Hot working and crystallographic texture analysis of magnesium AZ alloys

Abstract

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10^?3 and 5 s^?1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol^?1 and Z values range from 10 to 10 s^?1. Textural examinations show basal texture as the predominant orientation.     

Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

The effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED_?9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED_?9.4% specimens are similar, and they experience twinning exhaustion → slip and detwinning exhaustion → slip during each loading cycle. At a medium strain amplitude of 1%, twinning‐detwinning is involved in the cyclic deformation, but different deformation mechanisms were observed in the 3 different specimens. Partial twinning‐complete detwinning mode dominates the cyclic deformation in the ED specimen, while partial detwinning‐retwinning mode occurs in the ED_?9.4% specimen. For the TD specimen, both basal slip and tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED_?9.4% specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens.     

The influence of aluminium alloy anisotropic texture on crack-tip plasticity

Crack‐tip plasticity in textured aluminium alloys was numerically analysed using an anisotropic yield function and an isotropic hardening law as the material constitutive response. Four real textures obtained from extrusions of rectangular and square shapes as well as five ideal textures typical of rolled products were considered and predicted crack‐tip plastic zones were compared with those obtained using the isotropic von Mises yield criterion. The use of the anisotropic yield function revealed important differences in crack‐tip plasticity compared with the isotropic case. The plastic zone features obtained for different textures were compared to single crystal results published in the open literature and similar crack‐tip plastic strain localization was observed.     

The wettability of silicon carbide by liquid aluminium: the effect of free silicon in the carbide and of magnesium,silicon and copper alloy additions to the aluminium

Results from the sessile-drop method are reported for the effects on wetting angle, , of free silicon in the silicon carbide substrate and of alloy additions of silicon, copper or magnesium to the aluminium drop for the temperature range 700–960 or 1040 °C in a titanium-gettered vacuum (10^–4/10^–5 torr; 1 torr=133.322 Pa). Wetting angle, , was reduced by a factor as large as 2.8 for pure aluminium on reaction-bonded, compared with sintered silicon carbide, attributable to partial dissolution by the aluminium of the 18 wt% free silicon present in the reaction-bonded material. For wetting of reaction-bonded silicon carbide, the addition of 5 wt% silicon, copper or magnesium to the aluminium gave contact angles that decreased in the sequence SiCuMg, with the magnesium addition being the only one to result in wetting (i.e. <90 °) for all conditions studied. These results may have implications for design of conditions for joining or promotion of infiltration of silicon carbide parts, preforms or arrays with aluminium alloy melts.     

Influence of silicon and magnesium on grain refinement in aluminium alloys

Abstract

Grain refinement in Al–Si alloys with silicon contents in the range of 0·2–30 wt-% has been studied in detail with conventional as well as higher level additions of a Al–5Ti–1B master alloy. A poisoning effect was observed with Al–Si alloys containing ≥7 wt-%Si and the extent of poisoning increased with an increase in the silicon content. Silicon improves the grain refining behaviour of aluminium when added in small quantities (0·2%). Magnesium can counteract the poisoning effect of silicon. The optimum level of magnesium required to overcome the poisoning effect depends on the silicon content of the alloy. Higher level additions of a grain refiner could overcome the poisoning effect of silicon and the level required to achieve good grain refinement is a function of the silicon and magnesium contents of the alloy. The present paper also reports the influence of degasser and melt temperature on the grain refining response of Al–Si alloys.     

Joining of silicon nitride to silicon nitride and to Invar alloy using an aluminium interlayer

Si₃N₄ has been bonded to Si₃N₄ and to the Invar alloy using an aluminium interlayer at temperatures above the melting point of aluminium. Reaction was hardly observed at the interface between Si₃N₄ and aluminium up to 1223 K. The highest strength of the Si₃N₄-Al-Si₃N₄ joints was beyond 500 M Pa. In the Si₃N₄-Al-Invar joint, two main intermetallic compound layers were formed at the AI-Invar interface. The strength of the joints was between 150 and 200 MPa. It is expected that the aluminium layer and the reaction layer with the fine cracks growing perpendicular to the interface play an important role to compensate for the thermal expansion mismatch.     

Role of magnesium in cast aluminium alloy matrix composites

Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO₄), zirconia (ZrO₂), titania (TiO₂), silica (SiO₂), graphite, aluminium oxide (Al₂O₃) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.     

电场时效对2014铝合金织构的影响

用X射线衍射ODF分析方法解析了在电场时效过程中2014铝合金织构的形成和演变机理.结果表明:在电场时效过程中2014铝合金的主织构始终是{001}(210)织构,而非电场时效其主织构随着时效时间的延长由{001}(210)织构(6h和8h)转变为{001}(100)立方织构(10h),然后由{001}(100)立方织构转变为{011}(100)织构(12h).这主要是电场时效降低了固溶处理2014铝合金样品的储存能,从而抑制其回复所致.     

Optical anisotropy of a color-etched AZ91 magnesium alloy

In this paper, the optical anisotropy of a color-etched AZ91 magnesium alloy grain is studied. In the first part, the rinsing and drying conditions after etching were varied in order to improve the contrast between grains. A rinsing solution was selected and ellipsometric measurements were carried out to characterize the optical anisotropy of the etched surface. The wavelength, grain orientation and angle of incidence were varied. It was found that the reflection intensity at oblique incidence and the phase shift between parallel and perpendicular polarizations depend on the orientation of the etched surface of the grains. The optical contrast under diffuse light is explained by the morphology of the surface film deposited upon etching. The optical contrast under polarized light is attributed to form birefringence induced by the film texture. The birefringence, the fast axis, the slow axis, and the optical axis of the etched surface were also determined with the polarizing microscope.     

Kinetics of precipitation hardening phase in aluminium alloy AA 6110

Abstract

The solid solution treated aluminium alloy AA 6110 was investigated using a differential scanning calorimetry (DSC) with different heating rates. Kinetic parameters, e.g. the activation energy and the Avrami exponent, were calculated. The peak temperatures of the hardening phase β″ from each heating rate were collected to calculate the activation energy of the aluminium alloy AA 6110 using various mathematical models: the Kissinger, Ozawa and Boswell models. It was found that the activation energies from each model were within the range of 106–114 kJ mol^?1. These activation energies were found to be lesser than those for the bulk diffusion of aluminium, magnesium and silicon atoms in aluminium matrix. The average Avrami exponent, n of 2·36 was calculated using the developed Matusita model and corresponded to a one-dimensional linear growth of fresh nuclei of needle-like β″ in this aluminium alloy AA 6110.     

Kinetics of precipitation hardening in SiC whisker-reinforced 6061 aluminium alloy

Ageing behaviour at 180 °C of 6061 aluminium alloy-SiC_w composites, drawn from bars obtained in various extruded ratios, and 6061 aluminium alloy used as matrix, have been compared. These materials were dissolved in a salt bath at 529 and 557 °C for 2 h, quenched in ice-water, and aged at 180 °C in an oil bath for increasing periods. Ageing kinetics were studied with Brinell hardness measurements and differential scanning calorimetry (DSC). Various samples of the composite, deriving from bars with Φ20, Φ35 and Φ50 mm in diameter, and 6061 aluminium alloy, show the same ageing mechanism; however, the ageing rates results increased for composites. While 6061 aluminium alloy shows its maximum hardness value after about 4–5 h at 180 °C, the 6061-SiC_w composites reach theirs in 2–3 h. Moreover, for composites hardness abruptly decreases after 3 h, while aluminium alloy keeps its maximum value for an ageing time as long as 6 h. Thermal analysis allows us to put together a definite DSC trace for every microstructural state. The highest hardness values are obtained as a result of the formation of a Guinier Preston (GP) needle-shaped zones, which progressively become more thermally stable with protracted isothermal treatment at 180 °C. The different ageing process rates observed for composites and for the 6061 alloy are correlated with the sizes of the reinforcements. Dimensional analysis of whiskers has been performed by light scattering and scanning electron microscopy. Ordinarily the longer the average length of the whiskers in the samples, the faster the ageing process. Higher temperatures are required for composite solutions than for 6061 alloy. On the other hand, 6061-SiC_w samples solutionized at higher temperature and then quenched sometimes show microcrack formation in the materials.     

The effect of humidity on friction and wear of an aluminium–silicon eutectic alloy

The sliding wear of an aluminium–silicon eutectic alloy against cast iron counterface in 3–100% relative humidity range has been investigated. The results show that the moisture content has a significant effect on the friction and wear of the Al–Si alloy. The wear rate decreases by two orders of magnitude as the relative humidity increases from 3% to 100%. At low humidity conditions adhesive wear is predominant, whilst at high humidity conditions a layer of compacted oxide–metal debris film is formed on the Al–Si slider surface, which reduces the direct metal–metal contact. The friction coefficient is maximum at 3% and 100% relative humidity conditions. This revised version was published online in November 2006 with corrections to the Cover Date.