Papers by Keyword: GP Zone

Abstract: Aging phenomena of Al-Zn alloys have been much investigated and many studies have been made on the change of mechanical properties on aging. Appearance of saw-tooth yielding (serration) in the stress-strain curve of tensile deformation has been reported in several reports. In this study, the relationship between occurrence of serration and reversion heat treatment was studied by tensile test on Al-Zn alloys. Serration was observed for the furnace cooled specimen, but not for the as-quenched one and the shortly aged one, which result is difficult to be interpreted by the Cottrell theory. The stress amplitude in the serration increased with increasing the time of annealing for reversion and then stayed at a constant value which might be interpreted by the Cottrell theory. The stress amplitude in the serration decreased with increasing the annealing temperature, which is contrary to the prediction by the Cottrell theory. The specimen directly annealed without aging and the one annealed after aging agreed well with each other in the tensile strength and the stress amplitude in the serration, which suggests solute clusters as the cause of serration.

Abstract: Age-hardening of Al-Zn alloy after quenching develops inhomogeneously due to the effect of surface as a vacancy sink and grain boundary as an easy path. In this study, reversion of the age-hardened Al-Zn alloys, in which ellipsoidal GP zones were formed, was investigated by Vickers micro-hardness test. Ellipsoidal GP zones were reverted more quickly near the surface and grain boundary than in the interior, as spherical GP zones in Al-10%alloy did. It is considered that the surface and grain boundary plays a role of effective source for vacancies, in addition to the interior source such as dislocations, as in the case of the reversion of spherical GP zones.

Abstract: The effect of heat treatment conditions and small addition of Fe on occurrence of serration in Al-Zn alloys was investigated. Specimens were aged for various times up to 5Ms at 293K or 273K after quenching from various temperatures (T_Q), 398K to 853K, and tensile-tested at room temperature. Serration occurred more easily according as T_Q became lower and the aging time became shorter: in the case that T_Q=473K serration was observed even after aging for 2.6Ms, while in the case that T_Q=773K serration did not occur irrespective of aging conditions. Serration was also recognized when the specimens were furnace-cooled from 773K to room temperature. Thus, for the binary alloy serration was observed only when the aging period was short enough, but addition of Fe to the binary alloy prolonged the aging period where serration could be recognized. Aging rate measured by hardness was remarkably retarded with the increase of Fe addition. These results together with those obtained by the electrical resistometry suggest that the serration in Al-Zn alloys occurs in the early stage of aging where small GP zones or solute clusters are formed.

Abstract: 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.

Abstract: Aging behavior of Al-2.1Li-2.9Cu-0.12Zr(wt%) alloy has been studied as functions of aging time and temperature by using a differential scanning calorimetry(DSC) and transmission electron microscopy(TEM). The aged specimens at 130, 160, 190°Cwere compared with the as-quenched specimen in view of the aging behavior by observing the reaction enthalpy during the heating period of DSC experiments. DSC peak associated with the formation of GP zone is not observed at 130 and 160°C, but it does at 190°C, so it is found that the thermal stability of GP zone is changed at between 160 and 190°C. At the aging temperature of 130°C, the heat absorption corresponds to the formation of δ´ increases, while the heat evolution related with the formation of T1 reveals an opposite trend. In aging temperature of 160 and 190°C, aging time representing the drastic decrease of heat absorption of δ´ is coincident with the transition time showing the decreasing of heat evolution of T1. The micro-Vickers hardness of the specimens aged at 160°C and 190°C shows maximum values about 182 and 165 at aging times of about 72 hr and 25hr, respectively. From comparing reaction enthalpies for the dissolution of δ´ and the formation of T1 phases with the aging time showing the maximum hardness, it is found that δ´ phase rather than T1 phase markedly contributes to the hardening at an aging temperature of 160°C. In contrast, T1 phase plays an important role in hardening at an aging temperature of 190°C.

Abstract: Identification of precipitates appearing during DSC scan of Al-2.1Li-2.9Cu-0.12Zr(wt. pct) alloy has been conducted as a function of temperature by using a differential scanning calorimetry (DSC) and transmission electron microscopy(TEM). In the as-quenched specimen from 540°C, three couples of heat evolution and absorption peaks are observed during the heating period of DSC experiments. It is found from TEM works that these peaks are associated with the formation and dissolution of GP zone, δ‘, and T1(+θ') phases. The heat evolution peak appearing in the temperature range over 36~78°C is due to the formation of GP zone. Heat absorption peak appearing in 78~140°C is associated with the dissolution of GP zone. Heat evolution peaking at 166°C in the temperature range over 140~190°C and next heat absorption peak are attributed to the formation and dissolution of δ‘ phase. Heat evolution peaking at 288°C in the temperature range of 254~332°C and heat absorption at high temperature are attributed to the formation and dissolution of T1(+θ'). The hump in DSC curve at the temperature of 425°C is considered as the formation of hexagonal structure T2 phase.

Abstract: CuAlNi shape memory alloy with Cu-13.5Al-4.5Ni(wt%) composition was prepared by cross-rolling method and the aging effect, phase transformation characteristics, microstructural variation were investigated. Transformation temperature was greatly increased in aged specimens at 250°C. Transformation temperature was not changed after the second reversed transformation, and Ms point was same in most specimens with third reversed transformation cycle. The variation of Ms point was not seen with aging at 100°C, but it was decreased with aging at 250°C. Transformation temperatures appear to be constant, to an measurable extent, in specimens prepared by both hotrolling and cross-rolling at 900°C. Plate shaped-specimen with the thickness of about 1mm was prepared by cross-rolling treatment at 350°C. The transformation temperatures did not change after the second transformation-reverse transformation cycling, and specially transformation start temperature was the same in most specimens experiencing third thermal cycle and thereafter. Undesirable tweed-like structure was observed in 250°C-aged specimen after a cross-rolling at 350°C. On aging the specimen prepared by cross-rolling, G.P zone was formed easier than that of γ2 phase. This phase was transformed to plate-like θ phase during aging.

Abstract: The aging of Al-Zn alloys has been vastly studied for decades. In the previous paper, 0hta et al. studied carefully the hardness of the alloy during aging and revealed the existence of softer regions near the surface and the grain boundary than the interior of the specimen even after aging for a long time. Electrical resistivity measurement and X-ray small angle scattering experiment together with hardness test suggested that in these regions vacancy decay to the surface and grain boundaries was severe, thus the growth of GP zones were suppressed and therefore age hardening was retarded. Also, it is well-known that an addition of a small amount of Ag raised solvus temperature of GP zones. In this paper, soft surface layer formed in an Al-12mass%Zn alloy is studied by adding small amount of Ag by means of hardness test and resistometry. Addition of Ag more than 0.1% decreases the thickness of soft surface layer as well as accelerates age hardening rate and suppresses the formation of soft region near the grain boundaries. Higher quenching temperature also reduces the thickness of soft surface layer. Together with the behavior of aging curves of the specimen with various thicknesses, the origin of the soft surface layer is confirmed to be the effective role of surface as sinks for vacancies.

Abstract: Solid solution treatment at 450°C and 550°C and subsequent two step age hardening at 100°C and 150 °C up to 144 hrs. have been carried out for two conventional and four experimental 7xxx type of alloys with different Mg, Zn, Fe and Si content. The influence of silicon on phase and kinetics of age hardening zones and particles has been followed. Increase in silicon required higher solid solution temperature in order to achieve reasonable age hardening response. High silicon alloys, solid solution treated at high temperature, have tendency to recrystallize during aging. The GP-zone formation is affected by the ratio between Mg, Zn and Si. In alloys with Mg/Zn ratio in the range 1:2 GP(I)-type zones are formed, at higher solid solution temperature also GP(II); low Mg-content favor GP(II)- zones. In high silicon alloys GP-zones of b’’’-type (from the Al-Mg-Si) system contribute to age hardening. The precipitation kinetics of the main hardening phase h’, is influenced by the preceding GP-zone stage.

Abstract: A systematic study of the precipitation processes in high strength Al-Zn-Mg-Cu alloys has been conducted. A series of experimental alloys was prepared with varying Zn:Mg ratio, but equivalent total solute content, to be representative of those used in applications which demand a combination of strength, fracture toughness and resistance to environmentally sensitive cracking mechanisms. Artificial ageing curves were constructed for each alloy, based upon 7xxx series duplex treatments widely used in industry. Ageing kinetics were investigated beyond peak strength into the overaged condition, since this is known to promote the most favourable balance of properties. Differential scanning calorimetry (DSC) and transmission election microscopy (TEM) observations were made on the alloys throughout the ageing process to fully understand the precipitation events occuring. For a particular overageing treatment, higher Zn:Mg ratio alloys were consistently found to be at a more advanced stage of precipitation while higher strengths were retained at low Zn:Mg ratios. Grain boundary characteristics, such as particle size and precipitate free zone width, were also influenced by Zn:Mg ratio at a given strength level. This paper provides new understanding of precipitation kinetics and the control of important microstructural features which influence the balance of properties in 7xxx series aluminium alloys.