Phase field modeling of dendrite growth

Single dendrite and multi-dendrite growth for Al-2 mol pct Si alloy during isothermal solidification are simulated by phase field method. In the case of single equiaxed dendrite growth, the secondary and the necking phenomenon can be observed. For multi-dendrite growth, there exists the competitive growth among the dendrites during solidification. As solidification proceeds, growing and coarsening of the primary arms occurs, together with the branching and coarsening of the secondary arms. When the diffusion fields of dendrite tips come into contact with those of the branches growing from the neighboring dendrites, the dendrites stop growing and being to ripen and thicken.

Modeling of Solidification Microstructures Based on Fully Coupling of Macro-transport Phenomena with Cellular Automata

This paper has attempted to simulate the microstructure formation based on fully coupling of temperature field, concentration field and velocity field with micro-kinetics. The authors presented a new way, which is the combination of FDM and cellular automata (CAFD) to visualize the microstructure formation of the thin complex superalloy turbine blades cast by the vacuum investment process. The distribution, orientation and mechanism of the heterogeneous nucleation, the growth kinetics of dendrites and the columnar to equiaxed transition (CET) are considered. Capitalizing on these simulating schemes, the comprehensive influence of key process variables on the scale and uniformity of grains has been investigated quantitatively. The simulated grain size and morphology agree well with the experimental results.

Exploring the ultrahigh rolling contact fatigue life of M50 bearing steel by adjusting the cryogenic sequence

The influence of different cryogenic sequences on the rolling contact fatigue(RCF)life of M50-bearing steel has been studied.The results show that direct cryogenic treatment after quenching can effectively improve RCF life.The L_(10)life is strikingly 5 times longer than that with cryogenic treatment after tem-pering.This is caused by the distinct lattice construction of martensite and the transformation of retained austenite.More secondary nanocarbides and fine twins are formed via cryogenic treatment before tem-pering compared with cryogenic treatment after tempering.The improvement in the RCF life of the steel is attributed to the joint effects of the secondary nanocarbides and twin boundaries with a width of 5-13 nm,which delays significantly crack initiation and propagation.This study highlights a common method to improve the service life of high-carbon and high-alloy steels by adjusting the cryogenic se-quence.

Cerium refinement of grains and primary carbides during solidification of Cr_(4)Mo_(4)V bearing steel

The effect of cerium(Ce)on the solidification microstructure of Cr_(4)Mo_(4)V bearing steel was investigated via a combined experimental and theoretical method.With a trace amount(0.056 wt%)of Ce addition,the coarse columnar grains in as-cast microstructure transform into equiaxed ones,and the average diameter is reduced from 56 to 27μm.The network-like and bulky primary MC and M2C carbides at the interdendritic regions become disconnected and refined,and their volume percentage decreases from4.15 vol%to 2.1 vol%.Ce-inclusions acting as heterogeneous nucleation agents of prior-austenite grains and Ce atoms segregating at grain boundaries,both contribute to the refinement of grains.Thermodynamic calculations reveal that primary carbides are precipitated afterγ-austenite forms near the end of the solidification process.The modification of primary carbides in size and amount is mainly attributed to the isolated remaining melt separated by refinedγ-austenite grains in which the nucleation of carbides is promoted,while the growth is restrained owing to the less segregation of alloying elements.

Numerical Simulation of Stress and Deformation for a Duplex Stainless Steel Impeller during Casting and Heat Treatment Processes

A large-scale, thin wall duplex stainless steel impeller with complex geometry was deformed severely and unpredictably during casting and heat treatment processes resulted in dimensional failure for the final part. In this paper, the distortion of the impeller during casting and heat treatment was calculated. A commercial software, Experto-ViewCast, was used to simulate the transient heat transfer, solidification and mechanical behaviors during the casting and the heat treatment process. The coupled set of governing differential equations for mass, energy and mechanical balance were solved by finite control volume and finite element method. A thermoelastic-visco-plastic rheological model was used to compute the constrained shrinkage of the casting. At each time increment, a coupling of the heat transfer and mechanics was performed. Comparison of the experimental measurements with the model predictions showed good agreement. From the calculated displacements of key points of the blade, the proper inverse displacements were determined to provide an optimum casting pattern and to achieve a uniform and reasonable machining allowance for both faces of the blade.

Time Dependant Weld Shape in Ar-O_2 Shielded Stationary GTA Welding

The stationary gas tungsten arc welding （GTA） is carried out on SUS304 stainless steel under Ar-0.1%O2 and Ar-0.3%O2 mixed shielding to observe the evolution of the molten pool and investigate the role of Marangoni convection on the weld shape. After welding, the oxygen content in the weld metal was measured by using an oxygen/nitrogen analyzer. Small addition of oxygen to the argon based shielding gas can effectively adjust the weld pool oxygen content. Oxygen plays an important role as an surface active element in determining the pattern of Marangoni convection in the stainless steel weld pool. When the weld metal oxygen content is over the critical value, 0.01 wt pct, corresponding to the Ar-0.3%O2 mixed shielding gas, the Marangoni convection changes from outward to inward direction and the weld shape dramatically changes from wide shallow shape to narrow deep shape.

Research progress on preparation technology of oxide dispersion strengthened steel for nuclear energy

Nuclear energy is a low-carbon,safe,efficient,and sustainable clean energy.The new generation of nuclear energy systems operate in harsher environments under higher working temperatures and irradiation doses,while traditional nuclear power materials cannot meet the requirements.The development of high-performance nuclear power materials is a key factor for promoting the development of nuclear energy.Oxide dispersion strengthened(ODS)steel contains a high number density of dispersed nano-oxides and defect sinks and exhibits excellent high temperature creep performance and irradiation swelling resistance.Therefore,ODS steel has been considered as one of the most promising candidate materials for fourth-generation nuclear fission reactor cladding tubes and nuclear fusion reactor blankets.The preparation process significantly influences microstructure of ODS steel.This paper reviews the development and perspective of several preparation processes of ODS steel,including the powder metallurgy process,improved powder metallurgy process,liquid metal forming process,hybrid process,and additive forging.This paper also summarizes and analyzes the relationship between microstructures and the preparation process.After comprehensive consideration,the powder metallurgy process is still the best preparation process for ODS steel.Combining the advantages and disadvantages of the above preparation processes,the trend applied additive forging for extreme manufacturing of large ODS steel components is discussed with the goal of providing a reference for the application and development of ODS steel in nuclear energy.

Effects of Stabilization Heat Treatment on Microstructure and Mechanical Properties of Si‑Bearing 15Cr–9Ni–Nb Austenitic Stainless Steel Weld Metal

Two 15Cr–9Ni–Nb austenitic stainless steel weld metals with 2.5%Si and 3.5%Si(namely 2.5Si and 3.5Si samples,respectively)were designed and prepared through tungsten inert gas(TIG)welding and then hold at 800℃ or 900℃ for 3 h for stabilization.The microstructure and mechanical properties were investigated both for the as-welded and after-stabilization heat treatment(SHT)weld metals.There are 3.0–4.0%martensite and 2.5–3.5%δferrite in the 2.5Si as-welded weld metal and 6.0–7.0%δferrite in the 3.5Si as-welded weld metal.After SHT,a large amount of martensite formed in the 2.5Si weld metal,andδ→γtransition occurred during the SHT process both for the 2.5Si and 3.5Si weld metals.There were a large amount of coarse NbC and few nanoscale NbC in the as-welded weld metal.During the SHT,a large amount of nanoscale NbC formed in the matrix,while a large number of G phases formed at the austenite grain boundaries and theδ/γinterfaces.The decrease in solid solution C andδferrite content led to the decline of the yield strength of the weld metal after SHT.The martensite formed in 2.5Si weld metal after SHT had less effect on strength because of its low carbon content.The G phases formed during the SHT reduced the impact energy of the weld metal because it promoted the intergranular fracture,while theδ→γtransition reduced the amount of theδ/γinterfaces and avoided the intergranular fracture,which was beneficial for the impact toughness of the weld metals.

Effects of Primary Carbide Size and Type on the Sliding Wear and Rolling Contact Fatigue Properties of M50 Bearing Steel

The influences of primary carbide size and type on the sliding wear behavior and rolling contact fatigue (RCF) properties of M50 bearing steel were systematically investigated under oil lubrication condition. A major breakthrough was achieved in the influence of primary carbide on tribological behavior. The opposite effect brought by primary carbide size on the sliding wear resistance and RCF life of M50 bearing steel was determined. Wear resistance increased with an increase in the studied primary carbide size, whereas RCF life decreased significantly. Compared with the 0 R and R positions with a relatively small carbide size, the wear volume of the 1/2 R position with a large carbide size was the smallest. Compared with the 0 R and R positions, the L10 life of the 1/2 R position decreased by 82.7% and 84.8%, respectively. On the basis of the statistical correlation between primary carbide size and the two tribological properties, a critical maximum carbide size of 5-10 μm was proposed to achieve optimal tribological performance. This research suggests that the equivalent diameter of the primary carbide should be controlled to be smaller than 10 μm, but further decreasing primary carbide size to less than 5 μm is unnecessary. The influence of primary carbide type in M50 bearing steel on sliding wear resistance was also discussed. Results indicate that the MC-type carbides with higher elastic modulus and microhardness exhibit better wear resistance than the M2C-type carbides.

Effects of Hot Deformation on the Evolution of Microstructure in Pearlitic Steel Wire Rod

The influences of hot deformation parameters on pearlite grain nucleation and growth during austenite-pearlite phase transformation in a steel wire rod have been investigated through quantitative analysis of microstructure parameters such as austenite grain size,ferrite grain size,pearlite colony size,and lamellar spacing.During hot deformation,the austenite grain size decreases due to recrystallization,providing extra nucleation sites for pearlite phase transformation,which decreases the ferrite grain size and pearlite colony size.Moreover,the stored strain energy in undercooled austenite accelerates carbon diffusion during pearlite phase transformation,which facilitates ferrite grain growth and increases pearlite colony size.Consequently,the competing influence of recrystallization and strain energy provides flexibility in adjusting ferrite grain size and colony size by hot deformation.This study highlights the critical role of hot deformation in determining the microstructure of pearlitic steel.

Numerical Simulation of Macrosegregation for an Fe-0.8 wt pct C Alloy

Macrosegregation in Fe-0.8 wt pct C alloy solidifying with equiaxed morphology was numerically simulated. Based on a two-phase volumetric averaging approach, heat transfer, melt convection, composition distribution, nucleation and grain evolution on the system scale were described. A weak-coupling numerical procedure was designed to solve conservation equations. Simulations were conducted to study the effects of cooling rate and nuclei density on the macrosegregation pattern. The relative infliJence of thermal buoyancy- and solutal buoyancy-induced flows on macrosegregation was identified. Calculated results indicate that a higher cooling rate establishes a more homogeneous composition. More uniform solute distributions are formed with increasing nuclei density. In addition, it is noted that the direction of channel segregates depends on the relative strength of thermal and solutal buoyancy forces.

Synergic Evolution of Microstructure‑Texture‑Stored Energy in Rare‑Earth‑Added Interstitial‑Free Steels Undergoing Static Recrystallization

Synergic evolution of microstructure-texture-stored energy in interstitial-free(IF)steels has been investigated to elaborate the effect of dissolved rare-earth(RE)elements on static recrystallization.Grain size,texture fraction and geometrically necessary dislocation distribution of IF steel samples annealed for different times were compared,suggesting that RE elements could postpone recrystallization nucleation but accelerate grain coarsening.The visco-plastic self-consistent model was primarily adopted and verified,then used to calculate the relative activities of different slip systems.It was proved that the compatible deformation of IF steels was very sensitive to dissolved RE elements,in particular the{110}6<111>2 slip systems became extremely inactive,leading to anα-fibre textures rich configuration of RE-IF steels.Although both IF steels have the same stored energy sequence of whichγ-fibre takes precedence in nucleation followed byα-fibre,the nucleation rates ofα/γ-fibres driven by the reduced stored energy slowed down in RE-IF steels.Further nucleation-path analyses revealed that shear bands withinγ-fibre mainly sacrificed for grain nucleation of{111}<110>orientation,whileα-fibre especially prior grain boundaries therein preferred supplying nucleation sites for{554}<225>grains,which accounting for the competitive growth ofγ-fibre textures in RE-IF steels rather than being dominated by a single orientation.After grain growth,the major texture of Normal-IF steels had been transferred to{554}<225>from{111}<110>,while{554}<225>in RE-IF steels still inherited the orientation advantage and grew up rapidly,thus inducing the grain coarsening.As this work offers a significant understanding of RE microalloying effect on static recrystallization,it will provide references for alloy design and industrial application of IF steels.

Role of Solute Rare Earth in Altering Phase Transformations during Continuous Cooling of a Low Alloy Cr–Mo–V Steel

Effects of solute rare earth(RE)on continuous cooling transformation of a low-alloy Cr–Mo–V bainitic steel are investigated in detail by dilatometry,optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Microstructures appeared in thermal dilatometric samples of both low-alloy Cr–Mo–V(RE)steels are composed of quasi-polygonal ferrite(QPF),degenerate pearlite(DP),granular bainite(GB),lath bainite(LB),and martensite(M)depending on cooling rate.When cooling rate is lower than 2°C/s,the addition of RE suppresses QPF transformation,and thereby inducing a broader transformation region of GB.When cooling rate ranges from 2 to 100°C/s,the addition of RE decreases the start temperature of bainitic transformation distinctly,which results in finer bainitic ferrite grain size and higher dislocation density.The addition of RE can enhance the hardness of the low alloy Cr–Mo–V steel by affecting the aforementioned diffusional and/or partly displacive transformation.However,when cooling rate increases up to 150°C/s,two steels have the same hardness value of about 435 HV due to only martensite obtained by displacive transformation.

Effects of Boron on the Microstructure,Ductility-dip-cracking,and Tensile Properties for NiCrFe-7 Weld Metal

The distribution of boron and the microstructure of grain boundary（GB） precipitates（M23（C,B） 6 and M 2B）have been analyzed with their effects on the susceptibility of ductility-dip-cracking（DDC） and tensile properties for NiCrFe-7 weld metal,using optical microscopy（OM）,secondary ion mass spectroscopy（SIMS）,scanning electron microscopy（SEM）,and transmission electron microscopy（TEM）.The results show that boron segregates at GBs in NiCrFe-7 weld metal during the welding process.The segregation of boron at GBs promotes the formation of continuous M23（C,B） 6 carbide chains and M 2B borides along GBs.The addition of boron aggravates GB embrittlement and causes more DDC in the weld metal,by its segregation at GBs presenting as an impurity,and promoting the formation of larger and continuous M 23（C,B） 6 carbides,and M 2B borides along GBs.DDC in the weld metal deteriorates the ductility and tensile strength of the weld metal simultaneously.

Microstructure evolutions and interfacial bonding behavior of Ni-based superalloys during solid state plastic deformation bonding

As an advanced solid state bonding process,plastic deformation bonding(PDB)is a highly reliable metallurgical joining method that produces significant plastic deformation at the bonding interface of welded joints through thermo-mechanical coupling.In this study,PDB behavior of IN718 superalloy was systematically investigated by performing a series of isothermal compression tests at various processing conditions.It was revealed that new grains evolved in the bonding area through discontinuous dynamic recrystallization(DDRX)at 1000–1150℃.Electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM)results revealed that the bonding of joints is related with interfacial grain boundary(IGB)bulging process,which is considered as a nucleation process of DRXed grain under different deformation environments.During recrystallization process,the bonded interface moved due to strain-induced boundary migration(SIBM)process.Stored energy difference(caused by accumulation of dislocations at the bonding interface)was the dominant factor for SIBM during DRX.The mechanical properties of the bonded joints were dependent upon the recrystallized microstructure and SIBM ensued during PDB.

Effects of surface roughness on interfacial dynamic recrystallization and mechanical properties of Ti-6Al-3Nb-2Zr-1Mo alloy joints produced by hot-compression bonding

The influence of surface roughness on the interfacial dynamic recrystallization kinetics and mechanical properties of Ti-6 Al-3 Nb-2 Zr-1 Mo hot-compression bonding joints was systematically investigated.It is found that for the bonding interface of rough surfaces,elongated fine grains are formed at the bonding interface due to shear deformation of the interfacial area.As the surface roughness increases,the proportion of elongated grains drastically decreases as they further reorient to form equiaxed grains along the bonding interface of rougher surfaces resulting from severe incompatible deformation of the interface area.Meanwhile,high-density geometrically necessary dislocations accumulate around the interfacial recrystallization area to accommodate the incompatible strain and lattice rotation.A rotational dynamic recrystallization mechanism is thereby proposed to rationalize the formation of fine interfacial recrystallization grains during bonding of rough surfaces.In contrast to that of rough surfaces,bonding interface of polished surfaces exists in the form of straight interface grain boundaries without fine grains under the same deformation conditions.While with the increase of deformation strain,small grain nuclei form along the bonding interface,which is associated with discontinuous dynamic recrystallization assisted by strain-induced boundary migration of interface grain boundaries.Moreover,the bonding joints of rough surfaces show lower elongation compared with that of polished surfaces.This is because the formation of heterogeneous fine grains with low Schmid factor along the bonding interface of rough surfaces,leading to worse compatible deformation capability and thereby poor ductility of bonding joints.

Solidification microstructure of Cr4Mo4V steel forged in the semi-solid state

Semi-solid forging of iron-based alloys during solidification has unique characteristics distinct from those of the classical hot forging.With the aim of acquiring precise knowledge concerning the microstructural evolution of bearing steel Cr4 Mo4 V in this process,a series of semi-solid forging experiments were carried out in which samples were wrapped in a designed pure iron sheath.The effects of forging temperature and forging reduction on the grain morphology and liquid flow behavior were investigated,respectively.By forging solidifying metal(FSM),bulky primary dendrites were broken and spheroidal grains with an average shape factor of 0.87 were obtained at 1360?C.With the decreasing forging temperature to 1340?C,the microstructural homogeneity can be improved.On the other hand,it shows that a higher forging reduction(50%)is essential for the spheroidization of grains and elimination of liquid segregation.Those microstructural characteristics are related to different motion mechanisms of solid and liquid phases at different forging temperatures.Additionally,the effect of semi-solid forging on the eutectic carbides was also investigated,and the results demonstrate that the higher diffusion capacity and less liquid segregation jointly lower the large eutectic carbides and consequently cause its uniform distribution during FSM.

Ultra-Fine Heterogeneous Microstructure Enables High Strength-Ductility in a Cold-Rolled Medium Mn Steel

The objective of the present study is to develop heterogeneous microstructure in cold-rolled medium Mn steels(MMSs)annealing strategy.The cold-rolled Fe-4.7Mn-0.15C(wt%)steel is annealed twice at different temperatures to produce an ultra-fine heterogeneous microstructure with lath-shaped and granular-shaped retained austenite.Excellent mechanical behavior of significant strength enhancement with negligible ductility loss can be achieved.The high strength-ductility properties are attributed to the active transformation induced plasticity effect over a broad strain range owing to dispersive mechanical stabilities of the heterogeneous austenite.Furthermore,the typical yield point elongation phenomenon which is commonly observed in cold-rolled MMSs can be effectively reduced by this microstructural strategy.

Dynamic recrystallization behavior and interfacial bonding mechanism of 14Cr ferrite steel during hot deformation bonding

In this study, hot compression bonding was first applied to join 14Cr ferrite steel at temperatures of 950–1200°C and strains of 0.11–0.51 under strain rates of 0.01–30 s^(-1).Subsequently, tensile tests were performed on the joints to evaluate the reliability of the joints formed.Detailed microstructural analyses suggest that two different competing dynamic recrystallization(DRX) mechanisms occur during the bonding process depending on the strain rate, and the joints obtained at different strain rate exhibits distinct healing effect.At a low strain rate(0.01 s^(-1)), continuous DRX occurs, as expected in high-stackingfault-energy materials, and is characterized by the progressive conversion of the sub-boundaries into larger-angle boundaries, which involves very limited grain boundaries migration.In addition, straininduced precipitation(SIP) is sufficient under this condition, further impeding the healing of bonding interface.Hence, the joints obtained at low strain rate fractured at the bonding interface easily.Whereas discontinuous DRX is activated at high strain rates(10 and 30 s^(-1)).Under this condition, the formation of sub-boundaries is severely suppressed, resulting in the piling-up of dislocations and hence the storage of a greater amount of stored energy for nucleation and subsequent nuclei growth via the long-distance grain boundaries migration.Meanwhile, the SIP process is sluggish, making the conditions much more favorable for grain boundaries migration which plays a key role in the healing of the original bonding interface.Thus, the joints can be successfully bonded when a high strain rate is applied, with the joints exhibiting tensile properties similar to that of the base material.

Cellular automaton modeling of austenite formation from ferrite plus pearlite microstructures during intercritical annealing of a C-Mn steel

A mesoscopic cellular automaton model was developed to study the microstructure evolution and solute redistribution of austenization during intercritical annealing of a C-Mn steel. This model enables a depiction of three-stage kinetics of the transformation combined with the thermodynamic analysis:(1) the rapid austenite growth accompanied with pearlite degeneration until the pearlite dissolves completely;(2) the slower austenite growth into ferrite with a rate limiting factor of carbon diffusion in austenite;and(3) the slow austenite growth in control of the manganese diffusion until the final equilibrium reached for ferrite and austenite. The effect of the annealing temperature on the transformation kinetics and solute partition is also quantitatively rationalized using this model.