Atom Probe Tomography Study of Fe Segregation at Phase Interface in Zr–2.5Nb Alloy

β-Nb is a typical second phase in Zr–Nb-based alloys used as fuel claddings in water-cooled nuclear reactors. The segregation of alloying element Fe may affect the corrosion resistance of Zr–Nb-based alloys. In this work, the Fe segregation at the interface between β-Nb phase and α-Zr matrix in Zr–2.5 Nb alloy was studied using atom probe tomography and focused ion beam. The results suggested that the Fe concentration was much lower than Nb concentration in α-Zr matrix, while Fe selectively segregated at the β-Nb/α-Zr phase interface, leading to a Fe concentration peak at some interfaces. The peak Fe concentration varied from 0.4 to 1.2 at.% and appeared at the position where Zr concentration was approximately equal to Nb concentration. The selective segregation of Fe should be affected by the heat treatment and structure defects induced by cold rolling.     

Formation of an Ordered Structure in the 40Au–25.4Pd–34.6Cu Alloy (wt %)

Physics of Metals and Metallography - This paper presents the results of a study of the formation of the structure and properties of the 40Au–25.4Pd–34.6Cu alloy (wt %) in the course of...     

Electron Microscopic Study of the Structure of Tetragonal Martensite in In–4.5% Cd Alloy

In this work, the formation of a packet structure composed of colonies of lamellar plates separated by twin boundary {101}_fct in In–4.5 wt % Cd alloy upon cooling below the fcc → fct martensitic transition temperature has been shown using the methods of metallography, X-ray diffraction, transmission electron microscopy, and EBSD analysis. Two neighboring lamellae differ from each other by the direction of their tetragonality axes. Using ЕВSD analysis, it has been established that neighboring packets always contain three types of tetragonal martensite lamellae, which are in twin positions and differ from each other by the direction of their tetragonality axes. In turn, each martensite lamella consists of a set of smaller lamellae, which are in twin positions. After the cycle of fct → fcc → fct transitions, the alloy recrystallizes with a decrease in the grain size by several times compared with the initial structure such that the size of packets and the length and width of martensitic lamellae in a packet correlate with a change in the size of an alloy grain.     

Role of activation energies of individual phases in two-phase range on constitutive equation of Zr–2.5Nb–0.5Cu alloy

Dominant phase during hot deformation in the two-phase region of Zr–2.5Nb–0.5Cu (ZNC) alloy was studied using activation energy calculation of individual phases. Thermo-mechanical compression tests were performed on a two-phase ZNC alloy in the temperature range of 700–925 °C and strain rate range of 10^?2–10 s^?1. Flow stress data of the single phase were extrapolated in the two-phase range to calculate flow stress data of individual phases. Activation energies of individual phases were then calculated using calculated flow stress data in the two-phase range. Comparison of activation energies revealed that α phase is the dominant phase (deformation controlling phase) in the two-phase range. Constitutive equations were also developed on the basis of the deformation temperature range (or according to phases present) using a sine-hyperbolic type constitutive equation. The statistical analysis revealed that the constitutive equation developed for a particular phase showed good agreement with the experimental results in terms of correlation coefficient (R) and average absolute relative error (AARE).     

Microstructure Evolution During Solidification of Cu–Zr–Ti Alloy Forming B2 Phase Particles Embedded in a Glassy Matrix

Microstructural evolution during injection casting Cu₅₀Zr_50?xTi_x (x?=?0–8) alloys has been investigated using X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. Cubic CuZr(Ti) B2 phase is competing against the glass transition during solidification for all the alloys and the primary B2 phase has transformed into the martensitic phase for x?<?6 alloys during cooling after solidification. The formation of spherical morphology and spatially inhomogeneous distribution of B2 phase in a glassy matrix can be rationalized in terms of reduced interface kinetics of solid/liquid interface and polymorphic nature of the primary solidification taking place without solute partition. The partial replacement of Zr with Ti improves not only glass forming ability but also suppresses the martensitic transformation of B2 phase, enabling the fabrication of BMG composites consisted of the B2 phase embedded in a CuZr(Ti) glass matrix. However, due to local cooling rate change during solidification, development of non-uniform microstructure in the BMG composites seems to be inevitable, which may be an obstacle in future application of the BMG composites.     

Nucleation of the lamellar decomposition in a Ti–44Al–4Nb–4Zr alloy

The onset of the lamellar decomposition (α⇒α₂+γ) in a titanium aluminide alloy containing Nb and Zr has been studied by transmission electron microscopy. Samples water-quenched from the solution-treatment temperature of 1350 °C show fault-like features resembling those reported previously as the precursors for the formation of the γ lamellae. High-resolution lattice images obtained from such features have revealed that the “faults” are actually embryonic γ lamellae, just a few atomic layers in thickness, which clearly exhibit the ordered L1₀ structure. This implies that the γ phase is formed directly, rather than via some intermediate disordered face-centred-cubic phase as suggested previously. Moreover, the character and configuration of the interfacial defects is consistent with this occurring in a diffusive-displacive manner with short-range fluxes across the risers of mobile perfect interfacial disconnections.     

Effect of Aging Treatment on the Formation of α Precipitates in β-Type Ti–6Mo–6V–5Cr–3Sn–2.5Zr Alloys

The microstructural evolution of a novel β-type Ti–6Mo–6V–5Cr–3Sn–2.5Zr (wt%) alloy subjected to different aging treatments was investigated. The normalized intensity of the α precipitates reached a peak value at 450 °C. A nanoscale orthorhombic phase was observed to coexist with α precipitates in the β matrix, which followed the Burgers orientation relation of 〈\(1120\)〉_α//〈111〉_β and {0001}_α//{110}_β. Fine α precipitates were formed with metastable O and β′ phases, and the β phase was spinodally decomposed to β and β′ phases. The maximum hardness value of the specimen was obtained after aging at 450 °C. Compositional partitioning of Mo, V, and Cr elements occurred with the depletion of fine acicular α precipitates upon aging 450 °C.     

Effects of β-Dendrite Growth Velocity on β→a Transformation of Hypoperitectic Ti–46Al–7Nb Alloy

Solidification characteristics of Ti–46Al–7Nb melts were studied by the electromagnetic levitation technique.A maximum melt undercooling up to 240 K has been achieved. When the undercooling is lower than the critical value DT* = 205 K, the alloy possesses typical hypoperitectic solidification characteristic which can be evidenced by a peritectic layer observed in the as-solidified microstructure. However, the Widmansta¨tten structure can be observed at large undercooling regime of DT C DT*, where peritectic reaction cannot proceed and c lamellar precipitation within a plates is suppressed. Based on the BCT dendrite growth model, the dendrite growth velocities were calculated as a function of undercooling. Theoretical analysis indicates that the growth mechanism of the primary b phase transforms from solutaldiffusion-controlled to thermal-diffusion-controlled in the undercooling range of 188–205 K, which can be attributed to the onset of solute trapping at the critical undercooling. Meanwhile, with increasing undercooling, the solute trapping effect becomes more dominant as a consequence.     

Investigation on the High Temperature Internal Friction of a Subperitectic Al-Zr Alloy in Precipitating Process of ZrAl_3

An inverted torsion pendulum has been used to study the high temperature internal friction of an Al-0.22wt% Zr alloy.Theexperimental result has shown that the grain boundary internal friction peak shifted to lower temperature,the peak height des-cended and the activation energy related to this peak increased in the precipitating process of ZrAl_3 particle in the alloy.Mean-while,the high temperature background descended so obviously as to be separated from the grain boundary internal friction.Theinternal friction peak caused by diffusion around ZrAl_3 particles has been observed.The mechanism of improvement of proper.ties of aluminium alloys containing a small amount of zirconium has also been discussed.     

Explaining absence of texture development in cold rolled two-phase Zr–2.5 wt% Nb alloy

In the present study, two distinct starting microstructures of Zr–2.5 wt% Nb have been used: (1) single-phase α hcp martensitic structure; and (2) two-phase, 10% bcc β and rest hcp α, Widmanstätten structure. In the second case, two types of α were present—near grain boundary predominantly single-phase α (about 5% of the total α) and α plates in an apparently continuous β matrix. Both (1) and (2) had similar starting crystallographic texture of the hcp α phase and were deformed by unidirectional and cross rolling. In the two-phase structure the changes in the bulk texture on cold rolling was found to be insignificant, while in the single-phase material noticeable textural changes were observed. Taylor type deformation texture models predicted textural changes in single-phase structure but failed to predict the observed lack of textural development in the two-phase material. Microtexture observations showed that α plates remained approximately single crystalline after cold rolling, while the β matrix underwent significant orientational changes. Relative hardening, estimated by X-ray peak broadening, was observed mainly in β phase; while aspect ratio of α plates remained unchanged with cold rolling—indicating absence of effective macroscopic strain in the hcp α plates. Based on microstructural and microtextural observations, a simple model is proposed in which the plastic flow is mainly confined to the β matrix within which the α plates are subjected to ‘in-plane rigid body rotation’. The model explains the observed lack of textural developments in the two-phase structure.     

Changes in the Defect Structure of the Pd–5.3 at % In–0.5 at % Ru Alloy Subjected to Electrolytic Hydrogenation and Prolonged Relaxation

Variations in the reduced integral intensities of diffraction maxima in X-ray diffraction patterns taken for the Pd–5.3 at % In–0.5 at % Ru alloy foil, which was subjected to electrolytic saturation with hydrogen and subsequent prolonged relaxation at room temperature and atmospheric pressure, have been analyzed. The degree of texture of the sample has been found to decrease substantially, and first-class defects in the alloy matrix dominate.     

Influence of Severe Plastic Deformation on the Structure and Properties of Al–Li–Cu–Mg–Zr–Sc–Zn Alloy

The structural and phase transformations in the Al–Li–Cu–Mg–Zr–Sc–Zn alloy have been studied by the electron microscopy after the aging for the maximum strength and in the nanostructured state after severe plastic deformation by high-pressure torsion. It has been shown that severe plastic deformation leads to the formation of a nanostructured state in the alloy, the nature of which is determined by the magnitude of deformation and the degree of completeness of the dynamic recrystallization. It has been established that deformation also causes a change in the phase composition of the alloy. The influence of the structural components of the severely deformed alloy on the level of mechanical properties, such as the hardness, plasticity, elastic modulus, and stiffness has been discussed.     

Effect of Al_4C_3 Particle Size Distribution in a Al–2.5C Master Alloy on the Refining Efficiency of the AZ31 Alloy

The Al–2.5C master alloy is prepared to investigate the effect of the Al_4C_3 particle size distribution on the refining efficiency of the AZ31 alloy. The results indicate that the Al_4C_3 particles are potent nucleation substrates for primary a-Mg grains. With 1.0 wt% master alloy addition, the grain size is reduced from 204 to 70 lm. The grain refining efficiency of the Al_4C_3 particles on the AZ31 alloy is calculated to be 0.04%–0.75%. Such low refining efficiency is mainly attributed to the size distribution of the Al_4C_3 particles. The particle sizes are in the range from 0.18 to 7.08 lm, and their distribution is well fitted by a log-normal function. The optimum particle size range for significant grain refinement is proposed to be around 5.0–7.08 lm in the present conditions.     

Role of silicon in accelerating the nucleation of Al3(Sc,Zr) precipitates in dilute Al–Sc–Zr alloys

The effects of adding 0.02 or 0.06 at.% Si to Al–0.06Sc–0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al₃(Sc,Zr) precipitation kinetics in dilute Al–Sc-based alloys. Precipitation in the 0.06 at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4 h at 275 and 300 °C, compared with the 0.02 at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si–Sc clustering, which is attributed to attractive Si–Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si–Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al₃(Sc,Zr) (L1₂) precipitates in the high-Si alloy subjected to double aging (8 h/300 °C for Sc precipitation and 32 days/400 °C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al₃(Sc,Zr) (L1₂) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al₃Sc; and (iii) Si partitions preferentially to Al₃Sc (L1₂) rather than Al₃Zr (L1₂).     

Electron microscopic investigation of aging in the Cu–0.06% Zr alloy

The decomposition of supersaturated solid solution in the Cu–0.06 wt% Zr alloy has been investigated. Upon aging of the initially quenched alloy the homogeneous precipitation of particles is dominating. The decomposition begins from the precipitation of a metastable copper–zirconium phase, the particles of which have the shape of nanodimensional disks. An increase in the aging temperature results in the formation of coarser rodlike particles of the Cu₅Zr equilibrium phase. Aging of the deformed alloy is characterized by the predominance of the heterogeneous precipitation of particles at subboundaries and dislocations, and the decomposition begins at a lower temperature. The particle size is less by an order of magnitude than that in the quenched state. The precipitation of nanodimensional particles at dislocations retards the formation of recrystallization centers.     

Effect of Scandium Content on the Structure and Properties of Alloy Al – 4.5% Zn – 4.5% Mg – 1% Cu – 0.12% Zr

Metal Science and Heat Treatment - The effect of scandium content on the structure and properties of alloy Al – 4.5% Zn – 4.5% Mg – 1% Cu – 0.12% Zr is determined. The...     

Effect of torsion conditions under high pressure on the structure and strengthening of the Zr–1% Nb alloy

The effect of temperature and degree of deformation upon severe plastic deformation by torsion under a high pressure on the structure, phase composition, and microhardness of the industrial zirconium Zr–1% Nb alloy (E110) has been studied. The high-pressure torsion (HPT) (with N = 10 revolutions) of the Zr–1% Nb alloy at room temperature results in the formation of grain–subgrain nanosize structure with an average size of structural elements of 65 nm, increase in the microhardness by 2.3–2.8 times (to 358 MPa), and α-Zr → β-Zr and α-Zr → ω-Zr phase transformations. The increase in the HPT temperature to 200°C does not lead to a decrease in the microhardness of alloy owing to the increase in the fraction of ω-Zr phase, though the average size of structural elements increases to 125 nm. The increase in the temperature to 400°C during HPT with N = 10 revolutions leads to the grain growth in the α-Zr grain structure (~90%) to 160 nm and a decrease in the microhardness to 253–276 HV.     

The Study of the Oxidation of a Ti–45Al–2Nb–2Mn–0.2B Alloy in the Investment Casting Using an Yttria Face Coat Mould

Investment casting is an economical method to manufacture near net-shape metal components. Due to the very high thermal and chemical inertness, yttria has been widely used as the mould face coat material for the investment casting of titanium alloy for many years. An investigation was undertaken to study the oxidation behaviour of TiAl alloy during casting in a mould using pure yttria as the face coat. This research shows that the TiAl alloy was still oxidized in the mould during casting when using yttria as the face coat. During high temperature casting, the yttria in the face coat was dissolved by high temperature molten metal flow. The oxygen from the yttria face coat diffused into TiAl and interacts with TiAl to form different microstructure and phases (e.g. precipitates such as oxygen enriched Ti₃Al and Al₂O₃ phases). Meanwhile, the dissolved yttrium was then re-precipitated at the metal interfacial area as yttria inclusions after metal cool down.