熔体保温时间对含铝Monel合金组织与性能的影响

通过OM,SEM,TEM和HB-3000布氏硬度计研究了在真空下,熔体保温时间对含铝Monel合金组织与性能的影响.结果表明,在不同的熔体保温时间下,含铝Monel合金组织均由树枝晶状的γ初晶和γ'相组成.其中γ'相有2种存在形式,一种为在枝晶干上单个颗粒弥散分布的γ1相和成花瓣状分布在枝晶干与枝晶间过渡区的γ'相;另一种为枝晶间的离异共晶γ'相.随着熔体保温时间的延长,树枝晶状的γ初晶先细化后粗化,二次枝晶臂间距先减小后增大,枝晶间的共晶γ'相先增多后减少,且弥散于枝晶干上的γ'相的直径先减小后增大;同时,随着熔体保温时间的延长,含铝Monel合金的布氏硬度先增大后减小.当熔体保温时间为60 min时,合金组织中树枝晶较细小,枝晶间的共晶γ',相较多,枝晶干上的γ'相直径较小,合金的布氏硬度较大.     

Si-Ni合金中小晶面枝晶生长的相场法模拟

相场法作为一种有效预测过冷熔体凝固过程的数值模拟计算方法,在模拟枝晶生长方面得到了广泛应用。应用界面能的校正形式,建立二元合金小晶面枝晶生长的相场模型,对Si-Ni二元合金小晶面枝晶生长进行了模拟。结果表明,当各向异性强度系数超过临界值υ=1/15时,枝晶生长表现出小晶面生长形态,尖端曲率效应消失,呈现出棱角状的不连续结构;同时在枝晶主支上产生大量侧向分支。实验观察验证了小晶面枝晶生长相场模型的有效性。     

粗轧机球墨铸钢水平辊开裂原因分析

某材料为球墨铸钢的粗轧机水平辊在使用很短时间后即发生径向开裂,采用宏观检验、化学成分分析、硬度试验、金相检验等方法对水平辊的开裂原因进行了分析。结果表明:不良的组织状态是导致水平辊开裂的主要原因;水平辊生产过程中未进行高温扩散退火,导致水平辊组织偏析严重,呈枝晶状分布,枝晶间隙碳化物粗大,数量较多,而枝晶杆处部分碳化物呈断续网状分布;另外,铸造过程中钢液石墨化工艺控制不佳,导致石墨数量少,球化效果差。     

钛合金中TiC晶体的配位多面体生长基元与生长习性

铸造钛合金中存在八面体、棱面枝晶及光滑树枝晶等不同形态的TiC.本文结合定向凝固试样组织,用配位多面体理论分析了钛合金中TiC的生长基元及生长习性.认为TiC的配位多面体生长基元为六配位的TiC₆,TiC₆生长基元堆积时, {100}面族和{111}面族上的生长速率不同,容易形成以{111}面为显露面的八面体状TiC晶胚.TiC晶胚在熔体中生长时,在生长速率最快的<100>方向上生长出二次枝晶臂,并在生长过程中始终保持小面界面生长特征,最终形成棱面枝晶状TiC.如TiC晶体生长时生长速率慢的面族上形成大量的螺旋位错等缺陷,则棱面消失,形成树枝晶TiC.     

常规凝固条件下Mg-Zn-Y三元稳定准晶的研究

对常规凝固条件下Mg-Zn-Y系中稳定二十面体准晶相Mg3Zn6Y1的形成进行了研究,分析了按照严格化学计量比配制的大体积单相准晶的组织形态、凝固过程中的相变点、以及准晶在稀溶液中自由生长时的形貌变化.实验发现,按照严格化学计量比配制的准晶合金组织中准晶相占有较大的体积分数,在自由生长的情况下该准晶形貌为花瓣状,此外还有部分拉长的六边形及五边形组织.该准晶是在624.95℃通过包晶反应形成的.     

两种型壳温度对DD9单晶涡轮叶片凝固组织的影响EI北大核心CSCD

为了研究1500℃和1540℃两种型壳温度对第三代单晶高温合金DD9叶片截面凝固组织的影响,采用光学显微镜、扫描电子显微镜对叶片典型截面凝固组织进行分析。结果表明:随着型壳温度的增加,DD9单晶涡轮叶片凝固组织的枝晶花样呈细小趋势,二次枝晶呈发达趋势。相同型壳温度下,叶片叶身部位的枝晶比榫头部位的枝晶更细小。随着型壳温度的增加,枝晶干和枝晶间的γ′析出相尺寸和分散度均减小,并且γ′析出相尺寸分布遵循正态分布规律。相较枝晶间区域,枝晶干区域的γ′析出相的平均尺寸减小了61%。相同型壳温度下,叶片叶身部位的γ′析出相尺寸比榫头部位的γ′析出相尺寸更细小。与截面积变小相比,提高型壳温度会使γ′析出相变小更显著。随着型壳温度的增加,γ-γ′共晶尺寸和含量减小,γ-γ′共晶组织呈葵花状和光板状两种形貌特征。     

CrFeCoNiMo高熵合金与304SS的耐蚀性对比研究

为了研究CrFeCoNiMo高熵合金的耐蚀性,采用熔铸法制备了Cr19Fe22Co21Ni25Mo13高熵合金,测试了该合金的铸态显微组织、物相组成并将该合金与304SS在耐蚀性方面进行了对比.结果 表明:CrFeCoNiMo高熵合金显微组织呈典型的树枝晶形态,其中树枝晶为典型的单相FCC结构固溶体,而枝晶间为包含有1个FCC相和1个FCC1相的混合结构固溶体;与相同腐蚀条件下的304SS相比,CrFeCoNiMo高熵合金在硫酸、盐酸、硝酸和氯化钠溶液中的耐蚀性均表现较好,腐蚀速率均较低.高熵合金内树枝晶区域无明显腐蚀,而枝晶间区域腐蚀严重,这主要是由于二者间的晶体结构存在差异.     

Al-Cu-Fe-Cr准晶增强铝基复合材料的研究

采用常规铸造法制备了四元系Al-Cu-Fe-Cr准晶,利用XRD、SEM和TEM等分析手段,分析了准晶和其类似相的微观组织以及在热处理后的相转变。以ZL101合金为基体,Al-Cu-Fe-Cr准晶颗粒为增强体,通过机械搅拌的方法制备了Al-Cu-FeCr准晶/ZL101复合材料,分析复合材料的微观组织和成分,并研究其力学性能。结果表明:铸态准晶合金组织中含有Al65Cu20-Fe10Cr5相(I+d)、准晶I相、λ-Al3Fe相以及少量的η-AlCu和θ-Al2Cu相,经880℃×30min热处理水淬后,λ相消失,得到了高纯度的Al65Cu20Fe10Cr5相和准晶I相;复合材料中由于准晶和基体之间的相互扩散,使得准晶相失稳发生分解;随着准晶含量的增加,复合材料的抗拉强度增加,延伸率减小,但由于Cr元素的加入使针状的β-Fe相转变成汉字状或骨骼状的α-Fe相,对复合材料的延伸率有提升作用。     

超高分子量聚乙烯冲击断裂形态分析EI

用辊压和模塑方法加工了超高分子量聚乙烯（ＵＨＭＷＰＥ）试样，用扫描电镜研究了它的带缺口断裂形态和断裂方式。结果表明，ＵＨＭＷＰＥ形成了一种类串晶形态，断裂是以两种方式进行的：晶间断裂和串晶自身断裂，而无定形部分呈云朵状，发白，同时断裂变形具有与金属非常相似的力学孪晶作用。     

单辊旋铸CuCr25合金的显微组织及性能

研究了单辊旋铸CuCr25合金的显微组织和性能.结果表明,单辊旋铸CuCr25合金的显微组织呈现出球状富Cr相与枝晶状富Cr并存于Cu基体中的形貌,且富Cr相的大小随条带厚度的变化而变化,分析表明,单辊旋铸合金的快速凝固过程中发生了亚稳态的液相分离.与真空感应熔炼CuCr25合金的显微硬度相比,单辊旋铸CuCr25合金的显微硬度增大了57%,这主要是由于固溶度增加和晶粒细化的缘故.     

Phason dynamics in nonlinear photonic quasicrystals

We study the dynamics of phasons in a nonlinear photonic quasicrystal. The photonic quasicrystal is formed by optical induction, and its dynamics is initiated by allowing the light waves inducing the quasicrystal to nonlinearly interact with one another. We show quantitatively that, when phason strain is introduced in a controlled manner, it relaxes through the nonlinear interactions within the photonic quasicrystal. We establish experimentally that the relaxation rate of phason strain in the quasicrystal is substantially lower than the relaxation rate of phonon strain, as predicted for atomic quasicrystals. Finally, we monitor and identify individual 'atomic-scale' phason flips occurring in the photonic quasicrystal as its phason strain relaxes, as well as noise-induced phason fluctuations.     

Covalency, semiconductor-like and thermoelectric properties of Al-based quasicrystals: icosahedral cluster solids

A unified picture of icosahedral cluster solids obtained by comparing Al-based icosahedral quasicrystals to boron-based cluster solids, which consist of icosahedral clusters of the same group III element, B, is discussed. Metallic-covalent bonding conversion in icosahedral clusters of Al and B was demonstrated by molecular orbital calculations. According to the electron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al₁₂Re, α-AlMnSi, α-rhombohedral boron). The covalent bonds observed by the electron density analysis give rise to extremely low carrier density and small density of states at E_F in α-AlMnSi and perhaps in icosahedral quasicrystals. In β-rhombohedral boron, the electronic structure and properties depend on slight variation in atomic environment of the icosahedral multiple-shell structure of the crystals. As metallic atoms are doped into specific atomic sites of β-rhombohedral boron, the atomic structure, the electrical conductivity and its temperature dependence approach those of AlLiCu icosahedral quasicrystal and equal those of AlPdRe icosahedral quasicrystal. From the absolute density measurement, it was revealed that the concept of rigid sphere packing, i.e. metallic bonding, is not applicable to AlPdRe icosahedral quasicrystal. Various semiconductor-like behaviors of Al-based icosahedral quasicrystals were demonstrated by means of not only conventional conductivity and Hall coefficient measurements but also modulated photocurrent measurements. In addition, the thermoelectric properties of AlPdRe quasicrystal were estimated. The potential for application of the icosahedral quasicrystals as thermoelectric materials is also discussed.     

Covalency,semiconductor-like and thermoelectric properties of Al-based quasicrystals: icosahedral cluster solids

A unified picture of icosahedral cluster solids obtained by comparing Al-based icosahedral quasicrystals to boron-based cluster solids, which consist of icosahedral clusters of the same group III element, B, is discussed. Metallic-covalent bonding conversion in icosahedral clusters of Al and B was demonstrated by molecular orbital calculations. According to theelectron density distribution obtained using the maximum entropy method and the Rietveld method, the bonding conversion phenomenon occurs even in cluster solids such as some Al- and B-based icosahedral approximant phases (Al₁₂Re, α-AlMnSi, α-rhombohedral boron). The covalent bonds observed by the electron density analysis give rise to extremely low carrier density and small density of states at E_f in α-AlMnSi and perhaps in icosahedral quasicrystals. In β-rhombohedral boron, the electronic structure and properties depend on slight variation in atomic environment of the icosahedral multiple-shell structure of the crystals. As metallic atoms are doped into specific atomic sites of β-rhombohedral boron, the atomic structure, the electrical conductivity and its temperature dependence approach those of AlLiCu icosahedral quasicrystal and equal those of AlPdRe icosahedral quasicrystal. From the absolute densitymeasurement, it was revealed that the concept of rigid sphere packing, i.e. metallic bonding, is not applicable to AlPdReicosahedral quasicrystal. Various semiconductor-like behaviors of Al-based icosahedral quasicrystals were demonstrated by means of not only conventional conductivity and Hall coefficient measurements but also modulated photocurrent measurements.In addition, the thermoelectric properties of AlPdRe quasicrystal were estimated. The potential for application of the icosahedral quasicrystals as thermoelectric materials is also discussed.     

The role of defects in the crystal/quasicrystal transformation

The transformation between quasicrystals and related crystals, the so-called approximant phases, appears as a major point in the understanding of quasicrystal stability. Structural defects of approximant phases seem to be involved in the mechanism of crystal/quasicrystal transformation. Theoretical works as well as observations are supporting this point of view. We here report observations made by transmission electron microscopy on two systems which provide relevant examples for two types of mechanisms. In both cases, the approximant defects are identified as antiphase boundaries. In the first system (Al-Li-Cu), the transformation is due to a progressive organization of the approximant phase defects. In the second system (Fe-Cr-Mo), the vertex of intersecting defects exhibits key structural feature for the transformation in a quasicrystal.     

非均匀温度场Fe-0.06%C合金焊接接头凝固过程模拟分析

利用元胞自动机和有限差分(CA-FD)法,采用宏-微观两种尺度,将宏观温度场与微观枝晶生长过程耦合在一起,再现了Fe-0.06%C二元合金焊接熔池的凝固过程.同时,探讨了边界散热速率对焊接熔池中枝晶生长形貌及晶粒尺寸的影响;分析了形核基底数与枝晶生长间的关系;并用实验对模拟结果进行了验证.结果表明,在熔池凝固过程中,温度梯度沿散热边界向绝热边界方向不断减小,等温线弧度不断增大;熔池散热边界附近的液相中溶质浓度远远高于绝热边附近和模拟区域中心的液相溶质浓度;模拟区域内的温度梯度随着边界散热速率的增大而升高.此外,随着形核基底的增加,柱状晶数量基本不变;而等轴晶数量不断增多,分布范围逐渐扩大,但尺寸有所减小.模拟结果反映了焊接熔池的凝固过程,并与实验结果吻合,为实际焊接工艺的选取提供了一定的参考.     

准晶体分数维结构的键取向序和径向分布函数

给出了准晶体分数维结构的三维准晶格坐标表达式,在此基础上研究了准晶体分数维结构的键取向序,长程关联函数和径向分布函数。研究结果表明,准晶体分效维结构的廿面体长程关联很强。此外,从计算得到的偏径向分布函数与晶体的偏径向分布函数类似。     

Effect of cooling rate on the formation of metastable icosahedral quasicrystal phase in rapidly solidified Al-8.2 at % Mn alloy

The microstructure of rapidly solidified Al-8.2 at % Mn alloy was analysed by X-ray diffraction, transmission electron microscopy and energy-dispersive analysis of X-rays and the effect of cooling rate on the formation of the metastable icosahedral quasicrystal phase (IQP) was investigated. The formation of IQP was found to be sensitive to the cooling rate in a rapidly solidified alloy of a certain composition. A lower critical cooling rate at which metastable IQP starts to appear and an upper critical cooling rate at which IQP suppresses completely the stable crystalline phase exist. The fact that the amount and the manganese concentration of IQP change non-linearly with the cooling rate suggests that there is an optimum cooling rate at which both the amount of IQP and its solute concentration reach maximum values in an alloy of a certain composition.     

Structural stability and kinetics of hydrogen in TiZrNi quasicrystals

When hydrogen atoms slowly diffuse into a metastable structure, a coherence length of thin TiZrNi quasicrystal metallic ribbons is increased. Improved atomic order with addition of hydrogen is an uncommon phenomenon in metals, and may reflect the unordinary structural property of quasicrystals. Metastable TiZrNi quasicrystals prepared by rapid quenching of molten ingots were hydrogenated in a low pressure of hydrogen (lower than 1000 Torr) at high temperature. After completion of the pressure-composition-temperature measurements between 200 to 300 degrees C, X-ray diffraction data revealed that the full width at half maximum values of the diffraction peaks were decreased suggesting that the quasicrystal phase may stabilized by hydrogen inclusion. Annealing of the same sample at 200 degrees C without hydrogen yields no change of the relative intensity of diffraction peaks.     

准晶增强镁、铝基复合材料的研究进展

综述了利用准晶作为镁、铝合金增强相的研究进展。Mg3Zn6Y准晶的界面能低,与α-Mg基体之间形成稳定的界面结合,在高温变形过程中,准晶本身不发生粗化且可以抑制基体组织演化,因此,Mg3Zn6Y准晶增强变形Mg基复合材料具有中等强度和较高的室温、高温塑性。由于到目前为止发现的Al系稳定准晶都不与Al相共生,准晶增强Al基复合材料主要采用快速凝固的方法和外加的方法制备,如粉末冶金法、机械合金化法和液态搅拌法等。综述了采用以上方法制得的准晶增强Al基复合材料的组织特征及其力学性能。