The microstructure of a low alloy martensitic steel has been investigated using TEM. It was indicated that the as-quenched plate and lath martensites consist of ferrite matrix and high density of nanometer-scaled ultrafine particles embedded in the matrix. These particles were designated to beω phase with a primitive hexagonal crystal structure. Theω particles exhibit an orientation relationship with the ferrite (α-Fe) matrix as follows: [113]α//[2113]ω,(110)α //(1101)ω and (211)α//(0110)ω, with lattice parameters of aω=21/2aα , cω=31/2/2aα. The results of the present study suggested that the carbon atoms in the steel are not homogenously distributed in the martensites. The ferrite matrix possesses very low content of carbon, and most of the carbon atoms are concentrated in the ω phase.
Driven by a good combination of strength and ductility, austenitic stainless steels have attracted much interest in the past decade. These metastable alloys fall into the category of transformation induced plasticity (TRIP) steels in which high strength and excellent ductility can be achieved due to their strain--induced martensitic transformation at ambient temperature. However, there are few reports on the detail of promoting this phase transformation and enhancing the TRIP effect during deformation only by changing the loading mode. In present work, the effect of loading modes on mechanical property and microstructure of austenitic stainless steels was investigated under various temperatures. The tensile tests results reveal that cyclic tensile loading and unloading (CTLU) mode can strongly influence the deformation behavior of AISI 304 steel. There is no difference at high temperature tension by different loading modes. Compared with the conventional monotonic tensile loading (MTL) mode, the elongation has been slightly reduced by CTLU mode at cryogenic temperature. However, CTLU mode can improve both strength and ductility of AISI 304 steel at room temperature. An in situ Xray diffraction has been carried out to identify and evaluate strain-induced martensitic transformation by different loading modes at room temperature. Experimental results showed that the fraction of strain-induced martensite increases when unloading happens. It indicated that CTLU mode can enhance strain hardening in AISI 304 stainless steel, which prolongs the time to neck formation to a significant extent. Consequently the TRIP effect is enhanced.
In recent years, the MnFePGe compound has drawn tremendous attention not only for its excellent magnetocaloric effect (MCE), but also for its great commercial interest.Compared with other advanced MCE materials such as GdSiGe, MnFePAs, etc.,it possesses many practical advantages such as more abundant raw materials,lower fabrication costs as well as better environmental amity. In this work, Mn1.2Fe0.8P0.76Ge0.24 compound was prepared by mechanical milling and subsequent spark plasma sintering (SPS) technique, its microstructure was investigated by SEM, meanwhile the relationship between phase transition and the properties was investigated by neutrondiffraction, SQUID, DSC and XRD. The results show that the Mn1.2Fe0.8P0.76Ge0.24 compound is compact, and possess a hexagonal Fe2P-type crystal structure. Generally, either applied magnetic field or temperature change will induce the transformation between paramagnetic phase and ferromagnetic phase. When the applied magnetic field increased or temperature reduced, paramagnetic phase transformed to ferromagnetic phase and caused the magnetic entropy change to become larger. It is found that the magnetic entropy change of Mn1.2Fe0.8P0.76Ge0.24 compound is directly corresponding to the percentage of the phase transition.
A phase field model has been established to investigate grain growth of nanocrystalline AZ31 Mg alloy under realistic spatial-temporal scales. Most previous phase field models are limited to grain growth at micron scale. A set of rules as following has been proposed to determine the real physical value of all parameters in this new model. The expression of local free energy density function is modified due to the different initial state of grain growth process at nanoscale. The grain boundary range and grain boundary energy are studied to determine the correct gradient and coupling parameters, respectively, where the term of grain boundary range is to explain the physical backgrounds of the order parameter gradients at grain boundary and the diffusion grain boundary. The mobility constant of grain boundary for this model is originated by fitting a group of grain size from experimental results and then the values of grain boundary mobility at different temperatures are calculated by the Arrhenius equation combined with this mobility constant. The study aims especially to find out the mechanisms for nano-structural evolution by comparing the simulated results with experimental results in the literature and simulated results in micron scale. It is shown that the grain boundary range will cover two adjacent grains in nanoscale polycrystalline and the grain boundary energy is lower down to about a half than that in micron scale polycrystalline．It is found that the grain growth rate at nanoscale is slower than that at the micron scale, and these simulated results can be proved by the experimental results in the literature. Simulations expose that solute atoms would like to segregate at the grain boundaries more severely in nano-structure than in micron-structure, and this may be the reason why nano--structure shows a lower boundary mobility to result in a strange low grain growth rate in the first stage．It is found that the grain size fluctuation is more intensely in nano-sized grains than that in micron-sized grains by the quantitative analysis of the mixed degree of grains size in nano-structure and micron-structure in the models.
Underwater wet flux-cored arc welding (FCAW) has great potential prospects of wide application in ocean engineering due to its easiness of automation and high weld quality. However, the thermal process of underwater wet welding is more complicated: the arc energy distribution is more concentrated in high-pressure environment of underwater, the convection heat transfer coefficient of the weldment under water is much larger than that in air. This study focuses on establishing the numerical model for analyzing the thermal process and the temperature field in underwater wet FCAW by using the FEM software SYSWELD. Both the generalities and peculiarities of the conventional GMAW (gas metal arc welding) in air and underwater wet FCAW processes are taken into consideration, especially the two remarkable characteristics of underwater wet welding, i.e., the water compressing action to the arc, and the enhanced heat losses caused by the surrounding water. Based on the calculated temperature profiles, the weld bead shape and sizes are predicted in underwater FCAW, which lays the foundation for the process optimization. It is found that under 4 groups of typical welding conditions the calculated weld bead dimensions are in agreement with the experimental ones, which validated the energy distribution pattern of the heat source and the numeric model for underwater wet welding. Experiments showed that the weld bead was thinner and deeper in underwater wet welding than that in conventional GMAW under the same welding parameters, while the variation regularity of weld bead profile is similar.
Most of the familiar objects in modern society, from buildings and bridges, to vehicles, computers, and medical devices, could not be produced without the use of welding. Especially, with the rising development of advanced manufacturing industry, such as aircraft and aerospace industries, shipbuilding and marine industries and automotive industries, cost-effective high-efficiency high-quality welding processes are being progressively required for increasing performance requirements and enhancements in product quality. Thus, the plasma arc welding (PAW) provides a means for these process demands by using a high power density heat source. The keyhole effect is commonly recognized as the primary attribute to the deep-penetration welding. Compared to electron beam welding and laser welding, PAW is more cost effective and more tolerant of joint gaps and misalignment. However, the mechanism of keyhole formation in PAW process differs from that in other high power density welding processes. In PAW the keyhole is produced and maintained mainly by the pressure of the plasma arc, rather than by the recoil pressure of the evaporating metal in electron beam and laser welding. Considerable research has been focused on keyhole tracking and effective heat source models for PAW process. However, the existing models rarely can present the transient influences of the keyhole evolution on heat transfer and fluid flow in the weld pool. In this work, a three dimensional PAW model was established with the interaction between heat source and keyhole evolution considered. A combined heat source model was proposed to account for the transient energy propagation.It consists of a Gaussian heat flux model on the top surface and below a dynamic developing conical heat source, which continues rising in the wake of the keyhole growth. Volume of Fluid (VOF) method was applied to track the dynamic keyhole shapes, and the transient height of heat source model was simultaneously updated with the increasing keyhole depth. The transient evolution of heat density distribution concerning the keyholing effect was analyzed in details, and the corresponding temperature field was calculated and displayed to reveal the mechanism of heat transfer in the weld pool. The keyhole process and molten metal flow in the weld pool was also investigated. Finally, experiment was carried out on a stainless steel plate with thickness 6 mm, and the calculated results showed good agreement with the experimental data. It validated the mathematical model and the applicability of the dynamic heat source, which provides an insight into the understanding of the thermal process in the keyhole of PAW.
During the processing of hot working, the material undergoes shape and microstructural changes depending on the processing history. Therefore, the optimization of the processing parameters such as temperature, strain and strain rate is of great importance to achieve a defect-free component with desired microstructure. In order to optimize the hot working technology, the hot deformation behavior and hot workability of alloy 800H in the temperature range of 850-1100℃ and strain rate range of 0.01-30 s-1 were investigated with single-pass compression tests on MMS-300 thermo-simulation machine. The flow stress-strain curves of alloy 800H under different deformation conditions were plotted. The influence of precipitation on hot deformation below 950℃ at low strain rates was studied. The processing map was established based on dynamic materials model. The effect of processing parameters on hot workability and the mechanism of hot deformation in different regimes of processing map were analyzed combined with the observation of microstructural evolution. The results revealed that the adiabatic heating is generated obviously if strain rate is higher than 1 s-1 during hot deformation, which leads to flow softening via the mechanism of rotational dynamic recrystallization (DRX). Some ultrafine grains can be found in the core of shear bands. The magnitude of temperature rising and subsequent softening are supposed to be more significant at higher strain rates or lower deformation temperatures. The occurrence of DRX is inhibited by “Zener effect'' which pins the migration of grain boundaries and increases the activation energy of hot deformation. The alloy 800H exhibits a better workability in deformation temperature range from 975℃ to 1100℃ and strain rate range from 0.01 s-1 to 0.3 s-1 with the efficiency of power dissipation within the range of 35%—48%.
For the phase and microstructure selection during directional solidification of peritectic alloys, the solute distribution ahead of the solid-liquid interface plays a fundamental role. To study the solute distribution under convection condition, the convection factor (Δ) was introduced into the boundary layer model and Bower-Brody-Flemings model and the solute distribution ahead of the planar and cellular interfaces under convection condition during directional solidification were obtained. Based on these solute distributions, the nucleation and constitutional undercooling criterion and the assumption that the maximum interface growth temperature of phase growth is more stability during directional solidification, the nucleation conditions of new phase ahead of the growth interface under convection condition were analyzed and the phase and microstructure selection map in directionally solidified peritectic alloys under convection condition was developed. Compared with the Hunziker's model that is suit for the phase and microstructure selection of peritectic alloys under diffusive condition and the Karma's model that is used to explain the banded structure formation under convection condition, this model can return to the Hunziker's model under diffusive condition and include the Karma's model. Additionally, this model can predict the mixed banded structure and the coupled growth between the cellular primary phase and planar peritectic phase that can form under convection condition, which these models cannot display. In order to estimate the validity of this model, directional solidification experiments of Sn-1.6%Cd (mass fraction) peritectic alloy were carried out under different convection conditions. The results show that the experimental results are in agreement with the calculated results.That is, this model can explain the complex directional solidification microstructures of peritectic alloys appropriately.
The Pb-free plastic encapsulated ball grid array (BGA) packages have been assembled to printed-circuit boards (PCBs) with SnPb solder material using reflowing process. Isothermal aging with 4, 9, 16 and 25 d was applied to the assembled packages and the microstructures of the solder joints in the BGA packages were observed using SEM. The electrical performance test was conducted before and after different periods of isothermal aging. The result showed that there were no failure in the BGA devices. It was found that intermetallic compounds (IMCs) in the PCB substrate side were Cu3Sn and Cu6Sn5 while Ni-Cu-Sn ternary compounds were in BGA substrate side. The thicknesses of IMCs on both sides increased with the time of isothermal aging. The IMC growth rate at PCB side was significantly greater than that at BGA substrate side. The aggregation of Pb-rich phase, cracks along substrate, broken IMCs, and voids, which have negative effects on the joints reliability, were observed in a few solder joints after long-term isothermal aging process.
Powder metallurgy Ni-based superalloys are used extensively in hot section of advanced aeroγengines due to their excellent comprehensive properties. During creep,the deformation mechanism of Ni-based superalloy depends on the alloy chemistry,morphology and volume fraction of γ phase and service conditions. Generally, the microstructure of FGH95 Ni-based alloy is closely related to the heat treatment regimes and its creep mechanism and properties are mainly determined by alloy cooling at oil path or molten salt bath. The difference of creep mechanism of molten salt cooling alloy from oil cooling alloy is that the dislocation networks may be formed in the matrix and therefore decrease the steady strain rate during creep to prolong the creep lifetime of the alloy. Unfortunately, the formation mechanism of dislocation networks in FGH95 Ni-based superalloy during creep is still unclear up to now. For this reason, by means of creep property measurement, TEM observation and diffraction contrast analysis, the formation of dislocation networks in FGH95 Ni-based superalloy during creep has been investigated. The results show that the 1/2<110> dislocations are activated on the octahedral slip systems in the γ matrix of the alloy at initial stage of creep and then they continue to multiply through dislocation reaction. When the alloy enters into the steady stage of creep, two sets of slipping dislocations with different Burgers vectors would encounter on the same crystal plane to react and form a hexagonal dislocation network, or two sets of slipping dislocations on different planes would intersect to form a dislocation network with quadrangle cells. Generally speaking, the dislocation network formation can decrease the dislocation mobility and therefore restrain dislocation cross-slipping to enhance the creep resistance of the alloy. In the later stage of creep, the dislocations pile up near the regions of γ/γ’ interface and cause stress concentration, so that the deformed dislocations in the matrix shear and enter γ’ phase through damaged dislocation networks in γ’/γ’ interface,which may be decomposed to form the partials and stacking fault.
By means of heat treatment at different regimes, microstructure observation and XRD analysis, an investigation has been made into the influence of heat treatment on the phases constitution and distribution regularity of GH4169G alloy. The results show that under the experimental conditions, microstructure of GH4169G alloy consists of γ matrix, particle-like γ′,disc-like γ〞and δ phases, and the coherent interfaces are kept between the phases. Thereinto, microstructure of directional aging treatment ITF-DA-GH4169G alloy consists of a few γ′phase, lots of γ〞 and γ phases, however, long-time aging treatment ITF-DA-LTA-GH4169G consists of a few γ′,lots of γ〞, γ and needle-like δ phases. As the Nb atom diffuses into the lattice of γ′ phase during the aging treatment,γ′ -Ni3Al phase with L12 structure is transformed intoγ〞-Ni3Nb phase with DO22 structure when the Nb and Ni atoms on (001) plane of γ′ phase migrate along 1/2<110> direction. With the growth of γ〞 phase during the long term aging, the given crystal plane in the new parallelepiped migrates along the 1/6<112> direction, which makes the γ〞 phase transform into δ-Ni3Nb phase with DOa structure. Moreover, the γ〞 phase may grow up into the disc-like configuration along the c-axis direction due to the restriction of the a- and b-axis coherent interfaces. And it is a main reason that the δ-Ni3Nb phase grows into needle-like configuration along the (100) plane due to the {200}δ plane ofδ phase keeps coherent interface with {111}γ plane of γ matrix phase.
Nb-Ti-Si base ultrahigh temperature alloys that possess higher melting points, relatively lower densities and attractive high temperature strength have received worldwide attention for their potential applications as next-generation turbine blade materials. In this work, integrally directional solidification of an Nb-Ti-Si base ultrahigh temperature alloy was conducted at different withdrawing rates (2.5, 5, 10, 20, 50 and 100 μm/s) with a constant melt temperature of 2000℃. Effect of solidifying rate on the integrally directionally solidified eutectic microstructure and solid/liquid interface morphology of this alloy has been investigated by XRD, SEM and EDS, and its directional solidification behavior has been discussed. The results show that the directionally solidified microstructure is mainly composed of petal-like Nbss/α(Nb,X)5Si3 eutectic colonies (Eutectic I) and coupled grown lamellar Nbss/γ(Nb,X)5Si3 eutectic (Eutectic II) which distributed in the intercellular area. The Eutectic I and Eutectic II are both aligned straight and uprightly along the growth direction. When the solidifying rate increases from 2.5 μm/s to 100 μm/s, the microstructure becomes finer and finer, and the petal-like eutectic colonies evolve from round morphology to tetragonal morphology. Either silicides or fine eutectics locate in the centers of round eutectic cells, while cross-like Nbss locates in the centers of tetragonal eutectic cells. Eutectic II exhibits a well-aligned lamellar structure on longitudinal-section. The solid/liquid interface of the alloy undergoes an evolution from cellular dendrite, dendrite and finally to cellular dendrite morphologies.
Ni-based wrought superalloys are widely used in the hot section of aircraft gas turbine engines for their capability in retaining strength and resisting creep, fatigue, and oxidation at elevated temperature. With the development of the newer generation turbine disk alloys, it is highly imperative for aircraft engine manufacturers to substantiate the use of the materials by conducting a thorough examination of their mechanical properties. As these components are subjected to elevated temperatures and complex stress state in the service process where time dependent creep is the primary deformation failure mechanism and life—limiting factor for the component, it is of great importance to evaluate the relationship between microstructure, creep behavior and the underlying creep deformation mechanism. Therefore, the main objective of the present research aims at investigating the fundamental relationship between external creep condition and internal creep deformation mechanism in a new wrought superalloy with low stacking fault energy (SFE). In order to study the influences of the loading stress level and temperature on the creep deformation mechanism, stress range of 345—840 MPa and temperature range of 650—815℃ were selected to carry out the creep experiment. The results show that two kinds of γ′ with different diameters distributed in the matrix and the larger one began to coarsen when the creep temperature increased to 725℃. Under creep temperature of 650℃, the formation of SF resulted from the shearing of γ′ by dislocations dominated the creep deformation. When the temperature range was raised up to 725—760℃, SF and microtwins were the main microstructures after creep deformation. With further increasingthe temperature and load, instead of accommodating only in the γ′, the SF and microtwins penetrated thewhole γ′ and matrix area. When the temperature was increased to 815℃, the climb/bypass mechanism controlled the creep process.
With the development of continuous casting technology, the quality of billet has been paid more and more attention recently. It is very important in continuous casting to strictly control the cleanliness of molten steel, and to reduce the defects of billet. The control of flow pattern of molten steel in mold is one of the important means to increase casting efficiency and improve billet quality. Swirling flow in the submerged entry nozzle (SEN) has great effect on improving the uniformity and stability of the outflow from the nozzle in continuous casting of steel process. A new process for swirling flow generation in the SEN has been proposed. That is a rotating electromagnetic field is set up around the SEN to induce swirling flow in it by Lorentz force. In this research, the flow and temperature fields in the SEN and round billet mold with electromagnetic swirling are numerically simulated. The effects of the divergent angle of the SEN with electromagnetic swirling on the flow and temperature fields in the mold are investigated. The simulated results show that, with the increase of the coil current intensity, the magnetic flux density and the swirling flow velocity in the SEN increases. The largest swirling flow velocity in the SEN can reach about 3 m/s in coil current intensity 500 A, frequency 50 Hz. In a divergent angle of the SEN, such as 60℃, when the coil current intensity increases, the impinging depth of the outflow from the nozzle reduces, the upward flow velocity and the meniscus temperature increase. While the coil current intensity increases larger than 350 A, the meniscus temperature changes little. In a certain intensity of swirling flow, such as 350 A, 50 Hz, when the divergent angle of the SEN increases, the upward flow velocity and the meniscus temperature firstly increase and then decrease. In divergent angle 60℃, the upward flow velocity and meniscus temperature get the largest value. In a divergent angle 60℃, coil current intensity 350 A, frequency 50 Hz, with an artificial uneven flow of 0.5 m/s horizontal velocity at the inlet of the SEN,the uneven flow can be suppressed effectively.
Stress corrosion cracking (SCC) of 16Mn steel and its heat-affected zone (HAZ) in alkaline solution with sulfide and Cl- was investigated by electrochemical technology, slow strain rate tensile (SSRT) test and U-bent specimen immersing test. Results show that the original microstructure, the coarse grain structure acquired by air cooling treatment and the hardening microstructure obtained from quenching performed a passivation behavior in alkaline sulfide solution. Correspondingly, the passivation current density of them decreased gradually with the cooling rate increased. The corrosion potential of the quenching microstructure, air-cooling microstructure and original microstructure decreases sequentially, indicating that HAZ is the cathodic area and the fusion line as well as bulk steel anodic area. Based on the results, the corrosion is feasible to happen nearby the fusion line would increase, and thus the residual tensile stress area would be exposed to the electrolyte after long-term service, which results in SCC. The susceptibility of SCC was lowered down gradually among hardening microstructure, coarse grain microstructure and original microstructure. SCC mechanism of 16Mn steel in the alkaline solution containing sulfide was anodic dissolution (AD) in terms of intergranular fracture.
In the field of phase transformation in steels, much attention has been paid to the austenite decomposition transformation, while austenization has not been vastly investigated. However, most industrial heat treatment is related to this process, which is strongly influenced by initial microstructure. The austenization of lamellar pearlite is relatively complicated compared with spheroidized pearlite due to that in spheroidized pearlite, the angular component can be neglected, so only radius growth is considered. During the past years, much work has been done for austenite's one dimensional growth in spheroidized pearlite. However, less attention has been paid to the austenization of lamellar pearlite, which is a two dimensional growth process. Since the growth rates are different along parallel direction and vertical direction to the lamellar, it is necessary to treat growth behaviors individually in these two directions. In order to further investigate effects of substitutional alloying elements, an Fe-0.6C binary alloy and Fe-0.6C-1M (M=Mn, Cr) ternary alloys were studied. Equilibrium composition and thickness ratio of initial ferrite/cementite mixture microstructure was calculated with software Thermo Calc, and a single-layer structure model is employed. Growth of austenite vertical to lamellar direction is simulated by software DICTRA. It is found that ferrite dissolution was controlled by carbon diffusion in Fe-0.6C-1Mn alloy in 1073 K, while that of Fe-0.6C-1Cr alloy was controlled by Cr diffusion. The simulation results qualitatively agreed with experimental observation result. Compared with the parabolic growth pattern, austenite's parabolic growth coefficient is calculated for various alloys. For growth along lamellar direction, parabolic shape phase boundary is deduced based on a simplified assumption and microstructure observation. A general form of growth rate in this direction is derived for Fe--C binary system, and phase boundary shape is also modified to some extent. If a stable parabolic shape interface is reached, the growth rate parallel to the lamellar direction can also be derived for certain values of parabolic shape parameter p and lamellar spacing λ, which can be decided by microstructure observation. Given the pearlite colony size and ignoring the influence of nucleation incubation, it takes 0.053 s for a pearlite colony to transform to austenite, which is consistent with that pearlite austenization is a relatively fast process. In addition, taking interfacial energy into consideration, the formation of symmetric parabolic phase boundary is rationalized.
