Precipitation evolution in Al–Zr and Al–Zr–Ti alloys during aging at 450–600 °C

The transformation of Al₃Zr (L1₂) and Al₃(Zr_1−xTi_x) (L1₂) precipitates to their respective equilibrium D0₂₃ structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L1₂ precipitates nor their transformation to the D0₂₃ structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L1₂ precipitates transform to disk-shaped D0₂₃ precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the α-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D0₂₃ phase with α-Al. The transformation is very sluggish and even at 575 °C coherent L1₂ precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.     

Precipitation evolution in Al–0.1Sc,Al–0.1Zr and Al–0.1Sc–0.1Zr (at.%) alloys during isochronal aging

Precipitation strengthening is investigated in binary Al–0.1Sc, Al–0.1Zr and ternary Al–0.1Sc–0.1Zr (at.%) alloys aged isochronally between 200 and 600 °C. Precipitation of Al₃Sc (L1₂) commences between 200 and 250 °C in Al–0.1Sc, reaching a 670 MPa peak microhardness at 325 °C. For Al–0.1Zr, precipitation of Al₃Zr (L1₂) initiates between 350 and 375 °C, resulting in a 420 MPa peak microhardness at 425–450 °C. A pronounced synergistic effect is observed when both Sc and Zr are present. Above 325 °C, Zr additions provide a secondary strength increase from the precipitation of Zr-enriched outer shells onto the Al₃Sc precipitates, leading to a peak microhardness of 780 MPa at 400 °C for Al–0.1Sc–0.1Zr. Compositions, radii, volume fractions and number densities of the Al₃(Sc_1?xZr_x) precipitates are measured directly using atom-probe tomography. This information is used to quantify the observed strengthening increments, attributed to dislocation shearing of the Al₃(Sc_1?xZr_x) precipitates.     

Machining of a Zr–Ti–Al–Cu–Ni metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ metallic glass machining chips were characterized using SEM, X-ray diffraction and nano-indentation. Above a threshold cutting speed, oxidation of the Zr produces high flash temperatures and causes crystallization. The chip morphology was unique and showed the presence of shear bands, void formation and viscous flow.     

Influence of ultrasonic treatment on formation of primary Al3Zr in Al–0.4Zr alloy

The effect of ultrasonic treatment on the formation of primary Al₃Zr was investigated by applying ultrasound to an Al–0.4Zr alloy. Three temperature ranges were selected, i.e., 830 to 790 °C (above liquidus), 790 to 750 °C (cross liquidus) and 750 to 710 °C (below liquidus) for ultrasonication. Using the scanning electron microscopy, both the size and morphology of the primary Al₃Zr particles were examined. It was found that the size was significantly reduced and the morphology changed from large throwing-star shape to small compact tablet shape. The mechanisms for refinement of primary Al₃Zr were discussed. It is suggested that sonocrystallization theory via activation of aluminium oxide particles is responsible for the refinement of primary Al₃Zr when ultrasonic melt treatment (UST) is applied within the fully liquid state. The refinement of primary Al₃Zr particles when UST is applied in the slurry (growth stage) is due to the sonofragmentation.     

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.     

Composition optimization of the Al–Co–Zr bulk metallic glasses

Composition optimization for locating the composition with the largest glass forming ability in the Al–Co–Zr system is attempted in this investigation. The criteria that we have developed are respectively related to a specific conduction electron concentration, termed the e/a-constant criterion, and to a specific cluster structure, termed the e/a-variant criterion. For this system, the two criteria are incarnated into the composition line with constant e/a=1.5 and the Co₄Zr₉–Al composition line. Bulk metallic glasses are obtained by suction casting for compositions with e/a=1.3–1.5, with their thermal stabilities and glass forming abilities being increased with increasing e/a ratios. The crossing point of the e/a=1.5 line and the Co₄Zr₉–Al line gives the composition Al_23.5Co_23.5Zr₅₃ with the largest GFA (e.g. T_g/T_m=0.637), superior to the reported Al₂₀Co₂₅Zr₅₅ alloy with T_g/T_m=0.621.     

Zr–Cu–Ni–Al bulk metallic glasses with superhigh glass-forming ability

Zr–Cu–Ni–Al quaternary amorphous alloy compositions with varying glass-forming ability are developed by an efficient method of proportional mixing of binary eutectics. The critical diameter of the glassy sample is improved from 6 mm for Zr₅₃Cu_18.7Ni₁₂Al_16.3 to 14 mm for Zr_50.7Cu₂₈Ni₉Al_12.3 by straightforwardly adjusting the eutectic unit’s coefficients. The drastic improvement in GFA is attributed to balancing the chemical affinities of the Zr, Cu, Ni and Al components in the melt prior to solidification which makes the precipitation of competing crystalline phases more difficult. As the glass-forming ability increases, the concentration of Cu in the alloys exhibits a same trend. Based on synchrotron radiation high-energy X-ray diffraction analysis and Miracle’s structural model, it is envisioned that the substitution of additional Cu atoms for Zr atoms in the investigated alloys stabilizes the efficient cluster packing structure of the amorphous alloys, leading to the pronounced increase in their glass-forming ability.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Density change upon crystallization of amorphous Zr–Cu–Al thin films

Systematic deflection measurements with micro-cantilevers and a combinatorial-deposition method have been used to investigate the density change upon crystallization of amorphous Zr–Cu–Al thin films. It is found that, in general, the density change decreases from Cu-/Al-rich compositions to Zr-rich compositions, and the previously reported good glass-forming compositions are found to correspond to global minima of the density change inside respective local eutectic systems. Furthermore, we propose that the general trend of density change as a function of alloy compositions discovered in this work may have implications for the macroscopic plasticity of metallic glasses.     

Criteria for tensile plasticity in Cu–Zr–Al bulk metallic glasses

(Cu_0.5Zr_0.5)_100?xAl_x (x = 5, 6, 8) bulk metallic glasses (BMGs) were deformed in tension. Besides ductility up to 0.5%, the material shows work-hardening behaviour. Both effects are attributed to the deformation-induced precipitation of B2 CuZr nanocrystals and the formation of twins in the nanocrystals larger than 20 nm. The precipitation of the nanocrystals alters the stress field in the matrix and is expected to retard shear band propagation, which in turn allows stresses in the nanocrystals to rise. This stress build-up is more severe in the larger grains and might be responsible for the subsequent twinning. Both deformation-induced nanocrystallization and twinning consume energy and avoid crack formation and with it premature failure.     

Isothermal section of Al–Ti–Zr ternary system at 1073 K

Through alloy sampling combined with diffusion triple technique, phase equilibria in Al–Ti–Zr ternary system at 1073 K were experimentally determined with electron probe microanalysis (EPMA). Experimental results show that there is a solid solution β(Ti,Zr) which dissolves Al up to 16.3% (mole fraction). Ti and Zr can substitute each other in most Ti–Al and Al–Zr binary intermediate phases to a certain degree while the maximum solubility of Zr in Ti₃Al and TiAl reaches up to 17.9% and 4.0% (mole fraction), respectively. The isothermal section consists of 16 single-phased regions, 27 two-phased regions and 14 three-phased regions. No ternary phase was detected.     

Solidification structure and mechanical properties of Al–Li–Cu–Zr cast alloys

Abstract

The microstructure and properties of three Al–3Li–1Cu ternary alloys have been studied, in particular the effect of Zr additions on the microstructure, precipitation and mechanical properties. The results showed that, for these Al–Li casting alloys, Zr content up to 0.2 wt-% was acceptable, and the Zr additions appeared to refine the grain structure. During aging, the Zr rich phase provided nucleation sites for δ' phase and promoted δ' phase refinement and homogenisation. Under optimised conditions, the tensile strength and elongation to failure of the Al–Li–Cu–Zr casting alloys were 400 MPa and 2.5%, respectively.     

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).     

Superplastic behavior of friction-stir welded Al–Mg–Sc–Zr alloy in ultrafine-grained condition

The present work was undertaken to improve superplastic ductility of friction-stir welded joints of ultrafine-grained (UFG) Al–Mg–Sc–Zr alloy. In order to suppress the undesirable abnormal grain growth, which typically occurs in the heavily deformed base material, the UFG material was produced at elevated temperature. It was suggested that the new processing route could reduce dislocation density in the UFG structure and thus enhance its thermal stability. It was found, however, that the new approach resulted in a relatively high fraction of low-angle boundaries which, in turn, retarded grain-boundary sliding during subsequent superplastic tests. Therefore, despite the successful inhibition of the abnormal grain growth in the base-material zone, the superplastic deformation was still preferentially concentrated in the fully-recrystallized stir zone of the material. As a result, the maximal elongation-to-failure did not exceed 700%.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Strength and substructure of Al–4.7Mg–0.32Mn–0.21Sc–0.09Zr alloy sheets

Laws of the formation of substructure and of changes in the hardness and in the mechanical properties have been established for sheets of 1545K alloy obtained by tension according to different technologies at various accumulated strains. With an increase in cold deformation (e _cold) from 0 to 2.64, the yield stress of cold-worked sheets increases from 355 to 466 МPа and the relative elongation decreases insignificantly from 4 to 3.5%. The maximum strength with σ_0.2 = 410 МPа, σ_u = 460 МPа, and δ = 6.5% is provided by annealing at 150°C for 1 h of the sheets obtained via the technology with the maximum fraction of cold deformation (e _cold = 2.64). After annealing at 300°C for 30 min, a twofold increase in the plasticity is observed without a significant reduction in the strength characteristics a follows: σ_0.2 = 385 МPа, σ_u = 436 МPа and δ = 13%. It has been shown that the level of mechanical properties is determined by the substructure that is formed inside deformed grains during annealing.     

Change in primary phase from icosahedral quasicrystal to fcc Zr2Ni by mechanical disordering in Zr–Al–Ni–Cu–Pd glassy alloy

Change in the primary crystallization from a single icosahedral quasicrystalline phase into the fcc Zr₂Ni phase by mechanical disordering was investigated in a melt-spun Zr₆₅Al_7.5Ni₁₀Cu_12.5Pd₅ glassy alloy. The transition of the primary phase is attributed to the mechanical strain induced in the icosahedral local structure in the glassy state.     

Crack-resistance curve of a Zr–Ti–Cu–Al bulk metallic glass with extraordinary fracture toughness

For the emerging bulk metallic glasses (BMGs), damage tolerance is a key mechanical property needed for their practical applications. To reach a fracture toughness on a par with, or even better than, conventional engineering alloys, the only route reported so far is to compositionally base the BMG on high-cost palladium (Pd), which has a very high Poisson’s ratio (～0.42). Here we report the discovery of a Zr₆₁Ti₂Cu₂₅Al₁₂ (ZT1) BMG that has a toughness as high as the Pd-based BMG, but at the same time consists of common engineering metals and has robust glass-forming ability. The new BMG, while having an unimpressive Poisson’s ratio of 0.367, derives its high toughness from its high propensity for crack deflection and local loading-mode change at the crack tip due to extensive shear band interactions. The crack-resistance curve (R-curve) of this BMG has been obtained from fatigue pre-crack samples, employing standard “single-specimen” and “multiple-specimen” techniques.