A study on the β′ and β″ formation kinetics in AA6063 using differential scanning calorimetry

In this work the formation of β″ and β′ precipitates in a commercial AA6063 alloy was investigated using differential scanning calorimetry (DSC) and in situ heat treatments. The DSC used is equipped with fast heating and quenching facilities. The samples were solutionized, isothermally aged to induce formation of β″ or β′, rapidly quenched, and subjected to a DSC scan. The heat treatment cycle was performed entirely in the DSC, to improve the reproducibility of the dissolution and precipitation peaks. The effect of the aging treatment on the size and location of the β″ and β′ peaks was studied. It was found that after aging at 458 K (185 °C) the size of the β″ dissolution peak decreased with aging time, which is correlated with the amount of β″ already precipitated. Similar results were obtained for β′ precipitation at 523 K (250 °C) and 573 K (300 °C). An isothermal transformation diagram for β″ and β′ formation is presented, which is constructed from these experiments. The in situ method proved to be a viable method for studying the transformation sequence and kinetics in this type of alloy.     

Effects of retrogression heating rate on microstructures and mechanical properties of aluminum alloy 7050

The effects of the retrogression heating rate(340℃/min,57℃/min,4.3℃/min)on the microstructures and mechanical properties of aluminum alloy 7050 were investigated by means of hardness measurement,tensile properties testing,differential scanning calorimetry(DSC)and transmission electron microscopy(TEM).The results show that the retrogression heating rate significantly affects the microstructures and mechanical properties of the alloys treated by retrogression and re-aging(RRA)process, and it is found that the medium rate(57℃/min)leads to the highest mechanical properties.The strengthening phases in the matrix are mainly the fine dispersed η′precipitates and GP zones,and the grain boundary precipitates are coarse and discontinuous η phases.     

The effect of an addition of Sc and Zr on the precipitation behavior of AA6061 alloy

The effect of an addition of Sc and Zr on the precipitation behavior of AA6061 alloy was investigated. AA6061 alloy containing Sc and Zr showed different age hardening behavior compared to unadulterated AA6061 alloy. The hardness of the AA6061 alloy peaked when aged for 5 h at 190 °C due to the formation of β″ phases, whereas that of AA6061 alloy containing Sc and Zr peaked at 12 h of aging, and was greater than that of AA6061 alloy when aged for 24 h to 36 h. Thermally stable Al₃(Sc, Zr) phases with a L1₂ structure and low density of β″ phases were found in the Sc- and Zr-added AA6061 alloy aged for 5 h at 190 °C. Transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) revealed that the precipitation rate of the β″ phases was retarded by the addition of Sc and Zr to AA6061 alloy.     

Deformation and phase transformations during the cyclic oxidation of Ni−Al and Ni−Pt−Al

The reversible high-temperature γ′ to β phase transformation may be critical to explaining the unusual high-temperature oxidation behavior of (Ni,Pt)Al alloys and coatings. During high-temperature, high-frequency (1 h) cyclic oxidation in dry, flowing O₂, unprecedented macroscopic deformation was observed in two-phase (γ′+β) cast specimens of Hf-doped Ni−Al at 1,150°C and Hf-doped Ni−Pt−Al at 1,100° and 1,150°C, Outside of this two-phase field or when the cycle frequency was decreased to 100h, no deformation was observed. Using high-temperaturex-ray diffraction in an inert environment, the β-to-γ′ phase ratio was observed to increase above 1,000°C, causing a 2.5% volume change. The addition of platinum appeared to lower the transformation temperature consistent with the deformation observed in castalloys and rumpling of simple and platinum-modified aluminide coatings.     

Structure formation during thermomechanical processing of Ti-Nb-(Zr,Ta) alloys and the manifestation of the shape-memory effect

The formation of structure during thermomechanical processing by the regime of cold plastic deformation by rolling and postdeformation annealing (PDA) and its influence on the mechanical properties of Ti-Nb-(Zr, Ta) shape-memory alloys (SMAs) have been investigated. A moderate strain (e ≈ 0.3) leads to the formation of a developed dislocation substructure in the β phase. With going to severe plastic deformation (e ≈ 2), a nanocrystalline structure can locally be formed without the amorphization of the structure. There are also present α″-, α-, ω phases in the deformed alloys. When the PDA (1 h) is performed below 450°C, the structure of the β phase changes only slightly. Above 450°C, a polygonized substructure is formed in the β phase, which is nanosubgrained at an annealing temperature of 500°C and transforms completely into a submicron one at 600°C. In the case of severe plastic deformation to e ≈ 2, in this range of annealing temperatures, high-angle misorientations of blocks are also observed. The recrystallization of the β phase in the Ti-Nb-(Zr, Ta) SMAs develops at temperatures above 600°C. The presence of the ω phase is detected at temperatures of up to 550°C. The lattice parameters of the strain-induced α″ martensite formed in the Ti-Nb-Ta alloy are independent of the PDA temperature in the range from 600 to 900°C, where the polygonized substructure transforms into the recrystallized structure of the β phase. The range of PDA temperatures that are most favorable for the manifestation of the effect of superelasticity in the Ti-Nb-(Zr, Ta) alloys is in the vicinity of 600°C.     

Microstructure-properties relationship in two Al-Mg-Si alloys through a combination of extrusion and aging

Aluminum alloys containing magnesium and silicon as the major solutes are strengthened by precipitation of the metastable precursors (β″) of the equilibrium β (Mg₂Si) phase. In this study, dynamic aging of two Al-Mg-Si alloys—the 6061 (Al-1.34% Mg₂Si) and 6069 (Al-2.25% Mg₂Si) alloys—was conducted through equal channel angular extrusion (ECAE). Equal channel angular extrusion-assisted dynamic aging provides the potential for improving mechanical properties. The aging time scale is reduced from ∼1,000 min. for conventional static peak aging to ∼10 min. by using ECAE-assisted dynamic aging. Compared to the significant strengthening effect in static aging treatment, a notable further increase in ultimate tensile strength is achieved by dynamic aging: over 40 MPa for the 6061 alloy and 100 MPa for the 6069 alloy. Microstructures of both aged alloys were characterized using transmission electron microscopy; dislocation-assisted precipitation was observed to be the primary precipitate nucleation and growth mechanism during the dynamic aging process. It is concluded that ECAE-assisted dynamic aging is controllable and efficient in executing aging treatment that could result in superior mechanical properties of Al-Mg-Si alloys.     

Effect of Be on aging behavior of an Al-Si-Cu-Mg cast alloy

The effect of Be addition on the aging behavior of UNS 03370 (Al11Si3Cu0.3Mg) was investigated by micro-hardness measurement, differential scanning calorimetry (DSC) and transmission electron microscope (TEM) analysis. Age hardening analysis shows Be additions to an Al11Si3Cu0.3Mg alloy accelerates the age hardening rate and increases the peak hardness by 15 HV during aging at 160°C. DSC shows that Be additions lead to an endothermic peak corresponding to the dissolution of Gunier Preston zones (GP I) disappear with exothermic peaks corresponding to precipitation of GP II zones and the λ′ and/or ϑ′ phases shift to low temperature. DSC and TEM analyses show that GP II zones are more effective than λ′ and/or θ′ on hardening the alloy, and Be addition increases the homogeneous nucleation density of GP II zones. The possible Be atoms participating in the precipitation process during aging and the high Be-vacancy binding energy can explain the effect of Be on aging behavior of Al11Si3Cu0.3Mg alloy.     

Retrogression and re-aging treatment of Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr aluminum alloy

1 Introduction The 7000 series aluminum alloys have been widely used as aircraft structure material because of their high strength/density ratio. This series of alloys provide high strength in the T6 condition but are prone to stress corrosion cracking(SC…     

Precipitation Hardening of Cu-3Ti-1Cd Alloy

Precipitation strengthening of Cu-3Ti-1Cd alloy has been studied using hardness and tensile tests, electrical resistivity measurements, and transmission electron microscopy. The alloy exhibited a hardness of 117 Hv in solution-treated (ST) condition and attained a peak hardness of 288 Hv after aging at 450 °C for 72 h. Electrical conductivity increased from 7%IACS (International Annealed Copper Standard) in ST condition to 13%IACS on aging at 450 °C for 16 h. The alloy exhibited yield strength (YS) of 643 MPa and ultimate tensile strength (UTS) of 785 MPa in peak-aged (PA) condition. Strengthening in Cu-3Ti-1Cd alloy in PA condition is attributed to solid solution strengthening effect of cadmium (Cd) as well as fine scale precipitation of metastable and coherent β′-Cu₄Ti phase. On overaging at 450 or 500 °C, the alloy showed a decrease in hardness as a result of formation of equilibrium precipitate β-Cu₃Ti as continuous precipitation within the matrix and as discontinuous precipitation at the grain boundaries. While the tensile properties are better, the electrical conductivity of Cu-3Ti-1Cd alloy is less than that of binary Cu-2.7Ti alloy. The strengthening mechanism is the same in both binary and ternary alloys of Cu-Ti, i.e., precipitation of metastable and coherent β′-Cu₄Ti phase.     

Analysis of chill-cast NiAl intermetallic compound with copper additions

This study carried out a characterization of chill-cast NiAl alloys with copper additions, which were added to NiAl, such that the resulting alloy composition occurred in the β-field of the ternary NiAlCu phase diagram. The alloys were vacuum induction melted and casted in copper chill molds to produce ingots 0.002 m thick, 0.020 m wide, and 0.050 m long. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) performed in chill-cast ingots identified mainly the presence of the β-(Ni,Cu)Al phase. As-cast ingots showed essentially no ductility at room temperature except for the Ni₅₀Al₄₀Cu₁₀ alloy, which showed 1.79% of elongation at room temperature. Ingots with this alloy composition were then heat treated under a high-purity argon atmosphere at 550 °C (24 h) and cooled either in the furnace or in air, until room temperature was reached. β-(Ni,Cu)Al and γ′(Ni,Cu)₃Al were present in specimens cooled in the furnace and β-(Ni,Cu)Al, γ′(Ni,Cu)₃Al plus martensite-(Ni,Cu)Al were present in specimens cooled in air. Thermogravimetric analysis indicated that martensite transformation was the result of a solid-state reaction with M _s ∼ 470 and M _f ∼ 430 °C. Tensile tests performed on bulk heat-treated ingots showed room-temperature ductility between 3 and 6%, depending on the cooling media.     

Interdiffusion in multiphase,Al-Co-Cr-Ni-Ti diffusion couples

The interdiffusion in Co matrix/Al particle alloys and Co/Ni-based substrates was studied using electron probe microanalysis and was simulated with the software DICTRA. Alloys were prepared by mixing elemental powders and furnace melting under an inert atmosphere. The phases involved in the study were γ (Ni-based or Co-based), β-CoAl, and γ′. The alloys were single-phase (γ) as well as two-phase (γ + β and γ + γ′). Several equilibrium points in the Al-Co-Cr system were measured and compared with the calculated diagram at 1100 °C. The diffusion couples were prepared to produce combinations of selected alloys and were subjected to annealing at 1100 °C for times up to 72 h. The diffusion calculations made with DICTRA were performed using the TCNI1 thermodynamic database together with mobility data collected from different literature sources. A literature survey on diffusion data of this system was performed, and comparisons with available data were made. The validity of the selected mobility data was checked with the composition profiles measured on some single-phase γ/γ diffusion couples. In the γ + β/γ and γ/γ + γ′ diffusion couples, a regression of the dispersed phase (β or γ′) was observed due to the interdiffusion of Al. From combined experimental and theoretical results, the effects of temperature and coating thickness were determined as an input for a coating lifetime prediction model. Paper presented at Calphad XXXII, La Malbaie, Québec, Canada, May 25–30, 2003.     

Determination of the Fe-rich portion of the Fe-Ni-S phase diagram

The low-temperature, Fe-rich portion of the Fe-Ni-S phase diagram was determined from Fe-Ni-S alloys (2.5,5,10,20, and 30 wt.% Ni, 10 wt % S, balance Fe) heat treated at 100 °C intervals from 900 to 300 °C. The microstructure and microchemistry of the phases in the heat treated Fe-Ni-S alloys were studied using a high-resolution field-emission gun (FEG) scanning electron microscope (SEM), electron probe microanalyzer (EPMA), and analytical electron microscope (AEM). Tieline compositions were obtained by determining the average phase composition and by measuring compositional profiles across interphase interfaces with the EPMA and AEM. At 600 °C and below, at least one phase was <1 Μm in size requiring the use of the AEM for analysis. The measured α + FeS, γ+ FeS, and α + γ + FeS boundaries in the Fe-rich corner of the Fe-Ni-S isotherms are consistent with previous studies. However, two new phases were observed for the first time coexisting with γ and FeS phases: FeNiγ′′ (∼52 wt.% Ni) at 600 and 500 °C and Ni ₃ Fe, ordered Ll ₂,γ′ (∼64 wt.% Ni) at 400 °C. New ternary isotherms are given at 600,500, and 400 °C that include the newly determined γ+γ′′ + FeS and the γ + γ′ + FeS three-phase fields. The effects of S on the phase boundaries of the α + γ phase field and the application of the Fe-Ni-S phase diagram to explain the microstructure and microchemistry of the metallic phases of stony meteorites are also discussed.     

Grain Boundary Wetting by a Second Solid Phase in the Zr-Nb Alloys

Zr-Nb alloys play the important role in the energy production being the main material for the cladding of nuclear fuel in the nuclear power plants. The thermo-mechanical treatment of these alloys proceeds in the (αZr) + (βZr, Nb) two-phase area of the Zr-Nb phase diagram. Therefore, the morphology and the mutual arrangement of the (Zr) and (Nb) phases play an extremely important role. The microstructure of binary Zr-Nb alloys with 2.5, 4, and 8 wt.% Nb after long anneals (720 h) was studied between 660 and 810 °C in the two-phase (αZr) + (βZr, Nb) area of the Zr-Nb phase diagram. (βZr, Nb)/(βZr, Nb) grain boundaries (GBs) completely or incompletely wetted by the αZr phase were observed. The portion of the completely wetted (βZr, Nb)/(βZr, Nb) GBs increases from 10% (at 660 °C) to 60% close to the upper border of the (αZr) + (βZr, Nb) two-phase area of the Zr-Nb phase diagram (850 °C). The temperature of the beginning of the GB wetting phase transition of (βZr, Nb)/(βZr, Nb) GBs by the αZr phase is T _ws = 630 ± 10 °C. The αZr/αZr GBs completely wetted by a layer of (βZr, Nb) phase were not observed in the studied samples.     

The Effects of Retrogression and Reaging on Aluminum Alloy 2099 (C458)

The objective of this study was to investigate the feasibility of performing retrogression and reaging (RRA) heat treatments on 2099 aluminum-lithium alloy. The retrogression temperatures were 200-250 °C and retrogression times were 5-60 min. Half of the samples were exposed to a salt fog environment. Interestingly, the samples exposed to salt spray had consistently higher mechanical tensile properties than those which were not exposed.     

Redistribution and re-precipitation of solute atom during retrogression and reaging of A1-Zn-Mg-Cu alloys

The redistribution and re-precipitation of solute atom during retrogression and reaging of three different A1-Zn-Mg-Cu aluminum alloys were investigated. The results of hardness and tensile strength test indicate that after pre-aging at 100 ℃ or 120 ℃ and retrogressing at 200 ℃ for various time and re-aging treatment, the hardness and strength of the alloys are all larger than those under pre-aging condition, some of them even exceed the value under peak aging（T6） condition. TEM observation shows that the PFZ formed during retrogressing in short time becomes narrow and even disappears after re-aging treatment. However, the PFZ formed during retrogressing for a long time does not narrow after re-aging treatment. It is suggested that the redistribution and re-precipitation of solute atom during re-aging treatment result in the narrowing and even disappearance of the PFZ formed during retrogression, which reinforces the grain-boundaries and presents the value of tensile strength exceeding peak-aging strength in the RRA condition, while the precipitates in the matrix of the alloys still keep or even exhibit a more dispersed distribution, and a greater effect of precipitation strengthening is obtained.     

Structural characterization of Laves-phase MgZn<Subscript>2</Subscript> precipitated in Mg-Zn-Y alloy

Structural characterization of Laves-phase MgZn₂ precipitated in Mg-6 wt.%Zn-1 wt.%Y alloy was performed using a high-resolution transmission electron microscope (HRTEM). Plate- and rod-shaped precipitates with the Laves structure were observed in an aged sample at 200 °C. An individual rod of the β₂′ phase precipitated in the matrix contained several domains with various orientations. Numerous antiphase boundaries (APBs) and stacking faults (SFs) were observed in the β₂′ precipitates. The stacking sequence of the 14C structure partially changed to that of the C15 structure due to the presence of a stacking fault. The orientation relationship between the C14 structure and the C15 structure in the β₂′ precipitates was [11–20]_C14‖[1–10]_C15 and (0001)_C14‖(111)_C15. Modification of the stacking sequence based on the synchroshear mechanism was discussed.     

Analyzing As-Cast Age Hardening of 356 Cast Alloy

The as-cast age hardening behavior of 356 cast alloy has been investigated by micro hardness measurement, differential scanning calorimetry (DSC), transmission electron microscope (TEM), and electron probe micro analyzer. Age hardening results show that micro hardness value after as-cast aging treatment is almost the same as by T6 treatment, and the solidification rate has little effect on the as-cast age hardening response of 356 cast alloy. DSC and TEM analysis results show that the as-cast age hardening response of 356 cast alloy is attributed to the precipitation of β′ and β″ phases, the high Si concentration in α(Al) contributes about 10 HV to the micro hardness value for samples in as-cast and as-cast aging conditions.     

Comparison of molybdenizing and NiCrAlY coating on Ti and Ti-6Al-4V

Two surface treatments, molybdenizing and depositing NiCrAlY coating, were applied to improve the microhardness and the oxidation resistance of titanium and Ti-6Al-4V. Coupons were analyzed using optical microscopy (OM), scanning electron microscopy (SEM) with X-ray energy dispersive spectrometer (EDS), and X-ray diffraction (XRD). Vickers hardness and isothermal oxidation tests were carried out to evaluate the effects of these two surface treatments on the microhardness and oxidation resistance of the substrates. The post vacuum heat treatment of the NiCrAlY coating and the molybdenizing parameters were also discussed. It is found that molybdenizing can obviously increase the surface hardness of titanium due to the formation of β, α″, and α′ phases in the diffusion layer. As γ′ phase is formed after vacuum heat treatment, the NiCrAlY coating is effective in improving the surface hardness of Ti-6Al-4V. The NiCrAlY coating can obviously decrease the oxidation rate of Ti-6Al-4V at 700–900°C, which can be attributed to the formation of Al₂O₃ and Cr₂O₃ mixed scale during the oxidation.     

Microstructure investigation of a new type super high strength aluminum alloy at different heat-treated conditions

As a structural material with low density and high strength, super-high strength aluminum alloys have a future for wide application. However, its poor stress corrosion resistance (SCC) restricts further development. In present, retrogression and re-ageing (RRA) treatment, which can improve both strength and SCCR of 7XXX series alloy, is a best method to solve this problem. The effect of RRA treatment on the microstructure evolution of a new type low frequency electric-magnetic casting Al-9.0Zn-2.45Mg-2.2Cu-0.15Zr alloy was investigated using DSC and TEM technologies. The results show that the typical microstructure of the alloy at T6 condition is characterized by both fine η‘ and GP zone homogeneously distributed in the matrix and continuous η‘ particles occurred on the grain-boundary. After RRA treatment, the matrix precipitations are mainly fine and dispersed η‘ and η‘ phases, being coarser and more stable than that from T6 temper. While, the grain-boundary microstructure is very close to that resulting from T73 temper. High retrogression texture and long retrogression time leads to a more stable microstructare after re-ageing.     

The effect of pre-ageing temperature and retrogression heating rate on the strength and corrosion behaviour of AA7150

The effects of processing condition variations in the retrogression and re-ageing (RRA) treatment of AA7150 have been investigated. The results reveal that the corrosion resistance and strength of RRA-treated samples are sensitive to the pre-ageing temperature and retrogression heating rate. With an intermediate pre-ageing temperature of 60 °C and a slow retrogression heating rate of 5 °C/min, RRA-treated samples can be tailored towards a good combination of both corrosion resistance and strength, owing to a smaller potential difference between grain-boundary precipitates and the matrix, and a larger volume fraction of strengthening precipitates in the matrix.