Metals, Volume 8, Issue 9 (September 2018) – 83 articles

The objective of this paper is to investigate the mechanical response of EN08 steel at quasi-static and dynamic strain rates. Uniaxial tensile tests under quasi-static regime (from 0.0015 s⁻¹ to 0.15 s⁻¹) are conducted on EN08 steel at a range of temperatures between 298 K and 923 K. Dynamic compression tests are also performed by using a drop hammer and by considering different masses and heights to study the material response at strain rates up to 800 s⁻¹. Through the stress-strain responses of EN08 steel, a strong dependency of the yield stress as well as the ultimate strength on the strain rate and temperature is recognized. Furthermore, the strain hardening is highly affected by the increase of temperature at all levels of strain rate. The microstructure of the steel is also examined at a fracture by using SEM images to quantify the density of microdefects and define the damage evolution by using an energy-based damage model. Full article

17Cr2Ni2MoVNb steel is a new type of gear steel in the automotive industry; the rolling contact fatigue behavior is not well documented. In this study, some microscopic analysis methods (optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), and energy dispersive X-ray analysis (EDS)) were used to characterize material microstructure and damage surface. In addition, rolling contact fatigue behavior was evaluated by Weibull curve. The results show that the size of austenite grain increased while the quenching temperature increased. Moreover, the samples with a quenching temperature of 1000 °C exhibited the maximum mean life of 7.33 × 10⁵ cycles. In addition, the failure mode of quenched at 900 °C and 1100 °C were delamination, and pitting was the main failure mode of 1000 °C. Full article

This work presents the results of experimental and finite element modeling studies of impact behavior on the response of a high content of manganese steel blanks with a 1.2 mm thickness of sheets, known also commercially as Hadfield steel (an austenitic structure with a basic composition containing C 1.2% and Mn 12%). The study was done with a standard drop weight test device under certain variable parameters (velocity: 3 m/s and 5 m/s and temperature: room temperature, 70 °C, 100 °C, and 140 °C). In this study, the evolution of force and energy values were analyzed depending on the time in the case of impact. Special care was given to the evolution of peak stress counters of finite element simulation for different temperatures. The results of the force-time, energy-time, and force-displacement curves under different temperatures and impact velocities are compared experimentally and numerically. Then the discussion are built on the effect of the operational parameters on the damage behavior of this steel. Both of these works (experimental and finite element modeling) were compared and highly satisfying results were obtained. Full article

This paper presents the effect of laminated aluminium-steel panel with different configurations in a high-speed impact test. Layering aluminium plate with high strength steel has become an interest in reducing the overall density of armour vehicle body while improving the ballistic resistance. Different layering configurations differ in laminated panel performance. Two layering configurations of double-layered panel achieving 25% of existing panel weight reduction were tested using experiment and computational method to investigate their behaviours when impacted with 7.62-mm full metal jacket at velocity range of 800–850 m/s. The ballistic performance of each configuration plate in terms of ballistic limit velocity, penetration process and permanent deformation was quantified and considered. Laminated panel with aluminium as the front layer reduced the ballistic performance of existing panel to 50% and the other panel maintained its performance. Thus, the laminated panel with aluminium as the back layer can be used in designing a protective structure for armoured vehicle while maintaining the performance of the existing vehicle in achieving weight reduction. Full article

The phase transformation behavior of Ti-42Al-5Mn (at.%) alloy from different phase regions with various cooling rates was investigated based on electron probe micro analyzer-backscattered electrons (EPMA-BSE). It is shown that β→α₂′ takes place when this alloy is cooled at a high rate, such as water quenching (WQ), oil cooling (OC), from β single phase. With the decreasing cooling rate to air cooling (AC), β→α₂′ is restrained and β→γ is promoted by forming γ platelets. The room-temperature microstructure is β_o + α₂ when alloy cooled (WQ and OC) from (β + α) dual-phase. However, under AC, β→γ occurs and γ platelets form. It should be noted that α₂→γ happens when this alloy cooled from 1180 °C (>T_eut) by OC and AC, forming an incomplete lamellae (α₂/γ) structure in the α₂ phase. However, when the alloy cooled from 1100 °C (<T_eut), α₂/γ→β_o,sec occurs and complete lamellae generates in α₂ phase. Full article

A phenomenological constitutive model is developed to describe the uniaxial transformation ratcheting behaviors of the superelastic shape memory alloy (SMA) by employing a cosine–type phase transformation equation with the initial martensite evolution coefficient that can capture the feature of the predictive residual martensite accumulation evolution and the nonlinear hysteresis loop on a finite element (FE) analysis framework. The effect of the applied loading level on transformation ratcheting is considered in the proposed model. The evolutions of transformation ratcheting and transformation stresses are constructed as the function of the accumulated residual martensite volume fraction. The FE implementation of the proposed model is carried out for the numerical analysis of transformation ratcheting of the SMA bar element. The integration algorithm and the expression of consistent tangent modulus are deduced in a new form for the forward and reverse transformation. The numerical results are compared with those of existing models; experimental results show the validity of the proposed model and its FE implementation in transformation ratcheting. Finally, a FE modeling is established for a repeated preload analysis of SMA bolted joint. Full article

Additive manufacturing by selective laser melting (SLM) was used to investigate the effect of powder feedstock on 316L stainless steel properties include microstructure, relative density, microhardness and mechanical properties. Gas atomized SS316L powders of three different particle size distribution were used in this study. Microstructural investigations were done by scanning electron microscopy (SEM). Tensile tests were performed at room temperatures. Microstructure characterization revealed the presence of hierarchical structures consisting of solidified melt pools, columnar grains and multiform shaped sub-grains. The results showed that the SLM sample from the fine powder obtained the highest mechanical properties with ultimate tensile strength (UTS) of 611.9 ± 9.4 MPa and yield strength (YS) of 519.1 ± 5.9 MPa, and an attendant elongation (EL) of 14.6 ± 1.9%, and a maximum of 97.92 ± 0.13% and a high microhardness 291 ± 6 HV_0.1. It has been verified that the fine powder (~16 μm) could be used in additive manufacturing with proper printing parameters. Full article

In order to assess the dynamic tensile behavior of a newly developed three-step low-temperature-transformed bainitic microstructure, and a conventional one-step isothermal bainitic microstructure, and a quenched and tempered martensitic microstructure, a comparative research was conducted by using the dynamic tensile test. The results showed that more film-like retained austenite was obtained in the three-step low-temperature-transformed sample. The carbon content in the film-like retained austenite was found to be higher as compared to the blocky retained austenite. The dynamic tensile properties were all improved with an increase in strain rates (from 0.1 to 500 s⁻¹). However, the three-step bainitic low-temperature-transformed sample showed the most remarkable dynamic tensile properties, including ultimate tensile strength and total elongation. This is attributed to a better transformation-induced plasticity effect that is exhibited by the higher stability of the film-like retained austenite. Therefore, the present study suggests that performing a three-step low-temperature bainitic transformation promotes grain refinement and the formation of film-like retained austenite and improves the dynamic tensile properties. Full article

A Fe₆₁Cr₂Nb₃Si₁₂B₂₂ amorphous alloy rod sample of 8.8 mm diameter has been successfully prepared through explosive consolidation. The structure and thermal stability of the as-synthesized sample have been analyzed through X-ray diffraction (XRD) and differential scanning calorimeter (DSC) analysis. The results demonstrate that the sample still retains an amorphous structure, and the glass transition temperature (Tg), the crystallization onset temperature (Tx), the supercooled liquid zone (ΔTx) (Tx − Tg) and the reduced glass transition temperatures (Trg) (Tg/Tm) are 784 K, 812 K, 28 K, and 0.556, respectively. Its microstructure has been investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The average microhardness of the alumina compact is about 1069 HV. Full article

The refining process is one of the essential stages of aluminum production. Its main aim is to remove hydrogen, that causes porosity and weakens the mechanical and physical properties of casting aluminum. The process is mainly conducted by purging inert gas through the liquid metal, using rotary impellers. The geometry of the impellers and the processing parameters, such as flow rate of gas and rotary impeller speed, influence the gas dispersion level, and therefore the efficiency of the process. Improving the process, and optimization of parameters, can be done by physical modelling. In this paper, the research was carried out with the use of a water model of batch reactor, testing three different rotary impellers. Varied methods were used: visualization, which can help to evaluate the level of dispersion of gas bubbles in liquid metal; determination of residence time distribution (RTD) curves, which was obtained by measuring the conductivity of NaCl tracer in the fluid; and indirect studies, completed by measuring the content of dissolved oxygen in water to simulate hydrogen desorption. The research was carried out for different processing parameters, such as flow rate of refining gas (5–25 L·min⁻¹) and rotary impeller speed (3.33–8.33 s⁻¹). The obtained results were presented graphically and discussed in detail. Full article

One of the main in-service failure mechanisms of the work-rolls used in hot strip mill finishing stands is surface spalling. The indefinite chill double-poured rolls usually comprise of a peripheral working layer made of crushed Ni-hard cast iron and a grey cast iron core, mostly pearlitic matrix with spheroidal graphite. To enhance its wear resistance, the working layer can be alloyed with Mo and Nb. The possible cracking and spalling of the surfaces of these work-rolls is strongly influenced by the presence of carbides and the continuity of their network. The flexural and impact toughness tests are reliable testing methods to assess these properties. The aim of this paper is to identify those manufacturing factors that have a significant effect on the flexural strength and toughness of this material, correlating the results with the volume fraction of precipitated carbides. It is worth highlighting, among the analysed factors are the liquidus temperature, the %Si, the use of an inoculant with traces of Lanthanum, and inoculation with different amounts of FeB, SiCa and Mg. Inoculation with SiCa is found to have a positive effect on the toughness of the material, breaking up the continuity of the carbide network, while FeB is found to act as a heterogeneous nucleant for NbC precipitation. However, high FeB contents reduce flexural strength and do not have a significant effect on the hardness of the material. To enhance the fracture toughness of the working layer, a liquidus temperature in the 1270–1275 °C range is recommended, as well as inoculating the ladle with Mg, 3 kg/T FeB and 0.6 kg/T SiCa. Full article

Bio-ceramic coatings were synthesized on ZK60 magnesium alloys by micro-arc oxidation (MAO). The degradation behavior of the ZK60 alloys with and without MAO coating in the simulated body fluid (SBF) was studied. The samples were characterized by means of scanning electron microscopy (SEM), laser scanning confocal microscopy (CLSM), and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) was used to study the degradation behavior. The results showed that the porous MAO coating mainly consisted of MgO, Mg₂SiO₄, Mg₃(PO₄)₂, and CaCO₃. The pH values of both coated and uncoated samples increased over time. However, the pH values of the SBF for coated samples always maintained a lower level compared with those for the uncoated samples. Thereby, the coated samples showed a much lower degradation rate. After immersion in SBF for 5 days, corrosion product containing Ca and P was found on both samples, while the deposition was more active on the coated samples. The degradation models for the uncoated and coated samples in the SBF are also proposed and discussed. Full article

Joining γ-TiAl alloy to Ni-based superalloy Hastelloy using Ag-Cu sputtered coated Ti foil as brazing filler was investigated in this study. Brazing experiments were performed at 900, 950, and 980 °C with a dwelling stage of 10 min in vacuum. The microstructure and the chemical composition of the resulting interfaces were analyzed by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. Sound joints were produced after brazing at 980 °C, presenting a multilayered interface, consisting mainly of Ti-Al and Ti-Ni-Al intermetallics close to the γ-TiAl alloy, and of Ti-rich, Ti-Ni, and Cr-Ni-Mo rich phases near Hastelloy. The hardness of the interface, ranging from around 300 to 1100 HV0.01, is higher than both base materials, but no segregation of either Ag solid solution or coarse intermetallic particles was observed. Therefore, the developed brazing filler also avoids the need to perform post-brazing heat treatments that aim to eliminate detrimental extensive segregation of either soft phases or of hard and brittle compounds. Full article

In the present work, Cu-Pb-Sn and Q235 laminated composite were fabricated by a horizontal semisolid rolling procedure. The interfacial structure, elemental distribution, and properties of the composite were investigated. Finite-element simulation was conducted to analyze the temperature field and solidification process during the semisolid rolling. An appropriate semi-solid region was observed at a pouring temperature of 1598 K in the simulation, which would effectively kept fluidity and avoided casting defects. The experimental results showed that good interface between Cu-Pb-Sn alloy and Q235 steel was achieved by the proposed process at 1598 K, without casting defects or excessive deformation. The Cu and Fe alloys were bonded mainly by the diffusion of Fe into Cu matrix, and a handful of microscopic Pb-rich layer. Fine Pb-rich precipitates were uniformly distributed in the Cu-Pb-Sn alloy, and were considered to be advantageous to the self-lubrication property. The average tensile-shear strength of the interface was higher than 57.68 MPa at a pouring temperature of 1598 K, which fulfilled the requirements for a further extrusion process. Full article

Over the last decade, rare-earth elements (REEs) have become critical in the European Union (EU) in terms of supply risk, and they remain critical to this day. End-of-life electronic scrap (e-scrap) recycling can provide a partial solution to the supply of REEs in the EU. One such product is end-of-life neodymium (NdFeB) magnets, which can be a feasible source of Nd, Dy, and Pr. REEs are normally leached out of NdFeB magnet waste using strong mineral acids, which can have an adverse impact on the environment in case of accidental release. Organic acids can be a solution to this problem due to easier handling, degradability, and less poisonous gas evolution during leaching. However, the literature on leaching NdFeB magnets waste with organic acids is very scarce and poorly investigated. This paper investigates the recovery of Nd, Pr, and Dy from NdFeB magnets waste powder using leaching and solvent extraction. The goal was to determine potential selectivity between the recovery of REEs and other impurities in the material. Citric acid and acetic acid were used as leaching agents, while di-(2-ethylhexyl) phosphoric acid (D2EHPA) was used for preliminary solvent extraction tests. The highest leaching efficiencies were achieved with 1 mol/L citric acid (where almost 100% of the REEs were leached after 24 h) and 1 mol/L acetic acid (where >95% of the REEs were leached). Fe and Co—two major impurities—were co-leached into the solution, and no leaching selectivity was achieved between the impurities and the REEs. The solvent extraction experiments with D2EHPA in Solvent 70 on 1 mol/L leachates of both acetic acid and citric acid showed much higher affinity for Nd than Fe, with better extraction properties observed in acetic acid leachate. The results showed that acetic acid and citric acid are feasible for the recovery of REEs out of NdFeB waste under certain conditions. Full article

In many technical fields, high-strength steels like AISI 4140 are commonly used for highly dynamically loaded parts. Increasing demands on weight, performance and efficiency of the automotive industry lead to increasing demands on material properties. For surface conditioning, optimised machining processes are capable of improving the fatigue performance without increasing the production cost significantly. This paper compares the influence of three different sulphur contents and three different feed rates on the fatigue behaviour of deep hole drilled AISI 4140 fatigue specimens. The specimens were characterised regarding their surface condition, hardness and microstructure, and afterwards they were tested under fatigue loading for performance assessment. These tests were accompanied with Barkhausen noise analysis. The Barkhausen noise signal was detected by a custom-built sensor that is capable of detecting the magnetic values on the bore wall. Using this technique, a load-independent estimation of fatigue damage was established. Full article

Nanoindentation is a commonly used method to measure the hardness of surfaces with thin layers, and is especially useful in studying the change in mechanical properties of ion irradiated materials. This research compares two different methods of nanoindentation to study the changes in hardness resulting from ion irradiation of SS316 alloy. The samples were irradiated by He²⁺ ions at beam energies of 1, 2, and 3 MeV, respectively. The first method involves the indentation of the irradiated surface perpendicular to it using the continuous stiffness mode (CSM), while the second applies the indents on an oblique surface, accessing an inclined cross-section of the irradiated material. Finite element modelling has been used to further illuminate the deformation processes below the indents in the two methods. The hardness profiles obtained from the two nanoindentation methods reveal the differences in the outcomes and advantages of the respective procedures, and provide a useful guideline for their applicability to various experimental conditions. It is shown through an in depth analysis of the results that the ‘top-down’ method is preferable in the case when the ion irradiation energy, or, equivalently, the irradiated depth is small, due to its greater spatial resolution. However, the oblique cross section method is more suitable when the ion irradiation energy is >1 MeV, since it allows a more faithful measurement of hardness as a function of dose, as the plastic field is much smaller and more sensitive to local hardness values. Full article

The objective of the present study is to investigate the effects of nickel (Ni) on the microstructure and impact toughness of coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steel. It was observed that the microstructure of CGHAZ predominantly consisted of bainite and a small proportion of martensite-austenite (M-A) constituents and acicular ferrite (AF). With increased Ni content, the percentage of M-A constituent decreased and AF increased; consequently, the impact toughness of CGHAZ increased. The study revealed that a small addition of nickel significantly affected the formation of M-A constituents and AF; however, no obvious influence was observed on the bainitic microstructure of high-strength low-alloy steel. Full article

The use of lightweight materials is one possibility to limit the weight of vehicles and to reduce CO₂ emissions. However, the mechanical properties and weight-saving potential of mono-materials are limited. Material compounds can overcome this challenge by combining the advantages of different materials in one component. Lateral angular co-extrusion (LACE) allows the production of coaxial semi-finished products consisting of aluminum and steel. In this study, a finite element model of the LACE process was built up and validated by experimental investigations. A high degree of agreement between the calculated and experimentally determined forces, temperatures, and the geometrical shape of the hybrid profiles was achieved. In order to determine suitable parameters for further extrusion experiments, the influence of different process parameters on material flow and extrusion force was investigated in a numerical parametric study. Both the temperature and extrusion ratio showed a significant influence on the occurring maximum extrusion force as well as the material flow inside the LACE tool. The maximum force of 2.5 MN of the employed extrusion press was not exceeded. An uneven material flow was observed in the welding chamber, leading to an asymmetric position of the steel rod in the aluminum matrix. Full article

Carbon fiber-reinforced plastic (CFRP), which is a light and composite material, has a higher specific strength and stiffness than metal materials. However, owing to its low elongation, it is vulnerable to local impacts such as collision. Therefore, hybrid composite materials that can overcome the disadvantages of homogeneous materials by bonding CFRP and metal materials are increasingly popular. In this study, a physical surface treatment sandblast process was applied on a high tensile steel plate (CR980) manufactured by cold rolling to form another surface condition, and the bonding strength with CFRP was measured. In addition, spring-back due to the manufacturing process of the CFRP and CR980 hybrid composite material bonded with different surface roughness was observed. The bonding strength and the spring-back angle of the CFRP/CR980 hybrid composite material tended to increase with the increase in the surface roughness. Full article

This study demonstrates the potential of the use of Ni/Al nanolayers for joining dissimilar titanium alloys. For this purpose, a detailed microstructural characterization of the diffusion bonding interfaces of TiAl to Ti6Al4V, TiAl to TiNi and TiNi to Ti6Al4V was carried out. The nanolayers (alternated aluminum and nickel (Ni-7V wt.%) layers) were deposited onto the base material surfaces. Diffusion bonding was performed at 700 and 800 °C under pressures ranging from 5 to 40 MPa and at dwell times between 60 and 180 min. Microstructural characterization was performed using high resolution transmission and scanning electron microscopies. The results revealed that dissimilar titanium joints (TiAl to Ti6Al4V, TiAl to TiNi and TiNi to Ti6Al4V) assisted by Ni/Al nanolayers can be obtained successfully at 800 °C for 60 min using a pressure of 20 MPa. The bond interfaces are thin (less than 10 µm) and mainly composed of NiAl grains with a few nanometric grains of Al₈V₅. Thin layers of Al-Ni-Ti intermetallic compounds were formed adjacent to the base materials due to their reaction with the nanolayers. Full article

According to the coexistence theory of slag structure, a calculation model for the activity of FeO in the quaternary system SiO₂-CaO-Al₂O₃-FeO of depleted copper slag was established. The model was used to calculate and analyze the effects of temperature (T), basicity (B), and Al₂O₃ content on the activity of FeO (N_FeO). The results show that temperature has little impact on N_FeO. With increased basicity, N_FeO first increased slightly, then increased sharply, and finally decreased. It is easier for CaO to combine with SiO₂ than FeO to form calcium silicate, which replaces FeO in 2FeO·SiO₂ and increases N_FeO. However, when basicity is higher than 2.0, CaO not only reacts with SiO₂, but also combines with FeO to form calcium ferrate compounds to decrease N_FeO. In addition, the activity of FeO decreases with increased Al₂O₃ content because of the reaction between CaO and Al₂O₃. The results can be used as a theoretical basis to guide the carbothermal reduction process of copper slag. Full article

Although conventional tool steels have been heat treated on a routine basis for decades, the search continues for new ways to eliminate their troublesome sharp-edged primary chromium carbides, which impair toughness. One of the available techniques is semi-solid processing, which involves partial melting of the workpiece. The structure after semi-solid processing consisted of a austenite and carbide-austenite network. The network can be broken up and its fragments distributed uniformly by subsequent forming with appropriate parameters. In this experimental study, X210Cr12 tool steel was heated to a semi-solid state, and after cooling to a solid state, worked in a hydraulic press. Suitable soaking temperatures were sought within an interval between 1200 °C and 1280 °C. The workpieces were quenched from the forming temperature in water or oil. In order to improve formability and reduce hardness, tempering was tested as well. Additional experimental regimes included conventional quenching and tempering. Once the appropriate parameters were chosen, the elimination of primary chromium carbides was successful. The resultant microstructures were fine and consisted of M-A constituent with a size of approximately 1 μm, and very fine Fe₃C and Cr₇C₃ carbides. The hardness was in excess of 800 HV10. They were examined using optical and scanning electron microscopes. The carbides were characterized on transparent foils in a transmission electron microscope. Mechanical characteristics were determined in micro-tensile tests. Full article

Surface cracking seriously affects the quality of beam blanks, a relatively new blank in the continuous casting in China. In order to study the mechanism of the initiation and propagation of surface cracks, this study established a 2D micro-segmented model of the solidification process for a beam blank in the mold, with a user subroutine DFLUX written in Fortran. Using a contact algorithm, the stress in the shell of the beam was analyzed considering the mechanical properties of the material (Q235B), thermal stress, surface friction force and ferrostatic pressure. The results showed that at the center of the web, surface longitudinal cracks were most likely to initiate at a height of 180 mm from the meniscus; at the fillet, surface longitudinal cracks were most likely to initiate at a height of 200 mm from the meniscus. Moreover, the casting speed showed a greater effect on surface crack initiation than the pouring temperature did. This study reveals the cause of longitudinal crack initiation, and the most likely positions of cracks on the strand. Thus, it is instructive for controlling surface cracks in production. Full article

Zinc electroplating is a coating process controlled by several input process parameters. However, the commonly used input parameters for setting the process of zinc deposition are current density, temperature of the coating solution, zinc concentration, deposition time, and concentration of additives (conditioner and brightener). The power consumed in the zinc plating process, coating thickness, increase in coating mass, and corrosion resistance are considered to be outputs or zinc coating parameters. They are widely used when the zinc coating requirements are based on the coating process cost, coating process speed, corrosion resistance, and coating thickness. This paper seeks to determine regression models by the response surface method (RSM) that relate the zinc coating parameters to the input parameters in steel screws. When considering the coating requirements of cost, coating process speed, corrosion resistance, and coating thickness, the optimal input parameters were found by using a multi-response surface (MRS). Input parameters of 0.3 amps/dm², 20.0 °C, 13.9 g/L, 45 min, 28.5 mL/L, and 2.8 mL/L, respectively (relative to the commonly used input parameters detailed above), were obtained when considering the cost. Considering minimization of the deposition time, the input parameters obtained were 0.5 amps/dm², 24.6 °C, 13.9 g/L, 45 min, 26.9 mL/L, and 1.1 mL/L, respectively. The optimal inputs to maximize the corrosion resistance were 0.6 amps/dm², 32.4 °C, 14.0 g/L, 45 min, 28.7 mL/L, and 2.5 mL/L, respectively. Finally, when maximizing the coating thickness, the inputs were 0.7 amps/dm², 38.4 °C, 12.2 g/L, 45 min, 26.5 mL/L, and 1.5 mL/L, respectively. Full article

New metastable β titanium alloys are receiving increasing attention due to their excellent biomechanical properties and machinability is critical to their uptake. In this study, machining chip microstructure has been investigated to gain an understanding of strain and temperature fields during cutting. For higher cutting speeds, ≥60 m/min, the chips have segmented morphologies characterised by a serrated appearance. High levels of strain in the primary shear zone promote formation of expanded shear band regions between segments which exhibit intensive refinement of the β phase down to grain sizes below 100 nm. The presence of both α and β phases across the expanded shear band suggests that temperatures during cutting are in the range of 400–600 °C. For the secondary shear zone, very large strains at the cutting interface result in heavily refined and approximately equiaxed nanocrystalline β grains with sizes around 20–50 nm, while further from the interface the β grains become highly elongated in the shear direction. An absence of the α phase in the region immediately adjacent to the cutting interface indicates recrystallization during cutting and temperatures in excess of the 720 °C β transus temperature. Full article

Semisolid billet of the 7075 aluminum matrix composite reinforced with nano-sized SiC particles was first fabricated by an ultrasonic-assisted semisolid stirring method and rheoforming technology. Then it was thixoformed into a cylinder part under different remelting temperatures and soak times. The effects of the remelting temperature and soaking time on the mechanical properties and microstructure of the thixoformed composite part were investigated. The results show that parts of good quality were thixoformed successfully. The microstructure of the top side wall of the thixoformed part consisted of near spheroidal grains. A large quantity of elongated grains occurred in the medium and bottom side walls and the bottom itself. With increasing remelting temperatures, the size of the solid grains of the thixoformed parts showed a trend of first to increase and afterwards to decrease. High density dislocations were found in the microstructure when the remelting temperatures were 590 °C and 600 °C. When the soaking time was 15 min, the severest deformation occurred in the thixoformed part. High mechanical properties of the thixoformed parts were achieved under conditions such as a remelting temperature between 590 °C and 600 °C and a soaking time between 10 min and 15 min. The fracture mode of the thixoformed part changed from transgranular fracture to intergranular fracture when the remelting temperature was elevated from 580 °C to 610 °C. After the thixoformed parts were treated by T6, the ultimate tensile strength (UTS) and elongation of the side wall were improved to 552 MPa and 7.9%, respectively. Dispersed MgZn₂ precipitates created by T6 heat treatment led to an improvement of the mechanical properties. Full article

The mathematical nature of the flow rule for the strain gradient plasticity theory proposed by Nix and Gao (W.D. Nix and H. Gao, J Mech Phys Solids 46(3), 411(1998)) is discussed based on the paradigm developed by Gurtin and Anand (M.E. Gurtin and L. Anand, J Mech Phys Solids 57 (3), 405 (2009)). It is shown that, when investigated on the basis of Gurtin–Anand theory, the Nix–Gao flow rule is a combination of constitutive equations for microstresses, balance law, and a constraint. As an accessory, we demonstrate that the strain gradient term introduced in the model is energetic. The results are obtained by combining a virtual-power principle of Fleck and Hutchinson, and the free-energy imbalance under isothermal conditions. Full article

Simultaneously improving the strength, toughness, and ductility of cast steels has always been a difficult problem for researchers. Biphase TiC_x-TiB₂ nanoparticle-reinforced cast steels are prepared by adding in situ nanosized biphase TiC_x-TiB₂/Al master alloy during the casting process. The experimental results show that a series of significant changes take place in the microstructure of the steel: the ferrite-pearlite structure of the as-cast steels and the bainite structure of the steels after heat treatment are refined, the grain size is reduced, and the content of nanoparticles is increased. Promotion of nucleation and inhibition of dendrite growth by biphase TiC_x-TiB₂ nanoparticles leads to a refinement of the microstructure. The fine microstructure with evenly dispersed nanoparticles offers better properties [yield strength (1246 MPa), tensile strength (1469 MPa), fracture strain (9.4%), impact toughness (20.3 J/cm²) and hardness (41 HRC)] for the steel with 0.018 wt.% biphase TiC_x-TiB₂ nanoparticles, which are increased by 15.4%, 31.2%, 4.4%, 11.5%, and 7.9% compared with the 40Cr steels. The higher content of nanoparticles provides higher strengths and hardness of the steel but are detrimental to ductility. The improved properties may be attributed to fine grain strengthening and the pinning effect of nanosized carbide on dislocations and grain boundaries. Through this work, it is known that the method of adding trace (0.018 wt.%) biphase TiC_x-TiB₂ nanoparticles during casting process can simultaneously improve the strength, toughness, as well as ductility of the cast steel. Full article