AlFeCuCoNiCrTi_x高熵合金的退火组织及硬度变化

利用XRD、SEM和DSC方法研究了AlFeCuCoNiCrTix(x=0、0.5、1)高熵合金退火态的微观组织、相结构以及相转变,同时利用洛氏硬度仪测量了各退火温度下的硬度变化.结果表明,随着退火温度的逐渐升高,TiO合金的相组成大约在636℃以后会逐渐由原来的fcc+bcc结构变为fcc1+fcc2+bcc结构,其硬度在636℃会略微增加,在636～1112℃之间下降明显,在1112℃以后基本维持不变;对于Ti0.5合金,退火时其相组成基本没有影响,一直保持fcc+bcc1+bcc2的结构,其硬度在607℃会略微增加,在607～1092℃之间下降明显,在1092℃以后基本维持不变;而对于Til舍金,当退火温度达到800℃时,会有Fe2Ti型的Laves相析出,这有助于提高材料的硬度,当退火温度达到1200℃时,其硬度可以提高到51.3HRC.     

PdCu合金复合膜的制备与评测

采用化学镀方法在非对称氧化铝管状载体上制备出3μm厚的Pd59Cu41(质量分数,%)合金复合膜.考察了合金膜在473～953 K间的氢气渗透性能.在高于873 K和低于723 K温度区间内,氢气渗透都遵循单一的Arrhenius方程,活化能分别为30.0和9.8 kJ/mol.在723～873 K温度区间内,氢气渗透量随温度降低而增加的现象说明此时晶体结构为fcc和bcc的混合相,并且晶体结构正逐渐从fcc相向bcc相转变.0.5的压力指数说明在473～873 K温度区间内氢气扩散都遵循Sievert's方程.合金膜的厚度,组成和晶体结构分别用SEM,EDS和XRD进行表征.     

亚稳态奥氏体不锈钢的应力腐蚀开裂

研究了亚稳态奥氏体不锈钢的应力腐蚀开裂。试验结果指出:(1)裂纹尖端塑性区中的塑性形变会导致γ-α′相转变,所以应力腐蚀断日表面层主要为α′体;(2)这类亚稳态奥氏体不锈钢的SCC,实质上是α′体的类解理断裂,裂纹的扩展和断裂过程为: 塑性形变 阳极溶解 γ(fcc)——→α′(bcc)——→类解理断裂     

V系储氢合金及其合金化

概述了V系储氢合金的研究现状,涉及V系储氢合金的氢化物相结构、合金化元素及第二相对合金吸、放氢性能的影响:V系储氢合金随吸氢量的增加,氢化物结构发生bcc→bct→fcc转变,同时其稳定性呈降低趋势;合金元素通过改变V与H的亲和力以及氢化物的稳定性来影响合金的储氢性能;第二相的出现对合金电化学性能、吸放氢动力学有明显的影响作用.在上述分析的基础上,对V系bcc固溶体储氢合金今后的研究进行了展望.     

Fe52T2(T=Cr,Mn,Co,Ni)合金bcc与fcc相结构的第一性原理研究

运用基于密度泛函理论的第一性原理方法研究稀释掺杂的Fe_(52)T_2(T=Cr, Mn, Co, Ni)合金铁磁bcc相和反铁磁fcc相结构的晶格参数、磁性和两相的相对稳定性。结果表明:晶格参数和体模量随掺杂元素d壳层电子个数的变化关系不能用简单的d能带填充图像解释,说明FeT合金中存在较强的磁-结构耦合效应。FeT合金的铁磁bcc相比反铁磁fcc相稳定。反铁磁相呈四方结构,晶格常数c/a比值约为1.07,此相结构可能是一个亚稳态。晶格结构的变化引起电子的重新分布,导致不同磁结构和局域原子磁矩。     

Ti-V合金及其氢化物结构的研究

介绍了3种组成的Ti-V合金(Ti0.76V0.24、Ti0.51V0.49和Ti0.25V0.75)及其氢化物的晶体结构确定方法.通过XRD分析和理论计算,确定这3种合金为bcc结构,它们的纯β相氢化物是fcc结构,并测得了这两类结构的晶胞参数.研究发现,Ti-V合金吸氢性能比金属V有所改善,H原子数与合金原子数之和的比大于1.0才能发生α相氢化物向β相氢化物的转变.     

外压力下TiN电子结构的第一性原理研究

利用基于密度泛函理论的赝势平面波方法,研究了fcc-TiN在不同压力下的稳定性以及电子结构。通过焓压计算得到TiN由fcc结构到bcc结构的相变压力值约为350GPa,并研究了fcc-TiN的力学稳定性。对fccTiN以及bcc-TiN的电子结构进行计算,对得到的电荷密度、能带结构以及电子态密度进行综合分析和比较,发现随着压力的增加TiN的金属性降低。     

Pd-Cu合金复合膜的制备及表征

采用无电化学镀技术制备了Pd-Cu合金复合薄膜.复合膜的初始基体是0.2μm等级的316L多孔不锈钢圆片(PSS),经过改性的(含Pd)溶胶-凝胶ZrO2成膜技术对PSS表面进行了修饰.修饰后的PSS表面首先进行无电镀金属Pd膜,然后在Pd膜表面再镀金属Cu膜.最后把具有双金属镀层的膜片在773 K ,H2气氛(101kPa)下保持5～10h进行退火热处理,通过金属间分子热扩散把不锈钢基体上的金属Pd膜和金属Cu膜合金化为均匀的Pd-Cu合金复合薄膜.XPS确定表面组成为Pd90Cu10 (质量分数/%) 合金薄膜, 经过XRD分析结合确定为单相无序fcc结构;而Pd59Cu41合金膜是由平衡fcc 相和有序 bcc相组成.通过SEM观察到Pd90Cu10 合金薄膜(厚度5μm)表面存在一些针孔;而Pd59Cu41合金膜(厚度10μm)没有针孔存在,这种合金复合膜应该具有极高的透H2选择性.     

还原温度与SiO2壳层对Co/SiO2复合颗粒尺寸和相结构的影响

利用非均相沉淀-H2还原方法制得了平均粒径为20～30nm的Co/SiO2纳米复合颗粒.探讨了非均相沉淀-H2还原法的还原温度和SiO2壳层对Co/SiO2纳米复合颗粒尺寸和组织的影响.实验结果表明,在复合颗粒中,非晶SiO2壳层对纳米Co颗粒成功地进行了包覆.纳米尺度的fcc结构芯核Co可以在室温存在,但颗粒尺寸大于某一临界尺寸时,芯核Co将由fcc向hcp相转变.通过试验得出芯核Co由fcc向hcp相转变的临界尺寸,该试验结果和理论数据相吻合.并证明,还原过程中fcc结构Co的出现对应于一个临界温度,当还原温度低于该临界温度时,不能得到fcc结构的Co.     

矿物界面破碎特征分析与预测

根据不同组分矿物界面的差异性,利用盒子维法实现对矿物界面几何特征的描述、利用原位加载实验实现对矿物界面力学特征的描述,并利用反向传播(back propagation, BP)神经网络算法对矿物界面几何特征和力学特征进行映射预测。结果表明:所研究黑钨矿石中石英-钨界面分形维值大于石英-硅质岩界面的,且石英-钨界面分形维值范围为1.855 1～1.936 8、石英-硅质岩界面分形维值范围为1.163 3～1.371;不同组分粘结界面的力学性质存在差异,且与组成矿物的物理属性、形态特征等相关,其中石英-硅质岩界面最小破碎应力范围为1.178 5～1.482 6 GPa,石英-钨界面最小破碎应力范围为1.335 5～1.542 03 GPa; BP神经网络可实现矿物界面几何特征对力学特征的有效预测,在预测前期误差最大仅为4.14%,随着样本数据增多,预测精度越来越高,在预测后期误差最低仅为0.011%。     

Role of five-fold twin boundary on the enhanced mechanical properties of fcc Fe nanowires

The role of 5-fold twin boundary on the structural and mechanical properties of fcc Fe nanowire under tension is explored by classical molecular dynamics. Twin-stabilized fcc nanowire with various diameters (6-24 nm) are examined by tension tests at several temperatures ranging from 0.01 to 1100 K. Significant increase in the Young's modulus of the smaller nanowires is revealed to originate from the central area of quinquefoliolate-like stress-distribution over the 5-fold twin, rather than from the surface tension that is often considered as the main source of such size-effects found in nanostructures. Because of the excess compressive stress caused by crossing twin-boundaries, the atoms in the center behave stiffer than those in bulk and even expand laterally under axial tension, providing locally negative Poisson's ratio. The yield strength of nanowire is also enhanced by the twin boundary that suppresses dislocation nucleation within a fcc twin-domain; therefore, the plasticity of nanowire is initiated by strain-induced fcc→bcc phase transformation that destroys the twin structure. After the yield, the nucleated bcc phase immediately spreads to the entire area, and forms a multigrain structure to realize ductile deformation followed by necking. As temperature elevated close to the critical temperature between bcc and fcc phases, the increased stability of fcc phase competes with the phase transformation under tension, and hence dislocation nucleations in fcc phase are observed exclusively at the highest temperature in our study.     

Lattice Dynamics and Second and Third Order Elastic Constants of Iron at Elevated Pressures

We analyze the lattice dynamics of Fe in different crystal phases (bcc, fcc and hcp) by using density-functional theory. The study on equations of states indicates that bcc Fe is more stable than fcc and hcp Fe at low pressures. However, dynamical instabilities in lattice vibrations of bcc Fe predict a phase transformation from bcc to hcp at higher pressures. We reported a complete set of second-order and third-order elastic constants of Fe in these three phases. We observed a linear variation in the values of second order elastic constant as a function of increased pressures. The phonon spectra were also analyzed to understand the stability of Fe in different phases.     

Metastable phase formation in undercooled Fe-Co melts under terrestrial and parabolic flight conditions

Soft magnetic Fe-Co alloys display primary fcc phase solidification for>19,5 at% Co in conventional near-equilibrium solidification processes. Undercooled Fe-Co melt drops within the composition range of 30 to 50 at% Co have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolution Siphotodiode. Melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterize the solidification sequence and microstructure. The transition from stable fcc phase to metastable bcc primary phase solidification has been observed after reaching a critical undercooling level. The critical undercooling increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. Under microgravity conditions, a much smaller critical undercooling and an increased life time of the metastable bcc phase were obtained. This result was validated with TEM investigations. The appearance of Fe-O particles gives an indirect hint for an intermediate fcc phase formation from the metastable bcc phase at elevated temperature.     

Coalescence of martensite under uniaxial tension of iron crystallites by atomistic simulations

Molecular dynamics simulations are used to study the effects of tensile loading on nucleation and subsequent growth of bcc phase in pure fcc iron. The results show that orientation variant selection occurs during the stress-induced fcc-to-bcc transformation, which leads to the coalescence of neighbouring bcc platelets with identical orientation. The bcc phase nucleates mainly following Nishiyama–Wassermann and Kurdjumov–Sachs orientation relationships with the parent fcc phase. The present simulations contribute to a better understanding of mechanisms controlling mechanically induced martensitic transformation as well as coalescence of bcc platelets in steels.     

The pressure-volume relation of ytterbium up to 9 GPa

The pressure-volume relation of ytterbium has been determined up to 9 Gpa using tungsten carbide opposed anvil high pressure x-ray camera. The fcc phase of ytterbium is observed between one atmosphere and 4 GPa and the bcc phase above 3·5 GPa. The bcc phase can be metastably retained down to 1 GPa by gradually decreasing the pressure from a region where only bcc phase alone is observed. The bulk modulus,B ₀, at zero pressure and the pressure derivative of the bulk modulus,B’ ₀, are determined by fitting Murnaghan equation to the pressure-volume data. The following values were obtained:B ₀=16·3 GPa andB’ ₀=3·6 for the fcc phase, andB ₀=14·7 GPa andB’ ₀=1·5 for the bcc phase. Based on the present data it is suggested that the thermodynamic equilibrium pressure for fcc ⇆ bcc transformation in ytterbium is below 3·5 GPa. The valence change under pressure has been discussed.     

Embedded atom potentials in fcc and bcc metals

A new embedded atom potential has been proposed in this paper. The potential is expressed by simple functions and is applicable to the molecular dynamics simulations of large atomic systems. The potential parameters are determined from the experimental data using the cohesive energy, Born stability, elastic constants, C₁₁,C₁₂ and C₄₄, the formation energy of a vacancy. In case of fcc the stacking fault energy is also used to fit parameters. The potential functions for copper, silver and gold for fcc metals and for bcc metals Nb, Ta and Va are presented.     

Electronic properties of Cu–Zn–Al alloys

The stability of Cu–Zn–Al was studied by total-energy calculations using the KKR–CPA method. Mixing and elastic and chemical energies, solubility lines, and Fermi surfaces were computed for the fcc and bcc structure. We find that the martensitic transformation can be considered as a transition from a disordered bcc to a disordered fcc structure. Fermi-surface nesting leads to DO₂₃ ordering while the van Hove singularities contribute to the instability of the fcc structure.     

Effect of crystal lattice and degree of dispersion of alloy constituents on the resistance to damage due to abrasive wear

Abrasive wear tests carried out under industrial conditions on facing plates of dies for pressing refractory parts showed that the structure of alloys has a substantial effect on their wear resistance. The first stage of the disintegration of an alloy in abrasive wear is the formation of submicroscopic cracks due to interaction of dislocations in two intersecting slip planes. From the standpoint of energy considerations, this interaction of dislocations occurs more readily in bcc than in fcc metals. It was established that a body-centered cubic lattice has a lower resistance to plastic formation so that the degree of plastic deformation preceding fracture is smaller in bcc than in fcc metals.     

On the stability of intermetallic phases

A general methodology using atomic clusters is applied to three problems connected to the study of alloy phase stability. The cluster method proposed by Allen and Cahn is applied to non-ideal hcp structures under tetrahedral approximation using multiatom interactions. The possible ground-state structures which are stable at absolute zero temperature are obtained. A geometrical representation in 4D parameter space of the possible strengths of multiatom interactions permitted for these structures is illustrated in terms of a 2D analogue. Extending these ideas, the cluster variation method (CVM) proposed by Kikuchi is applied to fcc structures under tetrahedral approximation to find the effect of multiatom interactions on the topology of the coherent phase diagrams in which all the phases present are derivable by mere rearrangement of atoms on the parent disordered structure. In addition, the possible invariant reactions are identified in such coherent phase diagrams. Finally the CVM is applied for calculating a model incoherent phase diagram, that of Ti-Zr system, where disordered hcp and bcc phases are present. The free energies of hcp and bcc phases are formulated using CVM procedures respectively under tetrahedral-octahedral and tetrahedral approximations. The CVM is shown to be in better agreement with the thermodynamic data and to be able to reproduce the correct value of measured enthalpy of transformation compared to that given by the regular solution model, which significantly overestimates the same.     

Tight-binding based non-collinear spin model and magnetic correlations in iron

We have extended a first principles tight-binding total energy model to include magnetic correlations. We have tested the validity of the model against ab-initio calculations for bcc and fcc Fe. We find that our model can quantitatively describe the pressure dependence of magnetic moments and magnetization energy for bcc and fcc Fe. Moreover, in fcc Fe with increasing pressures it is able to capture the transitions from ferromagnetic to non-collinear spin to antiferromagnetic and finally to the non-magnetic state. PACS numbers: 71.55 Ak, 64.30 +t. This revised version was published online in June 2006 with corrections to the Cover Date.