Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated.One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining.Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel.The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.

Simultaneous refinement of α-Mg grains and β-Mg_(17)Al_(12) in Mg-Al based alloys via heterogeneous nucleation on Al_(8)Mn_(4)Sm

Due to the significant differences in the formation temperature and crystal structure between the primaryα-Mg and eutecticβ-Mg_(17)Al_(12),it is a great challenge to achieve simultaneous refinement of the primary and eutectic phases in Mg-Al based alloys via heterogeneous nucleation.Surprisingly,we found that theα-Mg andβ-Mg_(17)Al_(12) in the AZ80 alloy can be simultaneously refined after 0.2 wt.%Sm addition,with the grain size decreasing from∼217±15μm to∼170±10μm and theβ-Mg_(17)Al_(12) morphology changing from a typical continuous network to a nod-like or spherical structure.The simultaneous refinement mechanism is investigated through solidification simulation,transmission electron microscopy(TEM),and differential thermal analysis(DTA).In the AZ80-0.2Sm alloy,many Al8Mn4Sm particles can be observed near the center of theα-Mg grains or inside theβ-Mg_(17)Al_(12).Crystallographic calculations further reveal that the Al8Mn4Sm has good crystallographic matching with both theα-Mg andβ-Mg_(17)Al_(12),so it possesses the potency to serve as heterogeneous nucleation sites for both phases.The promoted heterogeneous nucleation on the Al8Mn4Sm decreases the undercooling required by the nucleation of the primary and eutectic phases,which enhances the heterogeneous nucleation rate,thus causing the simultaneous refinement of theα-Mg andβ-Mg_(17)Al_(12).The orientation relationships between the Al8Mn4Sm and Mg/Mg_(17)Al_(12) are identified,which are[1210]_(Mg)//[010]_(Al8Mn4Sm),(1010)_(Mg)//(301)_(Al8Mn4Sm) and[112]_(Mg_(17)Al_(12))//[010]_(Al8Mn4Sm),(110)_(Mg_(17)Al_(12))//(301)_(Al8Mn4Sm),respectively.Furthermore,the refinement of theβ-Mg_(17)Al_(12) accelerates its dissolution during the solution treatment,which is beneficial for cost saving in industrial applications.Other Al8Mn4RE compounds such as Al8Mn4Y might have the same positive effect on the simultaneous refinement due to the similar physicochemical properties of rare earth elements.This work not only proves the possibility of simultaneously refining the primary and eutectic phases in Mg-Al based alloys via heterogeneous nucleation,but also provides new insights into the development of refiners for cast Mg alloys.

Effects of on-line solution and off-line heat treatment on microstructure and hardness of die-cast AZ91D alloy

The effects of on-line solution, off-line solution and aging heat treatment on the microstructure and hardness of the die-cast AZ91D alloys were investigated. Brinell hardness of die-cast AZ91D alloy increases through on-line solution and off-line aging treatment but decreases after off-line solution treatment. By X-ray diffractometry, optical microscopy, differential thermal analysis, scanning electron microscopy and X-ray energy dispersive spectroscopy, it is found that the microstructures of the die-cast AZ91D magnesium alloy before and after on-line solution and off-line aging are similar, consisting of α-Mg and β-Al12Mg17. The precipitation of Al element is prevented by on-line solution so that the effect of solid solution strengthening is enhanced. The β-Al12Mg17 phases precipitate from supersaturated Mg solid solution after off-line aging treatment, and lead to microstructure refinement of AZ91D alloy, so the effect of precipitation hardening is enhanced. The β-Al12Mg17 phases dissolve in the substructure after off-line solution treatment, which leads to that the grain boundary strengthening phase is reduced significantly and the hardness of die cast AZ91D is reduced.

Study of the intrinsic mechanisms of nickel additive for grain refinement and strength enhancement of laser aided additively manufactured Ti–6Al–4V

It is well-known that grain refiners can tailor the microstructure and enhance the mechanical properties of titanium alloys fabricated by additive manufacturing(AM). However, the intrinsic mechanisms of Ni addition on AM-built Ti–6Al–4V alloy is not well established. This limits its industrial applications. This work systematically investigated the influence of Ni additive on Ti–6Al–4V alloy fabricated by laser aided additive manufacturing(LAAM). The results showed that Ni addition yields three key effects on the microstructural evolution of LAAM-built Ti–6Al–4V alloy.(a) Ni additive remarkably refines the prior-β grains, which is due to the widened solidification range. As the Ni addition increased from 0 to 2.5 wt. %, the major-axis length and aspect ratio of the prior-β grains reduced from over 1500 μm and 7 to 97.7 μm and1.46, respectively.(b) Ni additive can discernibly induce the formation of globular α phase,which is attributed to the enhanced concentration gradient between the β and α phases. This is the driving force of globularization according to the termination mass transfer theory. The aspect ratio of the α laths decreased from 4.14 to 2.79 as the Ni addition increased from 0 to2.5 wt. %.(c) Ni as a well-known β-stabilizer and it can remarkably increase the volume fraction of β phase. Room-temperature tensile results demonstrated an increase in mechanical strength and an almost linearly decreasing elongation with increasing Ni addition. A modified mathematical model was used to quantitatively analyze the strengthening mechanism. It was evident from the results that the α lath phase and the solid solutes contribute the most to the overall yield strength of the LAAM-built Ti–6Al–4V–x Ni alloys in this work. Furthermore, the decrease in elongation with increasing Ni addition is due to the deterioration in deformability of the β phase caused by a large amount of solid-solution Ni atoms. These findings can accelerate the development of additively manufactured titanium alloys.

Numerical and physical simulation of new SPD method combining extrusion and equal channel angular pressing for AZ31 magnesium alloy

A new serve plastic deformation(SPD) including initial forward extrusion and subsequent shearing process(ES) was proposed.The influence of the ES forming on the grain refinement of the microstructure was researched.The components of ES forming die were manufactured and installed to Gleeble1500D thermo-mechanical simulator.The microstructure observations were carried out on the as-extruded rods(as-received) and ES formed rods.From the simulation results,ES forming can increase the cumulative strain enormously and the volume fraction of dynamic recrystallization.From the physical modeling results,the microstructures can be refined.

IMPROVEMENT IN TENSILE PROPERTIES OF α+β TYPE Ti ALLOYS BY HYDROGEN TREATMENT

A new process to refine the microstructure of α+βtvpe Ti alloys was developed by using hydrogen as a temporary alloying element.It involves hydrogenating the alloys below the hydrogenated β transus temperature about 0-40 K,air cooling to room temperature and then dehydrogenating at 948 K.Ti-6AI-4V and Ti-5AI-2.5Fe alloys treated hy this new process show much refined microstructures,and the yield strength,ultimate strength as well as the elongation increase 8—15,5—13 and 7—14% respectively

Effects of solution treatment and aging process on microstructure refining of semi-solid slurry of wrought aluminum alloy 7A09

A new method was exploited using solution treatment and aging process as a pretreatment in preparing semi-solid slurry with fine microstructure before isothermal treatment of wrought aluminum alloy 7A09.Parameters of pretreatment were optimized by orthogonal experiment design and proper precursor was prepared.The evolution of microstructure of semi-solid slurry during isothermal treatment was analyzed and the mechanism of microstructure refining was discussed.The result of orthogonal experiment design shows that the optimum parameters are 462 ℃for solution temperature,40min for solution time,132 ℃for aging temperature and 14 h for aging time.Microstructure of isothermal treatment is fine,homogenous,with globular solid grains and a solid fraction between 50%and 70%,which is qualified for later semi-solid forming process.Mechanism of microstructure evolution includes the agglomeration ofα-phase and Ostwald ripening.Precipitations prepared by solution and aging treatment prevent the grains from coarsening and promote the grain ripening to globular shape.

DEFORMATION ENHANCED FERRITE TRANSFORMATION IN PLAIN LOW CARBON STEEL

The microstructure evolution during deformation enhanced transformation of undercooled austenite of a plain low carbon steel has been investigated by means of hot compression simulation experiment under various conditions of strain rate, deformation temperature and strain. The effect of austenite grain size on the strain enhanced ferrite transformation has been studied. The ferrite dynamic recrystallization involved in successive hot deformation has been explored.

Optimization of RPC technique for refining the intermediate transformation microstructure

The influence of processing parameters ofrelaxation-precipitation-controlling phase transformation (RPC) technique, finish rollingtemperature, reduction ratio and relaxing time on the microstructure was studied bythermo-simulation for a low carbon Nb and Ti containing micro-alloyed steel. The microstructure wasinvestigated by optical microscope, transmission electron microscope and electron back scatterdiffraction (EBSD). The statistical results of the packet size were calculated. It shows that, afterRPC process, the steel is a composite microstructure of bainite and matensite. The bestthermo-simulation process for refinement in this experiment is deformation for 30 percent at 850 degC, and then relaxing at this temperature for 60 s to 200 s. Increasing the reduction ratio from 30percent to 60 or decreasing the deformation temperature to 800 deg C would cause the best relaxationtime to become shorter, increasing the deformation temperature to 900 deg C would cause therefinement effect to be weak.

Influence of electromagnetic stirring on microstructure of AZ91-0.8%Ce magnesium alloy

Effect of electromagnetic stirring on microstructure of AZ91-0.8%Ce magnesium alloy was investigated. The results show that electromagnetic stirring causes a change of morphology of α-Mg phase from coarse dendrites to fine rosette-like or spherical shape. Grain size is significantly refined within the range of input voltage 75?125 V, moreover, the optimum input voltage corresponded to the minimum value (64 μm) of grain size is 125 V. Compared to the non-stirred condition, the amount of β-Mg17Al12 precipitate under the stirred condition obviously increases. The grain refinement of AZ91-0.8%Ce alloy is mainly attributed to multiplication of existing grains in the melt caused by forced fluid flow under electromagnetic stirring condition. Addition of 0.8% Ce results in the formation of ‘necking’ at secondary dendrite arm roots of α-Mg crystals, and consequently, it is helpful to increase the number of heterogeneous nucleation.

Physical simulation of hot deformation of TiAl based alloy

In order to establish a model between the grain size and the process parameters, the hot deformation behaviors of Ti 49.5Al alloy was investigated by isothermal compressive tests at temperatures ranging from 800 to 1?100 ℃ with strain rates of 10 -3 10 -1 s -1 . Within this range, the deformation behavior obeys the power law relationship, which can be described using the kinetic rate equation. The stress exponent, n , has a value of about 5.0, and the apparent activation energy is about 320 J/mol, which fits well with the value estimated in previous investigations. The results show that, the dependence of flow stress on the recrystallized grain size can be expressed by the equation: σ=K 1d rex -0 56 . The relationship between the deformed microstructure and the process control parameter can be expressed by the formula: lg d rex =-0 281?1gZ +3 908?1.

Simulation of electromagnetic-flow fields in Mg melt under pulsed magnetic field

The effects of a pulsed magnetic field on the solidified microstructure of pure Mg were investigated.The results show that microstructure of pure Mg is considerably refined via columnar-to-equiaxed growth under the pulsed magnetic field and the average grain size is refined to 260？？ under the optimal processing conditions.A mathematical model was built to describe the interaction of the electromagnetic-flow fields during solidification with ANSYS software.The pulsed electric circuit was first solved and then it is substituted into the magnetic field model.The fluid flow model was solved with the acquired electromagnetic force.The effects of pulse voltage frequency on the current wave and on the distribution of magnetic and flow fields were numerically studied.The pulsed magnetic field increases melt convection,which stirs and fractures the dendritic arms into pieces.These broken pieces are transported into the bulk liquid by the liquid flow and act as nuclei to enhance grain refinement.The Joule heat effect produced by the electric current also participates in the microstructural refinement.

Refinement Role of Electromagnetic Stirring and Calcium in AZ91 Magnesium Alloy

Effects of calcium addition and electromagnetic stirring on the microstructure of AZ91 magnesium alloy and refinement mechanism were investigated. The results show that calcium addition ranging from 0.1wt% to 0.3 wt% does not lead to formation of any new phases but cause the refinement of ascast microstructure.However, combined calcium alloying and electromagnetic stirring significantly decrease the grain size, change the morphologies of the β-Mg17Al12 phases,and reduce their volume percentage. The minimum grain size of AZ91 alloy is obtained in the case of the addition of 0.2 wt%Ca with exciting voltage of 100 V. The microstructural refinement is attributed to the increase of the degree of undercooling and nucleation temperature of primary α-Mg phases on the basis of DTA analysis results.

The Role of Calcium in Microstructural Refinement of AZ91 Magnesium Alloy

The effect of calcium addition on the microstructures of AZ91 magnesium alloy was investigated. It was found that a small amount of calcium in AZ91 allay produced a large decrease in the α- Mg grain size and the dispersed fine β-Mgt7 Alt2 phases. In addition, some Al4Ca particles were found to exist in the AZ91 alloy containing 0.5wt% Ca. EDS analysis and water-quenched technique revealed that the grain-refining mechanism of calcium for the AZ91 alloys was mainly attributed to the role of restricting growth of calcium in the primary α-Mg crystals.

Mechanical properties of fine-grained dual phase low-carbon steels based on dynamic transformation

The fine grained dual phase （FG-DP） steel with ferrite grains of 2-4.5 μm and martensite islands smaller than 3 μm was obtained through the mechanism of deformation-enhanced ferrite transformation （DEFT）. Mechanical properties of the steel were tested at room temperature. The results indicated that with a similar volume fraction of martensite （about 20vol%）,FG-DP steel exhibited a superior combination of higher strength and more rapid strain hardening at low strains compared with the coarse-grained dual phase （CG-DP） steel obtained by critical annealing. The combination of higher strength,large elongation,and more rapid strain hardening of FG-DP steel can be attributed to the fine ferrite grain and finely dispersed martensite islands. In addition,the uniformly distributed martensite islands in FG-DP steel have smaller interspacing compared with that of CG-DP steel. So,at the initial plastic deformation stage,the plastic deformation of ferrite was restrained and more pronounced load was transferred from ferrite to martensite. The plastic deformation of martensite in FG-DP steel started earlier.

Structure Transition Mechanism in Undercooled CuNi Alloys

Taking a Cu55Ni45 alloy as experimental alloy systems,we systematically studied the method of obtaining deep undercooling of alloy melt.Stable deep undercooling of alloy melt was obtained by the combination of molten glass purification and cyclic overheating.Combined with the nucleation and growth mechanism of undercooled melt,the microstructure evolution and grain refinement mechanism of the alloy were systematically studied in a wide undercooling range.The grain refinement solidification structure under large undercooling was analyzed by EBSD technology.Combined with the typical characteristics of recrystallization in metallographic pictures,it was finally confirmed that the grain refinement was caused by recrystallization.

Research on the microstructural refinement of sulfur in medium carbon microalloyed steel

Continuous rolling techniques were used in this study to produce steel with a medium level of carbon but a high level of sulfur. Controlled rolling techniques were used to control the shape of sulfide. The effect of sulfide on the microstructure of steel was also investigated in this study. The size and distribution of sulfide are determined through the use of an image analyzer and the statistics of the products ＆ service solution （SPSS） software. A comparison between the microstructures of normalized and quenched steel reveals that sulfide can pin the boundaries of the austenite grain boundary and refine the microstructure of the austenite grain. It also reveals that the size relationship between sulfide and austenite grain satisfies the Zener equation.

Effect of Extrusion Ratio on the Microstructure and Mechanical Properties in an Al-Cu-Mg-Ag Alloy

In order to examine the effect of extrusion ratio on the microstructure and mechanical behavior in Al-Cu-Mg-Ag alloy, the Al-6.3 Cu-0.48 Mg-0.4 Ag alloy was subjected to extruding with different extrusion ratios of 17, 30 and 67. The results indicate that the grains are refined and the strength is improved effectively with increasing extrusion ratio. However, further investigation shows that the extrusion ratio of 30 is more effective than the lower extrusion ratio（17） and the higher extrusion ratio（67） to refine the grains in the T6-temper alloy. Moreover, the sample with an extrusion ratio of 30 obtains more precipitates and superior mechanical properties after T6 treatment. This study supports the idea that there exists a critical extrusion ratio for grain refinement and improvement of mechanical properties for the T6-temper alloy. Recrystallization and precipitation during T6 treatment were introduced to explain the effects of extrusion ratio on the microstructure and mechanical properties of the Al-Cu-Mg-Ag alloys.

PPB structure elimination,DRX nucleation mechanisms and grain growth behavior of the3rd-generation PM superalloy for manufacturing aviation components

Previous Particle Boundary(PPB),as the detrimental structure in Powder Metallurgy(PM)components,should be eliminated by subsequent hot process to improve the mechanical properties.The objective is to investigate the Dynamic Recrystallization(DRX)nucleation mechanisms and grain growth behavior of the 3rd-generation PM superalloy with PPB structure.Microstructure observation reveals that PPB decorated with(Ti,Ta,Nb)C carbides belongs to the discontinuous chain-like structure.The elimination of PPB networks can be achieved effectively via hot deformation due to the occurrence of DRX.Four different DRX nucleation mechanisms were proposed and discussed in detail according to the special microstructure characteristics of the PM superalloy.Firstly,local lattice rotations can be detected in the vicinity of(Ti,Ta,Nb)C carbides during hot deformation and thus PPB structure serves as the preferential nucleation sites for DRX grains via Particle-Stimulated Nucleation(PSN).Then,Discontinuous-DRX(DDRX)characterized by grain boundary bulging dominates the microstructure refinement and Continuous-DRX(CDRX)operated by subgrain rotation can be regarded as an important assistant mechanism.At last,the initial Σ3 boundaries would lose their twin characteristics owing to the crystal rotation and then transform into the general High Angle Grain Boundaries(HAGBs).The distorted twins provide additional DRX nucleation sites,viz.,twin-assisted nucleation.Particular attention was focused on the grain growth behavior of the PM superalloy in subsequent annealing process.The recrystallization temperature was determined to be about 1110.C and 1140.C can be considered as the critical temperature for grain growth.The findings would provide theoretical support for microstructure refinement of the 3rd-generation PM superalloy,which is of pivotal significance for improving the mechanical properties of aviation components.

Microstructures and tensile properties of Mg-Zn-(Gd)-Zr alloys extruded at various temperatures

The microstructures and tensile properties of Mg-6.0 Zn-0.6 Zr(ZK60) and ZK60 + 1.8 Gd(ZEK620(RE = Gd)) alloys extruded at 290, 305, 320, 340 and360 ℃ were investigated. The results indicate that Gd addition can refine the microstructures of ZK60 alloy and the extrusion parameters such as the extrusion temperature will influence the tensile properties of both as-extruded ZK60 and ZEK620 alloys, although it is much slighter for ZK60 alloy. And the sensitivity is closely related to the textures developed during extrusion. In addition, Gd addition will also clearly improve the tensile strength at room temperature(24 ℃) and particularly at high temperatures(150 and 200 ℃), due to the much finer grains and smaller intermetallic particles. The highest strength is achieved in the as-extruded ZEK620 alloy with extrusion temperature of 320 ℃, with ultimate tensile strength(UTS) and tensile yield strength(TYS) of 405 and375 MPa, respectively, which are improved by approximately 9% and 11% than those of the as-extruded ZK60 alloy which owns the highest strength at extrusion temperature of 290 ℃.