Microstructural evolution of a Ti-4.5Al-3Mo-1V alloy during hot working

The study investigated the variation of microstructure of a Ti-4.5AL-3Mo-1V alloy during hot compressing in α + β phase field. By scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we systematically examined the influences of hot working parameters on the microstructural features including of the morphology of α grain, volume fraction of α phase and dynamic recrystallization (DRX). The experimental evidence showed that both α and β phases underwent DRX under the experimental conditions. Moreover, the recrystallization degree of α phase was more sufficient than that of β phase. The DRX was accelerated with increase of deformation temperature and decrease of strain rate. It was also noted that a certain degree of the (α → β) phase transformation occurred concurrently with the mechanical deformation. In accordance with the microstructural evolution, flow curves of the alloy were characterized by a linear increase until regular oscillations at high strain rates or single peak at low strain rates and softening was followed.     

Metadynamic recrystallization behavior of AZ61 magnesium alloy

Metadynamic recrystallization (MDRX) behavior of AZ61 magnesium alloy and its effects on flow behavior and microstructure evolution have been investigated in this study. Towards this end, a set of double-hit hot compression tests was conducted under strain rate of 0.1 s⁻¹ at 400 °C. To differentiate the static and metadynamic recrystallization dominant strain regions, the first stage of deformation was carried out up to the different pre-strains with a constant inter-pass annealing time of 200 s. The results indicated that the MDRX is predominant recrystallization mechanism where the pre-strains are higher than 0.35. Furthermore, to investigate the influence of MDRX on subsequent flow behavior and the related microstructure, an elaborated inter-pass annealing treatment was executed employing a range of inter-pass annealing time (2–500 s). The results show that the progress of MDRX leads to an increase in the flow stress as well as the rate of work hardening encountered in the subsequent deformation. Additionally, the microstructural examinations confirm that the observed hardening phenomenon is a consequence of grain growth evolved from MDRX and its direct effect on the onset of dynamic recrystallization at the second stage of deformation.     

Metadynamic recrystallization of 300M steel after isothermal compression

Isothermal compression tests of 300M steel were performed on a Gleeble-1500 thermo-mechanical simulator at deformation temperatures ranging from 1173 to 1373 K, strain rates ranging from 0.1 to 5.0 s^?1, and a strain of 0.69. Metadynamic recrystallization and grain growth after complete metadynamic recrystallization were investigated by isothermal compression with different inter-pass times. It was found that the inter-pass time, deformation temperature and strain rate markedly affected the austenite grains size of metadynamic recrystallization. The austenite grain size model and grain growth model of metadynamic recrystallization were determined based on the results of quantitative grain size. A good agreement between the predicted and measured austenite grain size and grain growth of metadynamic recrystallization was obtained, and the present models were effective to predict the austenite grain size and grain growth of metadynamic recrystallization in the isothermal compression of 300M steel.     

Microstructure evolution of the transitional region in isothermal local loading of TA15 titanium alloy

The microstructure of the transitional region determines the performance of TA15 component under isothermal local loading forming. To understand the characteristic of microstructure evolution in transitional region, a set of analogue experiments for isothermal local loading were designed and carried out to investigate the microstructure development under different temperatures and complex strain path. It is found that the isothermal local loading does not change the microstructural composition of TA15 alloy with initially equiaxed microstructure in the transitional region, though a small fraction of lamellar α phases appear at temperatures above 950 °C due to reheating and small deformation in the second loading step. Recrystallization takes place in the β matrix but not in the primary equiaxed α phases. The β grains are fine and equiaxed irrespective of strain path due to recrystallization. The existence of lamellar α refines the β grains. Kinked and disordered primary equiaxed α phases are produced under complex strain path. The grain size of primary equiaxed α phases increases slightly due to static coarsening and strain induced grain growth in multi-heat processing. The volume fraction of primary equiaxed α phases is not influenced by strain path but determined by working temperature.     

Effects of holding time and Zener-Hollomon parameters on deformation behavior of cast Mg-8Gd-3Y alloy during double-pass hot compression

Double-pass hot compression tests were carried out over a wide range of holding time (0–180?s) and Zener-Hollomon parameter (1.6E15–1.3E20) to study the deformation behavior of cast Mg-8Gd-3Y alloy. The flow curves show obvious work hardening and strain softening stages, leading to the peak stress of double-pass hot compression. Holding time and Zener-Hollomon parameter can significantly affect the second pass peak stress. It is found that increasing the holding time can cause a higher peak stress in the second pass deformation. The second pass stress reaches the peak stress of 71?MPa at Zener-Hollomom parameter of 1.6E15. When the parameter rises to 1.3E20, the second pass peak goes up to 237?MPa. In addition, the second pass peak stress is significantly higher than the unloading stress, which is opposite to the flow behavior of aluminum alloys. Residual stored deformation energy caused by the first pass deformation could be consumed by metadynamic recrystallization. Therefore, more strain energy is required for subsequent dynamic recrystallization, resulting in hardening behavior. A hardening fraction is defined to describe the deformation behavior quantitatively, which shows a positive correlation with the metadynamic recrystallization fraction. The metadynamic recrystallization leads to grain growth at the inter pass holding stage, diminishing dynamic recrystallization nucleation positions in the second pass deformation.     

A new method to predict the metadynamic recrystallization behavior in 2124 aluminum alloy

The metadynamic recrystallization behaviors in deformed 2124 aluminum alloy were investigated by isothermal interrupted hot compressive tests, which were carried out at the deformation temperatures of (653–743) K, strain rates of (0.01–10) s^?1 and inter-stage delay time of (30–180) s. A new approach, “peak stress method”, is proposed to calculate the softening fractions induced by the rapid metadynamic recrystallization. The kinetic equations were developed to predict the metadynamic recrystallization behaviors in hot compressed 2124 aluminum alloy. Both the experimental and predicted results show that the effects of deformation parameters, including strain rate, deformation degree and temperature, on the softening behaviors in the two-pass hot compressed 2124 aluminum alloy are significant. A good consistency between the experimental and predicted results indicates that the proposed kinetic equations can precisely estimate the softening behaviors and metadynamic recrystallization kinetics of the hot compressed 2124 aluminum alloy.     

Optimizing the hot-forging process parameters for connecting rods made of PM titanium alloy

In order to optimize the processing parameters of a new low-cost titanium alloy connecting rod made of powder forging, the deformation behavior of an α + β type Ti–1.5Fe–2.25Mo (wt%) alloy produced by elemental powder metallurgy (PM) route was studied using isothermal compression tests. The constitutive equations and a processing map were established to characterize the flow behavior and predict the optimum deformation parameters. The calculated apparent activation energy was 257.73 kJ/mol for deformation in the α + β phase region and 378.01 kJ/mol in the β phase region. Two deformation mechanism domains were found: α + β → β phase transformation and dynamic recrystallization. The results show that the optimum deformation parameters for the present alloy are (700–800 °C, 10^−1.7–1 s⁻¹) and (800–900 °C, 10⁻²–10 s⁻¹). Based on these results, a finite element method (FEM) simulation of the hot-forming of a connecting rod was conducted, and the simulated results have been successfully used in an industrial forging of the connecting rod.     

奥氏体不锈钢热轧加工性能的数学模型研究

钢的热加工性能是钢的热轧工艺设计的基础.奥氏体钢在热加工中涉及到众多的物理现象,如动态回复、动态再结晶、静态回复、亚动态再结晶、静态再结晶和晶粒长大.一个优秀的描述钢的热加工性能的数学模型可以优化热轧工艺,提高生产效率,改善产品质量.综述了奥氏体不锈钢在热加工中发生的各类物理现象及其相对应的数学模型,讨论了变形温度、变形参数与流变应力、再结晶以及再结晶晶粒度之间复杂的关系,并分析了在工业多道次轧制工艺中,如何应用这些数学模型模拟和预测轧钢过程中残余应变和其内部组织的演变过程.     

Mn18Cr18N护环钢静态再结晶组织及模型

在Gleeble-1500D热模拟机上,采用双道次热压缩试验研究Mn18Cr18N护环钢高温变形后不同停留时间内的静态软化行为,分析热变形温度、应变速率、变形程度以及初始奥氏体晶粒对静态再结晶行为的影响.通过应力补偿法计算静态再结晶软化率,并结合金相组织作了修正.建立其静态再结晶动力学模型,获得静态再结晶激活能249.3 k J/mol.研究表明:Mn18Cr18N钢静态再结晶软化曲线呈"S"形,符合Avrami方程.静态再结晶体积分数随着停留时间延长而增加,热变形温度越高,静态再结晶分数越大,而在较低温度和较小变形程度时,孕育时间较长,主要发生静态回复,将静态再结晶动力学模型的预测结果与实测值进行比较,二者吻合较好,为护环钢后续热镦粗工艺模拟提供更为详尽的模型.     

Microstructure characteristic for high temperature deformation of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy

Hot compression tests of a powder metallurgy (P/M) Ti–47Al–2Cr–0.2Mo (at. pct) alloy were carried out on a Gleeble-3500 simulator at the temperatures ranging from 1000 °C to 1150 °C with low strain rates ranging from 1 × 10⁻³ s⁻¹ to 1 s⁻¹. Electron back scattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to investigate the microstructure characteristic and nucleation mechanisms of dynamic recrystallization. The stress–strain curves show the typical characteristic of working hardening and flow softening. The working hardening is attributed to the dislocation movement. The flow softening is attributed to the dynamic recrystallization (DRX). The number of β phase decreases with increasing of deformation temperature and decreasing of strain rate. The ratio of dynamic recrystallization grain increases with the increasing of temperature and decreasing of strain rate. High temperature deformation mechanism of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy mainly refers to twinning, dislocations motion, bending and reorientation of lamellae.     

Prediction of metadynamic softening in a multi-pass hot deformed low alloy steel using artificial neural network

The metadynamic softening behaviors in 42CrMo steel were investigated by isothermal interrupted hot compression tests. Based on the experimental results, an efficient artificial neural network (ANN) model was developed to predict the flow stress and metadynamic softening fractions. The effects of deformation parameters on metadynamic softening behaviors in the hot deformed 42CrMo steel have been investigated by the experimental and predicted results from the developed ANN model. Results show that the effects of deformation parameters, such as strain rate and deformation temperature, on the softening fractions of metadynamic recrystallization are significant. However, the strain (beyond the peak strain) has little influence. A very good correlation between experimental and predicted results indicates that the excellent capability of the developed ANN model to predict the flow stress level and metadynamic softening, the metadynamic recrystallization behaviors were well evidenced.     

Microstructure evolution and alloying elements distribution between the phases in powder near-β titanium alloys during thermo-mechanical processing

In the present study, two powders near-β Ti alloys having a nominal composition of Ti-5Al-5Mo-5V-XCr-1Fe (X = 1–2, wt%) were studied. The alloys were produced via the blended elemental powder metallurgy technique using hydrogenated Ti powder. Microstructure evolution and the distribution of the alloying elements between the phases were investigated after each step of thermo-mechanical processing (TMP). Microstructures were refined through the TMP in both alloys. Porosity was reduced with deformation at 1173 K (900 °C) in the β phase field. The β → α phase transformation occurred during soaking at 1023 K (750 °C) in the α + β phase field. Fragmentation of the continuous grain boundary α occurred because of the 40 % deformation at 1023 K (750 °C). Variation in the concentration of the alloying elements in each phase took place through the diffusion during soaking in the α + β phase field, e.g. exit of β-stabilisers from the α-phase. However, the α phase remained supersaturated with β stabilisers. Deformation had no influence on the distribution of the alloying elements. An addition of 1 % Cr content slightly affects the amount of the α phase formed and β grain size, but it has no noticeable effect on the distribution of the alloying elements between the phases.     

Discontinuous precipitation of β-Ru phase in Ni–18Ru alloys

The precipitation of β-Ru phase upon aging of a solutionized Ni–18 at.% Ru alloy was studied by X-ray diffraction and electron microscopy. It was shown that: the β phase forms by discontinuous lamellar decomposition from nucleation sites on α grain boundaries; the phases exhibit well-defined orientation relationships; and the hardness of the alloy correlates strongly with the extent of decomposition. The implications for the use of heat treatment to control the microstructure and mechanical properties of the alloy are discussed.     

Analysis of work hardening and recrystallization during the hot working of steel using a statistically based internal variable model

A mathematical model has been developed which describes the hot deformation and recrystallization behavior of austenite using a single internal variable: dislocation density. The dislocation density is incorporated into equations describing the rate of recovery and recrystallization. In each case no distinction is made between static and dynamic events, and the model is able to simulate multideformation processes. The model is statistically based and tracks individual populations of the dislocation density during the work-hardening and softening phases. After tuning using available data the model gave an accurate prediction of the stress–strain behavior and the static recrystallization kinetics for C–Mn steels. The model correctly predicted the sensitivity of the post deformation recrystallization behavior to process variables such as strain, strain rate and temperature, even though data for this were not explicitly incorporated in the tuning data set. In particular, the post dynamic recrystallization (generally termed metadynamic recrystallization) was shown to be largely independent of strain and temperature, but a strong function of strain rate, as observed in published experimental work.     

Hot rolling simulations of austenitic stainless steel

The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.     

低碳钢奥氏体再结晶模型的建立

为了描述低碳钢变形过程的组织演化,建立了一套完整的奥氏体动态再结晶、静态再结晶、亚动态再结晶模型.本文利用Gleeble试验机研究不同初始晶粒度、变形温度、应变和应变速率对奥氏体再结晶量和晶粒尺寸变化的影响.流变应力模型考虑了变形条件对模型系数的影响.利用测得的应力-应变曲线及晶粒度由多元非线性回归得出了奥氏体再结晶模型系数,并且由模型计算的峰值应变、稳定应变、硬化区流变应力、再结晶体积分数、晶粒尺寸和实际接近.     

Dynamic and metadynamic recrystallization of Hastelloy X superalloy

Hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹ were performed on Hastelloy X superalloy in order to investigate the kinetics of hot deformation. An Arrhenius-type equation was used to characterize the dependence of the flow stress on deformation temperature and strain rate. The results showed that dynamic recrystallization (DRX) as well as metadynamic recrystallization (MDRX) occurred during hot working. A novel technique has been developed for calculating the DRX kinetics parameters on the basis of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isothermal transformation rate equations. The variation of grain size in the DRX and MDRX regimes correlated with the standard Zener–Hollomon parameter.     

Effect of process parameters on stir zone microstructure in Ti–6Al–4V friction stir welds

The effects of the main process variables on the stir zone microstructure in friction stir welds were investigated for Ti–6Al–4V. Welds were produced by employing varying welding speeds under a constant rotation speed or different rotation speeds at a constant welding speed. The stir zone microstructure was examined by optical microscopy and transmission electron microscopy. It was found that the stir zone microstructure was determined by the parameters controlling temperature and deformation history during the friction stir welding. A bimodal microstructure characterized by primary α and transformed β with lamellar α + β or a full lamellar microstructure composed of basket-weave α + β lamellae could be developed in the stir zone. The microstructural evolution mechanism in the stir zone was discussed.     

Strain effects on microstructure behavior of 7050-H112 aluminum alloy during hot compression

Effects of strain on microstructure behavior of 7050-H112 aluminum alloy was investigated by means of hot compression conducted at 450 °C and strain rate of 1 s⁻¹. The true stress–true strain behavior shows that it appears dynamic soft after acquired peak stress. The microstructure evolutions are mainly characterized by dislocation substructures with low misorientations which increase with deformation at low to medium strains, but decrease at high strain. It is concluded that the main soft mechanism is dynamic recovery with partial dynamic recrystallization at low to moderate strain, and then dynamic recrystallization at high strain. At last, the substructure behavior which is mainly affected by dislocation migrations is discussed in detail. At low deformation, dislocation migration can destroy grain boundaries and their junctions, resulting in formation of low angle boundaries. However, the interactions of dislocations increase with increasing of deformation, leading to a evolution of high-angle boundaries.     

The effect of metastability on room temperature deformation behavior of β and α + β titanium alloys

The deformation behavior of single-phase metastable β-titanium alloys and two-phase α+metastable-β alloys strongly depends on the degree of stability of the β-phase. Recently, it has been shown that the tensile deformation behavior, as well as the creep deformation behavior at low temperatures (<0.25T _m), is strongly influenced by the degree of metastability. For example, the titanium β-alloy Ti–13.0wt%Mn, which has higher stability than the titanium β-alloy Ti–14.8wt%V, deforms by slip only; whereas the latter deforms by slip and twinning. In addition to the mechanical properties, the deformation mechanisms also depend on the degree of metastability. Further, the deformation mechanisms of a given metastable β-alloy depend on whether the β-phase is present by itself as a single-phase alloy, or in the presence of α-phase in the form of a two-phase alloy. For example, it was found that a metastable Ti–V alloy deforms by slip and twinning when it is in the form of a single-phase alloy, but deforms by slip and martensitic transformation when the same metastable β-phase is present in a two-phase α + β alloy. The mechanical properties of the metastable β alloys in turn depend on these deformation mechanisms. These recent developments are reviewed in this article.