Microstructure refinement of a dual phase titanium alloy by severe room temperature compression

Microstructure refinement of a dual phase titanium alloy, Ti-3AI-4.5V-5Mo, by severe room temperature compression was investigated. Nanocrystalline grains were observed in the sample with 75% reduction, in which the grain sizes of a phase and β phase were approximately 50 and 100 nm. Conversely, the average thicknesses of a phase and β phase in as-received microstructure were measured to be 0.7 and 0.5 μm, respectively. TEM and XRD methods were used to analyze the microstructure and texture changes after severe deformation. Microstructure refinement was deduced to the complex interaction among slip dislocations in the a phase, the complex interaction among slip dislocations and martensites in the β phases. In addition, the interaction between the a phase and the β phase also contributed to the microstructure refinement.

Microstructure and damping properties of LPSO phase dominant Mg-Ni-Y and Mg-Zn-Ni-Y alloys

This work studied the microstructure,mechanical properties and damping properties of Mg_(95.34)Ni_(2)Y_(2.66) and Mg_(95.34)Zn_(1)Ni_(1)Y_(2.66)alloys systematically.The difference in the evolution of the long-period stacked ordered(LPSO)phase in the two alloys during heat treatment was the focus.The morphology of the as-cast Mg_(95.34)Ni_(2)Y_(2.66)presented a disordered network.After heat treatment at 773 K for 2 hours,the eutectic phase was integrated into the matrix,and the LPSO phase maintained the 18R structure.As Zn partially replaced Ni,the crystal grains became rounded in the cast alloy,and lamellar LPSO phases and more solid solution atoms were contained in the matrix after heat treatment of the Mg_(95.34)Zn_(1)Ni_(1)Y_(2.66)alloy.Both Zn and the heat treatment had a significant effect on damping.Obvious dislocation internal friction peaks and grain boundary internal friction peaks were found after temperature-dependent damping of the Mg_(95.34)Ni_(2)Y_(2.66)and Mg_(95.34)Zn_(1)Ni_(1)Y_(2.66)alloys.After heat treatment,the dislocation peak was significantly increased,especially in the alloy Mg_(95.34)Ni_(2)Y_(2).66.The annealed Mg_(95.34)Ni_(2)Y_(2.66)alloy with a rod-shaped LPSO phase exhibited a good damping performance of 0.14 atε=10^(−3),which was due to the difference between the second phase and solid solution atom content.These factors also affected the dynamic modulus of the alloy.The results of this study will help in further development of high-damping magnesium alloys.

Microstructure Characteristics and Possible Phase Evolution of the Coal Gangue-Steel Slag Ceramics Prepared by the Solid-State Reaction Methods

Industrial wastes such as steel slag and coal gangue etc.were chosen as raw materials for preparing ceramic via the conventional solid-state reaction method.With steel slag and coal gangue mixed in various mass ratios,from 100%steel slag to 100%coal gangue at 10%intervals,microstructure and possible phase evolution of the coal gangue-steel slag ceramics were investigated using X-ray powder diffraction,scanning electron microscopy,mercury intrusion porosimetry and Archimedes boiling method.The experimental results suggest that the phase compositions of the as-prepared ceramics could be altered with the increased amount of coal gangue in the ceramics.The anorthite-diopside eutectic can be formed in the ceramics with the mass ratios of steel slag to coal gangue arranged from 8:2 to 2:8,which was responsible for the melting of the steel slag-coal gangue ceramics at relatively high temperature.Further investigations on the microstructure suggested that the addition of the proper amount of steel slag in ceramic compositions was conducive to the pore formation and further contributed to an increment in porosity.

Investigation of Microstructure and Phase Composition of Chromic Oxide, AZS/Cr and High-alumina Refractories after Exposure of Basalt and Aluminaboronsilicate Glasses Melts

The microstructure and phase composition of high-alumina,chromic oxide,and AZS/Cr refractories containing 30%and 60%(by mass)Cr_(2)O_(3) after exposure to aluminaboronsilicate glasses and basalt melts depending on the type of melts and temperature have been studied.The mechanisms of refractory corrosion by the used melts and the factors contributing to the inhibition of corrosion development have been investigated by the method of petrographic analysis.On the basis of obtained results,the use of high-alumina,chromic oxide,and AZS/Cr refractories in the sections of glass furnace linings,experiencing the intensive impact of aluminaboronsilicate glasses and basalt melts,has been confirmed and scientifically substantiated.

Effect of continuous annealing parameters on the mechanical properties and microstructures of a cold rolled dual phase steel

A cold rolled dual phase （DP） steel with the C-Si-Mn alloy system was trial-produced in the laboratory, utilizing a Gleeble-3800 thermal simulator. The effects of continuous annealing parameters on the mechanical properties and microstructures of the DP steel were investigated by mechanical testing and microstructure observation. The results show that soaking between 760 and 820℃ for more than 80 s, rapid cooling at the rate of more than 30℃/s from the quenching temperature between 620 and 680℃, and overaging lower than 300℃ are beneficial for the mechanical properties of DP steels. An appropriate proportion of the two phases is one of the key factors for the favorable properties of DP steels. If the volume fraction of martensite and, thereby, free dislocations are deficient, the tensile strength and n value of DP steels will decrease, whereas, the yield strength will increase. But if the volume fraction of martensite is excessive to make it become a dominant phase, the yield and tensile strength will increase, whereas, the elongation will decrease obviously. When rapid cooling rate is not fast enough, pearlite or cementite will appear, which will degrade the mechanical properties. Even though martensite is sufficient, if it is decomposed in high temperature tempering, the properties will he unsatisfied.

Influence of silicon on the microstructures, mechanical properties and stretch-flangeability of dual phase steels

Uniaxial tension tests and hole-expansion tests were carried out to determine the influence of silicon on the microstructures, mechanical properties, and stretch-flangeability of conventional dual-phase steels. Compared to 0.03wt% silicon, the addition of 1.08wt% silicon induced the formation of finer ferrite grains （6.8μm ） and a higher carbon content of martensite （Cm≈ 0.32wt%）. AS the silicon level increased, the initial strain-hardening rate （n value） and the uniform elongation increased, whereas the yield strength, yield ratio, and stretch-flangeability decreased. The microstructures were observed after hole-expansion tests. The results showed that low carbon content martensite （Cm ≈ 0.19wt%） can easily deform in coordination with ferrite. The relationship between the mechanical properties and stretch-flangeability indicated that the steel with large post-uniform elongation has good stretch-flangeability due to a closer plastic incom- patibility of the ferrite and martensite phases, which can effectively delay the production and decohesion of microvoids.

Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and tensile test. The results show that Si can promote the transformation of austenite （γ） to ferrite （α）, enlarge the （α＋γ） region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite （M/A） constituents, as well as the decomposi- tion of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

Effects of pre-strain and baking parameters on the microstructure and bake-hardening behavior of dual-phase steel

In a typical process, C-Mn steel was annealed at 800℃ for 180 s, and then cooled rapidly to obtain the ferrite-martensite microstructure. After pre-straining, the specimens were baked and the corresponding bake-hardening （BH） values were determined as a function of pre-strain, baking temperature, and baking time. The influences ofpre-strain, baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the dual-phase steel were investigated systematically. It was found that the BH value apparently increased with an increase in pre-strain in the range from 0 to 1%; however, increasing pre-strain from 1% to 8% led to a decrease in the BH value. Furthermore, an increase in baking temperature favored a gradual improvement in the BH value because of the formation of Cottrell atmosphere and the precipitation of carbides in both the ferrite and martensite phases. The BH value reached a maximum of 110 MPa at a baking temperature of 300℃. Moreover, the BH value enhanced significantly with increasing baking time from 10 to 100 min.

Effect of hot-dip galvanizing processes on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel

A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel(DP600) was investigated using optical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength(YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient(n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength(UTS) and elongation(A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties(YS = 362 MPa, UTS = 638 MPa, A_(80) = 24.3%, n = 0.17) was obtained via process A.

Effect of intercritical annealing on microstructure,mechanical properties,and work-hardening behavior of ultrahigh-strength dual-phase steels with different silicon contents

In this study,three kinds of dual-phase(DP) steels were used to investigate the influence of silicon content and intercritical annealing temperature on their microstructures,mechanical properties,and work-hardening behaviors. By adding silicon and matching the critical annealing temperature,a new DP steel(1.0Si and intercritically annealed at 790 ℃) that exhibits an excellent combination of ultrahigh strength and adequate ductility was obtained. Variations in the strength,elongation,and fracture mechanism of the specimens with respect to different intercritical annealing temperatures were correlated to microstructural features. With an increase in the silicon content,there is no significant change in the martensitic band structure or ferrite morphology. At the same annealing temperature,the yield strength and yield strength ratio of the specimens decreased,but at different annealing temperatures,the tensile strength was reduced. The Hollomon analysis results indicate that the workhardening behavior obeys a two-stage work-hardening mechanism. With an increasing intercritical annealing temperature,the "transition strain"shifts to the left,and with an increasing silicon content,the "transition strain"shifts to the right. The surface exhibits ductile fractures characterized by a high density of microvoid dimples. With an increase in the silicon content,the average dimple size on the fracture surface decreases and the plasticity of the material increases.

Microstructure-Properties Correlation of Dual Phase Steels Produced by Controlled Rolling Process

The purpose of this research is to quantify the effects of compositional and processing parameters on the microstruc-ture and properties of dual phase steel produced directly by hot rolling and rapid cooling. Steels with the base composition of 0.1%C, 1.4%Si, and 1.0%Mn with additions of 0.5%Cr to influence hardenability, 0.04%Nb to retard recrystallization in the latter stages of rolling, or 0.02%Ti to inhibit grain growth during and after reheating were investigated. Investigation was made to predict microstructure evolution and to correlate microstructure with processing parameters. The effects of the important microstructure parameters such as ferrite grain size, martensite volume fraction (VM) and morphology (polygonal or fibrous) on the tensile and impact properties are discussed. Multiple linear regression analysis of the ultimate tensile strength has shown that, increasing VM and martensite microhardness and grain refinement of ferrite are the major contributions to increase the strength of the steel. It was found that the dual-phase steel produced by controlled rolling process, with a microstructure which consisted of fine grained ferrite (4 um) and 35%~40% fibrous martensite, presented optimum tensile and impact properties because of enhanced resistance to crack propagation.

Effects of overaging temperature on the microstructure and properties of 600MPa cold-rolled dual-phase steel

C–Mn steels prepared by annealing at 800°C for 120 s and overaging at 250–400°C were subjected to pre-straining（2%） and baking treatments（170°C for 20 min） to measure their bake-hardening（BH_2） values. The effects of overaging temperature on the microstructure, mechanical properties, and BH_2 behavior of 600 MPa cold-rolled dual-phase（DP） steel were investigated by optical microscopy, scanning electron microscopy, and tensile tests. The results indicated that the martensite morphology exhibited less variation when the DP steel was overaged at 250–350°C. However, when the DP steel was overaged at 400°C, numerous non-martensite and carbide particles formed and yield-point elongation was observed in the tensile curve. When the overaging temperature was increased from 250 to 400°C, the yield strength increased from 272 to 317 MPa, the tensile strength decreased from 643 to 574 MPa, and the elongation increased from 27.8% to 30.6%. Furthermore, with an increase in overaging temperature from 250 to 400°C, the BH_2 value initially increases and then decreases. The maximum BH_2 value of 83 MPa was observed for the specimen overaged at 350°C.

Microstructure evolution and mechanical properties of 1000 MPa cold rolled dual-phase steel

The microstructure evolution of 1 000 MPa cold rolled dual-phase (DP) steel at the initial heating stages of the continuous annealing process was analyzed. The effects of different overaging temperatures on the microstructures and mechanical properties of 1 000 MPa cold rolled DP steel were investigated using a Gleeble-3500 thermal/mechanical simulator. The experimental results show that ferrite recovery and recrystallization, pearlite dissolution and austenite nucleation and growth take place in the annealing process of ultra-high strength cold rolled DP steel. When being annealed at 800 ℃ for 80 s, the tensile strength and total elongation of DP steel can reach 1 150 MPa and 13%, respectively. The microstructure of DP steel mainly consists of a mixture of ferrite and martensite. The steel exhibits low yield strength and continuous yielding which is commonly attributed to mobile dislocations introduced during cooling process from the intercritical annealing temperature.

Effect of Holding Time on the Microstructure and Mechanical Properties of Dual-Phase Steel during Intercritical Annealing

Continuous annealing simulation tests were conducted by using a continuous annealing thermomechanical simulator. Holding times of 5, 60, 180, and 480 seconds for an intercritical annealing temperature of 820℃ were adopted to investigate the evolution of the mierostructure and mechanical properties of ferrite-bainite dual-phase steel. The ferrite-bainite dual-phase steel was characterized by high strength and low yield ratio due to the presence of the constituents （polygonal ferrite, bainite, martensite and retained austenite） of the steel microstructure. Specimen 3 exhibits the highest value of A50 （7.67%） and a product of Rm × A50 （10453MPa%） after a 180s holding. This is likely attributed to the presence of a C-enriched retained anstenite in the microstructure. And the effect of martensite islands and carbide precipitate is thought to be able to contribute in strengthening the present steel. It is expected that equilibrium of anstenite fraction would be reached for reasonable intercritical holding period, regardless of the heating temperature. The results suggest that long increasing holding times may not be needed because the major phase of the microstructure does not change very significantly. It is favorable for industrial production of DP steels to shorten holding times. Key words： ferrite-bainite dual-phase steel; holding time; martensite islands; mechanical properties

Microstructure of cast γ-TiAl based alloy solidified from β phase region

The microstructures and phase transformation of Ti-43Al-4Nb alloy in as-cast and heat-treated states were investigated by using optical microscopy, scanning and transmission electron microscopy as well as differential scanning calorimetry. The results show that a fine microstructure of the as-cast alloy can be obtained by solidifying through the β phase. γ grains can nucleate directly from the β phase. The coexistence of β phase and γ phase along primary α grain boundaries contributes to the decrease in the grain size of the as-cast alloy. The phase transformation sequence during solidification of the Ti-43Al-4Nb alloy is suggested as L→L＋β→β→α＋β→α＋βr→α＋γ＋βr→lamellae（α2＋γ）＋γ＋βr. The microstructure of the alloy after heat treatment at 1 250 ℃ for 16 h exhibits a certain coarsening compared with that of the as-cast state. The remnant β phase can be removed by the heat treatment process due to the diffusion of Nb and the non-equilibrium state of β phase.

Microstructure and phase precipitate behavior of Inconel 740H during aging

The microstructure and phase precipitate behavior and their effects on the room temperature hardness and impact toughness of Inconel 740H aged at 750 ℃ for 10000 h were investigated by SEM, TEM and mechanical analysis. The as-received alloy shows a low hardness value of HB 168 and a highest toughness value of 96 J. After an aging treatment at 800 ℃ for 16 h and cooled in air （standard heat-treated condition）, fine γ′ phase particles precipitate within the grains and small carbide particles are located at the grain boundaries. The hardness increases to HB 304 and the impact toughness decreases to 15 J after standard heat treatment. A maximum hardness value of HB 331 is achieved for the alloy aged at 750 ℃ for 300 h. With increasing the aging time from 300 to 10000 h, a decrease of the hardness and toughness is observed along with an enhanced quantity of M23C6 particles and the coarsening of γ′ phase.

A MICROSTRUCTURE-BASED ANALYSIS FOR CYCLIC PLASTICITY OF DUAL-PHASE PEARLITIC RAIL STEELS

Based on the assumption that a representative element of apearlitic steel is an aggregate of numerous spherical pearliticcolonies with randomly distributed orientations, and that each colonyis com- posed of many parallel fine lamellas of ferrite andcementite, a description for the dual-phase pearlitic steel isobtained by making use of a microstructure-based constitutiveequation for a single dual-phase pearlitic colony and the Hill'sself-consistent scheme. The elastoplastic response of dual-phasepearlitic steel BS11 subjected to asymmetrically cyclic loading isanalyzed, and a comparison with the experimental results showssatisfacto- ry agreement. The non-proportional cyclic plasticity ofBS11 is also analyzed, in which stress develops along a semi-circlein a biaxial tension/compression and shear stress plane, as istypically experienced by the sur- face elements in rolling andsliding contact.

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.

Microstructure and phase composition of as-cast Mg-9Er-6Y-xZn-0.6Zr alloys

The microstructure and phase composition of as-cast Mg-9Er-6Y-xZn-0.6Zr （x=1, 2, 3, 4; normal mass fraction in %） alloys were investigated. In low Zn content, aside from the major second phase of Mg24（Er, Y, Zn）5, there are a few lamellar phases that grow parallel with each other from the grain boundaries to the grain interior. With Zn content increasing, the Mg24（Er, Y, Zn）5 phase decreases, but the Mg12Zn（Y, Er） phase and lamellar phases continuously increase. When Zn content reaches 4% （normal mass fraction）, the Mg12Zn（Y, Er） phase mainly exists as large bulks, and some a-Mg grains are thoroughly penetrated by the lamellar phases. Moreover, the crystallography structures of the Mgl2Zn（Y, Er） and Mg24（Er, Y, Zn）5 phases are confirmed as 18R-type long-period stacking ordered structure and body-centred cubic structure, respectively.

Microstructure evolution and phase transformation of traditional cast and spray-formed hypereutectic aluminium-silicon alloys induced by heat treatment

Microstructural evolution and phase transformation induced by different heat treatments of the hypereutectic aluminium-silicon alloy, Al-25Si-5Fe-3Cu （wt%, signed as 3C）, fabricated by traditional cast （TC） and spray forming （SF） processes, were investigated by differential scanning calorimetry （DSC） and scanning electron microscopy （SEM） combined with energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that A17Cu2Fe phase can be formed and transformed in TC- and SF-3C alloys between 802-813 K and 800-815 K, respectively. The transformation from β-Al5FeSi to δ-Al4FeSi2 phase via peritectic reaction can occur at around 858-870 K and 876-890 K in TC- and SF-3C alloys, respectively. The starting precipitation temperature of δ-Al4FeSi2 phase as the dominant Fe-bearing phase in the TC-3C alloy is 997 K and the exothermic peak about the peritectic transformation of δ-Al4FeSi2→β-Al5FeSi is not detected in the present DSC experiments. Also, the mechanisms of the microstructural evolution and phase transformation are discussed.