Microstructures,micro-segregation and solidification path of directionally solidified Ti-45Al-5Nb alloy

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s^(-1) under the temperature gradients of 15-20 K·mm^(-1), and the primary phase is transformed into an α phase at relatively higher growth rates(V >20 μm·s^(-1)). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s^(-1) under a temperature gradient of 15 K·mm^(-1). The increase of temperature gradient to 20 K·mm^(-1) can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L+β L+β→αα+β_R β→ααα→γα+γα→α_2+γγ_R+(α_2+γ), in which β_R and γ_R mean the residual β and

Simulation of Fluid Flow, Heat Transfer and Micro-Segregation in Twin-roll Strip Casting of Stainless Steel

In twin-roll strip casting process, metal flow and temperature distribution in the molten pool directly affect the stability of the process and the quality of products. In this paper, a 3D coupled thermal-flow fenite element modeling （FEM） simulation for twin-roll strip casting of stainless steel was performed. Influences of the pouring temperature and casting speed on the temperature fields were obtained from the numerical simulation. The micro-segregation of the solutes during the strip casting process of stainless steel was also simulated. A developed micro-segregation model was used to calculate the micro-segregation of solutes in twin-roll casting of stainless steel. The relationship between the solidus fraction in solidification and temperature was given, which was used to determine the LIT （liquid impermeable temperature）, ZST （zero strength temperature） and ZDT （zero ductility temperature） in the period of non-equilibrium solidification. The effect of temperature on the micro-segregation was discussed. According to the computational results, the solidification completion temperature in the twin-roll strip casting of stainless steel was then determined, which can provide a basis for controlling the location of solidification completion temperature and analysing the crack of the casting strip.

Modeling the dendritic evolution and micro-segregation of cast alloy with cellular automaton

In order to precisely describe the dendritic morphology and micro-segregationduring solidification process, a novel continuous model concerning the different physicalproperties in the solid phase, liquid phase and interface is developed. Coupling the heat and solutediffusion with the transition rales, the dendrite evolution is simulated by cellular automatonmethod. Then, the solidification microstructure evolution of a small ingot is simulated by usingthis method. The simulated results indicate that this model can simulate the dendrite growth, showthe second dendrite arm and tertiary dendrite arm, and reveal the micro-segregation in theinter-dendritic zones. Furthermore, the columnar-to-equiaxed transition (CET) is predicted.

Phase field modeling for dendritic morphology transition and micro-segregation in multi-component alloys

By using the phase field model for the solidification of multi-component alloys and coupling with real thermodynamic data, the dendritic morphology transition and the dendritic micro-segregation of Ni-Al-Nb ternary alloys are simulated in two cases, i.e., varying the alloy composition at a fixed under-cooling and varying the undercooling at a fixed alloy composition. The simulated results indicate that with the increase of the dimensionless undercooling U (U=ΔT/ΔT0, where ΔT is the undercooling and ΔT0 the temperature interval between the solidus and liquidus), the dendritic morphology transfers from dendritic to globular growth in both cases. As to the dendritic micro-segregation, both cases present a regularity of increasing at first and then decreasing.

Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification:the Role of Nitrogen Content

The segregation and precipitation behavior of Alloy 690 containing 0.001-0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 ℃ have been investigated using optical microscopy （OM）,electron probe microanalysis （EPMA） and transmission electron microscopy （TEM）.The results indicate that the volume fraction of TiN-type nitride formed during isothermal solidification increases with the nitrogen content of Alloy 690.Segregation of Ti and Cr exists in samples solidified at 1370 and 1355℃.The Ti content in the residual liquid markedly decreases and the concentration of Cr increases when the nitrogen content of Alloy 690 increases.Furthermore,N and S also show segregation to some extent in the residual liquids at 1355℃.Accompanying by the segregation of Cr,Ti,C,N and S,sulfides and chromium nitrides form.In a low nitrogen content Alloy 690,sulfur segregates and precipitates in the form of Ti 4 C 2 S 2 and （Cr,Ti）S,but in the form of （Cr,Ti）S or CrS in a high nitrogen content Alloy 690.（Cr,Ti）N-type nitrides with an fcc crystal structure have been identified in a sample with 0.11 wt% nitrogen.

Coupled Simulation of Flow and Thermal Field of Twin-Roll Strip Casting Process

The first micro-segregation under conditions of twin roll strip casting was simulated.The relationship between the temperature and solid fraction in the mushy zone was given.The temperatures such as ZDT,LIT were got from this simulation.Then using the turbulent model,the flow field and thermal field in the pool of twin-roll strip caster was simulated.The speed and temperature at different casting speed was given,and the results were also explained.By these two simulations,the appropriate casting speed can be found.These simulations can provide effective data for controlling the twin-roll strip casting process.

Ultrasonic vibration assisted tungsten inert gas welding of dissimilar metals 316L and L415

Ultrasonic vibration assisted tungsten inert gas welding was applied to joining stainless steel 316 L and low alloy high strength steel L415.The effect of ultrasonic vibration on the microstructure and mechanical properties of a dissimilar metal welded joint of 316 L and L415 was systematically investigated.The microstructures of both heat affected zones of L415 and weld metal were substantially refined,and the clusters ofδferrite in traditional tungsten inert gas(TIG)weld were changed to a dispersive distribution via the ultrasonic vibration.The ultrasonic vibration promoted the uniform distribution of elements and decreased the micro-segregation tendency in the weld.With the application of ultrasonic vibration,the average tensile strength and elongation of the joint was improved from 613 to 650 MPa and from 16.15%to31.54%,respectively.The content ofΣ3 grain boundaries around the fusion line zone is higher and the distribution is more uniform in the ultrasonic vibration assisted welded joint compared with the traditional one,indicating an excellent weld metal crack resistance.

Modeling of Microstructure Evolution during Directional Solidification Process

A model was presented to describe the microstructure evolution during the directional solidification process. In this model, the problem of different properties in the solid and liquid phase was solved by making the properties continuous at the solid/liquid interface. Furthermore, a random noise was incorporated to reflect the anisotropic growth. Moreover, the averaging solute conservation was developed to keep the total solute conservation in the interface region. A simple ingot was simulated by this method, the model can represent the microstructure evolution, solute concentration redistribution, micro-segregation and the columnar-to-equiaxed transition.

Microstructure and mechanical properties of Ti43Al6Nb alloys with different zirconium contents

Ti43Al6Nb-xZr alloys with different additions of zirconium were prepared by vacuum arc melting furnace.The microstructure and compressive properties at room temperature(RT) were investigated.The microstructure shows dendrites with addition of 0 at%-2.5at% Zr,and the dendrites are refined with the primary dendrite arms spacing decreasing from 222.64 μm(0 at%Zr) to 92.57 μm(2.0 at% Zr).With Zr addition more than2.5 at%,the microstructure shows equiaxed grains surrounded by y phase.Zr is a y stabilizer and promotes the β/y transition,resulting in the change of microstructure morphology.Zr reaches the maximum solid solubility(about 6.5 at%) in y phase with addition of 2.5 at% Zr;moreover,γ phase increases in quantity,bringing about severe micro-segregation.With addition of Zr,the remained β phase turns into ω phase with B82 structure.Ti43Al6Nb-xZr alloys show brittle fracture.The maximum compressive strength is 2161.69 MPa with addition of 2.5at% Zr and the maximum compressive strain is 30.62%with addition of 0.5 at% Zr,improving by 9.24% and7.33%,respectively.The improvement of compressive strength results from fine-grain strengthening and solution strengthening.Severe micro-segregation is bad for compressive strength,and large solubility of Zr is detrimental to ductility.

Analysis of precipitation behavior of MnS in sulfur-bearing steel system with finite-difference segregation model

To further reveal the influence of micro-segregation on the precipitation behavior of MnS in sulfur-bearing steel system, a coupled model of micro-segregation and MnS precipitation was established by the finite-difference method based on various calculation domains and the solid diffusion degrees, and a new controlled diffusion equation with more stable convergence was also used. 49MnVS3 and 1215 steels were used to analyze the influence of calculation domain, segregation model and S content on the precipitation behavior of MnS. The calculation results were verified by a high- temperature confocal laser scanning microscope （HT-CLSM）. The results show that the domain has little effect on the precipitation temperature, precipitation solid fraction and precipitation amount of MnS, but affects the precipitation location and segregation of the solutes. For low- and medium-sulfur steels, the temperatures calculated by the diffusion control growth （DCG） model and the Lever model were nearly identical, whereas the temperature calculated by the Scheil model was lower. However, for high-sulfur steels, the precipitation temperatures calculated by three segregation models were nearly same. The precipitation solid fraction is more reasonable to describe the precipitation behavior of MnS. The precipitation behavior of MnS, observed by the HT-CLSM, matches well with that in the DCG model.

Limitations of the homogenization process of bulk martensitic mold steel-depending on the comprehensive manifestation of various strengthening

A homogenization treatment(1250 °C + 12 h) was carried out to minimize the micro-segregation of bulk 718H martensitic mold steel, as verified by advanced experimental characterization and kinetic model of diffusion. However, new research found that there are still limitations in the use of the homogenization process. The result indicates that the chemical heterogeneity can be significantly reduced after homogenization. The segregation ratio of Cr and Mo elements of sample decreased by 40.9% and 35.6% of the original level, respectively. Simultaneously, the test steel with higher strength and toughness is produced by controlling micro-segregation tempered from 540 °C to 650 °C. Importantly, it reveals that the impact energy is increased by up to 27.3%. The isotropy of impact energy in different directions can reach 0.89,resulting in an overall improvement in the isotropy. Toughness mainly depends on the orientation relationship between the crack propagation direction and the band segregation region. The chain carbides formed due to the decomposition of the micro-segregated region during tempering are considered the main source of cracks. The more evenly distributed the subsequent tempered carbides after homogenization, resulting in an increase in toughness. However, an abnormal phenomenon is found in which the yield strength after homogenization is lower than that of the untreated sample tempered at 700 °C. This result can be attributed to the combined influences of precipitation strengthening and fine grain strengthening by analyzing various strengthening mechanisms. The mutually restrictive strengthening effect leads to the limitations of the homogenization process of bulk martensitic mold steel.