Apparent activation energy and stress exponent in materials with a high Peierls stress

Equations are derived for the apparent stress exponent and activation energy in materials whose deformation is controlled by double kink nucleation and propagation on dislocations. The apparent activation energy (determined from Arrhenius plots of creep rate against inverse temperature) can easily be 10–20% lower than the true activation energy, depending on the stress. The apparent stress exponent (determined from the strain rate dependence of the stress or the stress dependence of the creep rate) is shown to be strongly dependent on stress, and therefore on temperature, and can decrease from values greater than 10 at low temperatures to unity at high temperatures, even though a single mechanism is controlling. Reasonable agreement has been found between the experimental and theoretical values of the stress exponent for yielding in single crystals of sapphire, spinel, Ni₃Al and NiAl, except that the calculated high temperature values tend to be somewhat lower than the observed values. The analysis should apply to yielding in many ceramics and intermetallics in single crystal form (or large grain polycrystals) where the double kink mechanism is rate-controlling.     

Ti53311S钛合金热变形的组织和机制分析

用Gleeble-1500型热模拟试验机对Ti53311S钛合金在温度为880～1080℃,应变速率为0.001～10 s-1的条件下进行高温压缩变形行为的研究.测试了其真应力.真应变曲线,采用双曲正弦本构方程计算出激活能,双相区为641 kJ/mol,β相区为244 kJ/mol.观察了变形后的显微组织,并分析了其变形机制.结果表明:该合金对温度和应变速率敏感,不同变形条件下应力值变化很大;应变速率敏感指数(m)随温度升高而降低,而变形激活能(Q)随温度升高而增大.合金的变形机制在双相区为晶界滑移和晶粒球化,在β单相区为动态回复.     

Temperature dependence of threshold stress for aluminum-based materials exhibiting high strain rate superplasticity

High strain rate superplasticity has been mostly demonstrated in aluminum-base powder metallurgy materials. The superplasticity has been illustrated by incorporating threshold stress, which decreases apparent flow stress, and is commonly interpreted as an extension of usual fine-grain superplasticity. That is, majority of deformation takes place in grain boundaries, meaning grain boundary sliding is the governing mechanism. Dispersion particles or whiskers seem to cause the threshold stress, whose value shows strong temperature dependence. In order to understand the exact nature of the temperature dependence, threshold stress data appeared in literature has been reanalyzed using linear and exponential functions of temperature. The modulus-compensated threshold stress data fit well with the Arrhenius plot, and two modes of thermally activated process are suggested. One is dominant at lower temperature ranges, and has an activation energy of about 50 kJ/mole and an interaction between moving dislocations and particles or interfaces is considered as the origin of the threshold stress. Another one is operational at very high temperatures near incipient melting condition, which exhibits considerably higher activation energy. Referring to the high activation energy and serrated flow curves, solute drag against gliding dislocations has been suggested as a possible source for the second threshold stress.     

含Ti微合金钢动态再结晶行为及模型研究

采用Gleeble 3800热力模拟实验机,研究了一种Ti微合金钢在850～1 150℃、0.1和1.0 s-1条件下的变形奥氏体的动态再结晶行为及再结晶后奥氏体晶粒尺寸的演化规律。结果表明:形变激活能与变形条件相关,随着应变速率的增加而增加;Avrami指数m同样与变形条件有关;对实验数据进行回归分析后建立的Ti微合金钢动态再结晶的特征应变、特征应力、动力学以及稳定状态晶粒尺寸的数学模型精度较高。     

EFFECTS OF MODIFICATION OF THE CARBIDE CHARACTERISTICS THROUGH GRAIN BOUNDARY SERRATION ON CREEP-FATIGUE LIFE IN AUSTENITIC STAINLESS STEELS

Modification of the carbide characteristics through the grain boundary serration is investigated, using an AISI 316 and 304 stainless steels. In both steels, triangular carbides were observed at straight grain boundaries while planar carbides were observed at the serrated grain boundaries. The serrated grain boundary energy is observed to be much lower than that of the straight one. Therefore, the carbide morphology is found to be changed from triangular to planar along the serrated boundary to reduce the interfacial energy between the carbide and the matrix. The creep-fatigue properties of these steels at 873K have been investigated. The creep-fatigue life of the sample with planar carbide at the serrated grain boundary was found to be much longer than that with triangular carbide at the straight one. These results imply that the planar carbides with lower interfacial energy have higher cavitation resistance, resulting in the retardation of cavity nucleation and growth to increase creep-fatigue life.     

MAGNETIC DOMAIN STRUCTURE AND RELATED MAGNETIC PROPERTIES OF(NdPr)_(16)Fe_(76)B_8 PERMANENT MAGNET

The magnetic domain structure and related magnetic properties of (NdPr)_(16)Fe_(76)B_8 permanentmagnet have been studied by colloid-SEM method.In thermally demagnetized state,the ma-trix grains in the magnet generally exhibit multidomain structure,i.e.,180° plate-like indi-vidual domain and a few spike-like or maze-like domains.The average width of the domainmeasured was 1.5μm.Grain size of single-domain was observed to be generally about 1μmand only a few up to 3μm.The results calculated are:the domain wall energy γ=36MJ/m~2,the exchange constant A=4.5×10~(-11)J/m,the domain wall width δ=15.7nm,and the critical grain diameter of single-domain D_c=0.5μm.The dependence of high magnet-ic field gradient at grain boundaries in different magnetization directions in the adjacent mag-netic domains located on both sides of grain boundaries and the effect of microstructure on thevalues of K,A and δ were discussed.     

熔盐电解制备Mg-Li-La合金(英文)

在温度为943K的LiCl-KCl-KF熔盐体系中添加不同浓度的La2O3和MgCl2直接电解制备Mg-Li-La合金。运用X射线衍射分析、扫描电子显微镜及能谱分析和金相显微镜对所得合金进行分析。结果表明,在熔盐体系中通过添加La2O3直接制备Mg-Li-La合金的方法是可行的。在电解过程中,温度不变,槽电压随着电流密度的增加而增加,而改变温度对于槽电压影响不大。X射线衍射分析结果表明,通过恒电流电解可以得到α+Mg17La2,α+β+Mg17La2和β+LaMg3三种不同镁、镧含量的Mg-Li-La合金。能谱分析结果表明,Mg元素在合金中分布均匀,然而La元素主要分布在晶界处与Mg形成金属间化合物。     

Creep of polycrystalline SrCo0.8Fe0.2O3−δ

In this study, a series of compressive creep tests on rectangular specimens of SrCo_0.8Fe_0.2O_3−δ of grain sizes in the range of 2.4–6.8 μm was performed in air, covering a temperature range of 850–975°C and a stress range of 2–80 MPa. The stress exponent has been found to be close to unity in the 10–20 MPa stress range. The apparent activation energy assumes two different values, 471 kJ/mole below 925°C and 275 kJ/mole above 925°C. The microstructural observations of crept samples revealed equiaxed grains and negligible grain growth indicating a diffusion-accommodated grain switching mechanism. The logarithmic plot of strain rate vs inverse grain size has been found to be non-linear. A possible explanation for this behavior is discussed through the incorporation of a threshold stress. The possibility of estimating the lattice and grain-boundary diffusion coefficients from the data is discussed.     

Serrated flow in aluminium alloys containing lithium

Serrated-flow characteristics have been studied in binary AlLi-alloys with different compositions. Load-elongation curves seem divided into two sections of which the first is weakly, the second heavily, serrated. The initial weak serrations, which diminish with ageing, arise probably from the interaction of dislocations with lithium atoms in solid solution. It is proposed that the larger serrations which appear later on the load-elongation curve (if the elongation to necking is large enough to allow them to develop), and which increase in size with progressive ageing, are connected with shearable δ′-precipitates. The effects of zirconium and magnesium additions to AlLi-alloys on serrations have also been investigated. These elements do not change the basic mechanisms of serrated flow. However, the apparent activation energies as calculated for the alloy containing magnesium from the onset strains do not make much sense, because two elements contribute now to serrated flow, and the situation becomes complex.     

INFLUENCE OF AGING ON SERRATED FLOW OF ALUMINUM ALLOY LC9

The serratedflow of alloy LC9 with different regimes of heat treatment has been investigatedat constant rate of stretching.The critieal strain ε_c at the starting of serrated flow increaseswith the extension of aging.The activation energy of the process is 0.63 e V which is consistentwith the diffusion activation energy of vacancies in A1.The ε_c decreases while the strain rate εincreases,i.e.,with negative strain rate relation.This relation is contrary to that of the an-nealed samples.It shows that the serrated 17ow of annealed sample is controlled bydeformation vaeancies,and that of aged sample is controlled by quenching vacancies.The den-sity and shape of the serration are changed by aging.The mechanism of the above mentionedresults is discussed.     

Thermal stability of nano-RuAl produced by mechanical alloying

The thermal stability of as-milled nano-RuAl has been studied by isothermal annealing at high temperatures. All three kinds of structural evolutions in as-milled powders upon high temperature exposure, namely reordering, strain relaxation and grain growth, show signs of stagnation. The total quantity of impurities, mainly 15 at.% Fe has been analyzed as being dissolved substitutionally in RuAl and also segregated to grain boundaries. Upon grain growth, lattice diffusion and segregation of impurity atoms in grain boundaries have been verified by the lattice parameter variation. The incremental apparent activation energy for grain growth at different temperatures is related to the accumulation of impurities in grain boundaries. Reordering and strain relaxation processes that accompany grain growth could consume a certain part of the driving force for grain growth.     

TC21合金的热压缩变形行为及变形机理

对TC21合金的热变形行为进行研究,通过对该合金变形过程中应变速率敏感性指数m值、热变形激活能Q、晶粒指数P的计算,得出不同应变速率和温度下m值、Q值和P值的变化规律。在绘制动态DMM模型热加工图的同时构建含位错数量的双相钛合金高温变形机理图。应用热加工图分析TC21合金热变形工艺,确定加工失稳区以及适合加工区域。运用双相钛合金高温变形机理图,根据不同温度下TC21合金柏氏矢量补偿的晶粒尺寸、模量补偿的应力值和位错数量预报该合金的热变形机理。     

EQUIAXED REFINEMENT OF GRAINS IN ALLOY TC11

The formation reason and elimination method of non-uniform microstructure defects in Ti al-loy TC11 bar have been studied.The coagulating and coarsening into block of the part ofgrain boundary α and secondary α seem to be caused by the ingot cogging and initial forgingtemperature in the β region as well as no more enough deformation and uneven distribution.The grain α,elongated α and blocky α may be finally eliminated by adopting the technique of(α+β)thermomechanical processing+β processing,W.Q.+recrystallization annealing,A.C.,thus the size of uniform and fine equiaxed α structure is believed to be reduced to 1.9258μm.     

Microstructure and high-temperature mechanical behavior of the NiAl–27 at.% Cr intermetallic composite

A NiAl–27 at.% Cr composite material was prepared by a powder metallurgical route, involving argon atomization and consolidation by hot isostatic pressing at 1350°C for 4 h at 400 MPa. The consolidated material exhibited a fine-grained microstructure consisting of a fine dispersion of Cr particles of about 1.7 μm in a NiAl matrix. The mechanical behavior at temperatures ranging from 650 to 1100°C was investigated by tensile-strain-rate-change tests. Analysis of the strain–stress data with both power law creep and Garofalo’s hyperbolic sine relation shows the transition to a low stress exponent creep regime with decreasing stress and/or increasing testing temperature. The measured activation energy for deformation of 300 kJ/mol is consistent with the activation energy for Ni self-diffusion in Ni–50Al. Experiments with coarse grain sizes established that the creep rate is independent of grain size which suggests that the deformation mechanisms must be associated with the motion of lattice dislocations.     

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior,microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach.The true stress–strain curves exhibited typical work hardening and flow softening,in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon(Z)parameter,and the activation energy was calculated to be 446.9 k J/mol.The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation.The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy.The coarse recrystallized γ and several massive γ phase were observed at grain boundaries.While in the case of Nb segregation,β/B2 phase harmonized bending of lamellae,combined with the growth of recrystallized γ grains and α+β+γ→α+γ transition under conditions of temperature and stress,leading to the breakdown of α_2/γ lamellar colony.During the hot compression process,gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism,leading to extensive c twins and cross twins in α/γ lamellae and at grain boundaries.In general,homogeneous microstructure during the hot deformation process can be obtained in Ti Al alloy with high Nb addition and low Al segregation.The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.     

Plastic flow of the mechanically alloyed Fe-0.6%O at temperatures of 550–700°C

Creep of steel Fe-0.6%O produced by the method of powder metallurgy has been studied in a temperature range of 550–700°C at flow stresses from 100 to 400 MPa. It has been shown that the creep of the material is characterized by high values of the apparent activation energy for deformation, which considerably exceeds the value of the activation energy for self-diffusion in α iron, and by high values of the stress exponent in the power law of creep. An analysis of the deformation behavior of the alloy showed that there are observed high threshold stresses as a result of retardation of moving dislocations by small incoherent particles of oxides. Taking into account the threshold stresses and the temperature dependence of the shear modulus, it has been established that the deformation behavior of the powder material is described by a power law of creep. The true values of the stress exponent were found to be approximately 8, and the values of the true activation energy for deformation, to be close to the activation energy for bulk (at T = 700°C) and pipe (at T = 550–650°C) self-diffusion.     

氯化物水溶液体系三价铬电沉积机理研究

采用循环伏安、恒电流阶跃和稳态极化等电化学研究方法和紫外分光度法,研究了Cr3+电沉积机理,结果表明其沉积过程分2步进行。计算得出Cr3+2步放电的表观活化能为34.9 kJ/mol和19.77 kJ/mol,表明Cr3+还原过程受电化学反应控制;初步拟定了甘氨酸作配体氯化物水溶液体系三价铬还原机理。     

Investigations of yield stress,fracture toughness,and energy distribution in high speed orthogonal cutting

This paper presents a novel prediction method of the yield stress and fracture toughness for ductile metal materials through the metal cutting process based on Williams' Model [38]. The fracture toughness of the separation between the segments in serrated chips in high speed machining is then deduced. In addition, an energy conservation equation for high speed machining process, which considers the energy of new created workpiece surfaces, is established. The fracture energy of serrated chips is taken into the developed energy conservation equation. Five groups of experiments are carried out under the cutting speeds of 100, 200, 400, 800 and 1500 m/min. The cutting forces are measured using three-dimensional dynamometer and the relevant geometrical parameters of chips are measured with the aid of optical microscope. The experiment results show that the yield stress of machined ductile metal material presents an obviously increasing trend with the cutting speed increasing from 100 to 800 m/min while it decreases when the cutting speed increases to 1500 m/min further. Meanwhile, the fracture toughness between the chip and bulk material displays a slightly increasing tendency. In high speed machining, the fracture toughness of the separation between the segments in serrated chips also presents increasing trend with the increasing cutting speed, whose value is much greater than that between the chip and bulk material. In the end, the distribution of energy spent in cutting process is analyzed which mainly includes such four portions as plastic deformation, friction on the tool–chip interface, new generated surface and chip fracture. The results show that the proportion of plastic deformation is the largest one while it decreases with the cutting speed increasing. However, the proportions of energy spent on new created surface and chip fracture increase due to the increasing of both the chip's fracture area and the fracture toughness.     

On the influence of gamma prime upon machining of advanced nickel based superalloy

Whilst gamma prime (γ′) phase is the strengthening phase in Ni-based superalloys its influence on machining has been seldom investigated. This paper reports for the first time on the effect of γ′ upon machining of Ni-based superalloys when cutting with parameters yielding different cutting temperature intervals which lead to strengthening/softening effects on the workpiece (sub)surface. In-depth XRD, SEM/FIB, EBSD analysis and unique micro-pillar testing in the workpiece superficial layers indicated that with the increase of γ′ fraction the grain plastic deformation significantly decreased, while specific cutting energy can switch from low to high values influenced by the real cutting temperature.     

Diffusion of Zn along tilt grain boundaries in Al:: pressure and orientation dependence

For the first time, a systematic investigation has been carried out on the pressure and orientation dependence of diffusion along grain boundaries. Zn diffusion has been measured in the B-kinetic regime in different sets of Al bicrystals at 553 K. Pressures up to 1.2 GPa have been applied. Three sets of symmetrical <001>, <011> and <111> tilt grain boundaries have been chosen with tilt angles around those, which correspond to a low coincidence lattice site parameter. The Zn concentration depth profiles have been measured by electron probe microanalysis and have been analysed using the Whipple equation. The orientation dependence of the diffusivity shows minima for the high coincidence grain boundaries at all applied pressures. Using known data for the bulk diffusion of Zn in Al, the activation volume for the triple product of grain boundary diffusion of Zn in Al has been determined for each grain boundary under investigation. The values of the activation volume range from 0.26 to 0.47 of the atomic volume of Al and show slight maxima for the high coincidence grain boundaries.