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采用电弧熔炼(CA)和机械合金化(MA)技术制备了晶粒尺寸差别较大的二元单相Fe-Si合金,并研究了其在900~1000 ℃纯O2中的高温氧化行为.结果表明2种方法制备的Fe-1Si和Fe-3Si合金氧化后均形成了Fe2O3→Fe3O4 FeO→FeO→FeO SiO2结构的氧化膜,均未形成连续的SiO2保护膜.机械合金化Fe-Si合金的氧化速度均大于相同温度下同成分的熔炼Fe-Si合金,这是因为细晶材料及其氧化生成的氧化膜中包含了更多的晶界,为合金各元素及氧提供了更多的短路扩散通道,使它们的扩散速度均有明显提高,SiO2富集区(FeO SiO2)的快速形成对Fe的进一步向外扩散起到了一定的阻挡作用,但这对提高合金抗高温氧化性能所起的正影响不及Fe的快速扩散对其造成的负影响. 相似文献
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采用机械合金化+热压烧结工艺制备Laves相NbCr2合金,研究球磨时间对热压合金晶粒尺寸及致密度的影响,探讨了晶粒细化对其1100 ℃氧化行为影响的作用机制.结果表明,随着Cr-Nb粉末球磨时间的延长,热压合金组织越来越致密,晶粒尺寸越来越小.当球磨时间达到100 h时,NbCr2合金的相对致密度及晶粒尺寸分别达到98.7%和35 nm.晶粒的细化有利于Cr原子扩散能力的提高,促进了Cr2O3膜的形成;但晶粒过细时,导致氧化膜内生长应力的增加,引起氧化膜的开裂、脱落,从而加剧了氧化. 相似文献
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采用晶粒表面的萃取碳复型技术,观察研究了钛在一种具有大晶粒尺寸的高温合金中的晶界析出行为-晶界(Ti,Mo)C脆性包膜的形成和效应;不同含量的铌对一种高温合金晶界析出相类型、形貌分布、成分和数量的影响;分析讨论了钨和钼等元素对某些高温合金晶界碳化物和硼化物的析出行为存在的直接作用和间接作用。 相似文献
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剧烈塑性变形对块体纳米金属材料结构和力学性能的影响 总被引:1,自引:0,他引:1
综述了剧烈塑性变形引起的块体纳米金属材料的结构和力学性能演变.以电化学沉积法制备的fcc结构纳米晶Ni-20%Fe(质量分数)合金为研究对象,通过对其进行不同应变量的高压扭转实验,系统分析了变形引起的结构和力学性能演变.结构表征结果表明:(1)变形引发纳米晶Ni-Fe合金晶粒旋转,实现晶粒长大.同时,晶粒长大过程伴随着位错密度、孪晶密度的演变;(2)存在一个最有利于变形孪晶生成的晶粒尺寸范围(45~100 nm),在这个晶粒尺寸范围之外,去孪晶起主导作用使原有的生长孪晶或变形孪晶消失;(3)位错密度是影响位错与孪晶反应的新的影响因素.当发生孪晶的晶粒内位错密度低时,位错可完全穿过孪晶界,部分穿过孪晶界,或被孪晶界吸收;发生孪晶的晶粒内位错密度高时,大量位错缠绕并堆积在孪晶界附近,形成应力集中,破坏孪晶界原有的共格性.为释放局部应力,将从孪晶界的另一侧发射不全位错形成层错和二次孪晶;(4)在塑性变形导致的晶粒长大过程中,原先偏聚于消失了的晶界上的C和S沿残留晶界扩散并继续偏聚于晶界上.结构与力学性能关系结果表明:随着应变量的增加,应变强化、应变软化交替出现.位错密度对硬度的演变起主导作用,其它结构演变(如孪晶密度的变化和晶粒尺寸变化)对硬度的演变起次要作用. 相似文献
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以双相亚稳Fe50Mn30Co10Cr10高熵合金为基体,通过添加Al元素,制备了(Fe50Mn30Co10Cr10)97Al3高熵合金。对其进行轧制及退火处理,研究了退火温度对合金再结晶行为、退火孪晶演变及力学性能的影响。结果显示,随着退火温度的升高,合金组织分别发生了部分再结晶、完全再结晶和晶粒长大现象。由于高熵合金具有严重的晶格畸变效应及迟滞扩散效应,使得合金在退火后表现出较高的再结晶温度(0.59 Tm)和抗晶粒粗化温度(700 ℃)。600~700 ℃退火态合金中形成大量退火孪晶,随着退火温度的进一步升高(800~900 ℃),由于晶界/孪晶界的迁移,退火孪晶界密度显著降低。拉伸试验结果表明,700 ℃退火态合金表现出良好的综合力学性能,抗拉强度为730 MPa,均匀延伸率为50.5%。同一退火温度下,单个晶粒中退火孪晶变体的数量与其晶粒尺寸有关,尺寸较小的晶粒中易形成单孪晶变体,尺寸较大的晶粒中易形成多孪晶变体。 相似文献
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Kyung-Tae Park Chong Soo Lee Dong Hyuk Shin Yong Shin Lee Won Jong Nam 《Metals and Materials International》2006,12(2):107-113
Grain boundary diffusion creep equations developed previously for nanocrystalline materials were reappraised in order to elicit
further understanding of plastic deformation of these materials in relation to grain boundary diffusion. From a mechanistic
viewpoint, the strain rate is inversely proportional to the second power of the grain size when the grain size is refined
to the same order of the grain boundary thickness. The presence of the threshold stress appears to be inherent, as a relatively
large volume fraction of the grain boundary region is associated with irregularities. 相似文献
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Progress on grain growth dynamics in sintering of nano-powders 总被引:1,自引:0,他引:1
Nanostructured materials, characterized by an ultrafine grain size, have stimulated much research interest by virtue of their unusual mechanical, electrical, optical, and magnetic properties. In this paper, the sintering process of nano-powders were reviewed, to which sintering of the traditional materials compared. The microstructural development, i.e., grain growth and densification during sintering as well as the mechanism of crystal surface diffusion and boundary migration were analyzed, and the dynamic models on sintering process were summarized by the relationship of grain growth and pores size, interface diffusion, densification rate, and sintering temperature. Finally, the research tendency of this major on the basis of above models was discussed. 相似文献
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综述了纳米面心立方金属的变形机制随晶粒尺寸的减小而发生的变化,即变形机制由晶界处发射不全位错、形成孪晶转变为晶界滑移、晶粒转动.当变形机制为晶界处发射不全位错、形成孪晶时,存在最佳孪晶形成晶粒尺寸范围,此时的孪晶形核应力最小.另一方面,随着晶粒尺寸的减小,在变形机制发生转变的临界晶粒尺寸附近存在韧-脆断裂方式的转变.提高孪晶密度、在纳米晶材料中加入微米晶相形成双峰晶粒材料可以提高纳米晶材料的塑性,得到更好的综合机械性能. 相似文献
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Superplastic-like viscous deformation of bulk metallic glass alloys around the glass transition temperature (Tg) was analyzed
based on the Nabarro-Herring creep model, a classical creep model, where the diffusional motion of atoms or vacancies through
the lattice (atomic configuration) is considered. The amorphous matrix of bulk metallic glasses that has a randomly-packed
atomic configuration was assumed to behave in a manner similar to the grain boundary in polycrystalline metals so as to approximate
the diffusivity of the major constituent element. In spite of rough approximation of the parameters in the Nabarro-Herring
creep equation, a reasonable value of the diffusion path (d) could be obtained from the experimentally-obtained metal flow
data, including the steady state stress and the strain rate. Due to the absence of vacancy sources such as grain boundaries
in homogeneous metallic glasses, the diffusion path, which, in polycrystalline materials, generally is the average distance
between vacancy sources such as grain boundaries, was considered in this work as the average distance between tunneling centers
in bulk metallic glass alloys. The calculated diffusion path was comparable to the density of tunneling centers around Tg,
proposed by M. H. Cohen and G. S. Grest based on free volume theory. The calculated diffusion path showed monotonous decrease
with temperature over Tg for Zr-based bulk metallic glass alloys. Based on this analysis, a schematic model for viscous deformation
of bulk metallic glass was proposed. 相似文献
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The effect of alloy grain size on the transient oxidation behavior of an alumina-forming alloy 总被引:5,自引:0,他引:5
In the early stages of alloy oxidation, diffusion of solute through the metal to the surface is important in determining the composition of the oxide scale that forms during the transient stage. Rapid solute diffusion to the interface will promote the formation of a protective scale, thereby suppressing the formation of base-metal oxide. The effect of alloy grain size on the formation of the transient oxide scale has been studied using a very fine grained NiCrAlY alloy produced by plasma spraying. The long-term oxidation behavior of this alloy was found to be independent of the grain size of the underlying alloy. However, the short-term, transient oxidation rate was found to decrease with decreasing alloy grain size. This is attributed to the rapid grain boundary transport of Al and Cr to the oxide/metal interface which promoted the formation of Cr2O3 and Al2O3. 相似文献
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NiO scales were formed on pure Ni and Ni-0.1 wt.% Y alloy by oxidation in pure O2 at 1200°C, and the grain boundary self-diffusion coefficients of Ni in the dense region of the scale were measured by the tracer-sectioning technique. The volume diffusion coefficient was also measured in the alloy scale. The grain boundary and volume diffusion coefficients and the grain boundary width were found to be the same for the two materials within the experimental uncertainty.Work supported by the Division of Basic Energy Sciences, U.S. Department of Energy. 相似文献
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氧化法可用于钢中晶粒度的测定。研究控制氧化时间对弹簧钢50CrV奥氏体晶粒度的影响。结果发现,氧化过程大致分为3个阶段:第一阶段,过剩铁素体相不断转变为奥氏体;第二阶段,基体与氧化膜之间的阴阳离子再次加快扩散,下层金属基体继续氧化脱碳,部分之前试样表面的奥氏体晶界被替代;第三阶段,一、二阶段的奥氏体晶粒逐渐长大,新旧氧化膜叠加生长,并在55SiCr、60Si2Mn、65Si2CrV钢得到验证。在不影响检测结果准确性的情况下,氧化时间可以由标准的60 min缩短至一半左右,再通过修正可得到晶粒度为10.96级,此结果与直接腐蚀法11.00级和氢致沿晶断口法11.09级相近。 相似文献
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The effect of grain refinement on the superplasticity of ingot-processed magnesium alloy was investigated. From the AZ61 material
with a linear intercept grain size of 5 μrn, which was obtained by the multi-rolling process at an elevated temperature, tensile
elongation over 400% could be achieved at 10-3s-1 at 400°C with a maximum value of 560% at 2x10-4s-1 at the same temperature. It was found that grain boundary diffusion controlled grain boundary sliding and pipe diffusion
controlled slip creep govern the plastic flow at low and high strain rate ranges, respectively. A deformation map for pure
magnesium was constructed to examine the effect of grain size and flow stress on deformation behavior at elevated temperature.
The superplastic formability of Mg alloys was demonstrated by forming an AZ61 sheet into a hemi-sphere. 相似文献