Microstructural Evolutions of AA7055 Aluminum Alloy Under Dynamic and Quasi-static Compressions

The microstructural evolution of AA7055 aluminum alloy under dynamic impact loading with the strain rate of 1.3 × 10^4 s^-1 controlled by a split Hopkinson pressure bar was investigated, and compared with that under quasi-static mechanical loading in compression with strain rate of 1.0 × 10^-3 s^-1. The quasi-static-compressed sample exhibited equiaxed dislocation cells, which were different from the elongated and incomplete dislocation cells for the alloy undergoing dynamic compression. The high strain-rate compression also induced the formation of localized shear bands in which the recrystallizations characterized as fine equiaxed grains were observed. The microstructural evolutions under both quasi-static and dynamic compressions are rationalized in terms of the dislocation cell model combined with the dislocation kinetics, in addition to the adiabatic temperature rise in shear bands at high strain rate.

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.

In-situ microstructural evolutions of 5Mn steel at elevated temperature in a transmission electron microscope

The microstructural evolutions of 5Mn steel during various heat treatments have been investiga- ted by in-situ transmission electron microscopy （TEM）. The specimen of 5Mn steel was pre- pared using focused ion beam （FIB） milling, which allowed the selection of specific morphology of interest prior to the in-situ observation, The complete austenization at 800 ℃ was verified at the atomic scale by minimizing thermal expansion and sample drift in a heating holder based on micro-electro-mechanical-systems. During annealing at 650 ℃, the formation of reverted austen- ite was dynamically observed, while the morphologies of austenite laths of 5Mn steel after in-situ heating were quite similar to that after ex-situ intereritical annealing. During annealing at 500 ℃, the morphological evolution of cementite and associated Mn diffusion were investigated. It was demonstrated that a combination of FIB sampling and high temperature in-situ TEM enables us to probe the morphological evolution and elemental diffusion of specific areas of interest in steel at high spatial resolution.

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.

Precise regulation of the phase transformation for pyrolusite during the reduction roasting process

The mechanism involved in the phase transformation process of pyrolusite (MnO_(2)) during roasting in a reducing atmosphere was systematically elucidated in this study,with the aim of effectively using low-grade complex manganese ore resources.According to single-factor experiment results,the roasted product with a divalent manganese (Mn^(2+)) distribution rate of 95.30% was obtained at a roasting time of 25 min,a roasting temperature of 700℃,a CO concentration of 20at%,and a total gas volume of 500 mL·min^(-1),in which the manganese was mainly in the form of manganosite (MnO).Scanning electron microscopy and Brunauer–Emmett–Teller theory demonstrated the microstructural evolution of the roasted product and the gradual reduction in the pyrolusite ore from the surface to the core Thermodynamic calculations,X-ray photoelectron spectroscopy,and X-ray diffractometry analyses determined that the phase transformation of pyrolusite followed the order of MnO_(2)→Mn_(2)O_(3)→Mn_(3)O_(4)→MnO phase by phase,and the reduction of manganese oxides in each valence state proceeded simultaneously.

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.

Processing and Microstructural Evolution of Superalloy Inconel 718 during Hot Tube Extrusion

The processing parameters of tube extrusion for superalloy Inconel 718 (IN 718), such as slug temperature, tools temperature, choice of lubricant, extrusion ratio and extrusion speed, were determined by experiment in this paper. An appropriate temperature range recommended for the slug is 1080-1120℃, and the temperature range recommended for the tools is 350-500℃. The microstructural evolution of superalloy IN 718 during tube extrusion was analyzed. With the increase of the deformation the cross crystal grains were slightly refined. While the vertical crystal grain is elongated evidently and the tensile strength increased along the axial rake. Glass lubricants have to be spread on the slug surface after being heated to 150-200℃, vegetable oil or animal oil can be used as the lubricant on the surface of the tools to reduce the extrusion force remarkably.

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.

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.

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 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 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 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.