T1 Phase Research Articles

How microstructure affects localized corrosion resistance of stir zone of the AA2198-T8 alloy after friction stir welding

In this study, the microstructure and corrosion resistance of the stir zone (SZ) of the AA2198-T8 Al-Cu-Li alloy welded by friction stir welding (FSW) were investigated by microscopy, immersion tests and electrochemical techniques such as measurements of open circuit potential variation with time, and scanning vibrating electrode technique (SVET) measurements. A low chloride-containing solution (0.005 mol L−1 NaCl) was employed in the corrosion studies and severe localized corrosion (SLC) was observed in the SZ related to intergranular attack. The results were compared to those of the non-affected areas by FSW, also known as base metal (BM). In the BM, SLC was found and the type of attack related to it was intragranular. In both zones, BM and SZ, SLC was due to precipitates of high electrochemical activity, specifically T1 (Al2CuLi) phase in the BM, whereas TB (Al7Cu4Li) / T2 (Al6CuLi3) in the SZ. Scanning vibrating electrode technique (SVET) analysis was very useful in the study of SLC in the AA2198-T8 alloy showing the development of high anodic current densities at the mouth of the SLC sites.

Effect of the structure and the phase composition on the mechanical properties of Al–Cu–Li alloy laser welds

Abstract The purpose of these investigations was to study the effect of heat treatment on the structure and the phase composition of laser welded joints of the Al-2.8Cu-1.7Li alloy (V-1461 grade), providing the maximum tensile strength. To study the phase composition of the weld metal, synchrotron radiation was applied using a ‘Mega Science’ facility. This enabled to assess the phase distribution across the weld metal before and after heat treatment. It was found using high-resolution scanning and transmission electron microscopy, energy dispersive X-ray analysis, and synchrotron X-ray diffractometry that the T1(Al2CuLi) and T2(Al6CuLi3) main phases had been formed in the weld metal and at the interfaces with the matrix. Ultimate tensile strength of the welded joints was about 341 MPa, which was 62% of that of the base metal. Subsequent heat treatment of the welded samples, included hardening and artificial aging, caused the homogenization of the Al–Cu–Li alloy solid solution, as well as the formation of the δ′(Al3Li) hardening phase. Also, the T1 and T2 phases were formed partially. After quenching, tensile strength of the welded joints improved and was about 85% of that of the base metal. After artificial aging, it was about 510 MPa and approached tensile strength of the base metal (93%).

Investigation on the Stress Relaxation Aging Behavior of 2195 Al-Li Alloy

2195 Al-Li alloy is famous for high strength, excellent fatigue strength and good stress corrosion resistance, which is widely used in the manufacture of high-performance aerospace components. The aim of this study is to validate how the stress relaxation aging behavior effect on the mechanical properties of 2195 Al-Li alloy. Through mechanical property test, the research was found that the performance after stress relaxation aging is higher than artificial aging (AA). In addition, the analysis of scanning electron microscopy SEM and TEM revealed that dislocations should be introduced by the stress relaxation aging process, which is more conducive to the precipitation of the T1 phase and strengthened the material with prolong ageing time. The results show that stress relaxation aging can significantly promote the precipitation of the T1. Therefore, this paper sheds new light on how SRA can improve mechanical properties and that SRA make better improve the distribution of precipitates in the grain boundary.

Sluggish precipitation strengthening in Al–Li alloy with a high concentration of Mg

The influence of different Mg concentrations on the mechanical properties and microstructure of an Al–Li alloy was studied. The highest Mg content was up to 1.1 wt.%. To the best of the authors knowledge, alloys with such a high Mg content has not been reported in the literature. The mechanical properties, yield strength and elongation at failure, of all of the alloys were investigated by tensile tests. Characterisation of the precipitates by analysing the dark field images and related diffraction patterns revealed that precipitation of the T1 phase (Al2CuLi) was retarded by high (1.1 wt.%) Mg concentration. We hypothesized that a competition between the precipitation of T1 phase and S' phase in high Mg containing alloy caused sluggish precipitation phenomenon and associated strengthening.

Microstructural characterization of arc-melted and directionally solidified near-eutectic molybdenum–silicon–boron alloys

Samples of near-eutectic Mo–Si–B alloys, with the nominal composition corresponding to the Moss + Mo3Si + T2 eutectic area, were prepared by arc-melting or by crucible-free zone-melting methods (directional solidification). In the directionally solidified (DSed) alloy, the volume fraction of the Moss + Mo3Si + T2 eutectic cells appeared to increase compared with that of arc-melted (AMed) alloy, and well-aligned and rod-shaped Moss and T2 phases were formed in a Mo3Si matrix of the eutectic cells. In the AMed alloy, only the T2 phase showed a <100] preferential growth orientation along the solidification direction, whereas the Moss phase displayed no obvious preferred orientation. The Mo3Si matrix showed a weak preference toward <001> growth along the solidification direction in the AMed alloy, and this became much stronger in the DSed alloy. Furthermore, the DSed alloy exhibited an apparent preferential growth orientation relationship of <001>Moss//<001>Mo3Si//<100]T2 along the solidification direction. However, no specific atomic structures were observed at the interfaces, indicating that the three phases grew independently from the liquid without the formation of any habit interface during solidification.

High-Temperature Equal-Channel Angular Pressing of a T6-Al-Cu-Li-Mg-Ag-Zr-Sc Alloy

Equal-channel angular pressing (ECAP) is known to induce significant grain refinement and formation of tangled dislocations within the grains. These are induced to evolve to form low-angle boundaries (i.e., cell boundaries) and eventually high-angle boundaries (i.e., grain boundaries). On the other hand, the precipitation sequence of age hardening aluminum alloys can be significantly affected by pre-straining and severe plastic deformation. Thus, ECAP is expected to influence the T6 response of aluminum alloys. In this study, a complex Al-Cu-Mg-Li-Ag-Zr-Sc alloy was subjected to ECAP following different straining paths. The alloy was ECAP at 460 K via route A, C, and by forward-backward route A (FB-route A) up to four passes. That is, ECAP was carried out imposing billet rotation between passes (route A), billet rotation by +90° between passes (route C), and billet rotation by +90° and inversion upside down between passes (FB-route A). The alloy was also aged at 460 K for different durations after ECAP. TEM microstructure inspections showed a marked influence of the different shearing deformations induced by ECAP on the alloy aging response. The precipitation kinetics of the different hardening secondary phases were affected by shearing deformation and tangled dislocations. In particular, the T1-Al2CuLi phase was the one that mostly showed a precipitation sequence speed up induced by the tangled dislocations formed during ECAP. The T1 phase was found to grow with aging time according to the Lifshitz-Slyozov-Wagner low-power regime.

Interface of a Al6061/Ti Composite Prepared by Field Assisted Sintering Technique

Architectured heterogeneous metallic composites consist of two dissimilar materials with a particular focus on spatial arrangement of constituents. This experimental study describes the application of Field Assisted Sintering Technique (FAST) for manufacturing of composite materials by sintering of a bulk reinforcement with a powder metal. Simple structure made of Ti wire (Ti Grade 2) was sintered with Al6061 alloy powder at 560 °C for 10 min. Successful material bonding and evolution of diffusion layer was thoroughly studied by scanning and transmission electron microscopy. Diffusion layer and adjacent precipitates are described as ternary Ti-Al-Si τ1 and τ2 phases. Si, as an alloying element in the Al6061 alloy, significantly affects the formation of the diffusion layer at the material interface due to its high inter-diffusion coefficient in both Al and Ti. Detailed TEM analysis also showed a modulated τ1/τ2 structure resembling a long-period stacking order (LPSO) phase, which has not been previously reported in the Ti-Al-Si ternary compounds. FAST is capable to manufacture composites from dissimilar constituents, which opens new possibilities for design and manufacturing of architectured materials.

Particle-stimulated nucleation and recrystallization texture initiated by coarsened Al2CuLi phase in Al–Cu–Li alloy

The particles stimulated nucleation (PSN) triggered by coarsened T1 phase (Al2CuLi) in Al–Cu–Li alloys is investigated to analyze the formation of recrystallization texture that is unusual from typical PSN. The specimens were uniaxially compressed to a series of compression amount and the recrystallization process is characterized by electron backscattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Results showed that the recrystallization occurred by progressive rotation of the matrix units that separated by the three-dimensional arranged T1 precipitates. The rotation of the units towards <001> was observed in the initial grains of various orientations all through the recrystallization process. The massively formed fine grains contribute to a strong <001> fiber texture. The fraction of recrystallization is 67% and the grains are averagely 2.5 μm in the specimen received a stain of 1.61. The PSN initiated by coarsened T1 phase fosters the feasibility to obtain textured and refined structure in Al–Cu–Li alloys which will facilitate the performance improvement of the fabricated components.

The effect of oxygen on phase formation in an industrial Zr based bulk metallic glass

Effect of oxygen on the phase formation and thermal stability of Zr50·75(Cu90Ni10)40·25Al9 bulk metallic glass (BMG) has been systematically studied. Our results show that τ3 phase (Zr50Cu29Ni2Al19 intermetallics) can be formed when the oxygen level reaches 350 ppm and its morphology changes from an undeveloped dendritic to dendritic structure with increasing oxygen content. It is demonstrated that oxygen is mainly concentrated in τ3 phase. Microstructure characterization indicates that τ3 phase possesses a face-centered cubic structure and its lattice parameter increases with increasing oxygen content. These findings would aid the development of Zr-based BMGs with industrial grade materials for commercial application.

Interactions between cadmium and Al3Li precipitates: A new mechanism of accelerating dissolution and transformation of phases in Al–Li–Cu alloy

Abstract The interactions between Cd and δ′-Al3Li precipitates and their impacts were reported for the first time. Transmission electron microscopy and X-ray diffraction results suggested that a trace (0.06 at. %) addition of Cd significantly altered the microstructural evolution and the hardening behavior of high-lithium aluminum alloys. The Cd addition not only led to a lower peak-hardness and shorter peak-aged time, but also caused a sharp decrease of hardness and the dissolution of δ′ precipitates in over-aged state. Atom probe tomography results and first-principles calculations showed that Cd atoms preferentially partitioned to δ′ precipitates, occupying the Al sublattice. Such configuration was shown to be energetically unfavorable, thus would accelerate the dissolution of δ′ precipitates, leading to the decrease of hardness. The dissolution of δ′ subsequently provided ample Li atoms in the matrix, promoting the formation of T2-Al6CuLi3 phase. Eventually, spherical δ′ precipitates completely dissolved, replaced by T2 phases of larger sizes and irregular shapes.

Effects of heat treatment on the microstructure, texture and mechanical property anisotropy of extruded 2196 Al-Cu-Li alloy

The mechanical properties and microstructures of the extruded 2196 Al-Cu-Li alloy plates under different heat treatment parameters are tested and characterized along different directions. It is found that the mechanical property anisotropy of 2196 Al-Li alloy is significantly affected by heat treatment. The anisotropy is mainly caused by the differences of grain morphology, microtexture, recrystallization degree and secondary particles characteristics in different directions. The specimens in 0° and 90°directions have high strength, while the specimen in 45° direction has the best ductility. The main microtexture of alloy are {111}<112>oriented A*1 and A*2 shear textures, and aging treatment has little effect on the grain morphology and microtexture. The decrease of mechanical property anisotropy after aging treatment is mainly due to the increase of number density and distribution uniformity of T1 phase. The elimination of PFZ and the increase of grain boundary strength are also conducive to the decrease of anisotropy. The dislocations introduced by pre-stretching is beneficial to the uniform precipitation of the fine phases, resulting in the optimal mechanical property homogeneity of the alloy. The ductility in different directions is also anisotropic, and the fracture mechanism changes from ductile to intergranular and cleavage fracture modes after heat treatment.

Characterising the Microstructure of an Additively Built Al-Cu-Li Alloy.

Al-Cu-Li alloys are famous for their high strength, ductility and weight-saving properties, and have for many years been the aerospace alloy of choice. Depending on the alloy composition, this multi-phase system may give rise to several phases, including the major strengthening T1 (Al2CuLi) phase. Microstructure investigations have extensively been reported for conventionally processed alloys with little focus on their Additive Manufacturing (AM) characterised microstructures. In this work, the Laser Powder Bed Fusion (LPBF) built microstructures of an AA2099 Al-Cu-Li alloy are characterised in the as-built (no preheating) and preheat-treated (320 °C, 500 °C) conditions using various analytical techniques, including Synchrotron High-Energy X-ray Diffraction (S-HEXRD). The observed dislocations in the AM as-built condition with no detected T1 precipitates confirm the conventional view of the difficulty of T1 to nucleate on dislocations without appropriate heat treatments. Two main phases, T1 (Al2CuLi) and TB (Al7.5Cu4Li), were detected using S-HEXRD at both preheat-treated temperatures. Higher volume fraction of T1 measured in the 500 °C (75.2 HV0.1) sample resulted in a higher microhardness compared to the 320 °C (58.7 HV0.1) sample. Higher TB volume fraction measured in the 320 °C sample had a minimal strength effect.

Microstructure evolution and mechanical properties of Al−3.6Cu−1Li alloy via cryorolling and aging

Abstract An Al−3.6Cu−1Li alloy was subjected to room temperature rolling and cryorolling to investigate their effects on microstructure evolution and mechanical properties. The microstructure and aging characteristics of the room temperature-rolled and the cryorolled alloys with 70% and 90% of thickness reductions were studied by microstructure analysis and mechanical tests. The samples subjected to cryorolling with 90% of thickness reduction have high strength and good toughness. This is mainly due to the inhibition of dynamic recovery and the accumulation of high-density dislocations in cryorolled samples. In addition, the artificial aging reveals that the temperature at which peak hardness is attained is inversely proportional to the deformation amount and directly proportional to the rolling temperature. Moreover, bright field images of cryorolled samples after aging indicate the existence of T1 (Al2CuLi) precipitates. This suggests that the high stored strain energy enhances the aging kinetics of the alloy, which further promotes the nucleation of T1 phases.

Microstructure evolution and mechanical properties of Al-Cu-Li alloys with different rolling schedules and subsequent artificial ageing heat treatment

Mechanical properties, precipitate microstructures and textures of AA 2195 Al-Cu-Li alloy with different rolling schedules and subsequent artificial ageing heat treatment have been investigated. The highest ultimate tensile strength (UTS) and yield strength (YS) were obtained in a longitudinal direction of the hot-rolled alloy at 280 °C due to a higher number density of the T1 phase and the increased volume fraction of the textures with rolling textures. In contrast, the second highest UTS and YS in the longitudinal direction and the highest UTS and YS in the transverse direction was clarified in the cold-rolled and hot-rolled alloys at 225 °C. This ‘anomalous’ anisotropy can be explained not only by a higher number density of the θ’ phase within fine and equiaxed recrystallized grains but also by grain boundary precipitates at low-angle grain boundaries. Therefore, textures control was suggested to be a useful method to improve mechanical properties in addition to the conventional precipitation strengthening.

Effects of Pre-stretching and Aging Treatments on Microstructure, Mechanical Properties, and Corrosion Behavior of Spray-Formed Al-Li Alloy 2195

In this study, different pre-stretching and aging processes were applied to spray-formed Al-Cu-Li alloy 2195. The effects of different processes on the microstructures, mechanical properties, and corrosion behaviors of the alloy were examined. The strengthening and corrosion mechanisms were investigated. The samples treated with pre-stretching and aging have higher strength than the only aged samples. However, the strength is not significantly improved with the increase in the amount of pre-stretching. The pre-stretching treatment can suppress the formations of grain boundary precipitates (GBPs) and precipitate-free zones (PFZ), and the intergranular corrosion (IGC) sensitivity of the alloy is therefore reduced. The increase in pre-stretching promotes massive precipitation of the T1 phase, which leads to the negative shift of open-circuit potential. The over-aging treatment after pre-stretching leads to the growth of the PFZ and GBPs as well as the coarsening of the T1 phase. These changes in microstructure cause a decrease in mechanical properties and the re-sensitivity to the IGC. Under the same pre-stretching deformation, the two-step aging treatment (110 °C/12 h + 160 °C/24 h) can result in more intensive precipitation of the T1 phase, which enhanced the strength to a higher level. Meanwhile, the two-step aged alloy was immune to the IGC. The evolution mechanism of IGC for the alloy with different pre-stretching and aging processes was proposed based on the TEM characterization and the observation of IGC morphology.

Investigation on the control of interfacial layer uniformity in laser-metal inert-gas hybrid welded-brazed Al/steel butt joint

Abstract Obtaining a homogeneous distribution of interfacial layer in the Al/steel butt joint could further improve the tensile properties of well-formed joint, which is of great significance to the application of Al-steel hybrid structures in engineering. This article successfully used laser-metal inert-gas (MIG) hybrid welding-brazing technology to obtained 2-mm-thick 6061-T6 aluminum alloy and 301 stainless steel dissimilar metals butt joint. The penetration ability of the hybrid heat source was improved by increasing the laser power within an appropriate range. The interfacial layer with the most uniform thickness (4.5 μm–5.2 μm) and phase composition (τ5 phase and θ phase) could be obtained at laser power of 2000 W. Numerical simulation results revealed that reducing the difference of peak temperature along Al/steel interface could effectively improve the uniformity of the interfacial layer. The smallest difference of interfacial temperatures in the joint produced at laser power of 2000 W was responsible for the most uniform interfacial layer. The tensile strength of the Al/steel butt joint with the most uniform interfacial layer could reach the maximum value of 198 MPa (about 70 % of the 6061-T6 tensile strength).

Effect of hot isostatic pressing on the microstructure and fracture toughness of laser additive-manufactured MoSiBTiC multiphase alloy

MoSiB-based alloys are extremely promising ultrahigh–temperature materials, owing to their high melting points, good thermal stability, and excellent elevated-temperature mechanical properties. However, their fabrication is limited to conventional manufacturing techniques. In this work, a MoSiBTiC multiphase alloy, mainly consisting of Mo solid solution (Moss), Mo5SiB2 (T2), and TiC phases, was fabricated by laser powder bed fusion (L-PBF) using ball-milled alloy powders; subsequently, the effect of hot isostatic pressing (HIP) on its microstructure and fracture toughness was investigated. Due to the non-equilibrium metallurgical characteristics of L-PBF, the as-built alloy included uniform and fine-grained structures, as well as internal microcracks. The length of microcracks was effectively reduced with the assistance of the TiC bridging effect during HIP. As revealed by a high-resolution transmission electron microscope, the TiC was uniformly dispersed and was in close contact with the Moss or T2 phase. Consequently, the MoSiBTiC multiphase alloy exhibited a high fracture toughness of 9.0 MPa(m)1/2 after HIP at 1973 K. This work could be an important step towards the fabrication of high-performance refractory components with advanced properties in ultrahigh–temperature use.

Microstructures and mechanical properties of a cast Al–Cu–Li alloy during heat treatment procedure

Al–Cu–Li alloy is a huge potential alloy for aerospace industry due to its low density and excellent comprehensive performances. In this study, the microstructures and mechanical properties of a novel cast Al–Cu–Li alloy during the heat treatment were investigated. Results showed that most of the secondary phases along grain boundaries in as-cast state were dissolved into the Al matrix after homogenization and solution treatment, while the Cu-rich phases were slightly segregated once again on grain boundaries after aging. A relatively good combination of strength and ductility (average ultimate tensile strength of 430 MPa and elongation of 8.9%) was obtained after natural aging. Transmission electron microscopy (TEM) results revealed that there was a complicated precipitate distribution in this cast alloy after natural aging. Compared with the similar alloys which introduced dislocations before aging, the density of T1 (Al2CuLi) phases was decreased but the size was increased. The low volume fraction of δ′ phase, medium density of needle-shaped precipitates and relativity narrow precipitates-free zone with varying widths near grain boundary are the reasons accounting for the improvement of performances.

Determination of pressurizing rate during hot gas forming with integrated heat treatment of Al–Cu–Li alloy: deformation and strengthening behaviors

Hot gas forming with an integrated heat treatment was utilized to form complex-shaped components in one operation that obtain full strength. The effects of the pressurizing rate on the deformation and strengthening behaviors were critical. Free-bulging tests were conducted at different pressurizing rates (0.0088–2.37 MPa/s) to reveal the corresponding deformation and strengthening behaviors of an Al–Cu–Li alloy sheet and for choosing appropriate pressurizing conditions. The profile and thickness were analyzed to describe the deformation behaviors. The Vickers hardness under different pressurizing and deformation conditions was measured to evaluate the strengthening behavior. The microstructure evolution was observed to reveal the deformation and strengthening mechanisms. An excellent bulging ability was obtained under the full solution condition, with a maximum strain of 0.88 at a pressurizing rate of 0.034 MPa/s. The thickness distribution became more uniform under rapid pressurizing conditions because of the increasing strain and strain-rate hardening abilities. The hardness increased with increasing deformation and was not affected by the pressurizing rate. Dynamic recovery was the main deformation mechanism, and dynamic recrystallization occurred at a low pressurizing rate. Fine precipitates comprising the T1 phase (Al2CuLi, hexagonal structure) were the main strengthening phase. The pressurizing condition was only considered to obtain the maximum improvement in formability and microstructure, regardless of its effect on strengthening during hot gas forming with an integrated heat treatment.

Effects of pre-strain and stress level on stress relaxation ageing behaviour of 2195 Al–Li alloy: Experimental and constitutive modelling

In the present work, the stress relaxation behaviour, mechanical properties and microstructural evolution of 2195 Al–Li alloy with different pre-strains (0%, 2% and 4%) have been experimentally investigated under different initial stress levels at 180 °C for 16 h. It is found that pre-strain can extend the duration of the first stress relaxation stage and significantly promote the stress reduction. Meanwhile, a new “multi-step stage” stress relaxation feature, containing a particular linear relaxation stage, is observed for the 2195 Al–Li alloy during stress relaxation ageing (SRA) process. In addition, the tensile strength of the alloy increases with the increase of pre-strains and initial stresses, while the corresponding elongation is opposite. The evolution of precipitates, including G.P. Zones, θ″, θ′ and T1 phases, have been characterized by transmission electron microscope (TEM). The number of T1 phases increases with the increase of the pre-strains and initial stresses, while the number of θ′ phases gradually decreases. Based on precipitation kinetics, stress relaxation mechanism and dislocation theory, a physically-based SRA constitutive model considering the evolution of dislocation density and relative volume fraction of T1 and θ′ precipitates is established. The results calculated by the constitutive model are in good agreement with the experimental data.