Mechanical properties and microstructural evolution of rheocast A356 semi-solid slurry prepared by annular electromagnetic stirring

Nowadays,having an effective technique in preparing semi-solid slurries for rheocasting process seems to be an essential requirement.In this study,semi-solid slurry of A356 aluminum alloy was prepared by three-phase annular electromagnetic stirring(A-EMS)technique under different conditions.The effects of stirring current,pouring temperature and stirring time on microstructural evolution,mean particle size,shape factor and solid fraction were investigated.The rheocasting process was carried out by using a drop weight setup and to inject the prepared semi-solid slurry in optimal conditions into the step-die cavity.The filling behavior and mechanical properties of parts were studied.Microstructural evolution showed that the best semi-solid slurry which had fine spherical particles with the average size of~27μm and a shape factor of~0.8 was achieved at the stirring current of 70 A,melt pouring temperature of 670℃,and stirring time of 30 s.Under these conditions,the step-die cavity was completely filled at die preheating temperature of 470℃.The hardness increases by decreasing step thickness as well as die preheating temperature.Moreover,the tensile properties are improved at lower die preheating temperatures.The fracture surface,which consists of a complex topography,indicates a typical ductile fracture.

Recent research progress on the phase-field model of microstructural evolution during metal solidification

Solidification structure is a key aspect for understanding the mechanical performance of metal alloys,wherein composition and casting parameters considerably influence solidification and determine the unique microstructure of the alloys.By following the principle of free energy minimization,the phase-field method eliminates the need for tracking the solid/liquid phase interface and has greatly accelerated the research and development efforts geared toward optimizing metal solidification microstructures.The recent progress in the application of phasefield simulation to investigate the effect of alloy composition and casting process parameters on the solidification structure of metals is summarized in this review.The effects of several typical elements and process parameters,including carbon,boron,silicon,cooling rate,pulling speed,scanning speed,anisotropy,and gravity,on the solidification structure are discussed.The present work also addresses the future prospects of phase-field simulation and aims to facilitate the widespread applications of phase-field approaches in the simulation of microstructures during solidification.

The effects of orientation control via tension-compression on microstructural evolution and mechanical behavior of AZ31 Mg alloy sheet

The grain orientation control via twinning activity on deformation features is of great significance to offer a key insight into understanding the deformation mechanism of Mg alloy sheets.The{10–12}twinning were performed by pre-strain paths,i.e.,tension(6%)and compression(5%)perpendicular to the c-axis along extrusion direction(ED),to investigate the microstructural evolution and mechanical properties of AZ31 Mg alloy sheets.The distinction in the texture evolution and strain hardening behavior was illustrated in connection with the pre-strain paths for the activities of twinning and slip.The result shows that the activation of the deformation mode was closely bound up with the grain orientation and the additional applied load direction.The{10–12}twin-texture components with c-axis//ED were generated by precompression,which can provide an appropriate alternative to accommodate the thin sheet thickness strain and enhance the room temperature formability of Mg alloy sheet.

Microstructural evolution,mechanical properties,and corrosion resistance of a heat-treated Mg alloy for the bio-medical application

During the recent years,some Mg based alloys have extensively been considered as a new generation of degradable and absorbable bio-medical materials.In this work,the Mg-2Zn-1Gd-1Ca(wt%)alloy as a new metallic bio-material was produced by the casting process followed by the heat treatment.The samples of the alloy were solution treated at temperatures of 500,550,and 600°C and then quench aged at temperatures of 125,150,and 175°C.The results of SEM-EDS examinations indicated that the alloy microstructure consists ofα-Mg matrix and the Ca_(2)Mg_(6)Zn_(3)and Mg_(3)Gd_(2)Zn_(3)secondary phases.With regard to the results of Vickers hardness test,the temperatures of 500°C and 150°C were selected as the optimum solutionizing and aging temperatures,respectively.Moreover,the dissolution of casting precipitates and production of lattice distortion occurring after the solution treatment led to the reduction in ultimate shear strength up to 21%.But,the precipitation hardening and morphological changes taking place during the aging treatment improved the ultimate shear strength up to 32%.Furthermore,the results of electro-chemical and weight-loss measurements in a simulated body fluid indicated that the heat-treated alloy is a promising candidate for the Mg based alloys recently considered for the bio-medical applications.

Microstructural evolution in Al–Zn–Mg–Cu–Sc–Zr alloys during short-time homogenization

Abstract： Microstructural evolution in a new kind of aluminum （A1） alloy with the chemical composition of AI-8.82Zn-2.08Mg- 0.80Cu-3.31Sc-0.3Zr was investigated. It is found that the secondary phase MgZn2 is completely dissolved into the matrix during a short homogenization treatment （470℃, 1 h）, while the primary phase A13（Sc,Zr） remains stable. This is due to Sc and Zr additions into the A1 al- loy, high Zn/Mg mass ratio, and low Cu content. The experimental findings fit well with the results calculated by the homogenization diffusion kinetics equation. The alloy shows an excellent mechanical performance after the short homogenization process followed by hot-extrusion and T6 treatment. Consequently, a good combination of low energy consumotion and favorable mechanical properties is obtained.

Microstructural evolution of Au-Sn solder prepared by laminate rolling during annealing process

The microstructural evolution and inteffacial reaction of the Au/Sn/Au/Sn/Au/Sn/Au couples were investigated during annealing at 453, 523, and 543 K for up to 240 h. The Au/Sn combination formed a rapid diffusion system. Even in rolled Au-Sn solder, three phases, such as AuSn, AuSn2, and AuSn4, were formed. After initial annealing at 453 K, the diffusion layers of AuSn, AuSn2, and AuSn4, which were formed after rolling, expanded gradually and then fully transformed into phase （containing Sn from 10% to 18.5%, mole fraction） and 6 （AuSn） phase. As a whole, the microstmcture of the couple was stable during annealing at 453 K. The solid-state interracial reaction was much faster at 523 K than at 453 K. After annealing at 523 K for 6 h, the AuSn, AuSn2, and AuSn4 were fully transformed into the phase and phase （AuSn）. In spite of the prolonged annealing time for up to 240 h, no significant change of the interfacial microstructure occurred, and the microstructure of the couple was stable during annealing at 523 K. When annealing at 543 K, however, the interfacial of Au/Sn was transformed into solid-liquid state, and the whole couple formed a eutecfic structure rapidly, causing the solder to be brittle. The study results clearly demonstrate that the service temperature of the Au-Sn solder should be lower than 543 K.

GLOBAL VIEW OF MICROSTRUCTURAL EVOLUTION:ENERGETICS,KINETICS AND DYNAMICAL SYSTEMS

An evolving material structure is in a non-equilibrium state, with free energy expressed by the generalized coordinates. A global approach leads to robust computations for the generalized thermodynamic forces. Those forces drive various kinetic processes, causing dissipation at spots, along curves, surfaces and interfaces, and within volumetric regions. The actual evolution path, and therefore the final equilibrium state, is determined by the energetics and kinetics. A virtual work principle Links the free energy landscape and the kinetic processes, and assigns a viscous environment to every point on the landscape. The approach leads to a dynamical system that governs the evolution of generalized coordinates. The microstructural evolution is globally characterized by a basin map in the coordinate space; and by a diversity map and a variety map in the parameter space. The control of basin boundaries raises the issue of energetic and kinetic bifurcations. The variation of basin boundaries under different sets of controlling parameters provides an analytical way to plot the diversity maps of structural evolution.

Microstructural Evolution of 2.25Cr-1.6W-V-Nb Heat Resistant Steel during Creep

2.25Cr-1.6W-V-Nb developed in Japan, is a low alloy heat resistant steel with good comprehensive properties. Influence of long term creep at elevated temperature on the structure of 2.25Cr-1.6W-V-Nb steel was studied in this paper, and the micromechanism of creep strength degradation was elucidated, too. Both TEM observation and thermodynamic calculation reveal that during creep the transformation occurs from M7C3 and M23C6 to M6C, which can be cavity nucleation sites. Besides, creep at 600癈 also leads to the decrease of dislocation density, the coarsening and coalescence of M23C6, the nucleation of cavities and development of cracks. The strength decrease of 2.25Cr-1.6W-V-Nb steel after long term creep is related to the decrease of dislocation hardening, precipitation hardening, solution hardening, the nucleation of cavities and development of cracks.

Insight into the microstructural evolution of anthracite during carbonization-graphitization process from the perspective of materialization

Materialization of coal is one of effective and clean pathways for its utilization. The microstructures of coal-based carbon materials have an important influence on their functional applications. Herein, the microstructural evolution of anthracite in the temperature range of 1000–2800 ℃ was systematically investigated to provide a guidance for the microstructural regulation of coal-based carbon materials.The results indicate that the microstructure of anthracite undergoes an important change during carbonization-graphitization process. As the temperature increases, aromatic layers in anthracite gradually transform into disordered graphite microcrystals and further grow into ordered graphite microcrystals, and then ordered graphite microcrystals are laterally linked to form pseudo-graphite phase and eventually transformed into highly ordered graphite-like sheets. In particular, 2000–2200 ℃ is a critical temperature region for the qualitative change of ordered graphite crystallites to pseudo-graphite phase,in which the relevant structural parameters including stacking height, crystallite lateral size and graphitization degree show a rapid increase. Moreover, both aromaticity and graphitization degree have a linear positive correlation with carbonization-graphitization temperature in a specific temperature range.Besides, after initial carbonization, some defect structures in anthracite such as aliphatic carbon and oxygen-containing functional groups are released in the form of gaseous low-molecular volatiles along with an increased pore structure, and the intermediates derived from minerals could facilitate the conversion of sp^(3) amorphous carbon to sp^(2) graphitic carbon. This work provides a valuable reference for the rational design of microstructure of coal-based carbon materials.

Microstructural evolution of Al-Cu-Li alloys with different Li contents by coupling of near-rapid solidification and two-stage homogenization treatment

Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density.In this regard,the Al-Cu-Li alloy castings with high Li content from 1.5 wt.%to 4.5 wt.%were prepared by near-rapid solidification,followed by two-stage homogenization treatment(490℃/16 h and 530℃/16 h).The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software.The results indicate that with the increase of Li content,the grain sizes decrease,the solution ability of Cu in the matrixα-Al phase increases,while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification.Additionally,by the coupling of near-rapid solidification and two-stage homogenization,the metastable precipitated phases(Al7Cu4Li and AlCu3)can be dissolved effectively in the alloys with Li content of 1.5 wt.%-2.5 wt.%;moreover,the stable precipitated phases(Al6CuLi3 and Al2CuLi)uniformly distribute at the grain boundaries in the alloys with Li content of 3.5 wt.%-4.5 wt.%.As a result,the refined and homogenized microstructure can be obtained.

Microstructural evolution of Al-8.59Zn-2.00Mg-2.44Cu during homogenization

The microstructural evolution and phase transformations of a high-alloyed Al-Zn-Mg-Cu alloy （Al-8.59Zn-2.00Mg-2.44Cu,wt%） during homogenization were investigated. The results show that the as-cast microstructure mainly contains dendritic α（Al）, non-equilibrium eutectics （α（Al） ＋ Mg（Zn,Al,Cu）2）, and the θ （Al2Cu） phase. Neither the T （Al2Mg3Zn3） phase nor the S （Al2CuMg） phase was found in the as-cast alloy. The calculated phase components according to the Scheil model are in agreement with experimental results. During homogenization at 460℃, all of the θ phase and most of the Mg（Zn,Al,Cu）2 phase were dissolved, whereas a portion of the Mg（Zn,Al,Cu）2 phase was transformed into the S phase. The type and amount of residual phases remaining after homogenization at 460℃ for 168 h and by a two-step homogenization process conducted at 460℃ for 24 h and 475℃ for 24 h （460℃/24 h ＋ 475℃/24 h） are in good accord with the calculated phase diagrams. It is concluded that the Al-8.59Zn-2.00Mg-2.44Cu alloy can be homogenized adequately under the 460℃/24 h ＋ 475℃/24 h treatment.

Microstructural evolution and mechanical properties of aging high nitrogen austenitic stainless steels

The microstructural evolution of 18Crl 8Mn2Mo0.77N high nitrogen austenitic stainless steel in aging treatment was investigated by optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）. The results show that hexagonal intergranular and cellular Cr2N with a=0.478 nm and c=0.444 nm and body-centered cubic intermetaUic X phase with a=0.892 nm precipitate gradually in the isothermal aging treatment. The matrix nitrogen depletion due to the intergranular Cr2N precipitation induces the decay of Vickers hardness, and the formation of cellular Cr2N and X phase causes the increase in the values. The impact toughness presents a monotonic decrease and SEM morphologies show the leading brittle intergranular fracture. The tensile strength and elongation deteriorate obviously except for the sample aged for 1 h in yield strength. Stress concentration occurs when the matrix dislocations pile up at the pre- cipitation and matrix interfaces, and the interracial dislocations may become precursors to the misfit dislocations, which can form small cleavage steps and accelerate the formation of cracks.

Microstructural evolution and mechanical properties of nanostructured Cu-Al-Ni shape memory alloys

The melt spinning technique, with an applied cooling rate of about 106 K/s, was used to produce a nanostructured Cu＋13.2Al＋ 5.1Ni （in wt%） shape memory alloy. The properties of nanostructured ribbons were then compared with those of conventional coarse struc- ture. The microstructural evolution was characterized using scanning electron microscopy （SEM）, atomic force microscopy （AFM） and X-ray diffraction （XRD） techniques. Microhardness measurements indicate a two-fold increase in hardness because of the produced nanos- lructure. Comparing to its coarse structure, the nanostructured Cu-A1-Ni shape memory alloy exhibited the enhanced mechanical properties including a ductility of 6.5% and a pronounced plateau in the stress-strain curve.

Microstructural evolution of die-cast and homogenized AZ91 Mg-alloys during dry sliding condition

Received 16 July 2016;revised 24 January 2017;accepted 7 February 2017 Available online 22 February 2017 Abstract Microstructural evolution of die-cast and homogenized AZ91 Mg-alloys was investigated during dry sliding wear condition.Tribological tests were performed using a pin-on-disc(EN8 steel)configuration with a normal load of 50 N at a constant sliding speed of 2.5 ms^(−1) under ambient environment.Delamination was recognized as a predominant wear mechanism in both of these materials.The die-cast AZ91 Mg-alloy exhibits lower coefficient of friction and higher wear rate.This can be ascribed to increase in the intensity of load bearing capacity of hardβ-Mg_(17)Al_(12) phase,and crack formation/de-cohesion at the interface between primaryα-Mg and discontinuousβ-Mg_(17)Al_(12) phases.On the contrary,the homogenized AZ91 Mg-alloy experiences higher coefficient of friction and lower wear rate.The friction-induced microstructural evolution(supersaturatedα-Mg to eutectic(α+β-Mg_(17)Al_(12)))tending to minimize the wear rate by providing barrier to material removal in the near surface region of homogenized AZ91 Mg-alloy.Therefore,experimental observation revealed that an inverse relationship exists between wear rate and coefficient of friction for the investigated materials.The analysis of worn surfaces and subsurfaces by electron microscopy provided evidence to delamination wear and microstructural evolution.

Melt Infiltration Ability and Microstructural Evolution of Fe40Al/ TiC Composites System

Pressureless melt infiltration is an economic route f or preparation of high-density ceramic/melt composites. In this study, the Fe40 Al iron aluminide intermetallic, a low cost material of excellent oxidation and corrosion resistance, was used as binder for fabricating Fe40Al/TiC composites b y pressureless melt infiltration. The wetting ability of liquid Fe40Al in porous TiC pre-form was studied by in-situ monitoring the melting and infiltration p rocess. The infiltration ability was investigated by observing the distance of l iquid Fe40Al intrusion in porous TiC pre-forms at different infiltration temper atures and times by using optical microscope. Porous TiC per-forms with density of 60%～88%TD (theoretical density), prepared under pre-defined sintering temp e rature cycles, were used for fabricating Fe40Al/TiC composites in the range of 1 2%～40% metal content by volume. Almost full dense Fe40Al/TiC composites were su c cessfully fabricated by this technique. Liquid Fe40Al exhibited excellent infilt ration ability, the distance of complete intrusion of liquid Fe40Al in the TiC s intered pre-form with density of 88%TD was over 7 mm after 5 min at the inf iltration temperature of 1 450 ℃. Microstructural observation by SEM and TEM also showed that liquid Fe40Al filled the very narrow gaps among TiC particles, the interfaces of TiC particles and F e40Al plastic ligaments being metallurgical bonded. TEM revealed that high densi ty of dislocations formed in Fe40Al ligaments during solidification, which favor the mechanical properties. Ti decomposed from TiC particles and dissolved into Fe40Al during infiltration. According to the compositional analysis of TEM-EDS, the concentration of Ti in Fe40Al ranges at 1at%～4at% depending on composite f a bricating conditions and the distance from the measuring point to the closest Ti C particles. XRD analysis indicated that the composites were composed of two pha ses, the original TiC and Fe 0.4Al 0.6 intermetallic. No new phase formed during infiltration, but the lattice parameter of Fe 0.4Al 0.6 was expended due to the Ti in the solid solution.

Microstructural evolution and castability prediction in newly designed modern third-generation nickel-based superalloys

The present research aims to establish a quantitative relation between microstructure and chemical composition （i.e., Ti, Al, and Nb） of newly designed nickel-based superalloys. This research attempts to identify an optimum microstructure at which the minimum quanti- fies of γ/γ＇ and γ/γ＂ compounds are achieved and the best castability is predicted. The results demonstrate that the highest quantity of inter- metallic eutectics （i.e., 41.5wt%） is formed at 9.8wt% （Ti ＋ A1）. A significant quantity of intermetallics formed in superalloy 1 （with a com- position of7 - 9.8wt% （Ti ＋ A1））, which can deteriorate its castability. The type and morphology of the eutectics changed and the amount considerably decreased with decreasing Ti ＋ A1 content in superalloy 2 （with a composition ofy - 7.6wt% （Ti ＋ A1）, 1.Swt% Nb）. Thus, it is predicted that the castability would improve for superalloy 2. The same trend was observed for superalloy 4 （with a composition of 7 - 3.7wt% （Ti ＋ A1）, 4.4wt% Nb）. This means that the amount of Laves increases with increasing Nb （to 4.4wt%） and decreasing Ti ＋ A1 （to 3.7wt%） in su- peralloy 4. The best castability was predicted for superalloy 3 （with a composition ofy - 5.7wt% （Ti ＋ A1）, 2.8wt% Nb）.

Deformation behavior and microstructural evolution during hot compression of an a+b Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy

The effect of processing parameters on the flow response and microstructural evolution of the a＋b titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si has been studied by conducting isothermal hot compressive tests at a strain rate of 0.01-10 s-1 at 860-1100°C. The true stress-true strain curves of the sample hot-compressed in the a＋b phase region exhibit a peak stress followed by continuous flow softening, whereas in the b region, the flow stress attains a steady-state regime. At a strain rate of 10 s-1, the alloy exhibits plastic flow insta-bilities. According to the kinetic rate equation, the apparent activation energies are estimated to be about 674-705 kJ/mol in the a＋b region and 308-335 kJ/mol in the b region, respectively. When deformed in the a＋b region, the globularization process of the a colony structure occurs, and a dynamic recrystallized microstructures are observed to show bimodal. Dynamic recrystallization can take place in the b region irrespective of starting deformed structures.

Microstructural evolution during ultra-rapid annealing of severely deformed low-carbon steel: strain, temperature, and heating rate effects

An interaction between ferrite recrystallization and austenite transformation in low-carbon steel occurs when recrystallization is delayed until the intercritical temperature range by employing high heating rate. The kinetics of recrystallization and transformation is affected by high heating rate and such an interaction. In this study, different levels of strain are applied to low-carbon steel using a severe plastic deformation method. Then, ultra-rapid annealing is performed at different heating rates of 200–1100°C/s and peak temperatures of near critical temperature. Five regimes are proposed to investigate the effects of heating rate, strain, and temperature on the interaction between recrystallization and transformation. The microstructural evolution of severely deformed low-carbon steel after ultra-rapid annealing is investigated based on the proposed regimes. Regarding the intensity and start temperature of the interaction, different microstructures consisting of ferrite and pearlite/martensite are formed. It is found that when the interaction is strong, the microstructure is refined because of the high kinetics of transformation and recrystallization. Moreover, strain shifts an interaction zone to a relatively higher heating rate. Therefore, severely deformed steel should be heated at relatively higher heating rates for it to undergo a strong interaction.