while grain orientations were evaluated in the nanocomposite using electron back scattering diffraction (EBSD). A fine equiaxed recrystallized grain structure with an average size of 2.1 μm was formed in the stir zone (SZ) after FSP while dispersing GNPs. Indentation revealed the hardness of the nanocomposite increased by 53% compared to the processed Al-Mg alloy. Yield strength also was improved by more than three times while preserving ductility which achieved 20% strain before fracture. Fractographic studies of tensile test specimens revealed a mixed ductile-brittle fracture behavior. Based on the micromechanics theory, three models considering the microstructural parameters (i.e., aspect ratio, mean size, and volume fraction of GNPs, grain size, and clustering during process) were developed to predict the strengthening effects of GNPs in the terms of elastic modulus, yield strength, and indentation hardness. Correlation between these predicted values and experimental data are discussed in detail.

Characterization of bainitic/martensitic structures formed in isothermal treatments below the M s temperature

Abstract: Advanced Multiphase High Strength Steels are generally obtained by applying isothermal treatments around the martensite start temperature (Ms). Previous investigations have shown that bainitic ferrite can form from austenite in isothermal treatments below Ms, where its formation kinetics is accelerated by the presence of the athermal martensite. That athermal martensite is tempered during the isothermal treatment, and fresh martensite may form during the final cooling to room temperature. The distinction between product phases present after the application of this type of heat treatments is difficult due to morphological similarities between these transformation products. The aim of this study is to characterize the structural and morphological features of the product phases obtained in isothermal treatments below the Ms-temperature in a low-carbon high-silicon steel. Multiphase microstructures, having controlled fractions of product phases, were developed by applying isothermal treatments above and below Ms, and were further studied by electron back scatter diffraction (EBSD) and scanning electron microscopy (SEM). The bainitic or martensitic nature of these product phases is discussed based on this characterization. Results showed that bainitic ferrite appears in the form of acicular units and irregularly shaped laths. Tempered martensite appears as laths with a sharp tip and as relatively large elongated laths with wavy boundaries containing protrusions.

Characterization of molybdenum particles reinforced Al6082 aluminum matrix composites with improved ductility produced using friction stir processing

Abstract: Aluminum matrix composites (AMCs) reinforced with various ceramic particles suffer a loss in ductility. Hard metallic particles can be used as reinforcement to improve ductility. The present investigation focuses on using molybdenum (Mo) as potential reinforcement for Mo(0,6,12 and 18 vol.%)/6082Al AMCs produced using friction stir processing (FSP). Mo particles were successfully retained in the aluminum matrix in its elemental form without any interfacial reaction. A homogenous distribution of Mo particles in the composite was achieved. The distribution was independent upon the region within the stir zone. The grains in the composites were refined considerably due to dynamic recrystallization and pinning effect. The tensile test results showed that Mo particles improved the strength of the composite without compromising on ductility. The fracture surfaces of the composites were characterized with deeply developed dimples confirming appreciable ductility.

Microstructure, mechanical properties and in vitro degradation behavior of novel Zn-Cu-Fe alloys

Abstract: Zn and Zn-based alloys as biodegradable material have drawn more and more attention in recent years due to their good mechanical properties, lower degradation rate than Mg and acceptable biocompatibility. However, too low degradation rate is a challenge for future application of Zn-based alloys. In this study, Zn-3Cu-xFe (x = 0, 0.5 and 1 wt%) alloys were proposed as candidate biodegradable materials. The microstructure, mechanical and in vitro biodegradable properties were investigated systematically. The Zn-3Cu alloy was composed of Zn matrix and CuZn5 phase, while the FeZn13 phase was newly formed with the addition of Fe. Due to the micro-galvanic effect produced by FeZn13 as a cathodic phase to the Zn matrix, in vitro degradation rate in simulated body fluid solution (SBF) was greatly increased by about 52.1%, from 45.3 ± 8.22 for Zn-3Cu alloy to 68.9 ± 7.34 μm/year for Zn-3Cu-1Fe alloy. Although the mechanical properties were decreased slightly due to the introduction of hard and brittle FeZn13 secondary phase to the Zn matrix, Zn-3Cu-0.5Fe alloy still exhibits good combined mechanical properties and the YS, UTS and elongation are 232 MPa, 284 MPa and 33%, respectively. Taken together, the mechanical properties and in vitro degradation behavior of the Zn-3Cu-xFe alloys are more suitable than Zn-3Cu alloy as candidate biodegradable materials.

Characterization and strengthening effects of γ′ precipitates in a high-strength casting Mg-15Gd-1Zn-0.4Zr (wt.%) alloy

Abstract: 1 wt.% Zn addition introduced additional basal γ′ precipitates to the Mg-15Gd-0.4Zr (wt.%) alloy. Based on the detailed analysis of the precipitates and tensile properties evolution of the alloys, the composite β′ and γ′ precipitates provided much stronger strengthening effects than the only β′ precipitates, even when the latter one had higher number density. The results imply that even a few γ′ precipitates can introduce a composite strengthening effect with the β′ precipitates. Meanwhile, the composite β′ and γ′ precipitates effectively strengthened the grain interior and hindered the twining during the tensile tests. Finally, the alloy containing the composite precipitates exhibited yield strength (YS) of 288 MPa and ultimate tensile strength (UTS) of 403 MPa which were only 232 MPa and 296 MPa, respectively, in the alloy containing only β′ precipitates.

Size dependence of microstructure of AlSi10Mg alloy fabricated by selective laser melting

Abstract: In order to investigate the size dependence of microstructure of the AlSi10Mg alloy fabricated by selective laser melting (SLM) combined with a powder bed technique, plate-shaped AlSi10Mg alloy samples with various widths (ranging from approximately 10 mm to 0.3 mm) were produced in this study. All the fabricated samples exhibited characteristic microstructural morphologies consisting of melt pools with columnar α-Al grains surrounded by fine eutectic Si particles, whereas their sample sizes had a relatively small effect on the texture and average grain size of the α-Al matrix. However, the formation of fine Si precipitates inside the columnar α-Al grains was observed more often for the smaller-sized samples, which could be a possible dominant contributor to the observed softening of the sample by reducing sample size. The obtained results were utilized for discussing the size dependence of the strength of the SLM-manufactured porous alloy samples.

Effect of electric current on recrystallization kinetics in interstitial free steel and AZ31 magnesium alloy

Abstract: The effect of electric current on recrystallization kinetics in interstitial free (IF) steel and AZ31 magnesium alloy was investigated based on Vickers hardness measurements and microstructural observation. Electropulsing treatment (EPT) and furnace heat treatment (HT) for these work-hardened metals were carried out under various temperature and time conditions. The Vickers hardness value after EPT was clearly lower than that after HT at the same annealing condition in both IF steel and AZ31 alloy. The microstructural observation confirms that the reduced hardness value was caused by recrystallization. This implies that the recrystallization kinetics was accelerated by the athermal effect distinct from Joule heating. To identify the athermal effect of the electric current on the recrystallization kinetics, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation was adopted. In both IF steel and AZ31 alloy, the value of the Avrami exponent, which depends on the nucleation rate in recrystallization, was higher in EPT than in HT. In addition, it was observed that the activation energy for recrystallization was reduced in EPT.

Localized corrosion in AA2024-T351 aluminium alloy: Transition from intergranular corrosion to crystallographic pitting

Abstract: The relationship between intergranular corrosion and crystallographic pitting in AA2024-T351 aluminium alloy during immersion in 3.5 wt.% NaCl solution is investigated. It is found that intergranular corrosion occurs first and crystallographic pitting initiates from the crevice wall behind the intergranular corrosion front. Propagating away from the grain boundaries, crystallographic pitting develops into selected grain interior. Due to copper enrichment along the pit walls, the crystallographic pits in the AA2024 alloy are characterized with relatively irregular pit walls on a much finer dimension compared to the well-defined half-cube shape crystallographic pits in pure aluminium.

An effect of the rotation speed on microstructure and mechanical properties of the friction stir welded 2060-T8 Al-Li alloy

Abstract: In the present paper, a study of the 2060-T8 Al-Li alloy friction stir welded using the rotation speed range of 600–1000 rpm at a constant welding speed of 300 mm/min is summarized. The effect of rotation speed on microstructure and mechanical properties of the joint was investigated. The results show that with the rotation speed increasing, material in the thermo-mechanically affected zone (TMAZ) on the retreating side (RS) is extended increasingly to the weld nugget zone (WNZ). The microhardness of the WNZ is much lower than that of the base material (BM), consequently all joints were fractured in the WNZ. The maximum tensile strength of the joint is 440 MPa. The maximum strength was achieved at the rotation speed of 800 rpm. However, the maximum elongation is only 2.8%. A model is constructed to explain the mechanism of initiation and propagation of the crack in a joint of the considered material systems. Compared to the BM with various precipitates, only small amounts of T1, β′ and δ′ were detected in the WNZ. This leads to the reduction of mechanical properties. It was established that the grain refinement and the dislocation strengthening are the dominant strengthening mechanisms for the WNZ.

Microstructural control of FeCrAl alloys using Mo and Nb additions

Abstract: The effects of Mo and Nb additions on the microstructure and mechanical properties of two FeCrAl alloys were studied. Fine and uniform recrystallized grain structures (~ 20–30 μm) were achieved in both alloys through suitable annealing after warm-rolling. The formation of Fe2Nb-type Laves phase precipitates in the Nb-containing FeCrAl alloy effectively stabilized the deformed and recrystallized microstructures. The Mo-containing FeCrAl alloy exhibited strong γ texture fiber after annealing at 650–900 °C, whereas the annealed Nb-containing FeCrAl alloy had much weaker texture. Both strength and ductility decreased as the grain size increased in both alloys.

A novel ultrafine-grained Fe 22Mn 0.6C TWIP steel with superior strength and ductility

Abstract: A fully recrystallized ultrafine-grained (UFG) Fe 22wt.%Mn 0.6wt.%C twinning-induced plasticity (TWIP) steel with mean grain size of 576 nm was fabricated by cold rolling and annealing process. Tensile test showed that this UFG steel possessed high yield strength of 785 MPa, and unprecedented uniform elongation of 48%. The Hall-Petch relationship was verified from the coarse-grained (CG) regime to the ultrafine-grained (UFG) regime. The microstructures at specified tensile strains were characterized by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The microstructures and strain hardening behavior of the UFG TWIP steel were compared with the CG counterpart. The strong strain hardening capability of the UFG steel is supposed to be responsible for the high strength and good ductility.

A study on microstructure and mechanical properties of AA 3003 aluminum alloy joints by underwater friction stir welding

Abstract: The friction stir welding of AA 3003 aluminum alloy with different initial microstructures was carried out under different welding conditions. The microstructural evolution and mechanical properties of weld joints were investigated. The results showed that the size of recrystallized grains and the amount of second-phase particles in the weld nugget zone (WNZ) decreased with decreasing welding ambient temperature. At the same welding condition, the size of recrystallized grains in the hot bands was smaller than that in the annealed hot bands, while the volume fraction of (Fe,Mn)Al 6 particles in the hot bands was lower than that in the annealed hot bands. The relationship between the yield strength and grain size was characterized by the Hall-Petch equation. The values of σ 0 and k in WNZ for the hot bands were higher than those for the annealed hot bands.

Mercury intrusion for ion- and conversion-based battery electrodes – Structure and diffusion coefficient determination

Abstract: The electrochemical performance of a secondary battery shows a significant dependency on electrode pore structure since the ionic transport is, beside the electric transport, the crucial transport process in ion based battery systems. Thus, it is necessary to determine and understand the electrode pore structures in detail. One common method to characterize pore size distributions and porosities in solid materials is mercury intrusion. Since battery electrodes are composites of different materials (metal current collector, different fractions of active material and additive particles, binder, and others) the used measurement and calculation methods have to be adjusted. In this work we provide an overview of applying mercury intrusion to determine different electrode pore structure properties: coating porosity, pore size distribution, tortuosity, absolute and specific pore volume, pore volume distribution sum and the inner coating surface. To demonstrate the developed method, a pore structure analysis for a continuous manufactured graphite based electrode with varying compression rates is shown. Furthermore, we show that it is possible to determine a structure dependent diffusion coefficient using mercury intrusion.

Effect of cryogenic deformation on microstructure and mechanical properties of 304 austenitic stainless steel

Abstract: 304 austenitic stainless steel plates have been deformed (10 to 40%) by multi-pass cold rolling incorporating soaking at 0 °C and − 196 °C after each pass with an aim to correlate the microstructure and mechanical properties under cold/cryogenically deformed conditions. Characterisation of phase constituents, microstructure and mechanical properties of such steel specimens has been conducted after processing under different schedules. Rolling of the investigated steel at near cryogenic temperature results into the formation of extended stacking faults, e-martensite and α′-martensite in contrast to the formation of homogeneous dislocation structure along with α′-martensite in the samples rolled at 0 °C, which can be correlated with temperature dependent stacking fault energy. EBSD phase analysis reveals 46.3% and 69.2% α′-martensite in the austenitic matrix for 10% and 20% deformation at − 196 °C, respectively. Deformation twins are evident in all the samples rolled at 0 °C as well as − 196 °C. 40% cold deformation at 0 °C leads to high strength (1225 MPa) and 13% total elongation, whereas comparatively lower 10–20% deformation at − 196 °C leads to higher level of strength (1306–1589 MPa) with 15–9% elongation due to the formation of the higher volume fraction of strain induced martensite (e/α′).

Nanostructure and surface roughness in the processed surface layer of Ti-6Al-4V via shot peening

Abstract: Shot peening process of Ti-6Al-4V was performed at the air pressures ranging from 0.15 MPa to 0.35 MPa and processing durations ranging from 15 min to 60 min so as to obtain the effects of the shot peening on the structure characteristic and surface roughness of Ti-6Al-4V. The experimental results showed that the nanocrystalline layer in Ti-6Al-4V could be fabricated via shot peening at the air pressure above 0.25 MPa and the processing duration above 30 min. The thickness of deformation layer and the degree of grain refinement increased with an increase in the air pressure and processing duration, but the corresponding surface became rougher. In order to obtain a good combination between the grain refinement and the surface roughness, shot peening of Ti-6Al-4V should be performed at a smaller air pressure and a longer processing duration.

Critical resolved shear stress for slip and twin nucleation in single crystalline FeNiCoCrMn high entropy alloy

Abstract: High entropy alloys is an emerging class of materials with superior mechanical properties down to cryogenic temperatures. At 77 K, and unlike traditional metallic alloys, an increase in strength, strain hardening rate, and ductility has been reported. This enhancement in properties has been attributed to the activation of twinning as an additional deformation mechanism at low deformation temperatures. The tendency for the formation of twinning and the hardening response dependence on crystal orientation has not been fully explored. This study is dedicated to explore the deformation evolution across several crystal orientations for the equiatomic FeNiCoCrMn high entropy alloy at room temperature (RT) and 77 K. The works aims to establish the critical resolved shear stresses (CRSS) for slip and twinning and study the orientation and temperature dependence in these magnitudes. The experimental results have revealed a strong temperature dependence in the CRSS for slip, increasing from 56 MPa at RT to 153 MPa at 77 K, with negligible orientation dependence. At 77 K, not all crystal orientations developed twinning even at high levels of deformation. The lack of twinning has been attributed to differences in the hardening response resulting in low stress levels below the twinning CRSS of 153 MPa, as established in this work. No twinning was observed in any of the crystal orientations deformed at room temperature, regardless of the level of hardening and the achieved stresses. Overall, the results discussed in this work enhances our understanding of the local deformation response in single crystalline FeNiCoCrMn high entropy alloy, particularly the nucleation of slip, nucleation of twinning, and the effect of crystal orientation and loading temperature on these deformation mechanisms.

Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging

Abstract: In this study, low volume fraction SiCp/AZ91 magnesium matrix nanocomposites billets intended for structural applications were synthesized using semisolid stirring assisted ultrasonic vibration, leading to the dispersion of SiC nanoparticles. Both the AZ91 alloy and nanocomposite billets were then subjected to isothermal multidirectional forging (IMDF). Micrographic observations illustrated that the mean grain size of the developed nanocomposite showed an initial increase after 3 IMDF passes, followed by a decrease after 6 IMDF passes compared to the AZ91 alloy. This indicated that the effect of dispersed SiC nanoparticles on the inhibition of the dynamic recrystallization grains growth was impaired due to the present high IMDF temperature. The improved yield strength of the nanocomposite could be attributed to Orowan strengthening effect related to the dispersed SiC nanoparticles as well as the second phases precipitated far from the dynamic recrystallization grains.

Improvement of fracture toughness of green concrete as a result of addition of coal fly ash. Characterization of fly ash microstructure

Abstract: Composites with the addition of coal fly ash (CFA) can be included in the sustainable and green concrete. Effective promotion of green concrete incorporating CFA is necessary to minimize the threat to the environment posed by CFA waste disposal and to reduce cement consumption thus cutting CO2 emissions. This study investigates the influence of the curing time on the fracture toughness of concrete produced with different concentrations of CFA. Concentrations of 20% – CFA-20 and 30% – CFA-30 of CFA were used and the results were compared with a reference mixture with 100% Ordinary Portland Cement (OPC) – CFA-00. Compressive strength – fcm and fracture toughness under mode I – KIcS (tension at bending), were determined after: 3, 7, 28, 90, 180 and 365 days. The results obtained lead to the conclusion that, it is possible to make green concrete containing CFA with high fracture toughness. Furthermore, this is one of the ways to reduce cement industry CO2 emissions. 20% additive of CFA guarantees high fracture toughness in mature concretes, whereas concrete with 30% CFA additive is characterized by highest dynamic increase of the parameter KIcS. The experimental program was completed by the analysis of microstructure of CFA by using SEM. These studies showed that the main morphological forms in the CFA are single grains, such as: pyrospheres, cenospheres, plerospheres, multispheres, ferrospheres, grains of irregular shapes, and amorphous grains. Grains of CFA may also occur in larger quantities, as: jointed grains, clusters and agglomerates.

In situ EBSD observation of grain boundary character distribution evolution during thermomechanical process used for grain boundary engineering of 304 austenitic stainless steel

Abstract: Electron backscatter diffraction (EBSD) was used to examine the microstructural evolution in a one-step thermomechanically processed 304 austenitic stainless steel specimen during the thermomechanical process of grain boundary engineering. Solution-treated materials were cold-rolled to 3% reduction and subsequently annealed at 1220 K for different annealing times. The EBSD observation of the specimen showed an increase in the frequency of coincident site lattice (CSL) boundaries and a decrease in the percolation probability of random boundaries. Additionally, the specimen exhibited heterogeneous growth of clusters of grains that contained a high frequency of CSL boundaries. These clusters of grains were developed in the entire observed area by strain-induced grain growth according to the results of grain orientation spread analysis. The details of the growth of the clusters and the disconnection of random boundaries were successfully observed in situ using EBSD and a heating stage. The frequency of CSL boundaries increased with the growth of the clusters. Disconnection of random boundaries between the clusters was achieved by the formation of annealing twins through the impingement of the growing clusters during the thermomechanical process. Twin variant selection to introduce CSL boundaries into a random boundary network was observed by the in situ EBSD observation.

Influence of equal channel angular pressing routes on texture, microstructure and mechanical properties of extruded AX41 magnesium alloy

Abstract: The influence of three different equal channel angular pressing (ECAP) routes (A, Bc and C) on the grain size, texture, dislocation structure and mechanical properties in pre-extruded AX41 magnesium alloy was investigated. It was found that during the first passes, the rate of grain fragmentation strongly depends on the processing route. After 8 passes, despite the almost identical values of the dislocation density (0.7 × 10 14 m − 2 ), the average grain size varied in the range of 2.0–4.5 μm for the individual ECAP routes. Macroscopic texture measurements revealed a gradual formation of very strong textures, which were significantly different for the various processing routes. The strength and the ductility of the samples were investigated by tensile test carried out parallel to the outgoing channel axis. Route A was found to be the most effective processing route for grain refinement. In tensile tests carried out at room and elevated temperatures, the highest strength was observed for the sample processed via route A for 8 passes, due to the highest texture hardening and the smallest grain size.

Phase transformation and microstructure evolution of an ultra-high strength Al-Zn-Mg-Cu alloy during homogenization

Abstract: Homogenization treatment of an ultra-high strength Al-Zn-Mg-Cu alloy was studied by SEM, TEM, DSC, hardness and electrical conductivity test. The second phases in as-cast 7A56 alloy are AlZnMgCu, Al 2 Cu and Al 7 Cu 2 Fe. AlZnMgCu phase has a similar lattice structure of MgZn 2 . After 380 °C homogenization treatment, transformation from AlZnMgCu to Al 2 CuMg occurred; after 470 °C homogenization treatment, AlZnMgCu were mainly directly dissolved into the matrix with no Al 2 CuMg discovered. That phenomenon was caused by the diffusion behavior of Zn and Cu at two different temperatures that Zn diffuses rapidly even at 380 °C while Cu shares a much lower diffusion rate. Much of the AlZnMgCu phase can be dissolved at 470 °C for a short time while at 380 °C it was seldom eliminated. After homogenized at 470 °C for 24 h, the dissolution process gradually reached into a balance with more than 95% had been eliminated and prolonging time imposes no evident influence on promoting the dissolution. Hardness and electrical conductivity are highly consistent with the vibration of AlZnMgCu phase, with the dissolution of which the hardness increases while electrical conductivity decreases.

Growth kinetics and thickness prediction of interfacial intermetallic compounds between solid steel and molten aluminum based on thermophysical simulation in a few seconds

Abstract: The growth kinetics of intermetallic compounds (IMCs) generated by initial interfacial reaction (interaction time 2 Al 5 ) and θ (Fe 4 Al 13 ) phase layers in the isothermal experiments in seconds, it is confirmed that the reaction diffusion controls the η phase growth and the θ phase precipitates from the liquid phase during cooling. The growth kinetics models of the η phase maximum and average thickness are proposed as well. The results showed that the maximum growth was governed by the interfacial reaction, whereas the average thickness was governed by a diffusion and interfacial reaction process. Considering the application of the kinetics model in welding-brazing of Fe/Al dissimilar metals, a mathematical model to predict the average thickness of η phase was established using the finite difference method in thermal cycles as well. The predicted results are consistent with the experimental results, indicating that the model is of good accuracy.

Microstructural evolution and mechanical properties enhancement of a cold-sprayed CuZn alloy coating with friction stir processing

Abstract: An innovative hybrid process combining two very effective solid-state techniques which are Cold Spray (CS) and Friction Stir Processing (FSP), was proposed to fabricate a high-strength ultrafine-grained Cu Zn coating. Results show that the cold-sprayed coating obtained exhibits an elongated microstructure with 78.42% of low-angle grain boundaries. Following FSP, there appear ultrafine grains with 90.47% of high-angle grain boundaries and a blend of α, β″ and γ phases, while the as-sprayed coating is mainly α-phased. Significant mechanical properties enhancement is achieved, i.e. with the ultimate tensile strength increasing from 87.2 MPa to 257.5 MPa and the fracture elongation increasing from 0.17% to 0.81%. The precipitates have a significant effect on the fracture behavior of friction-stirred coating.

Microstructure evolution and mechanical properties of carbon nanotubes reinforced Al matrix composites

Abstract: Carbon nanotubes reinforced pure aluminum (CNT/Al) composites were fabricated by combined ball milling and powder metallurgy (PM) techniques The distribution of CNTs, the evolution of the average Al grain size of the powder mixtures and as-prepared composite bulks were investigated, and the mechanical properties of the composites were also tested With increasing ball milling time, the entangled CNTs were broken, gradually achieving a uniform dispersion within the Al matrix The microstructure became denser and the Al grains in the powder mixture and extruded composites got significantly refined Some small-sized Al 4 C 3 needles along the Al grain boundaries were observed using transmission electron microscopy (TEM) The in-situ formed Al 4 C 3 rods have an orientation relation with the Al matrix as Al 4 C 3 [110]//Al [ 1 ¯ 12 ], which strongly improved the Al-CNT interface bonding The yield and the ultimate tensile strength of the composites significantly increased, when the ball milling time increased from 2 to 12 h, finally reaching about 210 ± 42 MPa and 253 ± 37 MPa, respectively, for the composite milled for 12 h The enhancement of mechanical properties mainly stems from the uniform distribution of CNTs, the grain refinement of the Al matrix and the in-situ formed Al 4 C 3

Effect of deformation induced precipitation on grain refinement and improvement of mechanical properties AA 7055 aluminum alloy

Abstract: An improved TMT (thermomechanical treatment) double step hot rolling (DR) including low-temperature pre-deformation, intermediate short-term annealing and final hot rolling, was proposed to manufacture fine grained AA7055 sheets. Microstructural analysis results show that the low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates which can exert great drag force to the migration of grain boundaries and dislocations so as to promote the formation of high density dislocation cells and accommodate sufficient deformation storage energy subsequently. Then, these dislocation cells may turn into polygon sub-grains by short-period intermediate annealing. With the final hot rolling, low angle grain boundaries (LAGBs like sub-grain boundaries) were gradually transferred into high angle grain boundaries (HAGBs) and fine-grained structures were obtained. Tensile testing results reveal that samples processed by the optimal DR processing (300 °C/60% + 430 °C/5 min + 430 °C/50%) display superior tensile plasticity than the conventional hot rolling (CR) samples without strength loss, which could be correlated with the fine grained structure of the DR sample. It indicates that the present DR processing may be a good alternative to efficiently produce fine grain structured heat-treatable aluminum alloys.