Investigation Of Microstructure Evolution Research Articles

An investigation of microstructural evolution in 304L austenitic stainless steel warm deformed by cyclic channel die compression

Microstructural evolution during warm severe plastic deformation of 304L austenitic stainless steel is investigated in the present work by employing Electron backscattered diffraction (EBSD) analysis. A total effective plastic strain of 4.8 is imparted through cyclic channel die compression technique at 500 °C. Macrostructural studies revealed distinct flow lines characterized by heterogeneous deformation bands indicative of deformation inhomogeneity. EBSD based microstructural analysis indicated substantial grain refinement wherein the grain size is decreased from ∼15 μm in the initial condition to ∼0.27 μm in warm deformed condition. Continuous dynamic recrystallization (CDRX) is identified as the dominant grain refinement mechanism evidenced by strain induced boundaries, subgrains bounded by low angle grain boundaries and thin deformation microbands. The role of deformation bands in facilitating grain refinement is evidenced. CDRX driven grain subdivision/fragmentation through the evolution of spatially distributed misoriented subgrains is corroborated to the significant increase in low angle grain boundaries in deformed condition. Considerable improvement in strength is evidenced from the evolution of microhardness and tensile behaviour. Two-fold increase in microhardness and three-fold increase in yield strength is observed. Towards the end, the grain size dependence on yield strength and microhardness is suitably described by Hall-Petch behaviour.

Toward vacuum sintering of YAG transparent ceramic using divalent dopant as sintering aids: Investigation of microstructural evolution and optical property

Transparent YAG ceramics were fabricated by solid state reaction sintering using divalent dopants (CaO and MgO) as sintering additives without TEOS doping, and the effects of divalent dopants on their microstructure evolution and optical properties were investigated. It was found that CaO was more effective with respect to inhibiting grain growth than MgO, but not as effective as MgO in promoting densification. Fully dense, transparent YAG ceramics with excellent optical qualities could be achieved by optimizing the doping concentrations of CaO and MgO; the transmittance at 1064nm was as high as 84.5% for 3mm thick sample at the molar ratio of Ca: Mg=1:4, after sintered at 1840°C for 8h in vacuum.

Investigation of microstructure evolution after post-weld heat treatment and cryogenic fracture toughness of the weld metal of AA2219 VPTIG joints

In this work, the effect of post-weld heat treatment (solid solution+artificial aging) on the microstructure and cryogenic fracture toughness of the weld metal of VPTIG welded AA2219 joints were investigated by using crack tip opening displacement (CTOD) test method. The samples were divided into three types: (i) the weld metal of as-welded joint, marked as sample A; (ii) the weld metal of post-weld heat treated joint, marked as sample B; (iii) the base metal used for comparison, marked as BM. The results showed that the solution treatment dissolved θ phases, and the artificial aging treatment re-precipitated θ' phases. After post-weld heat treatment, the strength of weld metal was improved, accompanied with a little bit of decrease in the plasticity and fracture toughness. No matter what kind of sample types, compared to 298K, the strength tested at 77K was higher due to the weaker lattice vibration frequency and lower energy fluctuation. Besides, the plasticity was also higher at 77K than that at 298K, because the Peierls-Nabarro force was not sensitive to temperatures. Thus, the fracture toughness was also better at 77K.

Investigation of microstructure evolution during self-annealing in thin Cu films by combining mesoscale level set and ab initio modeling

Microstructure evolution in thin Cu films during room temperature self-annealing is investigated by means of a mesoscale level set model. The model is formulated such that the relative, or collective, influence of anisotropic grain boundary energy, mobility and heterogeneously distributed stored energy can be investigated. Density functional theory (DFT) calculations are performed in the present work to provide the variation of grain boundary energy for different grain boundary configurations. The stability of the predominant (111) fiber texture in the as-deposited state is studied as well as the stability of some special low-Σ grain boundaries. Further, the numerical model allows tracing of the grain size distribution and occurrence of abnormal grain growth during self-annealing. It is found that abnormal grain growth depends mainly on the presence of stored energy variations, whereas anisotropic grain boundary energy or mobility is insufficient to trigger any abnormal growth in the model. However, texture dependent grain boundary properties, mobility in particular, contribute to an increased content of low-Σ boundaries in the annealed microstructure. The increased presence of such boundaries is also promoted by stored energy variations. In addition, if the stored energy variations are sufficient the coexisting (111) and (001) texture components in the as-deposited state will evolve into a (001) dominated texture during annealing. Further, it is found that whereas stored energy variations promote the stability of the (001) texture component, anisotropic grain boundary energy and mobility tend to work the other way and stabilize the (111) component at the expense of (001) grains.

Study on the characteristics and thermal stability of nanostructures in adiabatic shear band of 2195 Al–Li alloy

Adiabatic shear bands (ASB) were obtained by dynamic shearing with a split Hopkinson pressure bar in the hat-shaped specimens of 2195-T6 Al–Li alloy. TEM observations reveal that grains in ASB are mainly equiaxed with the grain size from 50 to 100 nm. The kinetics possibility of instant refinement of grains can well be explained with the rotation dynamic recrystallization mechanism. EBSD is used to investigate microstructure evolution in ASB after annealed at 100–400 °C for 1 h. Results show that grain size increases rapidly at higher annealing temperature, and grains grow from 0.22 μm at 300 °C to 1.77 μm at 400 °C. Microhardness measurement indicated that the microhardness value rises slowly with temperature increases and then drops quickly at 300 °C. The study indicates that the nanostructure in ASB is thermally stable below 300 °C.

Creep and microstructural processes in a low-alloy 2.25%Cr1.6%W steel (ASTM Grade 23)

A low-alloy 2.25%Cr1%Mo steel (ASTM Grade 22) has been greatly improved by the substitution of almost all of the 1%Mo by 1.6%W. The improved material has been standardized as P/T23 steel (Fe–2.25Cr–1.6W–0.25V–0.05Nb–0.07C). The present investigation was conducted on T23 steel in an effort to obtain a more complete description and understanding of the role of the microstructural evolution and deformation processes in high-temperature creep. Constant load tensile creep tests were carried out in an argon atmosphere in the temperature range 500–650°C at stresses ranging from 50 to 400MPa. It was found that the diffusion in the matrix lattice is the creep-rate controlling process. The results of an extensive transmission electron microscopy (TEM) analysis programme to investigate microstructure evolution as a function of temperature are described and compared with the thermodynamic calculations using the software package Thermo-Calc. The significant creep-strength drop of T23 steel after long-term creep exposures can be explained by the decrease in dislocation hardening, precipitation hardening and solid solution hardening due to the instability of the microstructure at high temperature.

Friction Stir Welding of Austenitic Stainless Steel: A Study on Microstructure and Effect of Parameters on Tensile Strength

Friction stir welding was performed on austenitic stainless steel plates on an indigenously retrofitted vertical milling machine. Machine was retrofitted with a robust work fixture capable of measuring welding force and a tool adapter. AISI-304 equivalent grade stainless steel was welded by FSW using tungsten carbide tools with tapered cylindrical (conical) pin. The experiments were performed using L4 Orthogonal Array of Taguchi design. The tensile tests revealed failure of test specimens outside the weld region. The results of ANOVA showed the order of importance in which the parameters have affected the UTS in terms of percent contributions i.e. welding speed with (56.83% contribution), shoulder diameter (27.44% contribution) and tool rpm (15.73% contribution). The microstructure study was performed to investigate microstructure evolution in Stir Zone (SZ), Thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ).The microstructure characterization in three distinct FSW zones showed: (i) Fine grain structure with a circular and non-uniformly distributed darkly etched band formation in SZ, (ii) Finer grains in TMAZ, and (iii) Refinement of grains in HAZ Micro-hardness distribution across the weldline was also obtained. The highest hardness was seen to occur in the TMAZ on advancing side.

Microstructural Evolution in Aluminum Alloy 7050 during Cooling from Homogenization

Microstructure of post-homogenization is significant to prevent particle stimulated nucleation of recrystallization. This paper investigated microstructure evolution in alloy 7050 during cooling from homogenization. Two intermetallic phases, Al7Cu2Fe and Mg2Si were insoluble during homogenization. Cooling to 460 °C and 400 °C follow by quenching, precipitates nucleated discontinuously on the grain boundaries, no precipitates was found within the grains. The precipitates were found near the grain boundaries when quenching from 350 °C, and precipitated uniformly within the center of the grains when quenching from 250 °C. The sizes of the precipitates increased with the decreased of the quenching temperature. Controlling of the post-homogenization microstructure is beneficial to prevent particle stimulated nucleation of recrystallization.

The Effect of Annealing Conditions and Alloying Elements on the Microstructure Stability and Mechanical Properties of Mg-Zn-Cesheets

In this work, four Mg-Zn-Ce sheets were processed on an industrial scale by “Helholtz-ZentrumGreesthacht” company. To study the effect of alloying elements and annealing conditions on the rolled sheets, these sheets were annealed at different conditions. Mechanical properties of these rolled and annealed sheets were determined in rolling and transverse directions. Optical microscopy was used to investigate microstructure evolution and stability as well as grain size after annealing. Annealing at 450°C, 1 hr for A and D sheets provides microstructure stability of these sheets. Whereas, annealing B and C sheets for one hour at 400 and 350°C, respectively, provide the optimum annealing conditions. In the studied alloys, the highest zinc (Zn) presence alloyed with magnesium significantly increases the grain size and the amount of precipitates. Whereas, the highest cerium (Ce) presence alloyed with magnesium, refines the grain size during the recrystallization process.

Defect formation and accumulation in CeO2 irradiated with swift heavy ions

We have investigated microstructure evolution in CeO2 irradiated with 210MeV Xe ions by using transmission electron microscopy to gain the fundamental knowledge on radiation damage induced by fission fragments in nuclear fuel and transmutation target. Analysis on the accumulation of ion tracks has revealed an influence region to recover pre-existing core damage regions of ion tracks to be 8.4nm in radius. Cross section observations showed that high-density electronic excitation induces both ion tracks and dislocation loops. At high fluences of 1.5×1019 and 1×1020ionsm−2, depth-dependent microstructure was developed with radiation-induced defects of ion tracks, dislocation loops (dot-contrast) and line dislocations. Formation of sub-divided small grains was found at shallow depth at a fluence of 1×1020ionsm−2. The microstructure evolution was discussed in terms of the accumulation of interstitials due to significant overlap of high density electronic excitation.

On the precipitates and mechanical properties of magnesium–yttrium sheets

Light-weight wrought magnesium alloys is an important part of the weight reduction in automobiles industry for improve their fuel efficiency. Yttrium containing magnesium alloy is a potential material in this perspective. In this work, two magnesium–yttrium alloys (C and D alloys) were cast and rolled to 2mm thick sheets. The mechanical properties of these hot rolled and annealed sheets were determined. Optical microscope and scanning electron microscope equipped with EDX were used to investigate microstructure evolution during thermo-mechanical processing in the studied alloys. Precipitates evolution during hot rolling and annealing processes were analyzed and compared with those calculated using thermo-chemical software (FactSage). Schiel phase distribution diagrams of C and D alloys were calculated using FactSage.

MICROSTRUCTURE EVOLUTION OF Al-Fe ALLOYS PREPARED BY MECHANICAL ALLOYING AND SPARK PLASMA SINTERING

Al-Fe alloys have wide potential applications in automobile and aerospace industries due to their high specific strength,high specific stiffness,good stability of microstructure and excellent high temperature strength.However,a wide variety of metastable phases can be formed in Al-Fe binary system,such as Al(Fe) supersaturated solid solution,amorphous and intermetallic phase.In order to better understand the phase formation in Al-Fe alloys,a systematic investigation of microstructure evolution is necessary.In this work,bulk dense Al-5Fe alloys were fabricated by mechanical alloying(MA) and spark plasma sintering(SPS).The phases,microstructures and morphologies of MA powders and the corresponding sintered samples were characterized by XRD,SEM and EDS.Special attention was paid to the effects of different milling times on structural change of phases during MA-SPS process.The results showed that during the MA,the size of alloy powders increased with increasing milling time(0—10 h),and then decreased with further milling time(10—20 h).The(111)_(Al) peaks in XRD spectra of MA powders shifted to higher angles with the increase of milling time,indicating the dissolution of Fe atoms into the Al crystal lattice. Homogeneous Al(Fe) solid solutions were obtained after MA for 20 h.Bulk samples sintered from MA powders of 0 and 10 h contained Al/Al_(13)Fe_4/Al_5Fe_2/Fe layer structure intermetallic phase and tiny Al_(13)Fe_4 phase in the Al matrix.However,bulk sample sintered from MA powders of 20 h contained only relatively small Al_(13)Fe_4 phase in the Al matrix.Based on thermodynamic analysis(effective heat of formation theory) and kinetic analysis(spherical shell model),the primary phase that formed on the interfacial layer of Al/Fe was Al_(13)Fe_4,and then Al_5Fe_2 can be formed by the reaction of residual Fe and Al_(13)Fe_4 for the lower Gibbs free energy of Al_5Fe_2 compared to that of Al_(13)Fe_4,leading to the formation of Al/Al_(13)Fe_4/Al_5Fe_2/Fe layer structure intermetallic phase.The absence of Al_5Fe_2 and Fe phases in sample sintered from MA powders of 20 h were attributed to the complete reaction between relatively small Fe particles and Al melt during SPS process.

Microstructure evolution to reach the single variant in an ordered Fe–55at.%Pd alloy

Abstract Recently, we reported single variant formation in an Fe–55at.%Pd is certainly realized from a disordered fcc-phase to an ordered L1 0 -phase by heat-treatment under magnetic field. In the present study, we have investigated microstructure evolution during the process of the single variant formation by an X-ray diffraction and an electron microscopy observation. As a result, followings are obtained: size of the ordered particles at the early stage of ordering is about 2 nm and the nucleation ratio of preferable variant, whose easy axis lies in the field direction, is higher than that of other variants. Each of the ordered preferable variant grows by consuming the order variants and finally come together to become a single variant. Based on the observation, a model is proposed for the single variant formation of the ordered L1 0 -phase under magnetic field.

Investigation of Microstructure of the Commercial Pure Aluminium in the ARB Process

The severe plastic deformation (SPD) of metallic materials has become attractive in recent years. Accumulative roll bonding (ARB) is a SPD process that can impose deformation to a sheet metal, as it develops fine grains of typically submicrometer or even the nanometer level in polycrystalline materials. The purpose of the present study is investigation of microstructural evolution and micro hardness behavior of the commercial purity Aluminium (AA1050) severely deformed by the ARB process. The microstructural evolution during 13 cycles of ARB process was investigated. With increasing ARB cycles the grains size reduced in nanometer range. Furthermore, Micro-Vickers hardness measurement was carried out throughout thickness of the ARB processed sheets.

Effect of Microstructure on Thermal Expansion Coefficient of 7A09 Aluminum Alloy

The relationship between microstructure evolution and coefficient of thermal expansion (CTE) of 7A09 aluminum alloy was investigated in this paper. Differential scanning calorimetry (DSC) was combined with transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) to investigate microstructure evolution taking place in 7A09 aluminum alloy during heating and cooling process. The corresponding CTE curves of the 7A09 alloy were recorded by thermal dilatometer. Results indicated that GPII zones and η′ phase were main precipitates in the highest strength tempered (T6) 7A09 alloy. The η phase was the main participate in 7A09 alloy during the cooling process. The nonlinear dependency existed between CTE and temperature in both changing temperature processes. During the heating process, obvious additional contraction of alloy volume was directly caused by phase transition, such as dissolution of η′ phase, transition from η′ to η phase and dissolution of η phase. The additional contraction could slow down the increase of CTE greatly and be expressed in the nonlinearity of CTE curve. Volume and energy changes of alloy system influenced the variation trend of CTE directly, which was caused by the precipitation of η phase during the cooling process. These effects were revealed by the corresponding nonlinear change of CTE.

Electrothermomechanical Finite-Element Modeling of Metal Microcontacts in MEMS

Three-dimensional fractal representations of surface roughness are incorporated into a finite-element framework to obtain the electrothermomechanical behavior of ohmic contacts in radio frequency (RF) microelectromechanical systems (MEMS) switches. Fractal surfaces are generated from the Weierstrass-Mandelbrot function and are representatives of atomic force microscope surface roughness measurements of contact surfaces in fabricated RF MEMS switches with metal contacts. A specialized finite-element scheme is developed, which couples the thermomechanical asperity creep deformations with the electromechanical contact characteristics to obtain predictions of contact parameters and their evolution as a function of time and loading. A dislocation-density-based crystal plasticity framework is also used to investigate microstructure evolution at microcontacts and its effects on contact parameters. Using this approach, simulations are made to investigate how surface roughness, initial residual strains, and operating temperature can affect asperity contact behavior. Based on these predictions, tribological design guidelines can be obtained to increase the lifetime of low-contact-resistance RF MEMS switches by limiting stiction and electrical resistance increase.

Simulation and Analysis on Microstructure Evolution of Large Generator Retaining Ring during Multi-Fire Forging

In order to investigate microstructure evolution of Mn18Cr18N retaining ring during the multi-fire forging, a series of constitutive equations for dynamic recrystallization, static recrystallization, meta-dynamic recrystallization and grain growth were developed and implemented into a Deform FE simulator. The single-axial hot upsetting test has been performed to investigate the process of microstructure evolution and to show validity and effectiveness of the developed program. Then based on the modified boundary condition, hot forging process for 300MW retaining ring was put into effect. The results have displayed that the microstructure prediction tool was validated by comparing the simulated grain structure with that of the experiment and it could provide a reference to optimize forging processes in the production of retaining ring.

An investigation of microstructure evolution in cement paste through setting using ultrasonic and rheological measurements

The response of hydrating cement paste through setting are monitored using rheological measurements and ultrasonic reflection measurements. Increases in the elastic modulus and yield stress of cement paste with time are obtained from the rheological measurements. Ultrasonic measurements are performed using horizontally polarized shear waves (SH) reflected off of the hydrating cement paste. Changes in the ultrasonic signal through setting are related with changes in the porosity and stiffness of an equivalent water-filled poroelastic material, which provides identical acoustic impedance. The measured changes in the shear modulus obtained from ultrasonic measurement are shown to correlate well with increase in elastic modulus obtained from rheological measurements. The increase in the shear modulus of the porous material obtained from the ultrasonic measurements is shown to correspond well with the observed increase in the yield stress of the cement paste. By combining the information from rheological and ultrasonic measurements, it is found that even in the fluid stage there is sufficient structural integrity in the arrangement of cement grains to support low-amplitude shear stress and the evolution of a continuously connected network of cement particles within the paste is coincident with a rapid increase in the shear modulus of the porous skeleton.

The difference in the types of intermetallic compound formed between the cathode and anode of an Sn–Ag–Cu solder joint under current stressing

An investigation of microstructural evolution with various current densities in a lead-free Cu/SnAgCu/Au/Cu solder system was conducted in this study. Current stressing induced migration of Cu toward the anode and resulted in the formation of Cu 6Sn 5 at the interface. The consumption rates of Cu were calculated to be 2.24 × 10 −7 μm/s and 5.17 × 10 −7 μm/s at 1.0 × 10 3 A/cm 2 and 2.0 × 10 3 A/cm 2, respectively, while the growth rates of Cu 6Sn 5 were 6.33 × 10 −7 μm/s and 7.72 × 10 −7 μm/s. The atomic fluxes of Cu were found to be 2.50 × 10 12 atom/cm 2 s and 5.88 × 10 12 atom/cm 2 s at the above-mentioned current densities. The diffusivities of Cu in Cu 6Sn 5 were 2.02 × 10 −11 cm 2/s and 2.38 × 10 −11 cm 2/s under 1.0 × 10 3 A/cm 2 and 2.0 × 10 3 A/cm 2 of current stressing. Current stressing effectively enhances the migration of Cu in Cu 6Sn 5 and results in a 1000-fold increase of magnitude in diffusivity compared to thermal aging. (Cu 1− x ,Au x ) 6Sn 5 compound was formed near the anode after a long period of current stressing.

Microstructure evolution during warm working of Ti–6Al–4V with a colony-α microstructure

A high-resolution electron backscatter diffraction technique was employed to investigate microstructure evolution during warm working of Ti–6Al–4V with a colony-α microstructure. Particular emphasis was paid to the specific mechanisms governing this process. Microstructure development was found to be driven mainly by the geometrical requirements of the imposed strain and by the kinking of α lamellae. For the most part, the lamellar microstructure was surprisingly stable during straining, with limited globularization observed only in kinked α colonies. The kinking process was shown to be closely linked with the development of shear bands within the colonies. These observations suggest that changes in strain path may be beneficial in promoting globularization during warm working.