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1.
This work concerns with the high temperature deformation of internally oxidized Al2O3/Cu composites. The investigation revealed that dispersive alumina can obstruct dislocation sliding and define the subgrain size thereby improve significantly the strength of the materials at high temperature. The sliding of dislocations is a main deformation mechanism in the given temperature range. The sliding of grain boundary and diffusive creep play important roles at high temperature and low strain rate. The dispersoids can raise the recrystallization temperature to higher than 1223 K. Dynamic recovery is a main softening way under the experimental conditions. Higher deformation rate and lower deformation temperature imply a higher flow stress. 相似文献
2.
Grain boundary sliding and accommodation mechanisms during superplastic deformation of ZK40 alloy processed by ECAP 总被引:1,自引:0,他引:1
Controlling mechanism during superplastic deformation of ZK40 alloy processed by ECAP was identified. Effects of twinning
and dynamic strain ageing (DSA) on superplasticity were analyzed. Amplitude in stress oscillation was correlated with solute
atom concentration theoretically. Twinning can be an enhancing factor in grain boundary sliding and DSA had apparent influence
on stress fluctuation; they were accommodation mechanisms for superplastic deformation through grain reorientation and interaction
between solute atoms and dislocations, respectively. The interaction between mobile and forest dislocations played a dominant
role for the occurrence of DSA, when dislocation density was relatively low in large grains. The effect of DSA became more
active with increasing temperature, although grain boundary sliding (GBS) was the controlling mechanism throughout the whole
process of superplastic deformation under elevated temperatures. 相似文献
3.
Transient creep data for high-purity polycrystalline alumina are examined at the testing temperature of 1150–1250 °C. The data are analysed in terms of the effect of stress and temperature on the extent of transient time and strain.In order to explain the observed transient creep, a time function of creep strain is proposed from a two-dimensional model based on grain boundary sliding. The grain boundary sliding is assumed to take place by the glide of grain boundary dislocations accommodated by dislocation climb in the neighboring grain boundaries. The time function for a creep strain obtained from the model is given in a form
which is similar to the previous empirical formula describing the experimental creep curves in metallic alloys. The model predicts that the transient creep strain T is approximately proportional to and the extent of transient creep time tT is inversely proportional to flow stress. The prediction is consistent with the experimental data in high-purity, fine-grained alumina at temperatures between 1150 and 1250°C. 相似文献
4.
超塑性Y-TZP的压缩塑性形变 总被引:1,自引:1,他引:0
通过恒定横梁速度和恒定载荷压缩试验,对超塑性3mol%Y2O3稳定四方ZrO2多晶体的压缩塑性形变进行了研究.测定了平均晶粒尺寸从0.30~1.33μm的3Y-TZP材料的塑性流动应力,应力指数和蠕交活化能;用扫描和透射电镜观察了试样的显微结构.结果表明,3Y-TZP材料塑性形变的机理为扩散适应的晶界滑移.随着晶粒尺寸由0.30μm增大至1.33μm,应力指数从3.2减小至1.4,活化能从580kJ/mol减小至500kJ/mol.形变机理随晶粒大小发生变化.对于晶粒较粗的3Y-TZP材料,当应变速率较高时,形变过程中在材料内产生晶间孔穴. 相似文献
5.
The edge dislocations near a cracked sliding interface were investigated. A continuous distribution of edge dislocations with
Burgers vector along the y direction was used to simulate a crack of finite length along the sliding interface. From the dislocation
distribution the stress field in the entire space was obtained. The stress intensity factors at both crack tips and image
force on the edge dislocation were derived. The effects of the dislocation source and shear modulus ratio on both stress intensity
factors and image force were also studied. Only mode I stress intensity factors at both tips were found in the composite materials
with a sliding interface. The edge dislocations with Burgers vector along the y direction emitted from the crack always shield
it to prevent propagation. The above results may reduce to an edge dislocation near a semi-infinite crack along a sliding
interface including a sliding grain boundary.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
6.
《Materials Science & Technology》2013,29(1):82-86
AbstractThe mechanical behaviour and microstructural evolution in tension of NiAl - 9Mo eutectic alloy at 1100 ° C and strain rates from 10-5 to 10-3 s-1 have been investigated. High values of strain rate sensitivity index, but relatively small elongations between 150 and 200%, have been observed. Tensile specimens with various strains were quenched in water to preserve the dislocation structures for TEM examination. The TEM results show that the dislocation configuration and density change significantly with an increase in strain rate, and therefore correspond to different deformation mechanisms. At a low strain rate (5.5 × 10-5 s-1), the dislocation density is relatively low and dislocation activity is regarded as an accommodation mechanism for grain boundary sliding. In contrast, the high density of dislocations as well as clear subboundaries observed in grains at a high strain rate (5.5 × 10-4 s-1) suggest that the dislocations are active directly in response to the applied stress as well as participating in the relaxation process of grain boundary sliding by subboundary formation. Thus, grain boundary sliding is mainly responsible for superplastic deformation at a low strain rate, while superlastic deformation at a high strain rate is controlled by the combined operation of both grain boundary sliding and dynamic recovery. 相似文献
7.
During creep of polycrystalline materials at elevated temperatures, a certain amount of the strain is accommodated by grain-boundary sliding (GBS). The relative importance of GBS depends on the stress and grain size and sometimes temperature. During high-strain deformation, dynamic recrystallization often occurs with the resultant grain size only related to the stress. In this situation the importance of GBS is then dependent only upon stress and sometimes temperature. In dynamically recrystallized Magnox Al80 deformed atT>0.8T
m, 16 to 23% of the imposed strain is accommodated by GBS. A comparison has been made between the experimental results and some theoretical models for the importance of GBS during creep, modified to take account of recrystallization. The best fit to the data is obtained with the modified form of Langdons model. Deformation mechanism maps constructed with this model suggest that dynamic recrystallization can cause a switch of mechanism from dislocation creep to dominant GBS at intermediate temperature (T<673 K) and low stress. Deformation mechanism maps have also been constructed for calcite based on the data of Schmidet al. These suggest that GBS is an important mechanism in calcite deformed under geological conditions. 相似文献
8.
《Materials Science & Technology》2013,29(10):1253-1258
AbstractDuring plastic deformation of a polycrystalline material, both the grain interior and the grain boundary regions exhibit distinctly different dislocation behaviours at a given strain and temperature. Studying the variation of experimental flow stress with temperature, it seems that the flow stress of a fine grained polycrystalline material is mainly controlled by dislocation dynamics at and in the vicinity of grain boundaries. At low temperatures in a polycrystalline material, the dislocations are piled up at grain boundaries and the density of dislocations increases significantly in the grain boundary region, while at high temperatures the annihilation of dislocations take place at and in the vicinity of the grain boundaries during deformation. Therefore, the flow stress behaviour of a polycrystalline material can be understood in terms of the process of accumulation and annihilation of dislocations at and in the vicinity of grain boundaries at a given strain and temperature. 相似文献
9.
The creep behaviour at elevated temperature of an austenitic stainless steel (25Cr-20Ni), both with and without antimony additions, has been reanalysed. Formerly, the creep behaviour was interpreted by considering creep mechanisms based on diffusional (Coble) creep and threshold stresses. In the present paper, it is proposed that an alternative mechanism of grain boundary sliding, accommodated by slip in grain boundary mantle regions, can in fact be used to describe more accurately the creep behaviour. Quantitative predictions, based on phenomenological equations for describing creep controlled by grain boundary sliding, are made of the influences of grain size, stress and antimony addition on creep rates, and of the influence of grain size on the activation energy for creep of 25Cr-20Ni stainless steel. Comparison of these predictions with those based on creep models incorporating only diffusional flow are made. Furthermore, the existence of a threshold stress in creep of single-phase, massive materials is strongly questioned. 相似文献
10.
Guillaume Bernard-Granger Christian Guizard Richard Duclos 《Journal of Materials Science》2007,42(8):2807-2819
Compressive creep tests in air have been performed on a polycrystalline submicron as sintered and slightly porous α-alumina
material. Two different deformation mechanisms, depending on the applied stress and creep temperature, have been identified
when the grain size becomes higher than a critical value 〈G
*〉.
For low temperatures and/or low applied stresses, deformation occurs by grain boundary sliding accommodated by an in-series
“interface reaction/diffusion of Al3+ cations” process, with the limiting step being the interface reaction. In this case increased densification of the samples
is observed after creep, compared to the as-sintered ones.
In contrast, for high temperatures and/or high-applied stresses, deformation occurs by grain boundary sliding accommodated
by the relocation and growth of preexisting cavities, the growth step being also controlled by the diffusion of Al3+ cations. In this case, a marked decrease of the relative density is measured on the crept samples compared to the as-sintered
ones.
Using these results, it is possible to identify the optimal conditions for superplastic forming of previously as-sintered
parts, leading to shaped objects with an increased final density. 相似文献
11.
李培恩 《材料科学技术学报》1992,8(2):119-122
The available experimental results have beensummarized concerning the effect of grain size onminimum creep rate.There are two types of creeprate-grain size relations.One is that there is a criti-cal grain size above which creep rate is independentof grain size,below which creep rate increases withthe decrease of grain size.The other is that there isan intermediate grain size at which creep resistanceis optimum.The first relation usually occurs athigher temperatures(>0.5 T_m),and intermediatestress ranges,while the second relation at interme-diate temperature ranges(0.4-0.5 T_m)and higherstresses.For the two types of creep rate-grain sizerelations,the increase of the creep rates with the de-crease of grain size for small grain sizes is all due tograin boundary sliding.For large grain sizes,a dis-location climb mechanism is dominant in creepdeformation for the first relation,while aHall-Perch grain boundary strengthening effect isbelieved to play an important role by dislocationglide mechanism for the second relation. 相似文献
12.
A comparison of elastic interaction of a dislocation and a crack for four bonding conditions of the crack plane was made.
Four cases of single crystalline material, sliding grain boundary, perfectly bonded interface, and sliding interface were
considered. The stress intensity factors arising from edge and screw dislocations and their image forces for the above four
cases were compared. The stress intensity factor at a crack tip along the perfectly bonded interface arising from screw dislocation
can be obtained from that in a single crystalline material if the shear modulus in the single crystalline material is replaced
by the harmonic mean of both shear moduli in the bimaterial. The stress intensity factor at a crack tip along the sliding
interface arising from edge dislocation in the bimaterial can be obtained from that along the sliding grain boundary in the
single material if the μ/(1−ν) in the single material is substituted by the harmonic mean of μ/(1− ν) in the bimaterial where
μ and ν are the shear modulus and Poisson's ratio, respectively. The solutions of screw dislocation near a crack along the
sliding grain boundary and sliding interface are the same as that of screw dislocation and its mirror image. Generally, the
effect of edge dislocation for perfectly bonded interface on the crack propagation is more pronounced than that for the sliding
interface. The effect of edge dislocation on the crack propagation is mixed mode for the cases of perfectly bonded interface
and single crystalline material, but mode I fracture for the cases of sliding interface and sliding grain boundary. All curves
of Fx versus distance r from the dislocation at interface to the right-hand crack tip are similar to one another regardless
of dislocation source for both sliding interface and perfectly bonded interface. The level of Fx for m=0 is larger than that
for m=−1.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
13.
《Materials Science and Engineering: A》1990,130(2):165-172
The helicoid spring specimen technique was applied to investigate creep of a Cu-14at.%Al solid solution alloy at homologous temperatures from 0.54 to 0.65 and stresses ranging from 0.2 to 5.0 MPa. At stresses lower than about 1 MPa, Coble-type creep was found to dominate, associated with a threshold stress apparently independent either of grain size or of temperature. At stresses above about 1 MPa, another creep mechanism obviously contributes to the measured creep rate. This mechanism operating in parallel with Coble creep is characterized by the fact that the steady state creep rate is proportional to the second power of stress and inversely proportional to the third power of grain size and is most probably grain boundary diffusion controlled. This mechanism, called the non-viscous mechanism in the present work, is similar to that considered by Gifkins and Kaibyshev et al. to result from the motion of grain boundary dislocations (grain boundary sliding) accomodated by slip of lattice dislocations in thin layers along grain boundaries, although these workers assumed the creep rate to be inversely proportional not to the cube but to the square of the grain size. 相似文献
14.
15.
Micro-grained γ-TiAl based alloy obtained via pulse current auxiliary sintering exhibits good room temperature ductility with the common influence of fine grain size and inner twinning microstructure. Superplastic behavior at relatively low temperatures is also observed. It is also noted that the tensile strength of the studied alloy manifests anomalous hardening from room temperature to approximately 600 °C as a result of the controlling of dislocations slip, and softening above 600 °C due to thermal activation. Based on calculation, the superplastic deformation mechanism in the present work is determined as the grain boundary sliding accommodated by grain boundary diffusion. 相似文献
16.
17.
(TiB + La2O3)/Ti composites were in situ synthesized and deformed with different deformation degrees. The influence of TiB whisker orientation and grain refinement on the creep properties of titanium matrix composites (TMCs) are discussed. The creep test reveals that the steady state creep rate of TMCs first decreases and then increases with the increase of deformation degree, which can be attributed to competing effects: TiB whisker rotating to the rolling direction, α plate grain boundary hindering and pinning dislocations can all decrease the creep rate, however, dislocation movement on the α plate grain boundary and dislocation emitting from the α plate grain boundary can both increase the creep rate. 相似文献
18.
19.
The activation areas for grain boundary sliding in Al, Pb, Sn, Zn, and Cu are compared with those for creep in the same materials. It is found that the activation area-stress relation for grain boundary sliding is similar to that for creep. This observation is consistent with a dislocation or ledge mechanism of grain boundary sliding. 相似文献
20.
Shehrazed Moulahem Sylvie Lartigue-Korinek Jacques Castaing 《Journal of Materials Science》2011,46(12):4420-4427
A bicrystal slightly deviated from the rhombohedral twin orientation is deformed by creep so as to activate two basal slip
systems in each grain. The tilt deviation in the starting bicrystal is accommodated by two parallel arrays of disconnections
with the smallest twin Burgers vectors and large associated steps. Dislocations from both grains interact with the grain boundary
(GB) and the tilt deviation increases by 3.6°, giving rise to an array of pure edge disconnections with no associated steps.
Interfacial defects are interpreted by reactions between incoming dislocations and GB disconnections. Decompositions of dislocations
into GB disconnections are followed by glide and climb of products that can annihilate or interact to give a perfect wall
of edge disconnections superimposed to the twin boundary perfect structure. Owing to their large Burgers vectors, the final
disconnections are always dissociated. 相似文献