新的具有应变软化特征的本构模型

基于蠕变方程,针对具有应变软化特征的材料提出了一个新的本构模型,该模型考虑了动态再结晶的软化效应。模型认为,由于变形温度决定了原子的扩散能力和位错移动的驱动力,而应变速率决定位错密度和晶界能的累积速度,因而峰值应力取决于变形温度和应变速率。由于再结晶过程是热激活过程,再结晶体积分数可通过唯象理论模型表示成应变的函数,而由峰值应力和再结晶分数可确定由于动态再结晶软化作用引起的应力的下降,因此可以认为,任意时刻的应力取决于峰值应力和应变。该模型表示了温度、应变速率和应变对应力的影响,适合具有动态再结晶的材料,如结构钢35CrMo、20CrMnTi及镁合金AZ31B,计算表明,新模型的预测值与实验值相一致。     

Textures in nanostructured metals processed by severe plastic deformation

This work demonstrates and analyzes the experimental results of the crystallographic texture formation processes in pure metals with a different type of crystal lattice, which were subjected to severe plastic deformation (SPD) applying various techniques such as high pressure torsion (HPT) and equal-channel angular pressing (ECAP). This article is based on a presentation made in “ICOTOM-13: Textures of Materials” held at Seoul National University, Seoul, Korea, August 26–30, 2002, organized by The Korean Institute of Metals and Materials BK21 Division of Materials Education and Research (Seoul National University) and Texture Control Laboratory (Seoul National University).     

超高强TB8钛合金高温塑性变形流变应力分析与本构方程

在Gleeble-3000热模拟试验机上进行等温恒速率热压试验(变形温度800~950℃,应变速率0.001~1.0 s-1),研究了TB8合金的高温塑性变形流变应力变化规律,建立了一个包含应变量的本构方程。结果表明,流变应力随变形温度的升高和应变速率的降低而减小;当ε·≤0.1 s-1时,TB8合金高温热压流变曲线为动态再结晶型流变曲线;热变形激活能Q、材料常数n、α、及ln A均与变形量有关;所建立的本构关系能较好的反应TB8合金高温低应变速率下的流变特征。     

35CrMoV钢高温塑性变形行为及其本构方程建立

为了研究35CrMoV钢的高温变形行为,借助Gleelble 3800型热模拟试验机,在应变速率为0.01~10 s^-1、变形温度为950~1150℃的条件下进行轴向单道次高温压缩试验,并根据试验结果绘制35CrMoV钢的流动应力-应变曲线。分析研究了变形温度、应变速率对流动应力的影响,计算了变形激活能Q及参数n、A、α的取值。试验结果表明:35CrMoV钢在950~1150℃进行压缩试验时,存在动态再结晶和动态回复两种流动应力-应变关系,当应变速率为0.01和0.1 s^-1时,其流动应力-应变曲线主要表现为动态再结晶型;当应变速率为1和10 s^-1时,其流动应力-应变曲线主要表现为动态回复型。在试验条件下获得35CrMoV钢的平均变形激活能Q为310.433 kJ·mol^-1,建立了用于描述35CrMoV钢流动应力、应变速率和变形温度三者之间关系的本构方程。     

Superstrength of nanostructured metals and alloys produced by severe plastic deformation

Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.     

316LN钢ESR材料热变形行为及高温塑性本构方程

为了研究铸态316LN钢ESR材料的高温变形行为,建立铸态316LN钢ESR材料高温塑性本构方程,采用Gleeble-1500D热模拟试验机对316LN钢进行等温压缩试验,研究了316LN钢ESR材料在变形温度为900~1200℃、应变速率为0.001~1 s~(-1)、最大变形量为55%条件下热变形行为,并测得相应的流动应力-应变曲线。结果表明,在高变形温度、低应变速率的条件下,更有利于动态再结晶的发生。通过对试验数据进行多元线性拟合计算,得到了316LN钢的热变形激活能,建立了316LN钢ESR材料的高温塑性本构方程。     

Microstructure development of steel during severe plastic deformation

The microstructure development during plastic deformation was reviewed for iron and steel which were subjected to cold rolling or mechanical milling (MM) treatment, and the change in strengthening mechanism caused by the severe plastic deformation (SPD) was also discussed in terms of ultra grain refinement behavior. The microstructure of cold-rolled iron is characterized by a typical dislocation cell structure, where the strength can be explained by dislocation strengthening. It was confirmed that the increase in dislocation density by cold working is limited at 10¹⁶m⁻², which means the maximum hardness obtained by dislocation strengthening is HV3.7 GPa. However, the iron is abnormally work-hardened over the maximum dislocation strengthening by SPD of MM because of the ultra grain refinement caused by the SPD. In addition, impurity of carbon plays an important role in such grain refinement: the carbon addition leads to the formation of nano-crystallized structure in iron.     

Accumulative channel-die compression bonding (ACCB): A new severe plastic deformation process to produce bulk nanostructured metals

This paper introduces a new severe plastic deformation process to produce bulk nanostructured metals: accumulative channel-die compression bonding (ACCB). In the ACCB process, which can be applied to thick billets, the procedure of cutting, stacking and compression bonding in a channel-die is repeated to provide an ultrahigh plastic strain. This process was trialed with high purity aluminum. A fully recrystallized aluminum sample was deformed by ACCB at room temperature for up to 10 cycles, corresponding to an equivalent strain of 8.0. The initially coarse grains were subdivided by deformation-induced high-angle boundaries, and the fraction of such high-angle boundaries increased with increasing strain. Several cycles of ACCB led to a quite uniform ultrafine structure dominated by high-angle grain boundaries. The average boundary spacing of the 10-cycles ACCB sample was as small as 690 nm. The maximum ultimate tensile strength of the ACCB samples was 130 MPa after 5 cycles. Further ACCB cycles, however, led to a slight decrease in strength due to enhanced recovery and boundary migration during the deformation process. It has been demonstrated that the ACCB process can be used to produce bulk nanostructured metals of relatively large dimensions. The results suggest that the ACCB process is equivalent to conventional rolling deformation at high strains.     

Single-mechanism rate theory for dynamic strain aging in fcc metals

A full thermal activation rate theory for dynamic strain aging is developed for the case where a single rate dependent strengthening mechanism controls dislocation motion in a material (e.g. solute diffusion). The analysis shows that negative strain-rate sensitivity (SRS) cannot be obtained within such a framework, a conclusion previously reached by Hähner [Hähner P. Mater Sci Eng A 1996;207:208]. However, the SRS can be greatly reduced over a range of strain rates, making the inverse behavior more accessible by other mechanisms. In addition, the aging mechanism naturally gives rise to an instantaneous positive SRS and stress relaxation behavior under strain-rate jump conditions, putting the concepts advanced by McCormick [McCormick PG. Acta Metall 1988;36:3061; Estrin Y, McCormick PG. Acta Metall Mater 1991;39:2977] on a quantitative footing. The results here set the stage for subsequent work wherein consideration of multiple strengthening mechanisms (solute and forest hardening) operating together can predict negative SRS in quantitative agreement with data in Al–Mg alloys.     

A study of the interactive effects of strain,strain rate and temperature in severe plastic deformation of copper

The deformation field in machining was controlled to access a range of deformation parameters—strains of 1–15, strain rates of 10–100,000 s^?1 and temperatures of up to 0.4 T_m—in the severe plastic deformation (SPD) of copper. This range is far wider than has been accessed to date in conventional SPD methods, enabling a study of the interactive effects of the parameters on microstructure and strength properties. Nano-twinning was demonstrated at strain rates as small as 1000 s^?1 at ?196 °C and at strain rates of ?10,000 s^?1 even when the deformation temperature was well above room temperature. Bi-modal grain structures were produced in a single stage of deformation through in situ partial dynamic recrystallization. The SPD conditions for engineering specific microstructures by deformation rate control are presented in the form of maps, both in deformation parameter space and in terms of the Zener–Hollomon parameter.     

Annealing-induced softening of copper and molybdenum for high strain rate deformation

The effect of annealing on the high strain rate deformation properties of copper and molybdenum was studied. Samples were extracted using spark erosion and were annealed under various conditions. High strain rate stress-strain curves at ∼700 s⁻¹ and 1500 s⁻¹ were measured using a split Hopkinson pressure bar. Recrystallization occurred for the copper and molybdenum at annealing temperatures of 300 °C and 1200 °C, respectively, and resulted in a significant softening of the samples compared to their unannealed state. Generally, copper and molybdenum are annealed at much higher temperatures and it is suggested that lower temperature annealing may provide cost savings during the manufacturing processes.     

A viscoplastic constitutive model for cyclic hardening in the dynamic strain aging regime

Negative strain rate sensitivity in certain temperature regimes is considered to be a manifestation of dynamic strain aging. Within the framework of unified state variable theory, viscoplastic constitutive equations incorporating the Mises yield criterion are presented to model negative rate sensitivity of the flow stress and its gradual variation with cyclic loading. The modeling capabilities are essentially accomplished by introducing a rate sensitivity parameter into the evolution law of the back stress. The influence of the drag stress and the isotropic stress as well as the rate sensitivity parameter both on the cyclic hardening and the variation of strain rate sensitivity is systemically investigated. Excellent agreement between experimental results reported in the literature and model simulations is obtained, confirming the efficacy of the proposed constitutive model.     

剧烈塑性变形制备微纳米材料的变形细化机理

对剧烈塑性变形法(SPD)制备微纳米材料变形细化机理进行了总结归纳,着重介绍了位错变形和热机械变形两种机制,详细论述了材料在剧烈塑性变形加工过程中的组织转变特点,同时给出了SPD变形细化机制等研究方向的个人见解。     

On possible mechanisms of nanostructure evolution upon severe plastic deformation of metals and alloys

Nondislocation mechanisms of deformation-induced fragmentation of nanostructures in metals upon plastic deformation are discussed. Conditions under which the refinement of nanograins can effectively occur via deformation twinning and/or deformation-induced phase transformations of a martensitic type are considered. It is shown that for each metal system and each deformation method, there exists a limiting nanostructure with a minimum possible average size of nanocrystallites.     

Formation of single and multiple deformation twins in nanocrystalline fcc metals

Deformation twins are often observed to meet each other to form multi-fold twins in nanostructured face-centered cubic (fcc) metals. Here we propose two types of mechanism for the nucleation and growth of four different single and multiple twins. These mechanisms provide continuous generation of twinning partials for the growth of the twins after nucleation. A relatively high stress or high strain rate is needed to activate these mechanisms, making them more prevalent in nanocrystalline materials than in their coarse-grained counterparts. Experimental observations that support the proposed mechanisms are presented.     

Severe plastic deformation (SPD) processes for metals

Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal. The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony. In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown. Moreover, the combined processes developed recently are also explained. Finally, the applications of the ultra-fine grained (UFG) metals are discussed.