Effect Of Initial Microstructure Research Articles

Effect of initial microstructure on austenite formation kinetics in high-strength experimental microalloyed steels

Austenite formation kinetics in two high-strength experimental microalloyed steels with different initial microstructures comprising bainite–martensite and ferrite–martensite/austenite microconstituents was studied during continuous heating by dilatometric analysis. Austenite formation occurred in two steps: (1) carbide dissolution and precipitation and (2) transformation of residual ferrite to austenite. Dilatometric analysis was used to determine the critical temperatures of austenite formation and continuous heating transformation diagrams for heating rates ranging from 0.03°C•s−1 to 0.67°C•s−1. The austenite volume fraction was fitted using the Johnson–Mehl–Avrami–Kolmogorov equation to determine the kinetic parameters k and n as functions of the heating rate. Both n and k parameters increased with increasing heating rate, which suggests an increase in the nucleation and growth rates of austenite. The activation energy of austenite formation was determined by the Kissinger method. Two activation energies were associated with each of the two austenite formation steps. In the first step, the austenite growth rate was controlled by carbon diffusion from carbide dissolution and precipitation; in the second step, it was controlled by the dissolution of residual ferrite to austenite.

Effect of initial microstructure on Ti–6Al–4V joint by friction stir welding

Ti–6Al–4V alloy plates with three types of initial microstructures, which have different densities of the high angle boundaries, were joined by friction stir welding (FSW), and the microstructure evolution in the stir zone by the FSW was clarified. Concerning the morphology of the microstructure, all three joints show similar characteristics; the top surface of the SZs exhibited a mixture of the lamellar structure with equiaxed α grains and the area fraction of the equiaxed α grain gradually increased with the increasing distance from the top surface due to the temperature gradient. On the other hand, the mean α grain size of the fully equiaxed α structure area decreased with the increasing initial density of high angle boundaries. This accelerated grain refinement was discussed based on the effect of the inhomogeneous microstructure on the recrystallization.

Thermal Fatigue Evaluation of Pb-Free Solder Joints: Results, Lessons Learned, and Future Trends

Thermal fatigue is a major source of failure of solder joints in surface mount electronic components and it is critically important in high reliability applications such as telecommunication, military, and aeronautics. The electronic packaging industry has seen an increase in the number of Pb-free solder alloy choices beyond the common near-eutectic Sn-Ag-Cu alloys first established as replacements for eutectic SnPb. This paper discusses the results from Pb-free solder joint reliability programs sponsored by two industry consortia. The characteristic life in accelerated thermal cycling is reported for 12 different Pb-free solder alloys and a SnPb control in 9 different accelerated thermal cycling test profiles in terms of the effects of component type, accelerated thermal cycling profile and dwell time. Microstructural analysis on assembled and failed samples was performed to investigate the effect of initial microstructure and its evolution during accelerated thermal cycling test. A significant finding from the study is that the beneficial effect of Ag on accelerated thermal cycling reliability (measured by characteristic lifetime) diminishes as the severity of the accelerated thermal cycling, defined by greater ΔT, higher peak temperature, and longer dwell time increases. The results also indicate that all the Pb-free solders are more reliable in accelerated thermal cycling than the SnPb alloy they have replaced. Suggestions are made for future work, particularly with respect to the continued evolution of alloy development for emerging application requirements and the value of using advanced analytical methods to provide a better understanding of the effect of microstructure and its evolution on accelerated thermal cycling performance.

Effect of initial microstructure on the work hardening behavior of plain eutectoid steel

Work hardening capability and tensile properties of the plain eutectoid steel rods are the key factors in the wire drawing process to fabricate high strength wire rods with a minimum failure. In the present research, the work hardening behavior of eutectoid steel with different initial microstructures of bainite, duplex bainite+pearlite, pearlite, partially spheroidized and spheroidized pearlite was assessed in terms of instantaneous work hardening exponent (n value) and work hardening rate (θ) using room temperature tensile test. The results show an inverse parabolic behavior for variation of instantaneous n value versus true strain, i.e., work hardening exponent initially increases up to a maximum value and then decreases. The bainitic microstructure exhibits the lowest n value, whereas the spheroidized pearlitic one shows the highest. It is shown that the fine pearlitic microstructure containing partially spheroidized regions exhibited the best combination of tensile properties, n value and work hardening rate.

Prediction of hot deformation behavior in Ni-based alloy considering the effect of initial microstructure

High temperature heat treatments were conducted for as-cast N08028 alloy to obtain various microstructures with different amounts of σ-phase, and then hot compression tests were carried out using Gleeble-3500 thermo-mechanical simulator in deformation temperature range from 1100 to 1200°C and strain rate range from 0.01 to 1s−1. For the same initial microstructure, the flow stress was observed to increase with increasing the strain rate and decreasing the deformation temperature, while for the same deformation condition, the flow stress was found to increase with increasing the amount of σ-phase in the initial microstructure. Moreover, dynamic recrystallization was found to be the main dynamic soften mechanism. On this basis, Arrhenius-type constitutive equations and artificial neural network (ANN) model with back-propagation learning algorithm were established to predict hot deformation behavior of the alloy. Furthermore, the parameters of constitutive equations were found to be dependent on the initial microstructure, which was also as one of the inputs for the ANN model. Suitability of the two models was evaluated by comparing the accuracy, correlation coefficient and average absolute relative error, of the prediction. It is concluded that the ANN model is more accurately than the constitutive equations.

Ti-6Al-4V合金のにおける摩擦攪拌接合部に及ぼす母材組織影響

Ti-6Al-4V合金のにおける摩擦攪拌接合部に及ぼす母材組織影響

Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C

The effect of initial microstructure on creep properties at 900°C has been investigated in a solid-solution-strengthened Haynes 230 superalloy. The proprietary direct aging combined with controlled cooling rate led successfully to the transition of serrated grain boundaries. The serrated grain boundaries were found to have plane normal terminated by low-index planes. The lamellar carbides formed by discontinuous precipitation process became more significant in the serrated specimen, as the aging temperature to which the specimen was slow-cooled from 1280°C decreased. The fine intragranular carbides which precipitate during initial stage of creep are responsible for the strong hardening effect by pinning the dislocations in the standard specimen. The serrated grain boundaries with stable planar M23C6 carbides enhanced highly creep ductility due to their high resistance to cavitation cracking. However, the serrated specimens exhibited a higher creep rate, which is attributed to the promotion of intragranular deformation due to the lack of fine intragranular carbides, as well as cavitation at the lamellar carbides. Based on the relationships between microstructural factors and creep behaviors at 900°C, the optimal microstructure is discussed.

Investigation on the Formation of Reversed Austenite in QT Treatment

Microstructures of 9Ni steel quenched at different temperatures were obtained, and a simulated tempering at different temperature was implemented with the DIL850A dilatometer, then the effect of initial microstructure before tempering on the formation of reversed austenite was studied. Results show that the formation of reversed austenite during tempering becomes easier with the decrease of quenching temperature and the increasing of tempering temperature, but the stability may deteriorates with the rapid increasing of reversed austenite in content. Overall, the amount of reversed austenite mainly depends on the tempering temperature, and its formation rate is affected by the quenching temperature because refined quenching microstructure can promote the formation of reversed austenite observably.

Effect of the initial microstructure on the deformation behavior of Ti60 titanium alloy at high temperature processing

Based on the stress–strain data from the isothermal compression test in a wide range of temperature and strain rate, the effect of initial microstructure on the deformation behavior of Ti60 titanium alloy at high temperature processing was analyzed. The beta heat treated specimen with fine acicular Widmanstätten microstructure and the alpha–beta forged specimen with duplex microstructure were tested. The constitutive equation and the processing map were established for this alloy with two different initial microstructures. The results show that the initial acicular and duplex microstructures of Ti60 alloy exhibit different deformation characteristics below beta transus and the similar deformation characteristics above beta transus. With the spheroidization of lamellar phase, the calculated activity energy of the Ti60 alloy with acicular microstructure is much larger than that duplex microstructure, and the processing map also exhibits a big difference. The research provides a basis for determination of the forging process of Ti60 alloy based on the different initial microstructures.

Development of ultrafine-grained microstructure in Al-Cu-Mg alloy through equal-channel angular pressing

Al-Cu-Mg alloys are extensively used for riveting applications in aerospace industries due to their relatively high shear strength coupled with high plasticity. The significant advantage of using V65 aluminum alloy ((Al-4Cu-0.2Mg) for rivet application also stems from its significantly slower natural aging kinetics, which gives operational flexibility to carryout riveting operation even after 4 days of solution heat treatment, in contrast to its equivalent alloy AA2024.Rivets are usually made by cold heading of wire rods. In order to form a defect free rivet head, grain size control in wire rods is essential at each and every stage of processing right from casting onwards upto the final wire drawing stage. Wire drawing is carried out at room temperature to reduce diameter as well as impart good surface finish. In the present study, different microstructures in V65 alloy bars were produced by rolling at different temperatures (room temperature to 523K) and subsequently deformed by equal channel angular pressing (ECAP) at 423K upto an equivalent strain of 7. ECAP was carried out to study the effect of initial microstructure on grain refinement and degree of deformation on the evolution of ultrafine grain structure. The refinement of V65 alloy by ECAP is significantly influenced by Initial microstructure but amount of deformation strongly affects the evolution processes as revealed by optical microscopy and transmission electron microscopy.

Effect of Initial Microstructure on Mg Scroll Forging under Warm Forming Condition

Magnesium (Mg) alloys have attracted considerable interest for various applications, including those in the automotive and electronics industries, because of their high specific strength, excellent thermal conductivity, and recycling capability in comparison with commonly used aluminum alloys and steels for structural materials. One of the core parts in the compressor unit of an automobile is the scroll rotor, which is assembled with two scrolls, a rotating orbit scroll on the fixed scroll, to compress the refrigerant. In this study, we proposed a net-shape forging process and an apparatus with the back-pressure technique for the Mg scroll at a forging temperature of 350°C with an Mg­6Al­1Zn extruded billet. The optimum amount of back-pressure is calibrated by finite element analysis using DEFORM-3D, which enhances the wrap height distribution in the Mg scroll uniformly. To elucidate the effect of the initial texture on the forgeability of the Mg scroll, twin-induced and dynamic recrystallization (DRX)-induced specimens are applied in Mg scroll forging, where the formability of the Mg scroll with the DRXinduced specimen is found to be noticeably improved in terms of the wrap height. [doi:10.2320/matertrans.M2013274]

The Effect of Initial Microstructure on the Dynamic Mechanical Behavior of Titanium Plate at Different Strain Rates

The dynamic mechanical property and microstructure evolution of commercial pure titanium with initial lamellar dislocation boundary structures were studied at different strain rate compression. The experiments were conducted to the cylindrical specimens using Gleeble-3500 thermal mechanical simulation machine at room temperature. With increasing of the strain rate, strain rate strengthening effect was found in the material. New dislocation boundary structures along impact direction were generated which perpendicular to initial dislocation boundary. Then the S bands boundary structure was formed by interaction between new and initial dislocations. It can be supposed that initial dislocation boundary are sheared and kinked by new dislocation slipping.

Effect of Initial Microstructure on Impact Toughness of 1200 MPa-Class High Strength Steel with Ultrafine Elongated Grain Structure

A medium-carbon low-alloy steel was prepared with initial structures of either martensite or bainite. For both initial structures, warm caliber-rolling was conducted at 773 K (500 °C) to obtain ultrafine elongated grain (UFEG) structures with strong 〈110〉//rolling direction (RD) fiber deformation textures. The UFEG structures consisted of spheroidal cementite particles distributed uniformly in a ferrite matrix of a transverse grain size of about 331 and 311 nm in samples with initial martensite and bainite structures, respectively. For both initial structures, the UFEG materials had similar tensile properties, upper shelf energy (145 J), and ductile-to-brittle transition temperatures 98 K (500 °C). Obtaining the martensitic structure requires more rapid cooling than is needed to obtain the bainitic structure and this more rapid cooling promote cracking. As the UFEG structures obtained from initial martensitic and bainitic structures have almost identical properties, but obtaining the bainitic structure does not require a rapid cooling which promotes cracking suggests the use of a bainitic structure in obtaining UFEG structures should be examined further.

Effects of initial microstructure on the microstructural evolution and stretch formability of warm rolled Mg–3Al–1Zn alloy sheets

Microstructural evolution and stretch formability of Mg–3Al–1Zn (AZ31) magnesium alloy sheets warm rolled at 225°C by a single differential speed rolling pass with two different initial microstructures were investigated. A cast ingot and that rolled by one pass at a high temperature of 550°C with average grain sizes of ~470μm and ~19μm, respectively, were used as starting materials. Both warm rolled sheets exhibit deformation microstructure in as-rolled condition and the occurrence of texture randomization during subsequent annealing as a result of static recrystallization. New recrystallized grains with a large orientation spread tend to persist in the original orientations of parent deformed grains and twin hosts. Regarding the nucleation sites at pre-existing grain boundaries and double twins, static recrystallization preferentially occurs at the sides of deformed grains and within double twins with their c-axes inclining toward the rolling direction (RD), resulting in a stable end annealing texture with a basal pole tilting toward the RD. For the sheet warm rolled from a cast ingot, incomplete recrystallization originating from less stored energy in the interiors of large deformed grains results in an insufficient texture weakening and an inhomogenous microstructure. Both sheets possess an improved stretch formability compared with conventional AZ31 alloy sheets due to the weakened textures. Benefiting from a weaker basal texture and a more homogenous microstructure, the sheet rolled at 550°C followed by warm rolling at 225°C exhibits a higher Erichsen value of 8.2 compared with that (6.1) of the sheet warm rolled directly from a cast ingot.

고탄소강의 구상화속도에 미치는 초기 미세조직, 냉간압연 및 온도의 영향

Abstract The spheroidization behavior of cementite in a SK85 high carbon steel was investigated in this study. Fine and coarse pearlite microstructures were obtained by appropriate heat treatments according to the TTT diagram of SK85 high carbon steel. oHot rolled plates of SK85 steel were austenitized at 800 C for 2 hrs and then put directly into a salt bath at either 570 o C or 670 C to obtain a fine pearlite (FP) structure and a coarse pearlite (CP) structure, respectively. Cold rolling was subsequently conducted on those specimens with reduction ratios from 0.2 to 0.4. Spheroidization heat treatments were conducted at the subcritical temperatures of 600 and 720 o C for 1 to 32 hrs to elucidate the effect of initial microstructures, heat treatment temperature, and cold reduction ratios on the cementite spheroidization rate. Spheroidization proceeded with fragmentation of cementite plates, spheroidization of the cementite platelets, and coarsening consecutively. Mechanical fragmentation of cementite by cold rolling expedited the rate of spheroidization. The spheroidization rate of FP was much more rapid than that of CP and the spheriodization rate increased with increases in the cold reduction ratio.

The Effect of the Initial Microstructure on Recrystallization and Austenite Formation in a DP600 Steel

The effects of initial microstructure and thermal cycle on recrystallization, austenite formation, and their interaction were studied for intercritical annealing of a low-carbon steel that is suitable for industrial production of DP600 grade. The initial microstructures included 50 pct cold-rolled ferrite–pearlite, ferrite–bainite–pearlite and martensite. The latter two materials recrystallized at similar rates, while slower recrystallization was observed for ferrite–pearlite. If heating to an intercritical temperature was sufficiently slow, then recrystallization was completed before austenite formation, otherwise austenite formed in a partially recrystallized microstructure. The same trends as for recrystallization were found for the effect of initial microstructure on kinetics of austenite formation. The recrystallization–austenite formation interaction accelerated austenization in all the three starting microstructures by providing additional nucleation sites and enhancing growth rates, and drastically altered morphology and distribution of austenite. In particular, for ferrite–bainite–pearlite and martensite, the recrystallization–austenite formation interaction resulted in substantial microstructural refinement. Recrystallization and austenite formation from a fully recrystallized state were successfully modeled using the Johnson–Mehl–Avrami–Kolmogorov approach.

Effect of initial microstructure on hot workability of 7085 aluminum alloy

The hot workability of 7085 aluminum alloys with different initial microstructures (as-homogenized and as-solution treated) was studied by isothermal compression tests at the deformation temperature ranging from 300 to 450 °C and the strain rate ranging from 0.0001 to 1 s−1. The strain rate sensitivity of the alloy was evaluated and used for establishing the power dissipation maps and instability maps on the basis of the flow stress data. The results show that the efficiency of power dissipation for the as-homogenized alloy is lower than that of the as-solution treated alloy. The deformation parameters of the dynamic recrystallization for the as-homogenized and as-solution treated alloy occur at 400 °C, 0.01 s−1 and 450 °C, 0.001 s−1, respectively. The flow instability region of the as-homogenized alloy is narrower than that of the as-solution treated alloy. These differences of the alloys with two different initial microstructures on the processing maps are mainly related to the dynamic precipitation characteristics.

Influence of initial conditions on the mechanical behavior of ion nitrided AISI 4340 steel

The influence of ion nitriding with surface roughness and initial microstructure on fatigue behavior and mechanical strength of AISI 4340 steel have been investigated under different process parameters including time and temperature. The first group fatigue and tension test specimens were not heat treated before ion nitriding and received in hot-rolled condition while the second group specimens were quenched and tempered. While the surface of the reduced section was being mechanically polished for the fatigue specimens, some specimens in the second group were not mechanically polished before ion nitriding to obtain the third fatigue test specimens group. While the effects of initial microstructure, surface roughness and ion nitriding conditions on fatigue behavior were investigated, the effects of initial microstructure and ion nitriding conditions on mechanical properties were investigated. First improvement in fatigue and mechanical properties were obtained with quenched and tempered. The obtained case depth of QT AISI 4340 steel is thicker than that of the hot-rolled AISI 4340 steel at all ion nitriding conditions and a linear relationship was obtained between fatigue strength of steel and case depth for both steels. Although the subsurface fatigue crack formation is dominant in failure initiation mechanism in ion nitrided steels, surface roughness effect on fatigue strength continued in a decreasing rate.

Cold-rolling and inter-critical annealing of low-carbon steel: Effect of initial microstructure and heating-rate

The effects of starting microstructure and heating rate on the inter-critical annealing treatment have been investigated by 80% cold-rolling of different initial microstructures namely ferrite–pearlite and ferrite–martensite with blocky and fibrous martensitic morphologies, followed by inter-critical annealing treatment using two different heating rates (∼0.5°C/s and ∼300°C/s). Sub-critical annealing has also been carried out to understand the effect of starting structures on cold-rolled and completely recrystallized microstructures. Due to the fine-scale lamellar structure comprised of alternate layers of ferrite and martensite, ferrite-fibrous martensite starting structure showed the finest ferrite grain sizes (3–6μm) and more uniform distribution of martenisitc islands compared to ferrite–pearlite and ferrite-blocky martensite after the inter-critical annealing. Slow heating resulted in a coarser ferrite grain size with more uniform distribution of martensite, compared to rapid heating. Recrystallization–transformation interaction and the avoidance of ferrite grain growth contributed to the finer grain sizes after rapid annealing. Nature and distribution of θ-particles and the austenite islands played an important role in controlling the ferrite recrystallization and grain growth. Finer microstructural constituents offered superior combination of strength, ductility and strain-hardening ability to ferrite-fibrous martensite starting structures after rapid annealing, compared to other structures. Rapidly heated samples showed higher strength than the slowly heated samples.

Modeling Flow Stress and Microstructural Development during Hot Working

A cellular automaton (CA) model was established to predict and control the microstructural evolution and flow stress characteristic during dynamic recrystallization (DRX). Values of model parameters were identified by a flow stress-based inverse analysis method; their variations with deformation conditions were estimated by the least square regression method and then integrated into the CA model. The effect of initial microstructure and deformation conditions on the microstructural evolution and flow stress behavior were investigated. The simulation agrees well with the experiment, which demonstrates the availability of the CA model.