Mg-5.5Zn-2Gd-0.6Zr铸造镁合金的蠕变机制

在ZM-1(Mg-5Zn-0.6Zr)合金的基础上,适量增加Zn的含量并加入重稀土元素Gd,设计了Mg-5.5Zn-2Gd-0.6Zr实验合金。采用砂型铸造工艺制备实验合金试样,在不同温度和应力条件下对该实验合金和ZM-1合金的蠕变曲线进行了测试。结果表明:在相同条件下,Mg-5.5Zn-2Gd-0.6Zr实验合金的稳态蠕变速率较ZM-1合金的降低了一个数量级;当施加应力为40 MPa时,实验合金的蠕变激活能Q200-250℃=142.0 kJ/mol,接近镁的自扩散激活能,蠕变受位错攀移控制,而ZM-1合金在相同应力下蠕变激活能Q200-250℃=88.5 kJ/mol,接近镁的晶界扩散激活能,蠕变受晶界滑移控制。合金在200℃条件下的应力指数n=4.21,而ZM-1合金的应力指数n=2.21。因此,认为加入重稀土元素Gd后实验合金的蠕变机制发生改变,200℃时的蠕变机制为位错攀移机制。     

Creep in grain coarsened ODS MA NiAl

NiAl based oxide dispersion strengthened (ODS) intermetallic alloys have been produced by mechanical alloying (MA) and consolidated by hot extrusion. Subsequent isothermal annealing was carried out to induce normal grain growth (NGG), and a thermomechanical treatment was performed to induced secondary recrystallization (SRx). SRx resulted in pronounced elongated grain growth without dispersoid coarsening, whereas concurrent equiaxed coarsening of grains and dispersoids occurred in NGG specimens. Creep properties of grain coarsened ODS MA NiAl were investigated, and the associated creep mechanisms were evaluated. The creep properties of SRxed MA NiAl are compared with those of as-consolidated MA NiAl and other counterparts. It has been shown that SRx results in improved creep resistance compared to NGG mechanism. The apparent activation energy and the stress exponent for creep indicate that SRx MA NiAl exhibits intermediate creep behavior of the two limiting dislocation creep modes; climb, controlled and viscous glide controlled. However, a grain size dependent creep has been shown, indicating that grain boundary sliding mechanism contributes to the overall deformation in MA NiAl.     

Characterization of creep properties and creep textures in pure aluminum processed by equal-channel angular pressing

High-purity aluminum was processed by equal-channel angular pressing (ECAP) and then tested under creep conditions at 473 K. The results show conventional power-law creep with a stress exponent of n = 5 which is consistent with an intragranular dislocation process involving the glide and climb of dislocations. It is demonstrated that diffusion creep is not important in these tests because the ultrafine grains produced by ECAP are not stable at this temperature. Texture measurements were undertaken using the high-pressure preferred orientation neutron time-of-flight diffractometer and they reveal significant differences in the evolution of texture during creep in pressed and unpressed specimens. These experimental measurements of texture are in excellent agreement with theoretical textures predicted using a visco-plastic self-consistent model that limits deformation to plastic slip. The calculations provide additional confirmation that creep occurs through an intragranular dislocation process.     

Steady-state creep behavior of a Ti-25al-10Nb-3v-lMo alloy

Creep tests were conducted on Ti-25Al-10Nb-3V-1Mo alloy in the the temperature range of 913 - 1093 K at stresses ranging from 40 to 600 MPa. The creep behavior of the Ti₃Al alloy under these testing conditions revealed three different stress exponent regimes. In the temperature range of 1033 to 1093 K at low applied stress levels, the stress exponent was equal to 1.5. At the intermediate stress range (10₃<σ/E<3x10^-3), a stress exponent of 3.3 was exhibited indicating that the creep deformation was controlled by a viscous dislocation glide process As the applied stress increase, the stress exponent changed from 3.3 to 4.4 The activation energy for creep was equal to 288 kJ/mole in the region where viscous dislocation glide was the dominant deformation mechanism (n=3.3) In view of the diffusion data, the rate-controlling species in the viscous glide region was assumed to be Ti lattice diffusion     

High-temperature compressive creep of liquid phase sintered silicon carbide

Creep of liquid phase sintered SiC has been studied at temperatures between 1575 and 1700°C in argon under nominal stresses from 90 to 500 MPa. Creep rates ranged from 3×10⁻⁸ to 10⁻⁶/s, with an activation energy of 840±100 kJ/mol (corresponding to carbon and silicon self-diffusion), and a stress exponent of 1.6±0.2. The crept samples showed the presence of dislocation activity, generally forming glide bands and tangles. Degradation of the mechanical properties due to cavitation or reaction of the additives was not detected. SEM and TEM microstructural characterization and analysis of the creep parameters leads to the conclusion that the creep mechanisms operating are grain boundary sliding accommodated by lattice diffusion and climb-controlled dislocation glide operating in parallel. Other possible operating mechanisms are discussed and the data are compared with published data.     

Stress-driven migration of simple low-angle mixed grain boundaries

We investigated the stress-induced migration of a class of simple low-angle mixed grain boundaries (LAMGBs) using a combination of discrete dislocation dynamics simulations and analytical arguments. The migration of LAMGBs under an externally applied stress can occur by dislocation glide, and was observed to be coupled to the motion parallel to the boundary plane, i.e. tangential motion. Both the migration and tangential velocities of the boundary are directly proportional to applied stress but independent of boundary misorientation. Depending on the dislocation structure of the boundary, either the migration or tangential velocity of the boundary can switch direction at sufficiently high dislocation climb mobility due to the dynamics of dislocation segments that can climb out of their respective slip planes. Finally, we show that the mobility of the LAMGBs studied in this work depends on the constituent dislocation structure and dislocation climb mobility, and is inversely proportional to misorientation.     

Studies of deformation mechanisms in ultra-fine-grained and nanostructured Zn

The temperature, strain rate, grain size and grain size distribution effects on plastic deformation in ultra-fine-grained (UFG) and nanocrystalline Zn are systematically studied. The decrease of ductility with the decrease of average grain size could be an inherent effect in nanocrystalline materials, that is, not determined by processing artifacts. The superior ductility observed in UFG Zn may originate from both dislocation creep within large grains and grain boundary sliding of small nanograins. The stress exponent for dislocation creep is about 6.6. The activation energy for plastic deformation in UFG Zn is close to the activation energy for grain boundary self diffusion in pure Zn.     

The high temperature tensile and compressive deformation characteristics of magnesia doped alumina

The mechanical characteristics of alumina have not yet been characterized completely in tension due in part to strain hardening accompanying grain growth and premature cavitation failure. Tensile tests were conducted on fine grained magnesia doped alumina over a range of strain rates, grain sizes and temperatures to evaluate the stress exponent, inverse grain size exponent and activation energy. Constant stress compression creep tests were also carried out under a similar range of experimental conditions. Extensive microstructural characterization after deformation indicated that there was considerable grain growth during deformation; however, the grains retained their initially equiaxed structure after significant deformation. Although a standard plot of strain rate versus stress indicated a stress exponent of ∼2, a complete analysis including the compensation of data for concurrent grain growth revealed that true stress exponent was ∼1, consistent with diffusion creep. It is argued that grain rearrangement processes accompanying grain growth will tend to mask the development of an elongated grain structure predicted by diffusion creep processes. In contrast to several ceramics with a significant amount of glassy phase, there is no significant difference between the elevated temperature tensile and compressive behavior of alumina.     

Creep properties and controlled creep mechanism of as-cast Mg-5Zn-2.5Er alloy

The creep behaviors of as-cast Mg-5Zn-2.5Er alloy(mass fraction,%) ,under various applied stresses(50-70 MPa) and creep temperatures(150-200℃) for 100 h,were investigated.The stress exponent n is in the range of 1.5-5.8,and the activation energy Qc is in the range of 28.3-77.1 kJ/mol.With respect to the calculated n and Qc as well as the microstructures after creep,it is suggested that there is a transition region between grain boundary sliding(GBS) dominated creep to dislocation creep mechanism(from n3 to n3) ,arising in the steady-stage creep rate value of 2.89×10-9 s-1.     

Nd对Al2O3f/AZ91D复合材料高温蠕变性能的影响

利用高温压缩蠕变实验研究了Nd对复合材料的高温蠕变性能以及压应力对濡变应力指数的影响.结果表明稀土元素Nd的加入可以明显改善复合材料的高温蠕变性能,试验中添加0.8％Nd的Al2O3f/AZ91D复合材料的抗高温蠕变性能最好;当应力为60～90 MPa与156～180MPa时复合材料的蠕变机理为基体和增强体之间的载荷传递,纤维的开裂和破断是其失效的主要机制;应力为90～156 MPa时复合材料的蠕变机理为位错滑移与位错攀移共同作用.     

Ti-6.5Al-2Zr-1Mo-1V钛合金低温应力松弛及组织演变

在500、550和600 ℃及不同初始应力下对Ti-6.5Al-2Zr-1Mo-1V钛合金进行了应力松弛实验。基于经典的Maxwell指数衰减函数,得到了应力松弛极限。提出了利用松弛稳定系数（C_S）和松弛速率系数（C_R）来描述Ti-6.5Al-2Zr-1Mo-1V合金的松弛特性,有利于制定残余应力消减工艺。根据Norton和Arrhenius方程计算了应力指数。通过应力指数和显微组织分析,阐明了应力松弛机理。在不同初始应力下,500 ℃时,位错的攀移和扩散主导了应力松弛过程;550 ℃时,位错滑移在应力松弛过程中起主要作用;600 ℃时,位错滑移、边界滑移和晶粒旋转控制着松弛过程。     

硅酸铝短纤维增强镁基复合材料的蠕变机制及蠕变本构模型（英文）

利用常应力拉伸蠕变实验法对体积分数为30%的硅酸铝短纤维增强AZ91D镁基复合材料及其基体合金AZ91D在不同温度和应力下进行蠕变测试。结果表明:2种材料的真应力指数均为3,真蠕变激活能均等于144.63kJ/mol;复合材料的蠕变是由基体的蠕变控制,以位错黏滞性滑移控制为主,晶界滑移控制为辅。由实验数据获得的蠕变本构模型的经验公式能较好地描述复合材料的蠕变变形规律。     

TC6钛合金的高温拉伸蠕变行为研究

通过高温拉伸蠕变实验,获得了TC6合金的蠕变应变-时间曲线,并计算了其不同应力与不同温度下的稳态蠕变速率、应力指数及在350～450℃范围内的蠕变激活能,借助OM、TEM等手段对合金蠕变前后的显微组织进行了观察和分析,并在此基础上研究了其蠕变变形机制.结果表明:TC6合金的稳态蠕变速率随温度或恒应力的增加而增大,该合金在此温度范围内的蠕变受位错和扩散双重机制的控制,晶界滑动对蠕变也有一定的作用.     

High-temperature grain boundary sliding behavior and grain boundary energy in cubic zirconia bicrystals

Misorientation dependence of grain boundary energy and grain boundary sliding at high temperature were examined in cubic zirconia bicrystals with [1 1 0] symmetric tilt boundaries, which were fabricated by diffusion bonding method from two cubic zirconia single crystals. High-resolution transmission electron microscopy observation revealed that the grain boundary in cubic zirconia bicrystals was clean and atomically sharp without any void or grain boundary amorphous layer. Grain boundary energy of the tilt boundaries was estimated from the dihedral angles on thermal grooved surface measured with atomic force microscope techniques. The misorientation dependence of the grain boundary energy in cubic zirconia bicrystals shows similar tendency to that of fcc metal such as aluminum and copper. Grain boundary sliding associated with intragranular dislocation slip in cubic zirconia bicrystals was observed for all specimens. The amount of the grain boundary sliding showed a good correlation with the misorientation factor of each boundary. Grain boundary migration also took place accompanying with the grain boundary sliding. The observed grain boundary sliding and migration can be explained based on a dislocation mechanism for sliding which is based on the movement of lattice dislocations along the grain boundary by a combination of climb and glide.     

An investigation of the deformation mechanism in grain size-sensitive Newtonian creep

Creep of polycrystalline materials at low stresses often shows a linear relationship between strain rate and stress, and an inverse dependence on grain size squared or cubed. Attribution of this behavior to diffusional creep or grain boundary sliding (GBS) has evoked much confusion and controversy in the literature. A model is proposed to unify these two creep mechanisms. The model predicts a change in dominant mechanism from diffusional creep to GBS accommodated mainly by diffusion or by GBS itself as the amount of matter moved by diffusion decreases. Corresponding to this change, the model also predicts a spectrum of creep rate with the absolute value being dependent upon the extent of diffusion accommodation. Although experimental data exhibit scattering, most of them are in very good agreement with the prediction of the GBS model. Therefore, it is suggested that the Newtonian creep behavior with grain size dependence be induced by GBS rather than by conventional diffusional creep as believed before.     

High temperature deformation mechanism and evolution of dislocation structure during creep of Ni-Base superalloy 713LC

Creep deformation of cast nickel base superalloy 713LC has been investigated in a temperature range of 723 to 982°C. The values of the stress exponent and activation energy for creep of the alloy vary with a combination of temperature and stress. Introduction of threshold stress for creep of the alloy provided an explanation of the high values of the stress exponent and the apparent activation energy. Microstructural evolution of the alloy with creep deformation has also been studied. The analysis of the creep mechanism has been supplemented by microstructural observations after deformation under various test conditions. The dislocation structure of the alloy at high temperature and low stress was different from that at low temperature and high stress. Shearing of γ′ particles by dislocation pairs was the dominant creep mechanism at low temperature and high stress whereas dislocation climb over γ′ particles was the rate controlling process of creep at high temperature and low stress.     

High-temperature deformation of Sr(FeCo)1.5Ox ceramics

Compressive creep of polycrystalline SrFe_1.2Co_0.3O_x and SrFeCo_0.5O_x ceramics has been investigated at 940–1000°C in constant-load and constant-displacement-rate experiments. At low stresses, the stress exponent was ≈1 and the activation energy was ≈110–135 kJ/mol. At higher stresses, a transition occurred and the stress exponent became ≈2.4–3.1 and the activation energy became ≈425–453 kJ/mol. At higher stresses, there was no dependence of the steady-state flow stress on oxygen partial pressure from 10–10⁵ Pa. The creep parameters and scanning and transmission electron microscopy observations of the deformed samples suggested that deformation was controlled by diffusion at low stresses and dislocation glide at high stresses.     

Creep deformation behavior of Sn-3.5Ag solder at small length scales

To adequately characterize the behavior of solder spheres in electronic packaging, their mechanical behavior needs to be studied at small-length scales. The creep behavior of single Sn-3.5Ag solder spheres on a copper substrate was studied between 25°C and 130°C using a microforce testing system. In the low-stress regime, the creep stress exponent changed from 6 at lower temperatures to 4 at higher temperatures, indicating creep by dislocation climb. The activation energy for creep was also found to be temperature dependent, and correlated with values for dislocation core diffusion and lattice diffusion in pure tin. A change in the stress exponent with increasing stress was also observed and explained in terms of a threshold stress for dislocation motion, due to the presence of obstacles in the form of Ag₃Sn particles. For more information, contact N. Chawla, Arizona State University, Department of Chemical and Materials Engineering, Ira A. Fulton School of Engineering, Tempe, AZ 85287-6006; e-mail nchawla@asu.edu.     

Creep behaviour of Fe3Al-based alloys in DO3 phase field

A study is made to bring out the effect of alloying with Cr, Ti or Mn on the creep behaviour of Fe₃Al. Impression creep experiments have been carried out in the DO₃ phase field. In all the alloys, power law creep behaviour is observed in the stress range covered. The stress exponent for steady state creep rate and the activation energy for creep indicate that the creep rate is controlled by the dislocation climb process. Among the alloying elements studied, addition of Ti is most effective in improving the creep resistance.