AZ61B镁合金热变形动力学的研究

采用等温压缩试验,测定了AZ61B镁合金材料在热变形条件下的流变应力,分析变形条件对流变应力的影响规律,借助于经验公式描述高温变形动力学,计算了合金的热变形激活参数,探明了AZ61B合金试验变形条件下主要的软化机制为动态再结晶.     

Continuous dynamic recrystallization of extruded NiAl polycrystals during the superplastic deformation process

The tensile deformation behavior and the associated mechanism of extruded Ni₅₀Al₅₀ polycrystals were systemically investigated. The superplastic deformation behavior was observed at certain conditions. The deformation microstructures consisted of subgrains, low-angle grains, and high-angle grains, meaning that a continuous dynamic recrystallization (CDRX) process was operating. This finding suggests that the relative balance between comparable work hardening (dislocation glide) and dynamic work softening (CDRX) results in the occurrence of superplastic deformation process.     

Observations of grain boundary sliding during superplastic deformation

Flat test specimens of a 10-micron grain size Al-33 pct Cu alloy were deformed in air over a range of strains at 450 °C (723 K) and at 10^-5 strain per second, and examined by SEM. Direct observations and measurements were also made in a SEM during superplastic deformation of a 5-micron Pb-Sn eutectic alloy at room temperature at a strain rate of about 10^-5 s^-1. Extensive grain boundary sliding was noted. Microcreep curves were obtained between initially adjacent grains and showed that deformation is cyclical in nature and varies by several orders of magnitude locally from point to point. Many surface grains are observed to change neighbors many times, to undergo some rotation and tilt, to show little change in size or shape, and yet to avoid intergranular cracking. Superplastic deformation is a highly heterogeneous process, not unlike that observed in the creep of conventional types of alloys at elevated temperatures, but differing in the extent of plastic deformation free of intergranular cracking, combined with extensive recovery in the form of boundary migration. Formerly a Graduate Student in the Department of Materials Science and Engineering at Massachusetts Institute of Engineering, then Research Manager of SAIL in Ranchi, India     

Control of superplastic deformation rate during uniaxial tensile tests

Precise determination of superplastic flow behavior involves imposing known and controlled strain rate during deformation of these alloys. Examination of tensile specimens after superplastic deformation has revealed variations in strain and strain rate occurring as a function of position and the difficulty of maintaining a constant strain rate during testing. To quantify these strain and strain-rate gradients within the specimens, interrupted tensile tests and tests on gridded tensile specimens were performed. It was observed that more uniform strain and strain rates could be achieved with longer gauge length specimens. While longer gauge lengths make it possible to have better control over the imposed strain rate by minimizing the effects of material flow from the specimen grip regions, it has been realized that for smaller specimen gauge lengths, typically used in most laboratories, a more complex control of crosshead speed (CHS) during a test is essential to characterize superplastic behavior. A mathematical model has been developed in order to gain better insight into this material flow and to provide an improved crosshead control schedule for constant strain-rate testing. The results of this analysis have been validated on a superplastic aluminum-magnesium alloy (5083 Al).     

Effects of ultrasonic vibration on plastic deformation of AZ31 during the tensile process

An investigation on the plastic behavior of AZ31 magnesium alloy under ultrasonic vibration(with a frequency of 15 kHz and a maximum output of 2 kW)during the process of tension at room temperature was conducted to reveal the volume effect of the vibrated plastic deformation of AZ31.The characteristics of mechanical properties and microstructures of AZ31 under routine and vibrated tensile processes with different amplitudes were compared.It is found that ultrasonic vibration has a remarkable influence on the plastic behavior of AZ31 which can be summarized into two opposite aspects: the softening effect which reduces the flow resistance and improves the plasticity,and the hardening effect which decreases the formability.When a lower amplitude or vibration energy is applied to the tensile sample,the softening effect dominates,leading to a decrease of AZ31 deformation resistance with an increase of formability.Under the application of a high-vibrating amplitude,the hardening effect dominates,resulting in the decline of plasticity and brittle fracture of the samples.     

On the mechanism of strain-enhanced grain growth during superplastic deformation

Superplastic deformation is often accompanied by grain growth, the rate of which depends on both strain and strain-rate, and is usually well in excess of that found in the absence of deformation. Two models for this process have been developed. In the first, which is most applicable to single phase materials, we assume that the deformation enhancement of grain growth is due to the damage created at triple junctions by grain boundary sliding. A geometrical model is used to show how the recovery of this damage by boundary migration, enhances the normal grain growth process. A second model, more suitable to microduplex alloys, is based on the postulate of Holm el al. [8, Acta metall.25, 1191 (1971)] that superplastic flow enhances the coarsening of particles which pin grain boundaries. Experimental data for a variety of materials give excellent agreement with these models.     

Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys

A study has been made of cavity growth during superplastic tensile deformation of two microduplex α/β nickel-silvers, one a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy. For cavities with radii of >0.5 /gmm, measured growth rates were found to be in good agreement with values calculated on the assumption that cavity growth was controlled by viscous flow of the matrix. For smaller cavity sizes a diffusional growth mechanism could predominate. Metallography revealed that cavity morphology changed with strain in a manner consistent with diffusion-controlled growth at small sizes, and matrix deformation controlled growth at intermediate and large cavity sizes. Density studies showed that the overall level of cavitation was independent of both strain rate and temperature, and was influenced only by strain.     

AZ31镁合金细丝拉拔工艺的研究

对AZ31镁合金细丝的拉拔工艺进行了实验研究,分析了拉拔速度、单道次变形量对细丝拉拔的影响,也分析了镁合金细丝的退火工艺。实验结果表明,室温条件下的冷拉拔和退火工艺相结合可以实现镁合金细丝的连续拉拔和晶粒细化的目的。     

AZ31镁合金的热变形加工图及其应用

为了确定AZ31镁合金轧制工艺参数,利用Gleeble-3500热模拟试验机进行热压缩试验以测试其热变形行为,并根据动态材料模型理论得到其热加工图.当变形温度为380~400℃、应变速率为3~12 s^-1时,功率耗散效率大于30%,属于动态再结晶峰区;在该区域进行异步轧制变形退火处理后得到平均晶粒直径为2.3μm的细晶组织,抗拉强度为322.7 MPa,延伸率为19.6%.当应变速率大于15 s^-1时,属于流变失稳区,250~300℃低温加工时合金的塑性显著降低,350~400℃高温加工时合金出现混晶组织.     

An analysis of cavitation occurring in near-&gg titanium aluminide during superplastic deformation

In order to understand the cavitation behavior of near-&gg titanium aluminide alloys under superplastic forming conditions, the uniaxial hot-tension behavior of a Ti-45.5Al-2Cr-2Nb (at. pct) rolled sheet material containing a microduplex structure was determined. Three initial microstructures were examined: as-rolled, and two coarser-grained rolled-and-heat-treated conditions (1177 °C/4 h or 1238 °C/ 2 h). The cavitation behavior was analyzed after isothermal constant-strain-rate tests were conducted at temperatures between 900 °C and 1200 °C and strain rates in the range of 10⁻⁴ to 10⁻² s⁻¹. Interrupted tests and strain-to-failure tests were conducted in order to track cavity growth with time. After testing at a given temperature and strain rate, as-rolled specimens developed fewer large-size cavities than heat-treated specimens, possibly due to the finer grain size in the as-rolled material. Cavity growth was found to be plasticity controlled; the largest cavity size and density of cavities increased with increasing strain or strain rate and decreasing temperature. Since the number of finest-sized cavities examined did not decrease with strain, it is believed that continuous cavity nucleation occurred. For all three initial microstructures, the optimum sheet-forming temperature in the regime examined was identified as 1200 °C, at which the lowest cavity growth rates and highest ductilities were observed.     

AZ31镁合金挤出棒锻造变形时的组织与织构的变化

为了解AZ31镁合金自由锻过程中织构的形成和变化,用ODF分析对AZ31镁合金在自由锻时的微曲向流变行为进行了分析;用光学显微镜对其显微组织和晶粒尺寸进行了测定．结果表明：锻造变形形成不完全动态再结晶,造成Mg17Al12相析出和组织不均匀．与锻压面相平行的均以{1217}和{1214}、{0115}面织构为主,并且随形变量变化而相互转换．当形变量为31％时{0115}面织构的ODF强度值达到最大．锻造产生的面织构将增加镁合金的各向异性,不利于改善镁合金的塑性变形能力和力学性能．     

Simulation of cavitation processes in superplastic deformation

Cavitation behavior during superplastic deformation is simulated by developing a three-dimensional model which incorporates the continuous nucleation, plastic growth, and coalescence of cavities. The cavity growth rate is determined by using an empirical relationship between the Poisson’s ratio and the cavity volume fraction, and cavities after coalescence are represented as overlapped spheres. The volumetric cavity growth-rate parameter (2.0 to 2.5) obtained from the simulation is consistent with the range of experimental observation. Comparison of the simulation with a modified Pilling’s model for cavity coalescence shows that the growth rates of the average cavity volume are consistent with each other at small strains, whereas they are higher in the former than in the latter at large strains. This is because multiple coalescence, rather than the pairwise coalescence assumed in the Pilling’s model, becomes predominant at large strains in the simulation. Between the simulation and experiments, close agreement is also found in the cavity-size distribution normalized with a maximum cavity size.     

Relationships between process parameters and temperature field of roll casting for wide sheet made of AZ61 magnesium alloy: Finite element method

In recent years, wide sheet made of AZ61 wrought magnesium alloys has been widely studied and applied in industry. Thin roll-casting technology for the new wrought magnesium alloy can provide acceptable quality wide and thin sheet made of AZ61 magnesium alloy. To study the influences of roll-casting process parameters on temperature field for wide and thin sheet made of AZ61 magnesium alloy plates, some simplification and assumptions have been done by characteristics of magnesium alloy. Two-dimensional FEM model for roll-casting has been established along casting direction. Simulations of temperature fields of the plates have been done by using finite element analysis ANSYS software. A series of researches on the temperature distributions under different process parameters (pouring temperature, heat-transfer coefficients and casting speeds) have been done. The simulation results and the literature about the casting process of the relevant theory are the same. The simulation results show that the process parameters of rapid-casting process for AZ61 magnesium alloy are mutual influenced on the temperature fields of wide sheet made of AZ61 magnesium alloy.     

Effects of calcium,samarium addition on microstructure and mechanical properties of AZ61 magnesium alloy

The microstructure and mechanical properties of AZ61 magnesium alloy with Ca, Sm addition were investigated. The results showed that the addition of 0.5 wt.% Ca reduced the quantity of Mg17Al12 phase, and formed a new Al4 Ca phase which is reticular in AZ61 alloy. With the addition of Ca and Sm, the microstructure was further refined and new Al-Sm rich phases were formed in AZ61 alloy with 0.6 wt.%–1.5 wt.% Sm addition, the TEM analysis confirmed that they were Al2 Sm and Al4 Sm. Tensile tests showed that 1.0 wt.% Sm addition contributed to the formation of the Al2 Sm and Al4 Sm and the improvement in the ambient strength, i.e., an ultimate tensile strength of 327 MPa and an elongation of 10.1%. However, excessive Sm addition led to the coarsening of Al2 Sm and Al4 Sm phases, thus resulted in the decline of strength and plasticity.     

A model for the evolution of grain size distribution during superplastic deformation

In a recent paper, we postulated that a grain size distribution in a polycrystal can result in mixed mode deformation during superplastic flow. Since diffusional flow is strongly grain size-dependent while power-law creep is not, it was inferred that large grains may deform by power-law creep, while concomitantly, the small grains deform by diffusional creep. Here, a first order model for dynamic change in the grain size distribution with strain is developed to explain the shape of the stress-strain curves obtained during superplastic deformation of aluminum alloys. The model is based on the simple assumption that regions deforming by diffusion creep suffer strain induced grain growth, while dislocation creep results in grain refinement. In spite of the approximation, the model correctly predicts the shape of the stress-strain curves. The possible significance of these concepts in classical dynamic recrystallization phenomena is also discussed.     

Simulation of deformation and failure process in bimodal Al alloys

Recent scientific interest in nanostructured materials stems from reports of attractive physical and mechanical properties. It has been proposed that the presence of multiple-length scales in a “nanostructured” matrix can alleviate the problem of low ductility by facilitating dislocation activity. One example of the concept of multiple-length scales is illustrated by a “bimodal” microstructure, e.g., containing a mixture of nanostructured and submicron grains. The present work reports on a numerical study of the tensile deformation and fracture of a nanostructured Al alloy with a bimodal microstructure. In the theoretical framework used in the present study, the elastic-plastic behavior, deformation, and fracture processes are approximated by the Ramberg-Osgood formula and finite-element method, respectively. The numerical results are found to be in reasonable agreement with the experimental behavior.