L12 Al3Ti Research Articles

Direct aging treatment of TiN modified Al–Mn–Sc alloy processed by laser powder bed fusion: On the microstructure evolution and mechanical property enhancement

In this study, the TiN nanoparticles modified Al–Mn-Sc composite and Al–Mn-Sc alloy have been fabricated by the laser powder bed fusion (LPBF). The microstructure, mechanical properties and strengthening mechanisms of these materials were discussed in details for both the as-built and artificial aged conditions. The results showed that the aged Al–Mn-Sc/TiN composite exhibits homogeneous nucleation with equiaxed grains ranging from 0.81 μm to 1.30 μm. After direct aging treatment of 300 °C, there are a volume fraction of secondary L12-Al3(Ti, Sc, Zr) nanoparticles precipitated in the composite sample, contributing to an ultra-high fracture strength of 701 ± 16 MPa. The increment in strength brought by TiN mainly comes from fine grains and high volume fraction of L12-Al3(Ti, Sc, Zr) nanoprecipitates. The fracture behaviors suggest that the uniform distribution of reinforced particles and less amount of micro pores can improve the strength and ductility of LPBF particle reinforced aluminum matrix composites.

Microstructure evolution and strengthening mechanisms of a high-performance TiN-reinforced Al–Mn–Mg–Sc–Zr alloy processed by laser powder bed fusion

In this work, a high-strength crack-free TiN/Al–Mn–Mg–Sc–Zr composite was fabricated by laser powder bed fusion (L-PBF). A large amount of uniformly distributed L12-Al3(Ti, Sc, Zr) nanoparticles were formed during the L-PBF process due to the partial melting and decomposition of TiN nanoparticles under a high temperature. These L12–Al3(Ti, Sc, Zr) nanoparticles exhibited a highly coherent lattice relationship with the Al matrix. All the prepared TiN/Al–Mn–Mg–Sc–Zr composite samples exhibit ultrafine grain microstructure. In addition, the as-built composite containing 1.5 wt% TiN shows an excellent tensile property with a yield strength of over 580 MPa and an elongation of over 8 %, which were much higher than those of wrought 7xxx alloys. The effects of various strengthening mechanisms were quantitatively estimated and the high strength of the alloy was mainly attributed to the refined microstructure, solid solution strengthening, and precipitation strengthening contributed by L12–Al3(Ti, Sc, Zr) nanoparticles.

Understanding the poisoning mechanisms of Si and Zr atoms on L12 Al3Ti (111) surface: A first-principles investigation

In order to reveal the poisoning mechanism, the adsorption and diffusion of Si and Zr atoms on the L12 Al3Ti (111) surface have been investigated using first-principles calculations. Based on the results of adsorption energy and work function, the most favorable adsorption sites for Si and Zr atoms are H4 and T1 of L12 Al3Ti (111) surface, respectively. It is revealed from electron density differences, density of states and Mulliken population that the Ti–Si covalent bonds in Si@Al3Ti(111) system the hybridization of Si 3p and Ti 3d orbitals while the Ti–Zr ionic bond in Zr@Al3Ti(111) system is come from the contribution of Ti 3d and Zr 4d orbitals. By calculating formation energy, it is found that the Si and Zr atoms prefer to incorporate the Al site of the first layer and Ti site of the second layer of L12 Al3Ti (111) surface, respectively. The energy barriers of Si and Zr atoms diffusion on or into L12 Al3Ti (111) surface are also evaluated.

Microstructures and mechanical properties of Al–Zn–Mg–Cu alloy with the combined addition of Ti and Zr

The effects of combined Ti and Zr addition on microstructures and mechanical properties are systematically investigated in Al–Zn–Mg–Cu alloy by microstructure characterizations and a physical-based model. The results show that combined addition of Ti and Zr can promote the precipitation of nano L12 Al3(Ti,Zr) dispersoids, while primary D022 Al3Ti and Al3(Ti,Zr) phases are formed during solidification when Ti addition is over 0.2 wt%. As compared to the 0Ti alloy, the ductility and toughness is enhanced markedly by 0.1–0.2 wt%Ti addition since the volume fraction of nano L12 Al3(Ti,Zr) dispersoids is increased. Both the strength and ductility are significantly decreased when Ti addition is more than 0.35 wt%. The strain hardening behavior and fractured morphology analyses suggest that the deterioration of mechanical properties is mainly due to the localized recrystallization and cracks caused by coarse primary phases, which cause the higher dislocation dynamic recovery rate. Combined addition of minor Ti and Zr elements may provide a simple approach to improve toughness and simultaneously reduce cost in developing high-strength Al alloys.

Unique microstructure evolution of a novel Ti-modified Al-Cu alloy processed using laser powder bed fusion

Numerous studies on laser powder bed fusion (LPBF) have already demonstrated the evolution of out-of-equilibrium microstructures with metastable phases. In the present work, a self-designed, pre-alloyed Al-Cu-Ti-Ag-Mg alloy is processed using LPBF. The solidification path, which is necessary to achieve sufficient supercooling to exceed the critical nucleation supercooling (ΔTn) required for heterogeneous nucleation on L12 Al3Ti nuclei, is derived from the microstructure. This unique microstructure can be divided into two areas: Area 1, with a thickness in the building direction of 5–10 µm, solidifies first and forms on the bottom of the semicircular melting pool. It is dominated by columnar α-Al grains, which contain numerous precipitated cube-shaped Al-Cu-Ti-Ag nanoparticles. During the solidification of Area 1, the constitutional supercooling (ΔTCS) and the thermal supercooling (ΔTtherm) gradually increase. The Ti and Al atoms in the residual melt react to form numerous primary L12 Al3Ti particles, which are activated for heterogeneous nucleation and serve as nuclei for α-Al grain growth once ΔTtotal (ΔTCS + ΔTtherm) exceeds ΔTn. Area 2, formed by heterogeneous grain refinement, occupies the remaining part of the melting pool and consists of fine equiaxed α-Al grains. The cube-shaped Al-Cu-Ti-Ag nanoparticles precipitated from the supersaturated α-Al in Area 1 cannot be observed in Area 2. The novel alloy with a fine-grained microstructure exhibits a tensile strength of 475 ± 7 MPa in combination with an elongation to fracture of 8.7 ± 0.5%.

Effect of tungsten mesh interlayer on microstructure and mechanical performance of A6061/Ti6Al4V dissimilar joints

A novel method of laser welding-brazing A6061/Ti6Al4V with the assistance of a tungsten mesh interlayer was proposed. The spreading and wetting abilities of the molten aluminum on the Ti6Al4V surface were improved by the addition of the tungsten mesh. Al5(Ti, W) and Al18Mg3W2 were observed as second phases in the aluminum weld, which was a benefit to refine the aluminum grains and improve the mechanical properties of the aluminum weld. The Al3(Ti, W) layer and the (Al, Si)3Ti layer were observed at the Ti/Al interface. W and Si atoms substituted Ti and Al atoms in Al3Ti, respectively. W element distributed unevenly in the Al3(Ti, W) layer and segregated at the grain boundaries of the Al3(Ti, W) phases. D022-Al3(Ti, W) was transformed into L12-Al3(Ti, W) at the grain boundaries resulting from the segregation. The c/a value of the (Al, Si)3Ti crystal lattice was higher than that of Al3Ti. Nanohardness and elastic modulus of the (Al, Si)3Ti phase were higher than those of the Al3(Ti, W) phase. The tensile shear capacity and ductility of the A6061/Ti6Al4V joints with tungsten mesh were better than those of the joints without tungsten mesh at the same welding parameters.

First-principles study of the ideal shear strengths and stacking fault energies of face-centered cubic Al3Ti

The ideal shear strengths and staking fault energies of face-centered cubic L12 Al3Ti have been systematically investigated by first-principles calculations. The stress-strain relations and lattice parameters variation for six common shear processes were obtained. The calculated results show that L12 Al3Ti would be most prone to deform in the {111}〈11-0〉 slip system by virtue of the lowest ideal shear strength. The γ-surfaces and the corresponding γ-curves of {0 0 1}, {1 1 0} and {1 1 1} planes for L12 Al3Ti were also obtained. It is shown that the nucleation and glide of perfect two collinear superpartials 1/2{0 0 1}〈1 1 0〉 bound to anti-phase boundary (APB) should be the main deformation mechanism in L12 Al3Ti. Due to the negative APB energy, by thermal activation or externally applied stress, the L12 structure would overcome the energy barrier and transform to other more stable configurations.

Role of intensive milling in mechano-thermal processing of TiAl/Al2O3 nano-composite

In this research a nano-composite structure containing of an intermetallic matrix with dispersed Al2O3 particles was obtained via mechanical activation of TiO2 and Al powder mixture and subsequent sintering. The mixture has been milled for different lengths of time and then as a subsequent process it has been sintered. Phase evolutions in the course of milling and subsequent sintering of the milled powder mixture were investigated. Samples were characterized by XRD, SEM, DTA and TEM techniques.The results reveal that the reaction begins during milling by formation of Al2O3 and L12 Al3Ti and further milling causes partial amorphization of powder mixture. DTA results reveal that milling of the powder mixture causes solid state reaction between Al and TiO2 rather than liquid–solid reaction. Also, it was observed that the exothermicity of aluminothermic reduction is reduced by increasing the milling time and the exothermic peak shifts to lower temperatures after partial amorphization of powder mixture during milling. Phase evolutions of the milled powders after being sintered reveal that by increasing the milling time and formation of L12 Al3Ti in the milled powder, intermediate phase formed at 500°C changes from D022 Al3Ti to Al24Ti8 phase.

Age Hardening of Ultrafine Grained Al-Ti-Cr Alloys Fabricated by Continuous Electron Beam Evaporation

Supersaturated Al-Ti-Cr alloys were produced by a continuous electron beam evaporation technique. The as-deposited alloy sheets were fcc supersaturated solid solution with an average grain size of about 400 nm. Due to the fine grain size, the hardness of the alloy was as high as 270 Hv. The sheet showed age hardening, and the peak hardness of 340 Hv was obtained after aging at 623 K for 900 s, in which ultrafine coherent precipitates of L12 Al3Ti phase were uniformly dispersed. As the annealing temperature increased up to 773 K, large D022 particles precipitate by the discontinuous precipitation mode and the hardness significantly decreased. Although Cr was confirmed to be enriched in the L12 particles, it did not completely stabilize the L12 phase probably due to the effect of oxygen.

Phase constitution of some intermetallics in continuous quaternary pillar phase diagrams

In order to express multicomponent phase relationships, a new type of phase diagram, named a “pillar phase diagram,” is proposed. The pillar phase diagram is a sort of quaternary and Brewertype phase diagram. A pillar phase diagram at a temperature is easily constructed by (1) composing three sides of Brewer-type ternary-phase diagrams along the periodic table and (2) piling triangles of general ternary phase diagrams along the periodic table. Using the pillar phase diagrams, systematic understandings of phase constitution and stability along the periodic table can be performed for (1) binary intermetallics containing ternary and quaternary additional elements and (2) ternary intermetallics containing quaternary elements. Thus, the diagrams are convenient in terms of alloy design of multiphase intermetallic alloys. In this paper, the concept and the construction method of the pillar phase diagrams are described, and some examples are shown for systems related with L12 Ni3Al, B2 NiAl, L21 Ni2AlTi, B2 NiTi, B2 FeTi, L10 TiAl, and L12 Al3Ti alloys.

Preparation of Al3Ti and L12 Al3Ti-Base Alloys Microalloyed with Fe by a Melting/Casting Rapid-Solidification Technique

Intermetallic compounds of Al3Ti and L12 (Al3Ti base) microalloyed with Fe have been obtained using a melting/casting rapid solidification technique. Microstructural characterization of these alloys has been carried out through analytical SEM observations and also x-ray diffraction studies. Contaminants in these alloys have also been studied and their nature explored. Other morphology structural features such as grain size have been studied.

Phase decomposition and hardening of Ag-modified L12Al3Ti

Abstract Transmission electron microscopy examinations of Ag-modified Al3Ti with an L12-ordered structure have revealed the various modes of precipitation of Al2Ti upon aging after quenching from higher temperatures. A fine uniform precipitation of Al2Ti occurs when the supersaturation is sufficiently high. On the other hand, a preferential precipitation at the antiphase boundaries can be observed in alloy with a low supersaturation. When L12Al3Ti is supersaturated with DO22Al3Ti, DO23Al11Ti5 with a multidomain structure is formed during aging. The L12 phase field in the TiAlAg system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. Appreciable hardening and overage softening during aging can be explained in terms of microstructural variations.

Alloy design and coarsening phenomenon of L12 precipitates in high-temperature Al-2 At. Pct (Ti,V,Zr) systems

Aging works of two melt-spun Al-2 at pct (Ti,V,Zr) alloys showed that metastable L12 Al3(Ti,V,Zr) precipitates were dominant and did not transform to stable D023 ones: the average radius was 3 to 4 nm and the interparticle spacing was 10 to 30 nm at 698 K up to 400 hours. Coarsening kinetics was found to be very sluggish and was coincident with the low lattice mismatch. Due to the low coarsening rate and the high thermal stability of the precipitated phase, rapidly so-lidified Al-Ti-V-Zr systems show promise as base of high-strength Al alloys for high-temperature applications.

Coarsening behavior of L12 precipitates in melt-spun AlTiVZr alloys

Aging studies of two melt-spun Al-2at.% (Ti,V,Zr) alloys showed that the would-be metastable L12 Al3 (Ti,V,Zr) precipitates did not transform to stable D023 ones, and the average radius is 5–7 nm and the interparticle spacing is 20–40 nm at 698 K up to 400 h. The coarsening rate of spherical Al3(ti0.2V0.4Zr0.4) precipitates was observed to be five times as fast as that of Al3(Ti0.1V0.4Zr0.5) precipitates. The coarsening behavior in both alloys obeyed the Lifshitz-Slyozov-Wagner (LSW) prediction well. Owing to the low coarsening rate and the high thermal stability of the precipitated phase, AlTiVZr systems show promise as bases for high-temperature high-strength Al alloys.

Comment on ‘Transmission electron microscopy of dislocation structures in iron-doped Al3Ti with the L12structure’

Abstract In deformed L12 Al3Ti, a transition of the dissociation of ⟨110⟩ superdislocations from two superpartials with ⅓⟨112⟩ Burgers vectors (mode II) at a low temperature to two superpartials with collinear ½⟨110⟩ Burgers vectors (mode I) at a high temperature, has been reported by Inui et al. in 1992. The weak-beam electron microscopy observations presented by these authors are re-examined. It is shown that the contrasts observed in the microstructure of deformation at a low temperature are fairly consistent with a narrow splitting into mode I rather than a dissociation according to mode II.