A positron study of the defect structures in the D03 and B2 phases in the Fe–Al system

The positron annihilation technique was used to identify the nature of the vacancy-type defects in the D0₃ and B2 phases of the Fe–Al system. Seven alloys with Al concentrations in the range 22.7–48 at.% Al and with different thermal treatments were examined. Positron lifetime calculations for the expected defects in the two phases were also performed in order to facilitate the defect identification. In the B2 phase, two types of defects were identified: a thermal complex formed by a Fe-divacancy and an Al antisite, and a Fe-vacancy. No constitutional vacancies were found in the D0₃ phase.     

First principles calculations of the stability of the T2 and D88 phases in the V–Si–B system

The crystal and electronic structures of D8_l-V₅SiB₂ and D8₈-V₅Si₃B ternary compounds have been investigated by means of first principle calculations. The calculated structural parameters are in very good agreement with the experimental data. The calculated values of the enthalpies of formation at T = 0 K of the D8_l-V₅SiB₂ and D8₈-V₅Si₃B ternary compounds are −67.1 and −62.1 kJ/mol of atoms respectively. The total and partial electronic densities of states show a strong hybridization between the B p states and V d states. The defect enthalpies of formation as well as the mixing enthalpies have been computed. These data are essential for the modeling of the D8_l and D8₈ phases in the V–Si–B ternary system. A partial V–VSi₂–VB isothermal section at 298 K is proposed.     

Evolution of ordered ω phases in (Zr3Al)–Nb alloys

Microstructural investigations on rapidly solidified Zr₃Al based alloys (binary Zr₃Al and ternary Zr₃Al–3Nb and Zr₃Al–10Nb) have revealed some unusual phase transformation sequences. The Zr₅Al₃ phase (D8₈ structure) has been found to occur in both the rapidly solidified ternary alloys unlike in the rapidly solidified stoichiometric Zr₃Al alloy in which the Zr₂Al phase (B8₂ structure) has been found to be present. The evolution of the D8₈ phase, which could be regarded as one of the ordered derivatives of the ω phase, could be described in terms of a superimposition of replacive and displacive ordering waves in the β phase. The orientation relationship between the β and the D8₈ phases has been established. The microstructural changes occurring in the rapidly solidified Zr₃Al–Nb alloys during aging have been examined. It has been found that on aging the D8₈ phase gets transformed into the B8₂ phase which, on continued aging, transforms to other metastable and equilibrium phases, depending upon the aging temperature. The observed sequence of phase transformations involving different structurally related phases has been along the direction of progressively close packed structures. The symmetry changes associated with the sequence of ω related transformations have been summarized in the form of a symmetry tree.     

On the fcc→D019 transformation in Co–W alloys

The continuous precipitation of D0₁₉ Co₃W from a highly supersaturated Co-rich fcc matrix has been investigated. A detailed analysis of planar defects generated in the precipitates has been carried out using conventional and high-resolution transmission electron microscopy. The spatial arrangement of stacking faults and antiphase boundaries revealed the actual defect formation mechanisms and disclosed important aspects of the phase transformation at an atomic level. The results point to a displacive/diffusional Widmanstätten type of transformation, which is not simply based on a common ledge growth and, furthermore, follows the path: fcc→hcp→D0₁₉. The α′+Co₇W₆→Co₃W peritectoid transformation has also been studied.     

Competition of the primary and peritectic phases in hypoperitectic Cu–Sn alloys solidified at low speed in a diffusive regime

Directional solidification experiments on hypoperitectic Cu–Sn alloys have been performed at very low velocity in a high thermal gradient to ensure planar front growth of both phases. The diameter of the sample has been reduced to 500 μm to strongly reduce convection. Lamellar and fibrous peritectic cooperative growth of the primary α- and peritectic β-phases has been observed on length spanning several millimeters. For the first time in a high solidification interval peritectic alloy, a quenched interface of both phases in contact with the liquid has been obtained. An unexpectedly high volume fraction of the primary phase, which furthermore fluctuates over time, has been observed. This is attributed to the transient state of the (α + β) growth front to a steady state and the associated evolution of the large diffusion layer ahead of the solid–liquid interface.     

Effects of Sn additions on microstructure and corrosion resistance of heat-treated Ti–Cu–Sn titanium alloys

The microstructure and corrosion properties of Ti7CuxSn (x?=?0–5?wt-%) alloys after solution treatment have been investigated. The alloys were solution-treated (ST) at 1000°C for 2?h, followed by quenching in water to room temperature. It was found that the microstructure of the ST Ti7Cu alloy had only a martensite structure, and that addition of Sn could refine the microstructure of Ti7CuxSn alloy. Notably, the pseudo dendritic α-Ti phase was formed in ST Ti7Cu5Sn alloys. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy data demonstrated that adding Sn improved the electrochemical corrosion behaviour of the Ti7CuxSn alloy.     

Effect of B and Ti on the directionally solidified microstructure of the Nb–Si alloys

With the guidance of the calculated phase diagram of the Nb–Ti–Si–B system, three alloys (M1: Nb–20Ti–14Si; M2: Nb–20Ti–14Si–4.5B; M3: Nb–24Ti–14Si–4.5B (at.%)) were selected to investigate the influence of alloying elements B and Ti on their microstructural formation. The alloys were prepared by arc-melting and then directionally solidified (DS) at a withdrawing rate of 10 μm/s. The DS microstructures in the near-solidification front zone, the mushy zone, and the steady-state zone were characterized by SEM, EPMA and XRD analyses. The experimentally observed solidified microstructures were then compared with the predicted solidification path calculated from the Scheil model. The good agreement between the experimental observations and the predicted solidification paths suggests that the computational models can be effectively used for the design of the Nb–Ti–Si–B alloys.     

Microstructural,mechanical and shape memory characterizations of Ti–Mo–Sn alloys

As a β stabilizing element in Ti-based alloys, the effect of Mo on phase constitution, microstructure, mechanical and shape memory properties was investigated. Different compositions of Ti–xMo–3Sn alloys (where x=2, 4, 6, at.%) were prepared by arc melting. A binary composition of Ti–6Mo alloy was also prepared for comparison. Ti–xMo–3Sn alloys show low hardness and high ductility with 90% reduction in thickness while Ti–6Mo alloy shows high hardness, brittle behavior, and poor ductility. Field emission scanning electron microscopy (FESEM) reveals round morphology of athermal ω (ω_ath) precipitates. The presence of ω_ath phase is also confirmed by X-ray diffraction (XRD) in both as-cast and solution-treated and quenched conditions. The optical microscopy (OM) and FESEM show that the amount of martensite forming during quenching decreases with an increase in Mo content, which is also due to β→ω transformation. The hardness trends reinforce the presence of ω_ath too. The shape memory effect (SME) of 9% is the highest for Ti–6Mo–3Sn alloy. The SME is trivial due to ω_ath phase formation; however, the increase in SME is observed with an increase in Mo content, which is due to the reverse transformation from ω_ath and the stress-induced martensitic transformation. In addition, a new and very simple method was designed and used for shape memory effect measurement.     

The coexistence of two S (Al2CuMg) phases in Al–Cu–Mg alloys

The decomposition sequence of the supersaturated solid solution leading to the formation of the equilibrium S (Al₂CuMg) phase in Al–Cu–Mg alloys has long been the subject of ambiguity and debate. This paper describes the identification of two distinct S phases with different lattice parameters, in a series of Al–2.5Cu–1.5Mg alloy samples, isothermally aged at temperatures between 200 and 400 °C for various times, using high-resolution synchrotron X-ray powder diffraction. The two S phases, denoted S1 and S2, form in competition with one another, with S1 being metastable with respect to S2. The lattice parameters of S2 are found to be in good agreement with those refined by Heying et al. (Z Naturforsch 2005;60B:491) for the equilibrium S phase. Despite the differences in their lattice parameters, both S1 and S2 appear to have the crystal structure proposed by Perlitz and Westgren (Ark Kemi Min Geol 1943;16B:1). The lattice parameters of both S1 and S2 are well defined, with little scatter between different samples treated for different times and temperatures, and there are no signs of the large range of lattice parameters reported by recent electron microscopy observations.     

Experimental Investigation of the Ti–V Side in the Ti–V–Sn Ternary System

Phase relations in the Ti–V–Sn system are of great importance for design of aerospace titanium alloys. However, reported Ti–V–Sn ternary phase diagrams present great differences. The isothermal section of the Ti–V side in the Ti–V–Sn system at 1073, 1173, and 1273 K was established using equilibria alloys. There are 11 two-phase equilibria and 3 three-phase equilibria, 9 two-phase equilibria and 3 three-phase equilibria, and 9 two-phase equilibria and 3 three-phase equilibria in the isothermal section at 1073, 1173, and 1273 K, respectively. In addition, remarkable ternary solubility in some binary compounds was detected, e.g., up to 21.18 and 22.23 at.% V in Ti₃Sn and Ti₂Sn, respectively, at 1273 K.     

Grain size refinement of magnesium composite alloys by addition of B2O3

The high performance magnesium alloy was investigated by adding B2O3 in magnesium and magnesium alloys. Experiments include adding B2O3 in Mg, Mg-AI and Mg-RE alloys, respectively, studying the effects of B2O3 on the microstructure, were studied measuring the change of grain size and microhardness of the materials, discussing the change of grain size, morphology and distribution. The results show that adding 3% or 6%（mass fraction） B2O3 in Mg can bring twinning in Mg, adding B2O3 in Mg-Al alloys and Mg-RE alloys can refine the alloy grain size. Adding 3?O3 in Mg-6Al alloys can refine the average grain size by about 5 pro, with the average hardness increased by 13.3% （53.3-60.4 HV0.O3）; adding 6?O3 in Mg-6Al alloys can refine the average grain size by about 13 pro, with the average hardness increased by 15.8% （53.3-61.73 HV0.O3）; adding 3% and 6?O3 into Mg-6RE alloys can refine the grain size by about 5 and 9/am, respectively, with the average hardness decreased to HV0.03 64.66 and HV0.O3 57.86, respectively from HV0.03 88.57. In the Mg-6Al alloy the content of aluminum is increased, while in the Mg-6RE alloy the content of oxygen is decreased. It can be concluded that it is beneficial to develop Mg-Al-B-O particle reinforce composite alloys, and it is feasible to develop nanometer crystallization of block material by Mg-B-O-RE.     

As-cast microstructures and solidification paths of the Nb–Si–Ti ternary alloys in Nb5Si3–Ti5Si3 region

Abstract The as-cast microstructures and solidification paths of the Nb-Si-Ti ternary alloys in the NbsSi3-TisSi3 region were investigated. Since there exist some isomor- phous compounds in the NbsSi3-TisSi3 region, such as aNbsSi3 with B3Cr5 prototype, 13NbsSi3 with Si3W5 pro- totype, 7NbsSi3 with MnsSi3 prototype, and TisSi3 with MnsSi3 prototype, the primary solidification areas of these compounds were not typically indentified in previous experiments. In the present paper, the microstructure observation, the phase identification, and the composition measurement were performed using scanning electron microscopy （SEM）, X-ray diffraction （XRD）, and electron probe microanalysis （EPMA）, respectively. No ternary compound is found. There exist three primary solidification areas, 13Nbs_x（Ti）xSi3, ~Nbs_x（Ti）xSi3, and Tis-x（Nb）xSi3 in the NbsSi3-TisSi3 region. Together with the literaturereported experimental data and optimization results, the liquidus projection of the whole Nb-Si-Ti ternary system is constructed, and totally ten primary solidification areas-- diamond-Si, Nb1-x（Ti）xSi2, Ti1-x（Nb）xSi2, Ti1-x（Nb）xSi, Ti5-x（Nb）xSi4, βNb5-x（Ti）xSi3,αNb5-x（Ti）xSi3, Ti5-x （Nb）xSi3, （Nb,Ti）3Si, and BCC--and nine transitional invariant reactions-L ＋ Nb1-x（Ti）xSi2 → Ti1-x（Nb）x Si2 ＋ Si, L ＋ Nb1-x（Ti）xSi2 → Ti1-x（Nb）xSi2 ＋ Ti5- （Nb）xSi4, L ＋ Ti5-x（Nb）xSi4 → Ti1-x（Nb）xSi2 ＋ Ti1-x （Nb）xSi, L ＋ 13Nb5-x（Ti）5Si3→ Nb1-x（Ti）xSi2 ＋ Ti5-x （Nb）xSi4, L ＋ βNb5-x（Ti）xSi3→b5-x（Ti）xSi3 ＋Ti5-x （Nb）xSi4, L ＋ αNb5-x（Ti）αSi3 → Ti5-x（Nb）xSi3 ＋ Ti5-x（Nb）x Si4, L ＋ αNb5-x（Ti）xSi3 →βNb5-x（Ti）xSi3 ＋ Ti5-x（Nb）xSi3, L ＋ βNb5-xTb-xSi3 → Ti5-x（Nb）xSi3 ＋ （Nb,Ti）3Si, and L ＋ （Nb,Ti）3Si → Ti5-x（Nb）xSi3 ＋ BCC are confirmed.     

Corrosion behaviour of Cu–2Ti and Cu–2Zr (wt%) alloys in neutral aerated sodium chloride solution

The corrosion behaviour of Cu–2Ti and Cu–2Zr (wt%) alloys has been assessed by open-circuit potential and electrochemical impedance spectroscopy measurements carried out in 3.5 wt% NaCl solution, at approximately neutral pH, without stirring and in contact with the air. For comparison, the electrochemical tests have also been carried out on unalloyed Cu. Electrochemical impedance results showed that Ti and Zr alloying additions significantly increased the corrosion resistance of copper, the best behaviour being observed for the Cu–2Zr alloy. This improvement may be ascribed to the formation of Ti- or Zr-enriched passive layer, which exhibits higher protective effectiveness compared with that of unalloyed Cu.     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Crystal structure and stability of complex precipitate phases in Al–Cu–Mg–(Si) and Al–Zn–Mg alloys

We demonstrate how first-principles total energy calculations may be used to elucidate both the crystal structures and formation enthalpies of complex precipitates in multicomponent Al alloys. For the precipitates, S(Al–Cu–Mg), η′ (Al–Zn–Mg), and Q(Al–Cu–Mg–Si), energetics were computed for each of the models of the crystal structures available in the literature allowing a critical assessment of the validity of the models. In all three systems, energetics were also calculated for solid solution phases as well as other key phases (e.g., equilibrium phases, GP zones) in each precipitation sequence. For both the S and η′ phases, we find that recently proposed structures (based on electron microscopy) produce unreasonably high energies, and thus we suggest that these models should be re-evaluated. However, for all three precipitates, we find that structures based on X-ray diffraction refinements provide both reasonable energetics and structural parameters, and therefore the first-principles results lend support to these structural refinements. Further, we predict energy-lowering site occupations and stoichiometries of the precipitate phases, where experimental information is incomplete. This work suggests that first-principles total energy calculations can be used in the future as a complementary technique with diffraction or microscopy for studying precipitate structures and stabilities.     

The in-situ Ti alloying of aluminum alloys and its application in A356 alloys

1. Introduction Titanium is one of the most effective grain refinement elements of aluminum alloys. The grain of aluminum alloys can be effectively refined if containing up to 0.2% titanium, which results in improvement of mechanical properties and performance [1-8]. Titanium is usually added into aluminum melt by melting Al-Ti or Al-Ti-B (Ti/B= 5:1) master alloy. This requires strict control in melting temperature and holding time, resulting in the complicacy to control the quality of allo…     

Stability of B2 phase in Ti–Ni–Fe alloys against MeV electron-irradiation-induced solid-state amorphization and martensite transformation

The amorphization tendency of a B2 phase in Ti₅₀Ni_50?xFe_x (x = 2 to 40 at%) intermetallic compound was investigated in order to determine the relationship between solid-state amorphization (SSA) and martensite transformation. SSA was observed in the B2 phase at 103 K and 298 K by high-voltage electron microscopy (HVEM). The total dose required for the amorphization increased with the Fe content. The replacement of Ni by Fe increased the phase stability of the B2 phase in Ti–Ni–Fe intermetallic compound and suppressed SSA as well as martensite transformation.     

Thermal stability of B2 CuZr phase,microstructural evolution and martensitic transformation in Cu–Zr–Ti alloys

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu₅₀Zr_50−xTi_x (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu₁₀Zr₇ and Cu(Ti, Zr)₂, of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu₁₀Zr₇, Cu(Ti, Zr)₂, and ZrTiCu₂ crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.     

Phase equilibria among α (hcp), β (bcc) and γ (L10) phases in Ti–Al base ternary alloys

Phase equilibria between the α(A3), α₂ (D0₁₉), β(A2 or B2) and the γ(L1₀) phases in the Ti–Al base ternary systems were investigated over the temperature range 1000–1300°C. The tie lines and the phase boundaries were determined by electron probe microanalysis using multiphase alloys. It was established that almost all the elements except Zr tended to partition into the β phase rather than into the α, α₂ or the γ phase, while Zr mostly partitioned into the γ phase. At 1000°C, in the equilibrium state between the α₂ and the γ phases, V, Cr, Mo, Ta and W partitioned to the α₂ phase rather than to the γ phase, whereas Mn, Fe, Co, Ni, Cu and Zr tended to concentrate into the γ phase. The partition coefficients for the alloying elements were only slightly dependent on their concentration. Based on these data, the relative stabilizing effects of alloying elements on the α, α₂, β and γ phases are discussed.