A TEM study of the combined effect of severe plastic deformation and (Zr), (Sc+Zr)-containing dispersoids on an Al–Mg–Si alloy

The microstructural evolution with strain was investigated in a Zr-modified 6082 Al–Mg–Si alloy and in the same alloy added with 0.117 wt.% Sc, subjected to a multi-pass equal-channel angular pressing, up to a true strain of ∼12. The role of fine Al₃(Sc_1-x ,Zr _x ) dispersoids, pertaining Al–Mg–Si–(Sc–Zr) alloy, and Al₃Zr dispersoids, pertaining to Al–Mg–Si–(Zr) alloy, was investigated by transmission electron microscopy techniques and discussed. Compared to the commercial parent alloy, Al–Mg–Si, block wall formation and propagation were favored by the presence of Sc–Zr containing dispersoids, while cell boundary evolution was less affected, Al₃Zr dispersoids affected the microstructure in a similar way, but in a lesser extent. Mean misorientation across block walls increased with strain much more in the Sc–Zr containing alloy, reaching a plateau, starting from a true strain of ∼8. Misorientation across cell boundaries continuously increased to ∼8° and ∼5° for the Sc–Zr and Zr containing alloy, respectively. The effect of the presence of Al₃(Sc_1-x ,Zr _x ) and Al₃Zr in the matrix, on the Mg₂Si particle shearing and Si shrinking phenomena with strain, was also addressed and documented.     

Characterisation of different types of dispersoids present in homogenised Al–4·5Zn–1Mg alloy containing Zr,Cr and Mn

Abstract

In the present paper, various types of dispersoids possibly formed during homogenisation of AA7020 alloy containing Cr, Zr and Mn are described. It shows that, in addition to the Zr containing dispersoids, three other types of dispersoids may be present in the homogenised microstructure of AA7020 aluminium alloy. These dispersoids are Cr and Mn containing ones and the one with a mixture of different elements. The Zr and Cr containing dispersoids are formed in the grain interior at all of the homogenisation conditions. However, the Mn containing ones form only in the grain boundary regions in the vicinity of Al₁₇(Fe_3·2,Mn_0·8)Si₂ particles at temperatures ?510°C and holding times longer than 4 h.     

Precipitation of Al3Zr Dispersoids under Various Homogenization and Its Effect on Recrystallization in 2198 Al–Cu–Li Alloy

Herein, the precipitation of Al₃Zr during one- and two-step homogenization processes is studied by the transmission electron microscopy method, and its effect on recrystallization is investigated by the electron backscattering diffraction technique after the same hot compression and solution treatment, respectively. The results show that when the temperature reaches 430 °C, just some small Al₃Zr dispersoids are visible in samples that undergo one-step treatment. As the first-step homogenization temperature increases from 365 to 430 °C, Al₃Zr radius increases from 9.4 to 19.1 nm while the number density decreases rapidly from 1201 to 183 μm⁻³. In addition, the average radius of Al₃Zr increases to 22.1 nm while the number density decreases to 76 μm⁻³ when the sample is homogenized at 500 °C/24 h. After a double-step homogenization with decreased first-step temperature, lower recrystallization fraction is obtained. Therefore, the optimal homogenization treatment is determined to be 365 °C/16 h + 500 °C/24 h, after which the most desirable Al₃Zr distribution and the smallest recrystallization fraction are obtained during thermomechanical process. This result can provide a more appropriate parameter for homogenization treatment in order to achieve a more dispersed distribution, which will result in stronger pinning force during the thermomechanical process.     

Microstructure and mechanical properties of as-extruded AZ80–xSn magnesium alloys

The effect of 0–2?wt-% Sn addition on AZ80 magnesium alloys after 350°C extrusion has been studied by analysing microstructure and mechanical properties. The results indicated that dynamically recrystallised grains were fine and homogeneous with less than 1?wt-% Sn addition. In AZ80–0.5Sn alloy, a large number of Mg₁₇Al₁₂ precipitated phases formed in grains and at grain boundaries during extrusion process. With more than 1?wt-% Sn addition, the size of dynamically recrystallised grains increased and the number of Mg₁₇Al₁₂ phases decreased. The strength of as-extruded AZ80–0.5Sn alloy enhanced largely as compared with that of the as-extruded AZ80 alloy. AZ80–0.5Sn alloy had the outstanding tensile and compressive properties.     

Precipitation behaviour of 7000 alloys during retrogression and reaging treatment

Abstract

Retrogression and reaging produces coarsening of grain boundary precipitates and thereby improves resistance to stress corrosion cracking. At the same time it causes pronounced heterogeneous precipitation on dispersoids of E (Al₁₈Cr₂Mg₃) phase inside the grain of 7075 alloy. Such heterogeneous precipitation does not occur on the coherent dispersoids of Al₃Zr phase in 7050 alloy and its absence leads to higher strength compared with 7075 alloy. Supplementary examination of laboratory alloys 7075-Zr and 7075-Cr differing only in transition metal content supports the above result. This effect is probably the reason why retrogression and reaging (T77 heat treatment) is recommended for alloys containing zirconium but not for those containing chromium.

MST/1898     

Atomic bonding and mechanical properties of Al–Li–Zr alloy

The atomic bonding of Al–Li alloy with minor Zr is calculated according to the “Empirical Electronic Theory in Solids”. The result shows that the stronger interaction between Al and Zr atoms, which leads to form the Al–Zr segregation regions, promotes the precipitation of Al₃Zr particles and produces a remarkable refinement of Al₃Li grains in the alloy. Because there are the strongest covalent Al–Zr bonds in Al₃Zr and Al₃(Zr, Li) particles, these covalent bonds can cause a great resistance for dislocation movement, and is favorable to strengthen the alloy. On the other hand, with precipitating the Al₃(Zr, Li) particles, it causes the coherent interphase boundary energy of Al/Al₃Li to decrease, and atomic bonding is well matched in between the interface of two phases.     

Growth of recrystallising grains in boron doped Ni76 Al24

Abstract

Cold rolled boron doped Ni₇₆ Al₂₄ was partially recrystallised at 850, 875, and 900°C. The recrystallised volume fraction and the interfacial area between recrystallised and unrecrystallised parts were determined as functions of time. The analysis of results showed the growth rates of recrystallised grains to be dependent on temperature, but independent of time. The activation energy for the transformation was 168 kJ mol^-1 and that for growth was 87 kJ mol^-1. The activation energy for growth was much less than that for diffusion and was dependent on the degree of cold working. This has been attributed to the strong interaction of boron with the structural defects produced by cold working.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Microstructure of SiCw reinforced Al–12Ti composites prepared by powder metallurgical technique

Abstract

Silicon carbide whisker reinforced Al–12Ti composites were fabricated by a powder metallurgical technique, and the microstructures were characterised by the means of X-ray diffraction, SEM, TEM, and energy dispersion X-ray analysis. It has been shown that secondary phase particles, Al₃ Ti, form in situ during hot pressing after premechanically ball milling, and a small amount of α-Ti is left because the in situ reaction between α-Ti and Al is not complete. High density dislocations including dislocation lines and dislocation loops exist in the coarse Al₃ Ti grains, while, hardly any dislocations can be found using TEM in the very fine (～150 nm) Al₃ Ti grains. In addition, nanometer equiaxed γ-Al₂O₃ and stick shaped Al₄ C₃ dispersoids form in the Al matrix as a result of the addition of a processing control agent. There is no fixed orientation relationships between γ-Al₂O₃ , Al₄ C₃ , and the Al matrix. Dislocations in the Al matrix are too sparse to be found even in the zones around SiC whiskers. Silicon carbide whiskers uniformly scatter in the Al matrix, and no reaction products are formed. A few microzones with nanosized (～20 nm) Al grains exist in the Al matrix, and an amorphous phase is usually found in the zones adjacent to SiC whiskers. The formation mechanism of the amorphous phase is discussed.     

Thermal stability and mechanical properties of mechanically alloyed Al-10Ti alloy

The mechanical properties of Al-10Ti alloy prepared by mechanical alloying and subsequent hot hydrostatic extrusion were evaluated at room and elevated temperatures. Transmission electron microscopy was used to characterize the microstructural changes of this alloy on heat treatment at 500 °C for various times. The results show that the mechanically alloyed Al-10Ti has high strength and high thermal stability at elevated temperature. The strength and stability of this alloy are attributed to its fine grain size and to the high volume fraction of small Al₃Ti intermetallic compounds dispersed in the aluminium matrix. After 50 h annealing at 500 °C, no serious coarsening of either the Al₃Ti dispersoids or the grains was observed.     

Development of Al3Ti and Al3Zr intermetallic particulate reinforced aluminum alloy AA6061 in situ composites using friction stir processing

Aluminum rich intermetallic particles are potential reinforcements for discontinuously reinforced aluminum matrix composites (DRAMCs). The objective of the present work is to produce AA6061/Al₃Ti and AA6061/Al₃Zr composites using in situ casting technique and applying friction stir processing (FSP) to enhance the distribution and morphology of Al₃Ti and Al₃Zr particles. AA6061/Al₃Ti and AA6061/Al₃Zr DRAMCs were produced by the in situ reaction of inorganic salts K₂TiF₆ and K₂ZrF₆ with molten aluminum. The microstructure was observed using optical and scanning electron microscopy. AA6061/Al₃Ti DRAMC exhibited clusters of Al₃Ti particles while the segregation of needle shape Al₃Zr particles was observed in AA6061/Al₃Zr DRAMC. The prepared composites were subjected to FSP. Significant changes in the distribution and morphology of Al₃Ti and Al₃Zr particles were observed after FSP. The changes in microhardness and sliding wear behavior of AA6061/Al₃Ti and AA6061/Al₃Zr DRAMCs before and after FSP is detailed in this paper.     

Thermomechanical ageing (TMA) of 2014 aluminium alloy for aerospace applications

Various thermomechanical ageing (TMA) treatments for 2014 Al-alloy have been developed which include partial peak ageing, warm rolling and further ageing to peak hardness at 160°C. Electron microscopic studies reveal that the TMA treatments affect substantially the ageing characteristics. The TMA-Ib treatment yields finest?′ needles having longitudinal dimensions of ～400 Å. TMA treatment leads to precipitate-dislocation network of different magnitudes. Among the TMA treatments, the TMA IIb treatment results in thickest precipitate-dislocation tangles. In addition to?′, two types of dispersoids Al₄CuMg₅Si₄ and Al₁₂ (Fe, Mn)₃Si have been observed. The density or concentration of these dispersoids is drastically reduced due to TMA treatments. Thus an optimum TMA treatment i.e. TMA-IIb has been developed which results in a significant improvement in the mechanical properties of 2014 Al-alloy.     

Predicting recrystallised volume fraction in aluminium alloy 7050 hot rolled plate

Abstract

An investigation into the partial recrystallisation that occurs during solution treatment of hot rolled thick aluminum alloy 7050 plate has been performed for three alloy variants with different zirconium concentrations. Electron backscattered diffraction has been used to study the grain and subgrain structure in the as rolled condition and after solution treatment. Localised recrystallisation in the grain boundary regions during solution treatment has been largely attributed to the stimulating effect of large intermetallic particles on the boundaries, combined with the low number density of pinning dispersoids close to the grain edges. A model that predicts the dispersoid distribution across a grain has been applied to predict the fraction of recrystallisation after solution treatment. It has been demonstrated that the recrystallised fractions predicted by the model show good agreement with those measured experimentally. Example calculations are presented showing the predicted effect of the zirconium level, homogenisation conditions, and subgrain size on the recrystallised fraction.     

Microstructure and bond strength of Ni–Cr steel/Al2O3 joints brazed with Ag–Cu–Zr alloys containing Sn or Al

Abstract

Sintered Al₂O₃ was joined to Ni–Cr steel by the active metal brazing route with Ag–Cu–Zr brazing alloys containing Sn or Al. A single ZrO₂ layer with a monoclinic structure was formed at the Al₂O₃ /brazement interface by the migration of Zr in the molten brazing alloy to the Al₂O₃ surface, followed by a redox reaction between the Al₂O₃ and Zr. The remainder of the brazement formed a Cu–Ag eutectic alloy. Precipitates CuZr₂ and Cu–Zr–Al were formed in the brazements of the Ni–Cr steel/ Al₂O₃ joints brazed with Ag–Cu–Zr alloys and Al containing Ag–Cu–Zr alloys, respectively. On the other hand, no precipitates were formed in the brazement of the Ni–Cr steel/Al₂O₃ joints brazed with Sn containing Ag–Cu–Zr alloys. The Ni–Cr steel/ Al₂O₃ joints brazed with Sn containing Ag–Cu–Zr alloys showed much higher fracture shear strengths than those brazed with Ag–Cu–Zr alloys or Al containing Ag–Cu–Zr alloys.     

Sc and Sc–Zr effects on microstructure and mechanical properties of Be–Al alloy

Herein, we investigated the effects of Sc and Sc–Zr on the microstructure and mechanical properties of Be–Al alloy, showing that Sc alloying resulted in Be grain refinement and reduced the secondary dendritic arm spacing (SDAS) of these grains by 1/3, whereas Sc–Zr alloying further decreased the SDAS to 7.5?µm and afforded equiaxed/cellular-like morphology with further refined Be grains. The above alloying resulted in the formation of intermetallic compounds (Be₁₃Sc, Be₁₃Zr, and Al₃(Sc_1–xZr_x)), increasing the macrohardness of the Be–Al alloy, with the microhardness and elastic modulus of the Be phase increasing to a larger extent than those of Al. Importantly, Sc–Zr alloying resulted in better microstructure modification and mechanical reinforcement than Sc alloying.     

Synthesis and ionic conductivities of M (Mg,Ba, Zr) and Al co-doped apatite-type lanthanum germanate electrolytes for IT-SOFC

In this paper, the structure and ionic conductivities have been investigated for M (Mg, Ba, Zr) and Al co-doped apatite-type lanthanum germanates. La_9.0.5Ge_5.5Al_0.5O_26±δ (M?=?Mg, Ba and Zr) were prepared by solid state reaction. The results of XRD analysis showed that the space group is P6₃/m. M (Mg, Ba, Zr) substituting La site and Al substituting Ge site increased the cell volume and improved the lattice distortion of the apatite structure. In addition, M (Mg, Ba, Zr) and Al doping facilitated sintering and improved the relative density. Analysis of AC impedance spectroscopy showed that M (Mg, Ba, Zr) and Al co-doping in lanthanum germanates introduced local distortion and improved the ionic conductivities. Moreover, the thermal expansion coefficient (TEC) of La_9.5M_0.5Ge_5.5Al_0.5O_26±δ (M?=?Mg, Ba and Zr) was improved after doping with M (Mg, Ba, Zr) and Al ions. La_9.5Mg_0.5Ge_5.5Al_0.5O_26.5 sintered at 1400 °C exhibited the highest conductivity (1.26?×?10^?2 S/cm, 800 °C) and improving TEC, which suggests that the (M (Mg, Ba, Zr), Al) doped apatite-type lanthanum germanate can be a potential material for the IT-SOFC electrolyte.     

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression was characterized in present work by high-temperature testing and transmission electron microscope (TEM) studies. The true stress–true strain curves exhibited a peak stress at a critical stain. The peak stress decreased with increasing deformation temperature and decreasing strain rate, which can be described by Zener–Hollomon (Z) parameter in hyperbolic sine function with the deformation activation energy 277.8 kJ/mol. The processing map revealed the existence of an optimum hot-working regime between 390 and 420 °C, under strain rates ranging from 0.1 to 1 s⁻¹. The main softening mechanism of the alloy was dynamic recovery at high lnZ value; continuous dynamic recrystallization (DRX) occurred as deformed at low lnZ value. The dynamic precipitation of Al₃Zr and Al₂₀Cu₂Mn₃ dispersoids during hot deformation restrained DRX and increased the hot deformation activation energy of the alloy.     

Investigation of primary Al3(Sc,Zr) particles in Al–Sc–Zr alloys

Abstract

This paper studied the primary Al₃(Sc,Zr) particles formed during solidification in three Al–Sc–Zr alloys with various Zr contents. It has been shown that the primary Al₃(Sc,Zr) particles formed during solidification since the Sc and Zr concentrations are above the solid solubility limit in Al matrix. Scanning electron microscopy line scanning results indicated that the distributions of Sc, Zr and Al atoms in the primary Al₃(Sc,Zr) particles differ a lot from those in the secondary Al₃(Sc,Zr) particles formed during annealing. In the primary Al₃(Sc,Zr) particles, both Sc and Zr contents are found from rim to the centre of the particles, with an increasing trend from the rim to the centre, while the Al content drops sharply on the rim of the particle and slightly from the rim to the centre.     

TEM study of the surface morphology of extracted ZrO2-Al2O3 fibres

The surface morphology and microstructure of a series of melt extracted ZrO₂-Al₂O₃ based fibres (ZA, ZAT and ZAS) have been imaged at the nanometre scale by transmission electron microscopy (TEM) using an advanced Pt/C replica technique. Growth characteristics of ZrO₂, Al₂O₃ and other crystalline phases formed upon heating up to 1550 °C are illustrated and described. Several grain morphologies including spherical and polygonal grains, as well as grains with rounded plate-like growth were observed indicating different active growth mechanisms. ZrO₂ particles on the surface of the fibres were almost spherical with some facetting and rounding of corners. These grains were very fine (< 50 nm) in the ZA and ZAS fibres while they were several microns in size in the ZAT fibres. Al₂O₃ grains were generally much larger (up to several microns) and exhibited two distinct growth morphologies of layered and rhombohedral type. Different grain morphologies of the ZA and ZAS fibres have been correlated to the phases identified by X-ray diffraction.     

Effect of minor Sc addition on microstructure and stress corrosion cracking behavior of medium strength Al–Zn–Mg alloy

Influence of Sc content on microstructure and stress corrosion cracking behavior of medium strength Al–Zn–Mg alloy have been investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy and slow strain rate test. The results indicate that the addition of Sc results in the formation of the quaternary coherent Al_3(Sc, Zr, Ti) dispersoids during homogenization treatment, which will inhibit the dynamic recrystallization behavior. The number density of Al_3(Sc, Zr, Ti) particles increases with the increase of Sc content, and thus the recrystallization fraction of hot-extruded alloy is reduced and the peak strength in two-stage artificial aging sample is enhanced. At the same time, the wide of precipitation free zone is reduced, and the content of Zn and Mg in grain boundary particles and precipitation free zone is increased with the increase of Sc content. In peak-aged state, the 0.06 wt% Sc added alloy shows the better stress corrosion cracking resistance than the Sc-free alloy because of the reduction of recrystallization fraction and the interrupted distribution of grain boundary precipitates along grain boundary. However, the further addition of Sc to 0.11 wt% will result in the deterioration of stress corrosion cracking resistance due to the increase of electrochemical activity of grain boundary particles and precipitation free zone as well as hydrogen embrittlement.