Precipitation Of T1 Phase Research Articles

Effect of laser shock peening and annealing temperatures on stability of AA2195 alloy near-surface microstructure

The thermal stability of surface microstructure as well as mechanical property induced by laser shock peening (LSP) in AA2195 alloy was investigated in the present work. Samples of the AA2195 alloy were firstly LSPed in once or three times and then annealed at 250, 300 and 350 °C for 1 h, respectively. The surface microstructures of LSPed and annealed samples were investigated by transmission electron microscopy (TEM), and the thermal stability of the mechanical properties was characterized by the changes of the residual stresses and microhardness at the top surface layer. Results showed that the critical temperatures of the sample with 3-times LSP leading to the notable grain growth and changes of mechanical properties were 350 °C and 300 °C respectively, and the corresponding temperatures of the sample with 1-time LSP were 300 °C and 250 °C respectively. Precipitates (eg. T1) in the surface layer of AA2195 alloy were dissolved after LSP, and the thermal stability of the structure and mechanical property was improved by the secondary precipitation of T1 phase during following annealing. The experimental results exhibited that the thermal stability of the sample with 3-times LSP was better than that of the 1-time LSPed one, which indicated that multiple times LSP enable benefit of the stability of structure and mechanical property of AA2195 alloy.

Pre-strain-dependent natural ageing and its effect on subsequent artificial ageing of an Al-Cu-Li alloy

The effects of pre-strain and natural ageing (NA) on the precipitation hardening behavior in an Al-Cu-Li alloy have been systematically investigated. Applying pre-strain significantly restricts the solute clustering-induced formation of fine GPI zones (single Cu-layer) and thus hardness increase during NA. NA before artificial ageing (AA) has a marginally positive effect on the age hardening in samples without pre-strain, giving rise to a strength improvement of 15–20 MPa without loss of ductility. NA slightly enhances the precipitation of T1 phase during AA while shows no obvious effect on θ′, σ, δ′ and X phases. The X phase hasn't been reported and is isostructural to the θ′ phase, but the habit plane is (011)Al instead of (001)Al. With the increase of pre-strain level, the T1 plates become dominant and finer. However, NA after pre-strain has no appreciable effect on the age hardening and microstructure of the alloy. The analysis by complementary methods suggests that the low stability of NA clusters and their inhibition by pre-strain are responsible for the observed NA effect on AA in Al-Cu-Li alloy.

Microstructural characteristics and mechanical properties of extruded Al-4Cu-1Li-0.4Mg-0.1Zr-xZn alloy

The microstructures and mechanical properties of cast and extruded Al-4Cu-1Li-0.4Mg-0.1Zr-xZn alloys were investigated and the amount of Zn additions was up to 1.1 wt%. The results showed that Zn addition could bring additional MgZn2 phases at the grain boundary and slightly refine the grain size in the as-cast alloy. The mechanical properties of extruded Al-4Cu-1Li-0.4Mg-0.1Zr-xZn alloy increased gradually with the Zn content under different conditions. The yield strength (YS) and ultimate tensile strength (UTS) of T8 treated (stretched by 4% and aged at 150 °C for 32 h) 1.1 Zn alloy could reach 639 MPa and 666 MPa, while elongation (EL) remained at 10.9%. For alloys peak-aged at 175 °C for 32 h (T6 treated), Zn could effectively promote the precipitation of T1 phases (Al2CuLi) but decrease the volume fraction of θ′ phases (Al2Cu) in the matrix. The coarsening of T1 phases could be obviously suppressed by the addition of Zn under overaged condition. However, the precipitate of T1 phases promoted by Zn under the T8 condition is not as obvious as the T6 condition.

Effect of trace amounts of added Sc on microstructure and mechanical properties of 2055 aluminum alloy

The effects of Sc additions and isothermal aging on the microstructure and mechanical properties of 2055 aluminum alloy have been investigated. The isothermal aging temperatures investigated were 175 °C and 200 °C. Results show that the aging temperature does not affect the type of precipitates in both Sc-free and Sc-added 2055 Al alloys. While remarkable changes in the size and number density of T1 (Al2CuLi) precipitates have been observed. For samples aged at 175 °C for 8 h, the average length of T1 precipitates was increased from 39.9 nm to 45.6 nm, the number density decreased from 5.77 × 1022 m−3 to 3.68 × 1022 m−3 after Sc addition. On the contrary, when samples were aged at 200 °C for 8 h, the length decreased from 59.89 nm to 43.32 nm and number density increased from 2.13 × 1022 m−3 to 3.00 × 1022 m−3 by adding trace Sc. Slight deterioration of tensile strength and yield strength with higher ductility was founded at the aging temperature of 175 °C for 8 h due to the larger size and lower number density of T1 precipitates along with the formation of a W (AlCuSc) phase during the casting or homogenization processes. When aging temperature increased to 200 °C, tensile strength and yield strength were significantly improved by Sc addition without losing ductility. Highly dense fine T1 precipitates take responsible for the strength improvement, and to some extent it compensated the strength loss caused by the formation of W phase. A “vacancy-prison” mechanism was adopted to explain the effect of Sc additions on the precipitation of T1 phase as a function of aging temperature.

Microstructural evolution and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment

The microstructure, electrical conductivity and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment were investigated. The results indicated that a population of secondary phases enriched along grain boundaries in as-cast studied alloy, most of which dissolved into the α-Al matrix after solution treatment. Profound enhancement in mechanical properties of the studied alloy was obtained after T6 heat treatment. TEM observations revealed competitive precipitation of T1 (Al2CuLi) and S′ (Al2CuMg) phases during ageing, while no θ′ (Al2Cu) phase was observed. The anomalous decrease in conductivity observed during ageing was attributed to the precipitation of T1 phases. Fine δ′ (Al3Li) particles were homogeneously distributed after quenching, gradual coarsening of which was observed with prolonged ageing time. As ageing proceeded, the δ′ coarsened and δ′-Precipitate-Free Zones (PFZs) developed, resulting in diminishing ductility observed from as-quenched to peak-aged state. Precipitation and continuous coarsening of T1 phases kept on at the expense of gradual δ′ dissolution. The improvements of ductility observed in over-aged samples arose mainly from the presence of T1, S′ phases, Al3(Li,Zr) composite precipitates and progressive δ′ dissolution. Moreover, continuous coarsening of T1 and S′ phases contributed primarily to the strength loss in over-aged studied alloy.

Microstructural evolution of 2099 Al[sbnd]Li alloy during friction stir welding process

Microstructural evolution of friction stir welded 2099 AlLi alloy was evaluated. Friction stir welding (FSW) resulted in significant grain refinement, dissolution of original phases T1 (Al2CuLi) and δ′ (Al3Li), and precipitation of ultrafine δ′ phases in the nugget zone (NZ). In the heat affected zone (HAZ), heavy precipitation of T1 phases led to a peak hardness of 105HV. A strong rotated Cube texture {013}〈100〉 was obtained in the base metal (BM). After FSW the NZ was characterized by shear texture {111}〈110〉, while Bass texture {110}〈112〉 and S texture {123}〈634〉 were developed in the thermo-mechanically affected zone (TMAZ). Compared with the BM, the dislocation density was decreased in the welding zones, which resulted from recovery and recrystallization.

Localised corrosion in AA 2099-T83 aluminium-lithium alloy: The role of grain orientation

The mechanism for localised corrosion in AA 2099-T83 alloy during immersion in 3.5% NaCl solution is investigated. It is found that localised corrosion tends to occur in the grain with relatively large Schmid factor. The localised corrosion is related to selective dissolution of T1 (Al2CuLi) phase that preferentially precipitates at grain/subgrain boundaries and dislocations within grain interiors. A model is proposed to explain the development of the localised corrosion in the alloy by taking into account heterogeneous plastic deformation during cold working and preferential precipitation of T1 phase at crystallographic defects within deformed grains.

The effect of thermal fatigue on the mechanical properties of the novel fiber metal laminates based on aluminum–lithium alloy

The effect of thermal fatigue on the mechanical properties of the novel fiber metal laminates (FMLs) based on aluminum–lithium alloy was investigated. The results indicated that no obvious delamination or defects were observed in the novel FMLs exposed to 1000 cycles. The samples treated with different cycles still exhibited stable and excellent interlaminar properties comparing with the as-manufactured ones. Furthermore, the tensile and flexural strength of the FMLs even increased with the thermal fatigue cycles owing to the positive age hardening behavior of aluminum–lithium layer. The homogeneous and fine precipitation of T1 phases dominated the strengthening effect of aluminum–lithium alloy. Besides, the novel FMLs after thermal fatigue treatments still possessed the similar resistance to fatigue crack growth (FCG) when compared with the as-manufactured ones. The slight changes in the properties of aluminum–lithium layers had no detrimental effect on the FCG.

Effect of age-forming on microstructure, mechanical and corrosion properties of a novel Al–Li alloy

The age-forming technology of Al–Li alloy has caused considerable attention in recent years. In this study, the effects of age-forming on microstructure, mechanical and corrosion properties of a novel Al–Li alloy are studied. Results show that age-forming in the novel Al–Li alloy enhances the mechanical properties including strength and elongation, meanwhile, improves the corrosion resistance. The stress during the aging does not change the morphology of the grains, but inhibits the precipitation of θ′ phases, promotes precipitation of T1 phases and reduces the dimension of T1 phases. The orientation effect for the θ′ phases is obvious, while for the T1 phases, this effect is not obvious under the load of 100MPa. At the same time, age-forming helps to inhibit grain boundary precipitate coarsening and the PFZ widening.

A New Ultrahigh Strength Al-Cu-Li Alloy

In this work, a new ultra-high strength Al-Cu-Li alloy was investigated. The ultimate strength, yield strength and elongation of the newly designed alloy by artificial aging are 647.2MPa, 609.4MPa and 7.3% respectively. Among the main strengthening phases of T1, θ′ and S′ in the experimental alloys, T1 is the dominant one. The combined addition of Mg and Ag promoted the precipitation of T1 and increased the strength of the new alloy greatly. Zn had a similar effect as Ag during the aging strengthening progress, when added with Mg. Among the three micro-alloying elements, Mg, Ag and Zn, Mg had the strongest influence on age strengthening. Compared with the combined additions of (Mg +Ag) and (Mg + Zn), (Ag + Zn) had the weakest influence on aging strengthening. Pre-deformation before aging promoted the precipitation of T1 phase which weakened the influence of micro-alloying elements (Mg, Ag and Zn) on strengthening the alloys and minished the strength difference between alloy containing (Mg + Ag + Zn) and alloys containing two of them.

Effects of Multiple Strengthening Treatments on Mechanical Properties and Stability of Nanoscale Precipitated Phases in an Aluminum-Copper-Lithium Alloy

Effects of multiple strengthening treatments (i.e. aging either at three or four consecutive temperatures) on mechanical properties and stability of nanoscale precipitated phases in an AA2090 alloy have been evaluated in this research. Various tests such as hardness, tensile, electrical resistance, differential scanning calorimetric (DSC), and transmission electron microscopy (TEM) have been performed. The results show that the ultimate tensile strength (UTS) and the yield strength of the samples aged at four consecutive temperatures (i.e. natural aging+190°C+150°C+100°C) can be increased approximately to 660 and 610 MPa, respectively. It is also found that precipitation of T1 phase occurs during multiple aging process of the alloy and the higher amounts of enthalpies shown in DSC charts are linked to higher volume fraction of this precipitate. Furthermore, TEM observations reveal that T1 phase has plate shape morphology and its crystal structure is in the form of hcp with lattice parameters of a=0.467 nm and c=0.878 nm.

The Effect of Precipitates on Anisotropy of Al-Li Alloys 2090 and 2090+Ce

The critical resolved stresses (CRSS) of two kinds of Al-Li alloys 2090 and 2090+Ce in which contained trace addition of cerium are examined. The three heat treatment conditions (solution, peak aging and reversion after peak aging) are used in order to identify the influence of precipitates to CRSS. The experimental results show that the strengthening contribution of T1 Phase is greatly different at various orientation with respect to rolling direction, and the precipitation of T1 phase can weaken the anisotropy of mechanical properties. The T1 phase precipitated in subgain boundaries in peak aged and reverted specimens produce a stronger tendency of delaminating along grain boundary, which is a direct reason for the decrease of anisotropic yield strength.

Effect of silver on precipitation response of Al–Li–Cu–Mg alloys

The precipitation response of silver bearing and silver free Al–Li–Cu–Mg alloys has been monitored using hardness and electrical conductivity measurements. The silver bearing alloy displayed enhanced age hardening with an anomalous decrease in conductivity. Transmission electron microscopy was used to investigate the evolution of microstructure in these alloys. The presence of small amounts of silver promoted the formation of GP zones and Al2Cu (θ′) precipitates, and greatly enhanced the precipitation of T1. The anomalous decrease in conductivity was associated with the precipitation of T1 phase. The initial age hardening in silver bearing alloy was due to δ′, GP zones and θ′. Precipitation of T1, δ′ and small amounts of θ′ and S′ were responsible during the later stages of aging. Precipitation of θ′ was not observed in the silver free alloy. In the overaged condition, dissolution of δ′ was more prominent in the silver free alloy.

A Calorimetric Study on the Precipitation of T<sub>1</sub> Phase during the Ageing of an Al-Li-Cu-Zr Alloy

The precipitation of T1 phase during the ageing of an Al- Li- Cu -Zr alloy sheet was studied by DSC (Differential Scanning Calorimetric) technique. Results showed that precipitation of T1 phase occurred in temperature range of 250°C to 300°C whereas its dissolution occurred within the temperature of 450°C to 530°C. Furthermore, activation energies for precipitation and dissolution of T1 phase were determined 122.1 kJ/mol and 130.3 kJ/mol, respectively.

The mechanical properties and precipitation behavior of an Al-Cu-Li-(in,Be) alloy

The effect of In and Be on the mechanical properties and precipitation behavior of an Al-Cu-Li alloy, which has applications in aerospace and aircraft, has been studied using hardness and tensile tests, differential scanning calorimetry (DSC), and transmission electron microscopy(TEM). The formation of GP zones and δ' phases was retarded, but the T1 and θ' phases formation were promoted by the addition of In to Al-Cu-Li alloy. However the formation of T1 and θ' phases, as well as the formation of GP zones and δ' phases, were accelerated by the combined addition of In and Be to Al-Cu-Li alloy. The additional precipitation of T1 phases which was formed at 160°C for a long aging time, was a main reason for the secondary peaks of hardness and tensile strength in Al-Cu-Li alloys added Be and In, or In. The Al-Cu-Li-In-Be alloy was suggested as a material of good combination of tensile strength, elongation and toughness.

Ageing of forged aluminium-lithium 8091 alloy

The microstructure, electrical conductivity and mechanical properties of cast Al-2Li-2Cu-0.5Mg-0.2Zr alloy during heat treatment were investigated. The results indicated that a population of secondary phases enriched along grain boundaries in as-cast studied alloy, most of which dissolved into the α-Al matrix after solution treatment. Profound enhancement in mechanical properties of the studied alloy was obtained after T6 heat treatment. TEM observations revealed competitive precipitation of T1 (Al2CuLi) and S′ (Al2CuMg) phases during ageing, while no θ′ (Al2Cu) phase was observed. The anomalous decrease in conductivity observed during ageing was attributed to the precipitation of T1 phases. Fine δ′ (Al3Li) particles were homogeneously distributed after quenching, gradual coarsening of which was observed with prolonged ageing time. As ageing proceeded, the δ′ coarsened and δ′-Precipitate-Free Zones (PFZs) developed, resulting in diminishing ductility observed from as-quenched to peak-aged state. Precipitation and continuous coarsening of T1 phases kept on at the expense of gradual δ′ dissolution. The improvements of ductility observed in over-aged samples arose mainly from the presence of T1, S′ phases, Al3(Li,Zr) composite precipitates and progressive δ′ dissolution. Moreover, continuous coarsening of T1 and S′ phases contributed primarily to the strength loss in over-aged studied alloy.