O-phase and carbides precipitation in intermetallics based on Ti−Al

In Ti−Al based intermetallics, precipitates of several ternary phases, such as the orthorhombic O-phase, perovskite P-phase and hexagonal H-phase structures, were experimentally identified during aging. These phases are believed to develop from the matrix of the α₂-phase as a result of the characteristic diffusion of the alloying elements that involves changes in chemical composition and lattice structure. In this paper, the formation mechanism of these phases was studied in the matrix of Ti−Al-based intermetallic compounds. The O-phase was found to form in the primary equiaxed α₂ grains in multivariant forms and as fine α₂+O mixtures in Ti-24A1-14Nb-3V-0.5Mo alloy during solution treatment at 1000°C/1h followed by water quenching and aging at 650°C and 850°C. The supersaturation of Nb, the β stabilizer, resulted in a segregation of the Nb-rich region and Nb-poor region, incurring the formation of the O-phase as a consequence of the α₂-phase decomposition. A crystallographic orientation relationship between the α₂-phase and the O-phase was observed. In Ti-46.6A1-2Mn-1.4Mo-0.3C alloy, P-phase carbides formed mainly at the α₂/γ interface whereas some H-phase carbides were also found within the γ grain and at prior α grain boundaries. The presence of ledges between the P-phase and α₂-phase interfaces indicated that the nucleation stage of the P-phase involved an interface-controlled process with a coherency relationship at the interface. This article is based on a presentation made in the “Symposium on Metastable Phases”, held at Korea Institute of Science and Technology, Seoul, Korea, November 10, 2000 sponsored by The Korean Institute of Metals and Materials.     

Atomistic mechanism of cyclic phase transitions in Nd–Fe–B based intermetallics

Mechanical milling induced atomic disorder and the mechanism of the cyclic crystal → amorphous → crystal phase transitions in a Nd₂Fe₁₄B intermetallic – based Nd₁₀Fe₈₅B₅ alloy were investigated. Least square first shell fitting of Extended X-ray Absorption Fine-Structure (EXAFS) data of mechanically milled samples revealed that phase evolution during mechanical milling of Nd₁₀Fe₈₅B₅ alloy occurred in two stages as a result of two dynamical effects: milling induced forced atomic mixing and enhanced diffusion due to continuous production of point defects. In the first stage of milling, atomic mixing of Nd and Fe atoms dominated, resulting in extensive chemical and structural disorder in the initially ordered intermetallic, followed by amorphization. The second stage of milling was characterized by a remarkable increase in point defect density. Enhanced diffusion resulted in redistribution of alloy atoms and annihilation of these point defects leading to the precipitation of α-Fe nanocrystals in the amorphous matrix. Thus the time evolution of defect structures formed during mechanical milling and their atomic scale interactions led to the cyclic phase transitions in the Nd₁₀Fe₈₅B₅ alloy. These milling induced changes in atomic order were found to strongly influence magnetic properties. Crystallization of mechanically milled alloy and effect of annealing parameters on magnetic properties was also analyzed.     

Analysis of electron structure of γ/α2 phase boundaries in ternary TiAl intermetallics

The electron structure of γ/α2 phase boundaries in lamellar colonies in Ti-47Al-2M（M=Nb, Cr, V） （mole fraction, %） alloys was theoretically investigated by Empirical Electron Theory of Solid and Molecules （EET） and the bond-length-difference （BLD） method. Average-Atom-Model was employed to calculate valence electron structure of TiAI intermetallics containing site substitution elements. On this basis, the boundary condition of electron movement was employed in the improved Thomas-Fermi-Dirac （TFD） theory to decide the continuity of the electron density of the lamellar colonies interface and it is found that of γ/α2 interface is continuous（△ρ〈10%）. Furthermore, it is found that adding alloying elements （including Nb, Cr, and V） can improve the electron density （ρ） of ）γ/α2 interfaces, and decrease the electron density difference（△ρ） of γ/α2 interfaces. Adding V element decreasing △ρ is more remarkable than other site substitution elements. According to electron structure study of γ/α2 interfaces in Ti-47Al-2M alloys, the added elements improve mechanical properties of the alloy in the following order： V〉Cr〉Nb.     

Oxidation of the intermetallics MoSi2 and TiSi2—A comparison

The oxidation behavior of two MoSi₂ variants, one Mo-rich and one Si-rich, and TiSi₂ was investigated between 1000 and 1400°C in air, oxygen and an 80/20-Ar/O₂ mixture. A protective SiO₂ scale develops on MoSi₂ in all atmospheres in the temperature range investigated. The SiO₂ modification changes around 1300°C from tridymite to cristobalite. This change in SiO₂ modification seems to cause an enhanced formation of SiO₂ and evaporation of MoO₃. The SiO₂ grows at the MoSi₂-scale interface. In air a two-layer scale grows on TiSi₂ between about 1000 and 1200°C with an inner inwards growing fine-grain mixture of SiO₂ + TiO₂ and an outer outward-growing TiO₂ partial layer. TiN formation in the transient oxidation is responsible for the formation of the inner mixed partial layer because in N -free atmospheres a scale of a SiO₂ matrix with some Ti oxide precipitates inside is formed. A one-layer scale structure similar as that in N-free atmosphere is found on TiSi₂ in air at T > 1200°C. In oxygen the TiO₂ precipitates grow as needles mostly oriented perpendicular to the surface. Due to the faster oxygen transport in TiO₂ compared with SiO₂, these TiO₂ needles act as oxygen pipes, causing an enhanced oxidation of TiSi₂ in front of these needles. The SiO₂ scale dissolves about 1–2% TiO₂. This doping causes a mixed oxygenand Si transport with the consequence that the SiO₂ scale on TiSi₂ grows partly by oxygen transport inwards and Si transport outwards. The SiO₂ modification is cristobalite over the entire temperature range investigated.     

Effect of microstructure on tensile properties and fracture behavior of intermetallic Ti2AlNb alloys

The tensile properties and fracture behaviors of Ti-22Al-27Nb and Ti-22Al-20Nb-7Ta alloys were investigated in the temperature range of 25-800℃ Three typical microstructures were obtained by ifferent thermomechanical processing techniques.The results indicate that the duplex microstructure has an optimum combination of tensile yield strength and ductility both at room and elevated temperatures.Adding Ta to Ti2AlNb alloy can improve the yield strength,especially at high temperature while retain a good ductility.The study on crack initiation and propagation in dedformed microstructure of Ti2AlNb alloys indicates that microstructure has ikmportant effect on the tensile fracture mechanism of the alloys.The cracks initiate within primary O/α2 grains along O/B2 boundaries or O phase laths in B2 matrix,and propagate along primary B2 grain boundaries for the duplex microstructure.The fracture mode is transgranular with ductile dimples for the duplex and the equiaxed microstructures,but intergranular for the lath microstructure.     

Calculation of activity coefficients for components in ternary Ti alloys and intermetallics as matrix of composites

Based on Kohler‘s ternary solution model and Miedema‘s model for calculating the formation heat of binary solution, the integral equation was established for calculating the activity coefficients in ternary alloys and intermetallics. The activity coefficients for components in alloy Ti-5Al-2.5Sn, Ti-6Al-4V and intermetallics TiAI, Ti3Al and Ti2 AlNb were calculated with the equations. The calculated data coincide well with the experimental ones found in literatures. According to the calculated activity coefficients and activities, it can be predicted that the interracial reaction in SiC/Ti3Al composite is more severe than that in composites SiC/Ti2 AINb and SiC/TiAl.     

Ti—Ti2AlNb功能梯度材料的激光立体成形研究

采用激光立体成形技术制备了从Ti到Ti2AlNb成分连续渐变,外形规则、高度为17 mm的梯度材料.分析了梯度材料的组织及相结构演变规律和硬度变化规律.随着Al和Nb成分的提高,Ti-Ti2AlNb功能梯度材料的相呈现α'→α β→α α'→α'→α β→α β/B2 α2→β/B2 α2→β/B2→B2 α2 O→B2的演变过程,实现了由α型钛合金经过α β型及β型钛合金向Ti2AlNb基合金的转变.硬度值HV从底部纯Ti的170连续渐变到顶端Ti2AlNb的470.基于钛合金富Ti区非平衡相图,并结合Al和Nb元索在钛合金中对α,β和α2稳定性的影响分析,对梯度材料在激光立体成形过程中的相演化过程进行了解释.     

Kinetics of ordering and disordering in the Fe–Al–Ti ternary alloy

The kinetics of ordering and disordering in the ternary b.c.c. Fe–Al–Ti alloy is investigated by means of the micro-master equation method. It was found that there are transient ordered states in the ternary b.c.c. Fe–Al–Ti alloy during the ordering from a quenched disordered state to the equilibrium ordered state. Two different mechanisms account for the occurrence of the transient states. In the kinetics of disordering, the evolution of the order parameters shows fluctuations.     

Anodic polarization of Ti in formic acid—I.Anodic behaviour of Ti in relation to anodizing conditions

The anodic behaviour of Ti in formic acid at various concentrations and temperatures hasbeen studied by means of V-t curves at constant i using a sensitive recorder to obtain the detailed shape of the curve. Depending on electrolytic conditions the Ti either remains passive, micropits or corrodes uniformly.A characteristic plateau at ∼ 7–12 V has been observed on the V-t curves in the micropitting region of concentration and temperature. Electron microscopy has revealed that this plateau corresponds with the formation of blisters in the oxide which rupture at the breakdown potential of ∼ 45 V. The uniform distribution of micropits on the metal surface has been explained on the basis of the different rates of growth of oxide on different crystal faces, resulting in a random distribution of blisters in the oxide film.     

Ti2AlNb基合金热处理中的相结构与相变预测

运用相图计算技术,预测了名义成分为Ti-22aAl-27Nb的Ti2AINb基合金在热处理过程中的相结构与相转变,得到了该合金的相转变温度、相比例和各相的成分等重要信息,将材料热力学中的亚晶格模型和第一性原理总能计算相结合,对Ti2AlNb基合金中的主要组成相-O相(有序正交相)中的合金元素在各个亚晶格上的占位分数和有序化行为进行了深入研究,表明O相的有序.无序转变属于具有连续变化特征的二级相变,预测结果与现有实验结果相当吻合.     

Effect of hydrogen on the formation of the structure and phase composition in the Ti2AlNb-based alloy

Methods of X-ray diffraction, electron microprobe analysis, electron microscopy, differential thermal analysis, and microdurometry were used to study the effect of alloying with 5.2 and 8.5 at % hydrogen on the occurrence of phase and structural transformations in the intermetallic alloy Ti—24.3 Al—24.8 Nb—1.0 Zr—1.4 V—0.6 Mo—0.3 Si (at %) upon heating in the temperature range of 700-1050°C. It has been established that hydrogen favors the stabilization of the ? solid solution, the suppression of the formation of the α ₂ phase, and the reduction of the temperature of heating for quenching, starting with which the formation of an ordered a phase is possible in the alloy structure. Procedures of calculation of the electron concentration in the alloys with hydrogen have been proposed. By the method of the differential scanning calorimetry, the temperatures of disordering have been determined in the alloys.     

激光立体成形Ti2A1Nb基合金组织演化

利用SEM、XRD和TEM研究了激光立体成形(LSF,lasersolid forming)Ti2A1Nb基合金的组织演化规律。结果表明:激光立体成形Ti2A1Nb基合金沉积态组织由B2/β0相(ordered-bcc)和O相(orthorhombic)组成。沉积态的相组成及微观结构从成形试样顶部→中部→底部呈现从B2/β0固溶体→B2/β0+O魏氏体→B2/β0固溶体的变化,进而使得激光沉积态Ti2A1Nb基合金中间部位的显微硬度值最大,顶部与底部相当。合金显微硬度随激光功率的增大而增大。     

Effect of compositional modification on Young's modulus of Ti2AlNb-based alloy

The Young's moduli of three Ti₂AlNb-based alloys were investigated. When they were heat treated to have a B2 single-phase structure, their Young's moduli were identical. In an O+B2 structure, the Young's modulus of O phase changed with alloy, and showed a correlation with the lattice parameter rate a/b.     

Ti2AlNb基合金表面渗铬层结构及其摩擦学性能

采用双层辉光等离子表面合金化技术对Ti2AlNb基合金进行表面渗Cr处理,结果表明：经过离子渗铬处理后可获得约25μm的表面合金层;渗层中Cr含量随渗层深度呈梯度变化,且由于原子扩散能力的差异,在渗层的不同区域形成不同相,结合XRD和XPS分析技术,证实表层以含Cr2Nb的Laves相和Al8Cr5相为主;渗层硬度值由外层的HV0.11 125逐渐过渡到基体的HV0.1400左右;渗Cr处理后合金室温摩擦因数由原来的0.24降低到0.15,由于硬质Cr2Nb和Al8Cr5相起到支撑载荷的作用,降低粘着磨损,室温磨损率降低60%以上;高温条件下由于氧化和摩擦磨损的共同作用,摩擦因数变化不大,磨损率降低20%。     

Effect of plasma surface tungstenising on the friction and wear of Ti2AlNb-based alloys

To obtain a strong bond between W coatings and the substrate, a novel graded tungstenised layer on Ti-Al-Nb alloys was produced using a double glow plasma surface alloying technology and a special graded tribological coating was designed. The microstructural results showed that the tungstenised layer was distributed in a graded manner and was mainly comprised of W- or Ti-rich Ti_xW_1?x phases. Varying the friction conditions indicated that an increase in the load and sliding speed led to an increase of the friction coefficient and wear rate of the tungstenised layer at room temperature. These changes were mainly caused by the graded distribution of the W composition and the change in surface contact status. The results indicated that the friction and wear properties of Ti-Al-Nb alloys were greatly improved by the surface tungstenising.     

High-temperature tribological behaviors of Ti2AlNb-based alloys by plasma surface duplex treatment

Plasma tungstening followed by carburization (W-C duplex treatment) was performed on the Ti₂AlNb-based (O phase) alloy by using the double glow plasma process to enhance its wear resistance. The microstructure and high-temperature tribological behaviors of the un-treated and W-C duplex-treated samples were investigated. The results show that the duplex-treated layer is mainly composed of W₂C or W₆C_2.54 phases and the contents of W and C elements in the alloyed layer change gradually along the depth by surface plasma duplex treatment. The diffusion depth of W is about 12 μm, while the carbon atoms most exist in the depth more than 12 μm. High temperature tribometer tests indicate that the friction coefficient of the W-C duplex-treated layer is approximately 1/6 that of substrate. The wear rate of the duplex-treated layer is about 28% that of the untreated one. So, plasma surface W-C duplex treatment can obviously improve the high-temperature tribological resistance of Ti₂AlNb-based alloy. The tribological mechanism of the duplex-treated layer is discussed by dividing the friction process of the duplex-treated layer into three fluctuate stages. The first stage is the formation of oxide film between W-C duplex-treated layer and counterface. The second stage is the detachment of oxide film, acting as “the third body”. The last stage is the period that the friction and wear occur between the compact particle layer and counterface.     

Influence of Ti and Zr additions on solidification of Fe base intermetallics precipitating Laves phase

Abstract

Fe–Al–Zr and Fe–Al–Ti alloys are potential candidates for high temperature structural applications. Ti and Zr additions strongly enhance the mechanical properties of Fe–Al based alloys by the precipitation of a Laves phase. The quaternary Fe–Al–Zr–Ti system combines the benefits of the precipitation of the Laves phase and reduction in weight due to the aluminium addition. Such alloys are promising candidates for the development of new casting alloys which could be substituted for heavier and more expensive stainless steels. This study aims to enhance the knowledge concerning the solidification of alloys belonging to the iron rich corner of the Fe–Al–Ti–Zr quaternary system. Before studying the solidification process of the quaternary alloys, it is mandatory to know how the corresponding ternary systems behave. Experimental results are presented concerning the equilibria in the iron rich corner of the Fe–Al–Zr and Fe–Ti–Zr systems to determine new data or clarify uncertainties in the literature. These results are then synthesised to help to understand the stable Laves phase structure in quaternary alloys.     

NiCoCrAlY coatings with and without an Al2O3/Al interlayer on an orthorhombic Ti2AlNb-based alloy: Oxidation and interdiffusion behaviors

The mechanism of oxidation protection of NiCoCrAlY overlay coatings on the orthorhombic Ti₂AlNb-based alloy (O alloy) Ti–22Al–26Nb (at.%) is described. While the bare alloy exhibited poor oxidation resistance at 800 °C, adding NiCoCrAlY coatings significantly improved the oxidation resistance. However, serious interdiffusion between the coatings and the substrate resulted in rapid degradation of the coating system. Several reaction layers were formed at the coating/substrate interface by interdiffusion, and non-protective scales mainly of Cr₂O₃ and TiO₂ were formed due to the degradation of the coating. In order to solve this problem, an Al₂O₃/Al interlayer was sandwiched into the coating system as a diffusion barrier. The isothermal and cyclic oxidation protection of the multilayer coating system on the Ti–22Al–26Nb substrate was evaluated at 800 and 900 °C. The results indicated that the interdiffusion was much suppressed, and the duplex coating system demonstrated improved oxidation resistance on the Ti–22Al–26Nb substrate, with a thin and adherent protective α-Al₂O₃ scale forming on the surface.