Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys

Differential scanning calorimetry (DSC) has been combined with transmission electron microscopy (TEM) to investigate the low-temperature decomposition processes taking place in an Al-5 wt pct Zn-1 wt pct Mg alloy. It was confirmed that two types of GP zones, i.e., GP(I) (solute-rich clusters) and GP(II) (vacancy-rich clusters), formed independently during decomposition of the supersaturated solid solution. The GP(I) zones form at a relatively low aging temperature and dissolve when the aging temperature is increased. The GP(II) zones are stable over a wider range of temperatures. To investigate the nature of the zones in the Al-Zn-Mg alloy, differential scanning calorimetry and transmission electron microscopy have also been carried out on binary Al-Zn alloys containing 5 wt pct and 10 wt pct Zn. In these Al-Zn alloys, GP zones formed rapidly during quenching, and they gave rise to characteristic electron diffraction patterns identical to those from GP(II) in the Al-Zn-Mg alloy system, implying that GP(II) zones in Al-Zn-Mg alloys are very similar to the zones formed in binary Al-Zn alloys. Thus, it is likely that GP(II) zones in Al-Zn-Mg alloys are zinc-rich clusters. In the Al-5 wt pct Zn-1 wt pct Mg alloy, both GP(I) and GP(II) were found to transform to η′ and/or η particles during heating in the differential scanning calorimeter. The η′ was also observed to form after prolonged isothermal aging of the Al-Zn-Mg alloy at 75 °C or after short aging times at 125 °C.     

Nanocrystalline structure formation in Al-Mg-Sc alloys during severe plastic deformation

The possibility of formation of a nanocrystalline structure (with a grain size smaller than 100 nm) in four Al-Mg-Sc alloys with 3.1–5.9% Mg during severe plastic deformation by torsion at a hydrostatic pressure of 6 GPa (high-pressure torsion (HPT)) has been studied. Room-temperature HPT of the alloys is shown to produce a nanocrystalline structure if the magnesium content is more than 4% (in the range 0.16–0.31% Sc). As the magnesium content increases, the grain size decreases and is minimal (40–50 nm) in a 01570 alloy with 5.9% Mg and 0.3% Sc. The structure of the HPT-processed 01570 alloy remains nanocrystalline upon heating to 200°C or at a deformation temperature as high as 200°C. Postdeformation heating is found to cause aging processes. The hardening of all the Al-Mg-Sc alloys is maximal after HPT at 20°C followed by aging at 300°C.     

Methods for Designing Concurrently Strengthened Severely Deformed Age-Hardenable Aluminum Alloys by Ultrafine-Grained and Precipitation Hardenings

The age-hardenings behavior and precipitate microstructures with high dislocation density and/or ultrafine grains have been studied for 6022Al-Mg-Si and 2091Al-Li-Cu alloys. The high-pressure torsion (HPT) specimen of the former alloy exhibited either suppressed age hardenings or even age softening, unlike in the cases of the undeformed and cold-rolled specimens, at room temperature (RT) to 443 K (170 °C). On the other hand, the HPT specimen of the latter alloy successfully increased the hardness up to >HV290 at 373 K (100 °C), suggesting that concurrent strengthening by ultrafine-grained and precipitation hardenings can be activated if both alloy system and aging temperature are optimally selected. The corresponding transmission electron microscopy (TEM) microstructures attributed such a high level of hardness to the transgranular precipitation of the nanometer-scale particles within ultrafine grains. From the results of in situ small-angle X-ray scattering (SAXS) measurements, methods to maximize the effect of the combined processing of severe plastic deformation (SPD) and the age-hardenings technique are proposed based on the underlying phase transformation mechanisms.     

Effect of Annealing on Thermal Stability, Precipitate Evolution, and Mechanical Properties of Cryorolled Al 7075 Alloy

The effect of annealing on microstructural stability, precipitate evolution, and mechanical properties of cryorolled (CR) Al 7075 alloy was investigated in the present work employing hardness measurements, tensile test, X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron backscattered diffraction (EBSD), and transmission electron microscopy (TEM). The solution-treated bulk Al 7075 alloy was subjected to cryorolling to produce fine grain structures and, subsequently, annealing treatment to investigate its thermal stability. The recrystallization of CR Al 7075 alloys started at an annealing temperature of 423 K (150 °C) and completed at an annealing temperature of 523 K (250 °C). The CR Al 7075 alloys with ultrafine-grained microstructure are thermally stable up to 623 K (350 °C). Within the range of 523 K to 623 K (250 °C to 350 °C), the size of small η phase particles and AlZr₃ dispersoids lies within 300 nm. These small precipitate particles pin the grain boundaries due to the Zener pinning effect, which suppresses grain growth. The hardness and tensile strength of the CR Al 7075 alloys was reduced during the annealing treatment from 423 K to 523 K (150 °C to 250 °C) and subsequently it remains constant.     

Mechanism of Effects of Rare Earths on Microstructure and Properties at Elevated Temperatures of AZ91 Magnesium Alloy

By using real-space recursion method,the energetics of the undoped and Al and/or RE atoms doped 7（1450）〈0001〉 symmetric tilt grain boundaries（GBs）in AZ91 alloys were investigated.Similar calculations were performed on undoped and doped bulk α Mg for comparison.The results showed that Al atoms segregated at GBs in AZ91 alloys.When RE atoms were added,they also segregated at GBs,and their segregation is stronger than Al atoms＇.Therefore,RE atoms retard the segregation of Al atoms.Calculations of interaction energy indicated that Al atoms repelled each other,and could form ordered phase with host Mg atoms.On the contrary to the case of Al,RE atoms attracted each other,they could not form ordered phase with Mg,but could form clusters.Between RE and Al,there existed attractive interaction,and this attractive interaction was the origin of Al11RE3 precipitation.Precipitation of Al11RE3 particles with high melting point and high thermal stability along GB improves high temperature properties of AZ91 alloys.     

The Effect of Solute Atoms on Aluminum Grain Boundary Sliding at Elevated Temperature

Grain boundary sliding (GBS) is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of solute atoms in near grain boundaries (GBs) on the sliding of Al bicrystals at 750 K (477 °C). The threshold stress for GBS is computed for a variety of GBs with different structures and energies. Without solute atoms, low-energy GBs tend to exhibit significantly less sliding than high-energy GBs. Simulation results show that elements which tend to phase segregate from Al, such as Si, can enhance GBS in high-energy GBs by weakening Al bonds and by increasing atomic mobility. In comparison, intermetallic forming elements, such as Mg, will form immobile Mg-Al clusters, decrease diffusivity, and inhibit GBS.     

The behavior of silicon in the solidification of Zn-55Al-1.6Si coating on steel

Silicon is an essential element in the Zn-55Al-1.6Si coating. It is added to promote the formation of an adherent coating and prevent the excessive growth of an intermetallic alloy layer at the steel/coating interface. The addition of silicon also results in the formation of a silicon phase distributed in the interdendritic region of the overlay, having a flowery pattern on the surface, and appearing needlelike when observed inside the overlay. The behavior of silicon during the solidification process of the Zn-55Al-1.6Si coating is examined in the current study. It is found that the coating solidification proceeds in three stages. At stage I, primary α-Al dendrites develop at about 566 °C to 520 °C, forming the framework of the coating structure. This is followed by stage II at about 520 °C to 381 °C, where the binary Al-Si eutectic reaction takes place, with the majority of the silicon phase forming at about 520 °C to 480 °C. At stage III the remaining molten phase undergoes a ternary Al-Zn-Si eutectic reaction forming the interdendritic zinc-rich network. The ternary Al-Zn-Si eutectic reaction is essentially equivalent to the binary Al-Zn eutectic reaction because of the very low level of silicon at the Al-Zn-Si eutectic point.     

Defect structures and interactions in Al-Zn eutectoid alloys

Defect structures in superplastic and nonsuperplastic Al-Zn eutectoid alloys were studied by transmission electron microscopy. Slowly cooled alloys develop a fine lamellar microstructure and are not superplastic. The equiaxed quenched and quench-aged alloys, however, are super-plastic under the proper conditions of temperature and strain rate. Quench-aging produces sub-boundaries in the aluminum-rich α phase and dislocation loops in the zinc-rich β phase. Subsequent room temperature deformation creates a typical cold-worked dislocation structure in the α phase. At higher deformation temperatures, the cold-worked structure is increasingly replaced by a recovered structure. At the same time, the dislocation loop density in the β phase decreases to lower values. During superplastic deformation at 250βC, the dislocation loops in the β phase are annihilated by interactions with glide dislocations. In the α phase, a continuous, or steady-state, dynamic recovery process appears to operate. A completely recovered structure is maintained with dislocation-free subgrains.     

The effect of molybdenum on γ′ coarsening and on elevated-temperature hardness in some experimental nickel-base superalloys

The coarsening of γ′ and the elevated-temperature hardness have been studied as a function of molybdenum content, time, and temperature in experimental wrought nickel-base superalloys. The alloys were selected from a systematic series containing 3, 4 1/2, and 6 wt pct Al and 1 wt pct Al plus 3 1/2 wt pct Ti. Each of the aluminum (plus titanium) series consisted of four alloys containing 0, 2, 5, and 8 wt pct Mo. The alloys were solution-treated plus aged up to 112 h at 1700°F (925°C) and up to 1000 h at 1400°F (760°C). Molybdenum retards the coarsening of γ′ on aging; this retarding effect is most pronounced in alloys containing 6 wt pct Al. The coarsening of γ′ particles follows Ostwald ripening kinetics. Hardness testingin vacuo at temperatures up to 1750°F (955°C) shows that molybdenum also increases the elevated-temperature hardness significantly. The relation of elevated-temperature hardness to the volume fraction of γ′ is considered, and the influence of aluminum and titanium contents is discussed.     

The effect of fatigue deformation on microstructural evolution in a superplastic aluminum-zinc eutectoid alloy

The microstructure of a superplastic aluminum-zinc eutectoid alloy that had been fatigue tested at 100 °C and 200 °C was examined. At 100 °C, in the aluminum-rich phase, precipitate-free zones (PFZs) formed beside (Al)/(Zn) interphase boundaries because of interphase boundary migration. Interphase boundary migration was due to phase growth, which proceeded more rapidly during fatigue deformation than during annealing. At 100 °C and 200 °C, PFZs beside (Al)/(Al) grain boundaries were asymmetrical owing to grain boundary migration. The precipitation of the equilibrium zinc-rich phase in the aluminum-rich phase proceeded more rapidly during fatigue deformation than during annealing. J. W. BOWDEN, formerly Graduate Student, Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON.     

Optimization of composition and heat treatment of age-hardened Pt-Al-Cr-Ni alloys

The objective of this work was to produce an alloy showing a microstructure similar to Ni-base superalloys, but with Pt as base metal. The Pt-base alloys with various contents of Al, Cr, and Ni were arc melted. Solution heat treatments at 1450 °C followed by water quenching lead to single-phase alloys. Ageing at 1000 °C resulted in the precipitation of Ll₂ ordered particles. An alloy with 11 at. pct Al, 3 at. pct Cr, 6 at. pct Ni, and Pt balance shows cuboidal precipitates with edge lengths of 200 to 500 nm along with a volume fraction of 23 pct and a lattice misfit of −0.1 pct. Aging at 1100 °C leads to coarsening of precipitates. Volume fraction and morphology of the precipitates were investigated by scanning electron microscopy and optical microscopy. X-ray diffraction as well as transmission electron microscopy (TEM) were applied to verify the crystal structure.     

Formation regularities of grain-boundary interlayers of the α-Ti phase in binary titanium alloys

The microstructure of polycrystalline alloys of titanium with chromium (2, 4, and 5.5 wt %), cobalt (2 and 4 wt %), and copper (2 and 3 wt %) is investigated. Series of prolonged isothermal annealing of these materials are performed in a temperature range from 600 to 850°C (in vacuum). Annealing temperatures fall in two-phase regions α(Ti,Me) + β(Ti,Me) of phase diagrams Ti–Cr, Ti–Co, and Ti–Cu. Temperature dependences of the fraction of grain boundaries β(Ti,Me)/β(Ti,Me) completely “wetted” by interlayers of the second solid phase α(Ti,Me) and average contact angle are measured. The results of microstructural investigations showed that the type and concentration of the second component in the alloy strongly affect the formation of equilibrium grain-boundary interlayers. A nonmonotonic temperature dependence of the fraction of grain boundaries completely wetted by interlayers of the second solid phase in the absence of ferromagnet–paramagnet phase transformations in the volume is revealed for the first time.     

Effect of Dislocation Mechanisms during Extrusion of Nanostructured Aluminum Powder Alloy

Nanostructured Al-3.0Fe-0.42Cu-0.37Mn powder alloy was deformed by extrusion over a temperature range of 375 °C to 425 °C, a ram speed range of 1 to 30 mm/s, and an extrusion rate of 10:1. Flow stresses and strain rates were calculated from the experimental ram pressures and speeds. The stress–strain-rate–temperature relationship in the extrusion of the nanostructured alloy was found to be similar to that in hot-worked conventional materials. The extrusion, torsion, compression, and creep data of nanostructured and conventional materials, extending over ten orders of magnitude of strain rate and over two orders of magnitude of stress, were correlated by a hyperbolic-sine constitutive equation, because the power and exponential laws lose linearity at high and low stresses, respectively. The hyperbolic-sine equation is widely used to correlate the hot-working behavior of conventional materials. It was concluded that the hot working of nanostructured powders is a thermally activated process in which the rate-controlling mechanism is the climb of edge dislocations. Microstructural changes in the consolidated alloys as a function of the extrusion conditions were investigated. An analysis was made of the dislocation behavior in very small grains of nanostructured metal by transmission electron microscopy (TEM) and we identified a dislocation structure and the different ways it appears in 40- to 100-nm Al-alloy grains. We also discuss the thermally activated propagation of dislocations and their interactions with shear bands/grain boundaries (SBs/GBs), and dislocation loops. Microstructural features including low-angle GBs, high-angle GBs, and equilibrium GBs and subgrain boundaries were observed. Dislocation structures under a deformation condition were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as dislocation tangle zones (DTZs) and dense-dislocation walls (DDWs), and the recovery process is discussed herein.     

Influence of Reaction Temperature and Reaction Time for the Manufacturing of Al–Ti–B (Ti:B?=?5:1, 1:3) Master Alloys and Their Grain Refining Efficiency on Al–7Si Alloys

In the present work, ternary Al?CTi?CB master alloys have been prepared in an induction furnace by the reaction between preheated halide salts (K₂TiF₆ and KBF₄) and liquid molten Al. A number of process parameters such as reaction temperature (800, 900, 1,000?°C), reaction time (45, 60, 75?min.) and compositions (Ti/B ratio: 5/1, 1/3) have been studied. The indigenously prepared master alloys were characterised by chemical analysis, particles size analysis, XRD and SEM/EDX microanalysis. Results of particle size analysis suggest that the sizes of the intermetallic particles [Al₃Ti and TiB₂ in Al?C5Ti?C1B and (Al, Ti)B₂ in Al?C1Ti?C3B] present in various Al?CTi?CB master alloys increases with increase in reaction temperature (800?C1,000?°C) and reaction time (45?C75?min.). The population of the particles decreases with increase in reaction time and temperature. Further, SEM/EDX studies revealed that different morphologies of the intermetallic particles were observed at different reaction temperatures and reaction times. Further, the performances of the above-prepared master alloys were assessed for their grain refining efficiency on Al?C7Si alloy by macroscopy, DAS analysis. Grain refinement studies suggest that, B-rich Al?C1Ti?C3B master alloy shows better grain refinement performance on Al?C7Si alloy when compared to Ti-rich Al?C5Ti?C1B master alloy.     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

Optimization of composition and heat treatment of age-hardened Pt−Al−Cr−Ni alloys

The objective of this work was to produce an alloy showing a microstructure similar to Ni-base superalloys, but with Pt as base metal. The Pt-base alloys with various contents of Al, Cr, and Ni were are melted. Solution heat treatments at 1450 °C followed by water quenching lead to single-phase alloys. Ageing at 1000 °C resulted in the precipitation of L1₂ ordered particles. An alloy with 11 at. pct Al, 3 at. pct Cr, 6 at. pct Ni, and Pt balance shows cuboidal precipitates with edge lengths of 200 to 500 nm along with a volume fraction of 23 pct and a lattice misfit of −0.1 pct. Aging at 1100 °C leads to coarsening of precipitates. Volume fraction and morphology of the precipitates were investigated by scanning electron microscopy and optical microscopy. X-ray diffraction as well as transmission electron microscopy (TEM) were applied to verity the crystal structure. M. Huller, formerly with Metallic Materials, University Bayreuth, D-95440 Bayreuth, Germany     

Ultrasonic Spot Welding of Aluminum to High-Strength Low-Alloy Steel: Microstructure,Tensile and Fatigue Properties

The structural applications of lightweight aluminum alloys inevitably involve dissimilar welding with steels and the related durability issues. This study was aimed at evaluating the microstructural change, lap shear tensile load, and fatigue resistance of dissimilar ultrasonic spot-welded joints of aluminum-to-galvanized high-strength low-alloy (HSLA) steel. Two non-uniform layers were identified in between Al and HSLA steel via SEM/EDS and XRD. One was an Al-Zn eutectic layer and the other was a thin (<2 μm) layer of intermetallic compound (IMC) of Al and Fe in the nugget zone. The lap shear tensile testing gave a maximum load of 3.7 kN and the sample failed initially in between the Al-Zn eutectic film and Al-Fe IMC, and afterward from the region containing Al on both matching fracture surfaces. The fatigue test results showed a fatigue limit of about 0.5 kN (at 1 × 10⁷ cycles). The maximum cyclic stress at which transition of the fatigue fracture from transverse through-thickness crack growth mode to the interfacial failure mode occurs increases with increasing energy input.     

Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 vol pct aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 °C to 1000 °C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al particle interfaces and concentrically around the original Al particles. Upon heating to the 500 °C to 600 °C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (600 °C to 1000 °C), diffusional reaction between NiAl and nickel produces (γ′)Ni₃Al. The final equilibrium microstructure consists of blocks of (γ′)Ni₃Al in a γ(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminides.