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Showing papers in "Metals and Materials International in 2019"


Journal ArticleDOI
TL;DR: The microstructure and properties of the novel heat resistant Al-3Ce-7Cu alloy produced by selective laser melting were investigated in this article, where the as-printed yield strength, ultimate tensile strength and elongation are 274 MPa, 456 MPa and 4.4% respectively.
Abstract: The microstructure and properties of the novel heat resistant Al–3Ce–7Cu alloy produced by selective laser melting were investigated. Fine Al11Ce3 and Al6.5CeCu6.5 eutectic phases were found in the microstructure. Annealing at temperatures in the 250–400 °C range leads to a decrease in the hardness. Hardness has larger values after annealing at 350 and 400 °C than at 250 °C due to the precipitation of nanosized particles. The low hardness after quenching and aging at 190 °C is caused by quench stress relief and the absence of aging hardening because of poor solid solution. The as-printed yield strength, ultimate tensile strength and elongation are 274 MPa, 456 MPa and 4.4%, respectively. High mechanical properties of the Al–3Ce–7Cu alloy were demonstrated by high temperature tension and compression tests.

73 citations


Journal ArticleDOI
TL;DR: In this article, the authors focused on the HIC related failure in pipeline steel, since the role of texture and grain boundary character has not been completely recognized, and the existing solutions for improving the steel resistance to HIC have been investigated based on the control of micro-alloy elements, texture andgrain boundary engineering.
Abstract: Currently, thousands of kilometers of pipeline steels are transferring hydrocarbon fluids such as oil and natural gas in the world. Due to the fact that these pipes transport corrosive and high-pressure fluids from harsh environments, they are damaged and eventually degraded. Previous studies showed that sulphide stress cracking, hydrogen induced cracking (HIC) and stress corrosion cracking are the main destructive factors in these types of pipes. This paper focused on the HIC related failure in pipeline steel, since the role of texture and grain boundary character has not been completely recognized. Moreover, if pipeline damage is occurred by hydrogen cracks, besides the environmental pollutions, it will cost a lot to repair or replace the damaged pipeline steels. In this research, the factors influencing the initiation and propagation of the HIC cracks, such as hydrogen traps, inclusions, precipitates, microstructure and texture of steel have been investigated. Also, the existing solutions for improving the steel resistance to the HIC have been investigated based on the control of micro-alloy elements, texture and grain boundary engineering. For instance, some special dominant texture components and coincidence site lattice boundaries decrease the HIC susceptibility by providing the resistant path for crack propagation.

58 citations


Journal ArticleDOI
TL;DR: In this paper, the microstructural characteristics of spot weld joints and LME cracking tendency were investigated using dye penetration tests, optical microscopy, scanning electron microscopy together with energy dispersive X-ray spectroscopy.
Abstract: Liquid metal embrittlement (LME) caused cracking of Galvanized transformation induced plasticity steels was investigated during resistance spot welding (RSW). Effect of electrode force (3–5 kN) for a weld time of 400, and 800 ms on crack resistance of TRIP steel was examined in relation to LME phenomenon. The microstructural characteristics of spot weld joints and LME cracking tendency were investigated using dye penetration tests, optical microscopy, scanning electron microscopy together with energy dispersive X-ray spectroscopy. It was found that as the electrode force increases, the crack size decreases due to a fast increase in crack tip temperature which rises further with more holding time up to 800 ms in high temperature zone. Least amount of crack size was observed at 5 kN electrode force and 400 ms of welding time. Finally, the experimental results have also been simulated by finite element modeling (FEM) to find suitable mechanism of crack formation, and a combination of 4 kN and 400 ms was suggested for the crack free and less thermal deformation in the spot welded TRIP steel.

56 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated additive manufacturing of polylactic acid by fused deposition modeling based on statistical analysis, where the honeycomb internal pattern was employed to build inside of specimens due to its remarkable capability to resist mechanical loads.
Abstract: In the present study additive manufacturing of Polylactic acid by fused deposition modeling were investigated based on statistical analysis. The honeycomb internal pattern was employed to build inside of specimens due to its remarkable capability to resist mechanical loads. Simplify 3D was utilized to slice the 3D model and to adjust fixed parameters. Layer thickness, infill percentage, and extruder temperature were considered as controlled variables, while maximum failure load (N), elongation at break (mm), part weight (g), and build time (min) were selected as output responses and analysed by response surface method. Analysis of variance results identified layer thickness as the major controlled variable for all responses. Interaction of infill percentage and extruder temperature had a significant influence on elongation at break and therefore, tough fracture of printed parts. The input parameters were optimized to materialize tow criteria; the first one was to rise maximum failure load and the second was to attain tough fracture and lessen build time and part weight at a time. Optimal solutions were examined by experimental fabrication to evaluate the efficiency of the optimization method. There was a good agreement between empirical results and response surface method predictions which confirmed the reliability of predictive models. The optimal setting to fulfill the first criterion could bring on a specimen with more than 1500 (N) maximum failure load and less than 9 (g) weight.

55 citations


Journal ArticleDOI
TL;DR: In this paper, a single FCC phase 40Fe−25Ni−15Cr−10Co−10V high-entropy alloy was designed, fabricated, and evaluated for potential cryogenic applications.
Abstract: A single FCC phase 40Fe–25Ni–15Cr–10Co–10V high-entropy alloy was designed, fabricated, and evaluated for potential cryogenic applications. The alloy forms a single FCC phase and exhibits higher yield strength, tensile strength, and elongation at cryogenic temperature (77 K) than at room temperature (298 K). The superior tensile properties at cryogenic temperature are discussed based on the formation of deformation twins during the tensile test at cryogenic temperature. In addition, a constitutive model reflecting the cryogenic deformation mechanism (i.e., twinning-induced plasticity) was implemented into the finite element method to analyze this behavior. Experimental results and the finite element analysis suggest that the increase in plastic deformation capacity at cryogenic temperature contributes to the formation of deformation twins.

54 citations


Journal ArticleDOI
TL;DR: In this article, an overview of different available materials and process parameters for FGMs based on their mechanical and wear characteristics is presented, and the findings in current work are classified according to material parameters such as the type of matrix material, reinforcing material (type, weight, and size), in addition to process parameters, such as mould rotational speed and pouring temperature.
Abstract: Mechanical characteristics of functionally graded materials (FGMs) can be optimized for specific application depending on the demands of different engineering sectors using appropriate combinations of material amounts and processing parameters. In recent years, many approaches to the production of functionally graded metal matrix composites (FGMMCs) using the centrifugal casting method have been proposed. This paper presents an overview of different available materials and process parameters for FGMs based on their mechanical and wear characteristics. The major conclusions in this paper are drawn from the previous studies on experimental investigations on mechanical properties and wear characteristics of FGMMCs. The findings in current work are classified according to material parameters such as the type of matrix material, reinforcing material (type, weight, and size), in addition to process parameters such as mould rotational speed and pouring temperature.

53 citations


Journal ArticleDOI
TL;DR: In this article, four different compositions of high entropy alloys based on Al-Co-Cr-Fe and AlCo Co-Cr−Fe-Ni systems were prepared using mechanical alloying and consolidation by spark plasma sintering.
Abstract: Four different compositions of high entropy alloys based on Al–Co–Cr–Fe and Al–Co–Cr–Fe–Ni systems were prepared using mechanical alloying and consolidation by spark plasma sintering. The chemical compositions of the studied alloys were experimentally selected to obtain a BCC solid solution and mixtures of BCC with FCC. The microstructure of the Al25Co25Cr25Fe25 (all in at%) high entropy alloy consisted of a matrix with a high concentration of Al, Co and Fe, in which spherical grains (50–200 nm) enriched in Cr were embedded. Both the matrix and grains had body centered cubic structures. The addition of nickel to a four-element system led to the formation of a multiphase composition. The microstructure of the Al20Co20Cr20Fe20Ni20, Al10Co30Cr20Fe35Ni5 and Al15Co30Cr15Fe40Ni5 HEAs consisted of fine grains measuring 50–500 nm composed of: AlNi-B2, BCC phase, FCC or BCC solid solutions and σ-sigma phase, respectively. The complex structure of the studied samples resulted in changeable mechanical properties. The highest compression strength of 3920 MPa was accompanied by an increased yield strength of 3500 MPa, and a low strain of 0.7%, for the Al25Co25Cr25Fe25 alloy. The addition of Ni led to the formation of plastic FCC phases responsible for a decrease in strength with increases in ductility, which, in the new non-equiatomic Al10Co30Cr20Fe35Ni5 high entropy alloy reached 6.3% at a yield strength of 1890 MPa and compression strength of 2230 MPa. The conducted abrasion studies revealed that non-equilibrium high entropy alloys have the highest abrasion resistance.

46 citations


Journal ArticleDOI
TL;DR: In this paper, the most key factors of shot peening (SP) process on the microhardness, grain size, and residual stress of AISI 1060 high carbon steel were detected.
Abstract: In the present study, the main purpose is to detect the most key factors of shot peening (SP) process on the microhardness, grain size, and residual stress of AISI 1060 high carbon steel. The specimens were treated using various types of SP process namely conventional shot peening and severe shot peening. Several experiments were performed to study metallurgical and mechanical properties of AISI 1060 steel. Almen intensity and surface coverage from one side and microhardness, grain size, and residual stress from the other side were considered as input and output parameters for the design of experiment methodology, respectively. The $$L_{18} (2^{1} \;and\;3^{1} )$$ mixed level of Taguchi orthogonal array design was used to study all cases. The test results were investigated by signal-to-noise ratio formula. It was identified that the surface coverage is the most key factors for shot peening process considering the affected depth. Also, the effect of this parameter on the microhardness, grain size and residual stress was obtained approximately 68, 89 and 57%, respectively. Eventually, the results obtained from all Taguchi sensitivity analysis indicated that it would be better to adjust the surface coverage factor in comparison with Almen intensity factor in order to create the surface compressive residual stress on the material and consequently to increase the fatigue lifetime of component using shot peening treatment.

40 citations


Journal ArticleDOI
TL;DR: In this paper, a nonlinear combined isotropic and kinematic hardening model was used to estimate the strength enhancement from back-stress hardening, and the simulation results were correlated with the experimental results.
Abstract: Although back-stress contributes to the mechanical properties of materials, the degree of strength enhancement from back-stress is not easy to estimate. In this research, back-stress hardening of twinning-induced plasticity (TWIP) + interstitial free (IF)-layered steel sheets were estimated by implementing a non-linear combined isotropic and kinematic hardening model. High back-stress evolution occurs due to plastic strain incompatibility between a TWIP-steel core and IF-steel sheath, and the strength of TWIP + IF layered steel sheath is greater than the strength estimated by the rule-of-mixtures. A non-linear combined isotropic and kinematic hardening model was used to estimate the strength enhancement from back-stress hardening, and the simulation results were correlated with the experimental results. This result shows that the back-stress evolution in heterogeneous materials contributes to their strength, and that the non-linear combined isotropic and kinematic hardening should be included to estimate the degree of back-stress hardening.

38 citations


Journal ArticleDOI
TL;DR: In this article, Ni32-x-Ti-Ta18+x (x 0, 2, 3, 4, 5, 6, 7, 8) shape memory alloys, produced by arc-melting method, were investigated using differential scanning calorimetry (DSC), optical microscopy (OM), x-ray diffraction (XRD), and Vickers micro-hardness measurements.
Abstract: In the present work, Ni32-x-Ti-Ta18+x (x = 0, 2, 3, 4, 5, 6, 7, 8) shape memory alloys, produced by arc-melting method. Then, differential scanning calorimetry (DSC), optical microscopy (OM), x-ray diffraction (XRD), and Vickers micro-hardness measurements were carried out to investigate thermodynamic parameters, microstructure, crystal structure, and mechanical properties of the alloys, respectively. The DSC results showed that, as the amount of Ta increased, the phase transformation temperatures of the specimens significantly changed. In addition, increasing of Ta composition raised the mass density and electron participation of NiTi alloy, and thus, the vibrational term of entropy overcomes the electron participation; consequently, the total entropy declined in the alloys. It is found that OM images possess a dendritic microstructure, where by increasing the amount of Ta, the dendrites length increase while random orientations decrease. Moreover, XRD patterns exhibited the existence of each austenite phase (B2), martensite phase (B19ʹ), and β-Ta riched phase in all samples.

37 citations


Journal ArticleDOI
TL;DR: The tensile properties and serrated flow behavior of as-cast CoCrFeMnNi high-entropy alloy were examined over a wide range of temperatures in this article.
Abstract: The tensile properties and serrated flow behavior of as-cast CoCrFeMnNi high-entropy alloy were examined over a wide range of temperatures. Tensile elongations were higher at 22 °C than at high temperatures above 700 °C. With the increase in the temperature from 500 to 700 °C, distinct serration flows occurred and evolved in the sequence of type A, type B, and type C serrations. Anisotropy in the tensile properties and serrated flow was identified. The average stress drop amplitude in the serrated flow was generally higher when tensile loading was applied perpendicular to the columnar grain growth direction. Moreover, 700 °C, the strain increment required for subsequent stress drops was significantly lower when tensile loading was applied perpendicular to the columnar grain growth direction. The start and finish temperatures of serration flow were slightly higher in as-cast HEA by 100–200 °C compared to those in wrought HEA.

Journal ArticleDOI
TL;DR: In this paper, the effect of process variables on microstructures, intermetallic compounds and their phases, and thereby on corrosion of the aluminum-steel welded joint was investigated. But the results of the experiments were limited.
Abstract: The use of aluminum in conjunction of steel can reduce the weight of structures but dissimilar materials welded structure results in the formation of intermetallic compounds and inhomogeneous distribution of grains. Since aluminum is more active than the steel, the structures made from such dissimilar materials can be affected from corrosion medium which needs to be investigated. In the present work, friction stir welding has been used to join AA6061-T6 and AISI304 in lap configuration, each having a thickness of 1 mm under varied process parameters. The detailed investigations have been made which includes understanding the effect of process variables on microstructures, intermetallic compounds and their phases, and thereby on corrosion of the aluminum-steel welded joint. SEM with integrated EBSD detector and XRD analyses have been carried out to characterize the weld interface that revealed the evolution of grain boundaries and existence of phases such as Fe2Al5 and AlCrFe2. The grain size of the weld zone has been found to be decreasing with increase in weld speed and plunge depth. The temperature profiles have shown a faster rate of heating and cooling with increase in welding speed and plunge depth which led to the refinement of microstructure. The evolution precipitates mainly comprised of Al, Mg and Si as the major elements. The corrosion rate was found to be increasing with decrease in grain size. Samples were corroded by pitting corrosion, inter-granular corrosion, and environmental corrosion. Severity of pits have been found to be non-uniform in the along weld cross-section.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the relationship of stretch-flangeability to microstructure and mechanical properties of ultra-high-strength dual-phase (DP) steels.
Abstract: To clarify the direction of microstructure design for improving stretch-flangeability, relationships of stretch-flangeability to microstructure and mechanical properties of ultra-high-strength dual-phase (DP) steels were investigated. Microstructure of relatively simple ferrite-martensite DP steels was modified by intercritical annealing, then the effects of microstructure modification on stretch-flangeability, tensile properties, and fracture resistance of the DP steels were systematically quantified. The hole-expansion ratio (HER) increased linearly with an increase the apparent fracture initiation energy, but was not significantly correlated with any individual microstructural properties of DP steels, which have been reported to correlate with HER (e.g., the fraction of martensite, the carbon content of martensite, or the hardness difference between ferrite and martensite). To increase the stretch-flangeability of an ultra-high-strength DP steels, its microstructure should be designed to increase its fracture toughness (i.e., microstructure with low mechanical heterogeneity).

Journal ArticleDOI
TL;DR: In this article, the activation energy of high manganese twinning induced plasticity (TWIP) steels was investigated by means of a single-hit compression test in the temperature range of 850-1000°C and the strain rate range of 0.1-10−s−1.
Abstract: High manganese twinning induced plasticity (TWIP) steel is an attractive material for automotive applications as its use could result in an improved vehicle fuel efficiency and a superior passenger safety. Due to the limited research on the hot deformation behaviour of High Mn steel, the selection of suitable operating conditions for the hot rolling process is challenging. The present contribution focusses on the hot deformation behaviour and the dynamic recrystallization kinetics of V micro-alloyed high manganese TWIP steel, by means of single-hit compression test in the temperature range of 850–1000 °C and the strain rate range of 0.1–10 s−1. The activation energy for hot deformation and the processing map of a V-free TWIP steel and a V-added TWIP steel were compared by analysing their stress–strain curves. The V-added TWIP steel exhibited a higher activation energy than the V-free TWIP steel, i.e. 383.4 kJ/mol versus 372.5 kJ/mol. Processing maps based on a dynamic material model indicated that the hot workability of TWIP steel was decreased by micro-alloying with V. The effect of V on the hot deformation behaviour of TWIP steels was also analysed by means of its effect on the microstructure using the SEM-EBSD technique. The V-added TWIP steel was characterized by a higher peak stress at a lower peak strain as compared to the V-free TWIP steel, indicating that the onset of dynamic recrystallization was accelerated by the addition of V. The rapid dynamic recrystallization kinetics resulted in a smaller recrystallized grain size in the hot deformed microstructure of the V-added TWIP steel.

Journal ArticleDOI
TL;DR: In this article, the kinetics of grain growth in the two-phase austenite plus ferrite region was studied based on the parabolic grain growth law and it was revealed that the thermally-activated grain growth of ferrite depends on the soaking temperature and the presence of Austenite islands, where these two factors compete with each other.
Abstract: The study of grain coarsening and its kinetics during intercritical annealing is of vital importance for the production of dual phase (DP) steels with appropriate microstructure and properties required for industrial applications. In the present work, the kinetics of grain growth in the two-phase austenite plus ferrite region was studied based on the parabolic grain growth law. It was revealed that the thermally-activated grain growth of ferrite depends on the soaking temperature and the presence of austenite islands, where these two factors compete with each other. As a result, by increasing the temperature, initially the rate of growth increases and the activation energy of grain growth (Q) was determined as 615 kJ/mol, which indicates the high-temperature dependency. However, after the formation of a certain amount of austenite and the formation of chain-network morphology of austenite, the increase of temperature results in a decreased growth rate with the Q value of − 258 kJ/mol. This suggests that the pinning effect counteracts the temperature effect in this stage. The effect of grain size on mechanical properties and work-hardening behavior was also discussed. Higher work-hardening rate was observed for the fine-grained DP microstructures, which was found to be responsible for the better strength–ductility trade off.

Journal ArticleDOI
TL;DR: In this article, a multi-level processing parameters optimization method based on particle swarm optimization (PSO) algorithm is developed to obtain the designed target microstructures of an aged superalloy during isothermal forging.
Abstract: To obtain the designed target microstructures of an aged superalloy during isothermal forging, a multi-level processing parameters optimization method is developed based on particle swarm optimization (PSO) algorithm. In the developed method, the accurate material models are used to characterize the microstructural evolution. Based on the designed target microstructures, the global and local optimality criterions are constructed to alternately optimize global and local multi-level processing parameters by the PSO algorithm with a linear decreasing inertia weight strategy. The optimized initial volume fraction of δ phase (δVF), deformation temperature and strain rate are 12.95%, 1000 °C and 0.001 s−1, respectively. According to these optimized parameters, the recrystallization volume fraction, average grain size and δVF are 100%, 11.2 µm and 2.1%, respectively, which well agree with the designed targets. Additionally, the processing parameters optimized by the developed method and traditional processing maps are compared. It is found that the developed method is more effective to control microstructures for the studied superalloy.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the prestrain effect on hydrogen embrittlement susceptibility of EH 36 steels using thermal desorption spectroscopy and in situ slow-strain-rate testing.
Abstract: Hydrogen can provide pure and clean energy; however, to use it as an energy source, facilities such as hydrogen carriers and recharging stations need to be constructed. Structural steels are affected by hydrogen embrittlement (HE), and their susceptibility to this needs to be investigated prior to their use in construction. Most structural steels are normally fabricated using thermomechanical controlled processing, which produces a large dislocation density to increase strength. This study investigated the prestrain effect on HE susceptibility of EH 36 steels using thermal desorption spectroscopy (TDS) and in situ slow-strain-rate testing. Hydrogen was electrochemically charged into specimens, and the reversible hydrogen content and that relating to trap sites were measured using TDS. With an increase in prestrain, there was increase in the diffusible hydrogen content; furthermore, with hydrogen charging, there was a drastic reduction in total elongation with an increase in prestrain. In addition, there was an increase in HE susceptibility with an increase in prestrain compared to when an air condition was employed. Specifically, there was an abrupt increase in HE sensitivity at a prestrain value between 10 and 15%; strain hardening was more dominant below a prestrain value of 10%; and HE was more dominant above a prestrain value of 15% for EH 36 steels.

Journal ArticleDOI
TL;DR: In this paper, electroless Ni-P-W coatings were applied on AZ91 sheets with various W contents and both the corrosion and wear resistance characteristics of these coatings are observed.
Abstract: Electroless Ni–P–W was coated on AZ91 sheets with various W contents and both the corrosion and wear resistance characteristics of these coatings were observed. The increase in W content resulted in a decrease in P content and an increase in the crystallinity of the electroless coatings. The corrosion resistance of the electroless coatings increased upon increase in W content of the electroless coatings up to approximately 5%. Above this content, however, the corrosion resistance tended to decrease due to the disturbed grain structure of the electroless coating. But both the surface hardness and wear resistance increased concurrently due to the positive contribution of W solid solution hardening. The applied heat treatment resulted in a decrease in the corrosion resistance due to the disappearance of the amorphous structure and increase in both the surface hardness and wear resistance due to the precipitation hardening. Al2O3 dispersion was better with the nonionic surfactant, as compared to the anionic and cationic surfactants. Composite coating reduced the corrosion resistance and increased the wear resistance of the electroless Ni–P–W coatings. The applied heat treatment was observed to have positive contribution to Ni–P–W coatings in getting the optimum corrosion and wear resistance combination.

Journal ArticleDOI
TL;DR: In this paper, a high-strength AZ91 alloy is produced via hot extrusion using flakes fabricated through the rapidly solidified flaky powder metallurgy, which exhibits an almost fully recrystallized microstructure with a very small average grain size of 1.2
Abstract: A high-strength AZ91 alloy is produced via hot extrusion using flakes fabricated through the rapidly solidified flaky powder metallurgy. The AZ91 alloy flakes have an extremely fine dendritic structure without any second-phase particles owing to the fast cooling rate during solidification; these microstructural features considerably promote dynamic recrystallization and precipitation behaviors during extrusion process. As a result, the AZ91 alloy extruded using the flakes exhibits an almost fully recrystallized microstructure with a very small average grain size of 1.2 µm owing to an increase in the number of nucleation sites for recrystallization, and it shows a high microstructural homogeneity owing to the numerous Mg17Al12 precipitates uniformly distributed throughout the material. This extruded AZ91 alloy has a tensile yield strength of 345 MPa, ultimate tensile strength of 417 MPa, and total elongation of 5.6%. These superior tensile strengths are mainly attributed to the combined effects of precipitation hardening caused by abundant fine precipitates and grain boundary hardening caused by fine recrystallized grains.

Journal ArticleDOI
TL;DR: The correlation among the strength, ductility and precipitates in a dual-phase Al0.5CoCrFeNi high-entropy alloy has been investigated in this paper.
Abstract: The correlation among the strength, ductility and precipitates in a dual-phase Al0.5CoCrFeNi high-entropy alloy has been investigated. The property of plastic was improved in the recrystallized Al0.5CoCrFeNi HEA prepared as the master alloy. Formation of the stable nanosized L12 phases after aging at 650 °C was revealed, along with the lath-like BCC phases precipitated in the grain, as the primary contribution of the strength enhancement. Excellent balanced tensile properties at room temperature were achieved through aging treatment, as the yield strength and ultimate tensile strength show 2.68, 1.63 times higher in 650 °C/140 h aged condition than that in initial states, respectively, superior to many high-entropy alloys and conventional alloys.

Journal ArticleDOI
TL;DR: In this paper, the progress of fabrication methods and properties of Mg microtubes for using as biodegradable stents are reviewed, and the authors classify the fabrication methods, and then investigate produced micro-tubes' properties from the perspective of mechanical, microstructural and biocorrosion properties.
Abstract: Magnesium and its alloys have attracted a great deal of attention in the field of biomedical applications, especially biodegradable stents. However, they have not been extensively used because of some inherent limitations such as poor mechanical properties and high corrosion rate. During the last decade, the selected fabrication methods for producing stent precursors is of great importance and can significantly affect the final stent’s properties. In this paper, the progress of fabrication methods and properties of Mg microtubes for using as biodegradable stents are reviewed. The paper will firstly classify the fabrication methods, and then investigate produced microtubes’ properties from the perspective of mechanical, microstructural and biocorrosion properties.

Journal ArticleDOI
TL;DR: In this article, an artificial neural network (ANN) model was used to predict flow stress correction from adiabatic heating at finer intervals of strain rates and temperatures, and the predicted isothermal flow stress values were utilized to construct processing maps for Ti-19Al-22Mo alloy at true strain of 0.4 and 0.6.
Abstract: The isothermal compression tests were carried out to study the hot deformation behavior and microstructure evolution of Ti–19Al–22Mo alloy. The samples were deformed in the temperature range from 1100 to 1250 °C with an interval of 50 °C, strain rate ranging from 0.01 to 1 s−1 and the height reduction of 50% using Gleeble-3800 thermal–mechanical simulator. By using this experimental data an artificial neural network (ANN) model was developed and evaluated with unseen data. Further, the developed ANN model was used to predict flow stress correction from adiabatic heating at finer intervals of strain rates and temperatures. The predicted isothermal flow stress values were utilized to construct processing maps for Ti–19Al–22Mo alloy at true strain of 0.4 and 0.6. The maximum efficiency was noticed at 1100 °C with the strain rate of 0.01 s−1 associated with dynamic recrystallization and dynamic recovery. The deformation conditions of the instability domains in processing map showed wedge cracking and flow localization. Using the processing maps safe working parameters for hot deformation of Ti–19Al–22Mo alloy was identified.

Journal ArticleDOI
TL;DR: In this paper, the effects of Mn and C content on the age hardening of Fe-Mn-Al-C lightweight steels, which have austenitic or duplex (austenite and ferrite) microstructures, were investigated.
Abstract: The effects of Mn and C content on the age hardening of Fe–Mn–Al–C lightweight steels, which have austenitic or duplex (austenite and ferrite) microstructures, were investigated. An increase in Mn content induced a delay of the age hardening that is caused by the formation of intra-granular κ-carbides. In order to interpret the effect of Mn content, first-principles calculations were conducted using the supercells of Fe24Al8C8, Fe24Al8C7, Fe24(Al7Mn)C8, and Fe24(Al7Mn)C7. The calculations showed that an increase in Mn content could be the source of the delay of the intra-granular κ-carbide formation by suppressing C atom’ occupation of the vacancy at the body-centered site of L12. An increase in C content accelerated the formation of intra-granular κ-carbides, which induced the intense age hardening, and coarse inter-granular κ-carbides, which resulted in significant decrease in impact absorbed energy due to inter-granular fracture.

Journal ArticleDOI
TL;DR: In this paper, the microstructure of the binary alloy consisted of the phases of aluminum rich α grains (dendrites), primary silicon, plate like β and eutectic Al-Si containing needle like silicon particles.
Abstract: In this study, a binary Al–12Si, eight ternary Al–12Si–Sr, and six quaternary Al–12Si–0.1Sr–(0.2–1)Mg alloys were produced by permanent mold casting. It was observed that microstructure of the binary alloy consisted of the phases of aluminum rich α grains (dendrites), primary silicon, plate like β and eutectic Al–Si containing needle like silicon particles. The ternary alloys have fine and globular (modified) eutectic silicon particles and higher volume fraction of α (Al) dendrites than binary alloys. They also contained Al4Sr phase after 0.02 wt% Sr, in addition to the phases in the binary alloy. This phase got coarse when the strontium ratio exceeded 0.1%. It was observed that the plate like β phase seen in the binary alloys transformed into the fibrous form δ phase in the ternary alloys. Magnesium addition resulted in transformation of δ phase into script like π phase, and the formation of lamellar like Mg2Si phase when the ratio of it in the quaternary alloys reached the 0.6 wt%. The lamellar like form of Mg2Si phase changed to Chinese-script type after the 0.6 wt% Mg. The results showed that hardness, yield and tensile strength of the Al–12Si–Sr alloys increased with increasing strontium content up to 0.1 wt%. The results also showed that hardness of the quaternary alloys increased with increasing magnesium content, while yield and tensile strength increased only up to 0.6 wt% Mg.

Journal ArticleDOI
TL;DR: In this paper, an attempt has been made to enhance the corrosion and biocompatibility performance of magnesium alloy AZ31 containing carbon nanotubes (CNTs) as reinforcement and evaluate its degradation and invitro mineralization performance in physiological medium.
Abstract: Magnesium (Mg) based implant materials are believed to be the perfect candidates for biomedical applications due to their versatile properties. However, regulating their corrosion/degradation rate in the biological surroundings is still a noteworthy task. Suitable strategies to overcome this task is to wisely select alloy elements with improved corrosion resistance and mechanical characteristics. An attempt has been made to enhance the corrosion and biocompatibility performance of magnesium alloy AZ31 containing carbon nanotubes (CNTs) as reinforcement and evaluate its degradation and invitro mineralization performance in physiological medium. Corrosion behavior of AZ31 alloy with CNTs reinforcement was investigated using electrochemical methods, weight loss, and hydrogen evolution in SBF during short and long-term periods. The obtained results revealed that the corrosion resistance of AZ31 alloy enhanced significantly due to the incorporation of CNTs. Hydrogen evolution test and weight loss tests revealed that the presence of CNTs improves the stability of the Mg(OH)2 and efficiently regulate the degradation behavior in SBF. Surface characterization after immersion in SBF revealed the rapid formation of bone-like apatite layer on the surface, validated a good bioactivity of the AZ31 nanocomposite samples.

Journal ArticleDOI
TL;DR: In this paper, a massive phase transformation and a thermal decomposition of an αm to fine lamellar α/β with an α lamella width of approximately 1.5 µm have been identified in additively manufactured Ti-6Al-4V alloy by directed energy deposition.
Abstract: The occurrence of a massive phase (αm) transformation and a thermal decomposition of an αm to fine lamellar α/β with an α lamellar width of approximately 1 µm have been identified in additively manufactured Ti–6Al–4V alloy by directed energy deposition. The β to αm phase transformation during additive manufacturing generated a high dislocation density, which was calculated by the kernel average misorientation. This caused a significant change in the Gibbs free energy of α, resulting in the nucleation of the β phase becoming preferable in the area with high dislocation density at temperature below the β-transus temperature. Subsequent annealing at 850 °C altered the massive morphology to lamellar due to the formation of a β phase between α lamellar boundaries.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the influence of the austenitization time on the structure and homogeneity of the spheroidal cast irons in the production of high-strength ADI.
Abstract: This paper considers the important factors of the production of high-strength ADI (Austempered Ductile Iron); namely, the austenitization stage during heat treatment. The two series of ADI with different initial microstructures were taken into consideration in this work. Experiments were carried out for castings with a 25-mm-walled thickness. Variable techniques (OM, SEM, dilatometry, DSC, Variable Magnetic Field, hardness, and impact strength measurements) were used for investigations of the influence of austenitization time on austempering transformation kinetics and structure in austempered ductile iron. The outcome of this work indicates that the austenitizing temperature has a very significant impact on structure homogeneity and the resultant mechanical properties. It has been shown that the homogeneity of the metallic matrix of the ADI microstructure strongly depends on the austenitizing temperature and the initial microstructure of the spheroidal cast irons (mainly through the number of graphite nodules). In addition, this work shows the role of the austenitization temperature on the formation of Mg–Cu precipitations in ADI.

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TL;DR: In this article, the dynamic recrystallization mechanism and microstructure evolution in an extruded AZ31 rod during hot torsion deformation at 150°C were investigated.
Abstract: The dynamic recrystallization mechanism and microstructure evolution in an extruded AZ31 rod during hot torsion deformation at 150 °C were investigated. It indicated that several dynamic recrystallization mechanisms were initiated during hot torsion deformation, including discontinuous DRX (DDRX), continuous DRX (CDRX) and twinning induced DRX (TDRX). With increasing strain, CDRX became the dominant DRX mechanism and contributed to a remarkable refinement of grains. A gradient distribution of dynamic recrystallization grains on the cross section of samples generated due to the gradient shear strain in twisted samples. Hot torsion can also arouse the c-axis of grains to rotate towards the extrusion direction. From low strain to high strain, the recrystallized grains exhibited a similar texture development with the deformed grains. The relevant mechanisms were revealed.

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TL;DR: In this article, an artificial neural network (ANN) model was applied to correct the non-isothermal flow stress in 10 wt% Cr steel, which exhibits a non-linear temperature dependence within a target temperature range of 750-1250 °C.
Abstract: An accurate processing map for a metal provides a means of attaining a desired microstructure and required shape through thermo-mechanical processing. To construct such a map, the isothermal flow stress, σiso, is required. Conventionally, the non-isothermal flow stress measured by experiment is corrected to σiso using whole-temperature-range linear interpolation (WRLI) or partial-temperature-range linear interpolation (PRLI). However, these approaches could incur significant errors if the non-isothermal flow stress exhibits a non-linear relationship with the temperature. In this study, an artificial neural network (ANN) model was applied to correct the non-isothermal flow stress in 10 wt% Cr steel, which exhibits a non-linear temperature dependence within a target temperature range of 750–1250 °C. Processing maps were constructed using σiso corrected by applying the WRLI, PRLI, and ANN approaches, respectively, and were then compared with the actual microstructures. The WRLI approach produced the highest minimum error of σiso (17.2%) and over-predicted the shear-band formation. The PRLI approach reasonably predicted the microstructural changes, but the minimum error for σiso (8.9%) was somewhat high. The ANN approach not only realized the lowest minimum error of σiso (~ 0%), but also effectively predicted the microstructural changes.

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TL;DR: In this article, the microstructural degradation and the creep fracture behavior of thermomechanically treated Grade 91 steel were investigated after performing small punch creep tests, showing that a remarkable reduction in creep ductility was observed for the samples thermomachaneally treated in comparison to those conventionally treated under the tested conditions of load (200 N) and temperature (700 N).
Abstract: The microstructural degradation and the creep fracture behavior of conventionally and thermomechanically treated Grade 91 steel were investigated after performing small punch creep tests. A remarkable reduction in creep ductility was observed for the samples thermomechanically treated in comparison to those conventionally treated under the tested conditions of load (200 N) and temperature (700 °C). A change in the fracture mechanism from a ductile transgranular fracture to a brittle intergranular fracture was observed when changing from the conventionally treated to the thermomechanically treated processing condition, leading to this drop in creep ductility. The change in the fracture mechanism was associated to the localized concentration of creep deformation, close to coarse M23C6 carbides, at the vicinity of prior austenite grain boundaries (PAGB) in the thermomechanically treated samples. The preferential recovery experienced at the vicinity of PAGB produced the loss of the lath structure and the coarsening of the M23C6 precipitates. The electron microscopy images provided suggest that the creep cavities nucleate in these weak recovered areas, associated to the presence of coarse M23C6. After the coalescence of the cavities the propagation of the cracks was facilitated by the large prior austenite grain size produced during the austenitization which favors the propagation of the cracks along grain boundaries triggering the intergranular brittle fracture. This fracture mechanism limits the potential use of the proposed thermomechanical processing routes.