Katsushi Matsumoto

Effects of Zn Addition and Aging Condition on Serrated Flow in Al-Mg Alloys

The serrated flow phenomena in Al-Mg alloys with and without Zn were investigated after aging on several conditions, focusing on the role of precipitates. Al-6mass%Mg-0~3mass%Zn alloys were solution treated at 753~803K, quenched, and then aged at room temperature. Further artificial aging at 323~573K for 86.4ks was performed for some of them after natural aging for 2.6Ms. The serrated flow behavior was evaluated by tensile test. Microstructure was characterized by differential scanning calorimetry, transmission electron microscopy, atom probe tomography, and positron annihilation lifetime spectroscopy. The increase in the amount of Zn addition and the natural aging time lead to a delayed onset of serrated flow. The artificial aging at higher temperatures after natural aging, on the other hand, decreases the onset strain. A large number of small coherent Zn-Mg clusters are formed during natural aging in the Al-Mg-Zn alloys, which are transformed to the larger incoherent meta-stable precipitates during subsequent artificial aging. These results suggest that the mechanism of interfering with serrated flow is related to the vacancy trapping effect, which is enhanced by the coherent clusters.

In situ SEM-EBSP observations of recrystallization texture formation in Al-3mass%Mg alloy

This study presents in situ observations of recrystallization texture formation in Al-3mass%Mg using SEM concurrent with electron back scattering pattern (EBSP) with hot stage. In the present discussion, the emphasis is on the characteristics of the preferred growth or the shrinkage of Cube and other oriented grains. The in-situ observations of recrystallization demonstrate clearly that the nucleation, growth and shrinkage of recrystallized grains occur simultaneously in each orientation in each region. The overall development of recrystallization texture depends on the balance of nucleation/growth and shrinkage/disappearance of each orientation during recrystallization. The preferential growth is determined by the grain boundary mobility between recrystallized grains or the clusters composed of several similar oriented grains, i.e. Cube clusters or S clusters, and neighboring deformed matrix, and the competitive growth with the surrounding grains. On the other hand, the isolated oriented grain and the strain-stored grains tend to shrink and disappear during recrystallization and grain growth.

Effect of secondary phase on grain growth process of α/β dual-phase CuZn alloy

Abstract Investigation of the effect of a secondary β phase on the grain growth process in a Cu-40mass%Zn alloy with a α/β dual-phase microstructure resulted in the following: (1) The α phase grains did not display marked texture even after grain growth advanced, while the β phase showed strong texture of (001)[110]at an early stage of annealing and then (111)[110] texture appeared with further annealing. The α phase matrix was separated into small regions by randomly distributed β phase and development of the texture in the α phase grains was disturbed by the β phase. (2) The GBCD of α/β grain boundaries did not show a remarkable change during annealing, while the frequency of random boundaries increased in β/β boundaries. (3) Ostwald ripening occurred for β phase grains in the α phase matrix. The supersaturation of solute atoms due to the dissolution of small β grains provided the driving force for the migration of α/β interphase boundary.

Effects of Cu or Li Addition and Multi-Step Aging Conditions on the Bake-Hardenability of an Al-Mg-Si Alloy

In recent years, automobiles with lower fuel consumption are required because the exhaust fume is severely regulated. The weight-saving is quite effective to realize such low fuel consumption, and therefore aluminum alloy becomes attractive as an alternative material of steels due to its high specific strength. 6XXX series Al-Mg-Si alloys exhibit good bake-hardenability during paint-bake treatment in the automobile manufacturing process, but to reduce further environmental impact, the paint-baking temperature is supposed to be lowered than the present temperature of about 443K. In this study, it was aimed to investigate the attained hardness after paint-bake treatment at various temperatures of 408-443K for an Al-0.55wt%Mg-0.90wt%Si alloy with/without microalloying elements of Cu and Li. The effects of multi-step aging conditions; e.g. pre-aging, natural aging and paint-bake treatments, were also investigated through Vickers hardness test, TEM observation and DSC analysis. From the obtained experimental results, it was clarified that the addition of Cu or Li to the Al-Mg-Si alloy increases the attained hardness even at a paint-baking temperature of 408K due to the increased volume fraction of precipitates. Furthermore, pre-aging treatment at 373K for 18ks was also effective in suppressing the increase in hardness during natural aging, resulting in the highest attained hardness among the investigated multi-step aging conditions; i.e. HV100 in the Li-added alloy paint-baked at 408K.

Effects of Sn Addition on Clustering and Age-Hardening Behavior in a Pre-Aged Al-Mg-Si Alloy

The effects of Sn addition on clustering and age-hardening behavior in an Al-0.6Mg-1.0Si (mass %) alloy were investigated. Addition of Sn delayed the age-hardening in single aging at 170 ̊C. On the other hand, Sn promoted the age-hardening response in 3-step aging process which comprises a pre-aging (PA) at 90 ̊C for 18ks followed by natural aging (NA) for 604.8ks and artificial aging (AA) at 170 ̊C. The characteristics of clusters formed during PA and NA were evaluated by differential scanning calorimetry (DSC) analysis and atom probe tomography (APT). The DSC results show that the endothermic peak at around 160 ̊C to 200 ̊C was observed in the Sn-free alloy. On the other hand, in the Sn-added alloy, endothermic peak was not observed. It is suggested that Sn addition suppresses the formation of the clusters formed during NA. The APT results show that the Sn addition decreases the number density of clusters, especially smaller clusters. No Sn precipitates were found in Mg-Si precipitates formed during AA at 170 ̊C for 3.6ks. It is speculated that suppression of smaller cluster formation by addition of Sn promotes the age-hardening response

Hot Deformation Behavior of Near-α Ti-Fe Alloy in (α+β) Two-Phase Region with Different Fe Content

In the present study, microstructure evolution of Ti-Fe alloys with different Fe content between 0.2-1.5mass% during hot deformation in (α+β) two-phase region is studied with focusing on effect of phase volume fraction at different deformation temperatures and strain rates. Hot deformation was conducted on the specimens quenched after β solutionizing at 1173K for 1.2ks at 1108, 1073 and 948K, by uniaxial compression by 50% at various strain rates ranged from 1 to 10-4 s-1. Initial structures are (α+β) lamellar structures of fine interlamellar spacing and colony sizes. Increase in Fe content results in increasing the fraction of the β phase at the given deformation temperature. Either colony size or interlamellar spacing is coarser at higher temperatures. At the higher deformation temperature where β phase fraction is larger, dynamic recovery of β phase is a major deformation mechanism while at a lower temperature, i.e., a higher α fraction, dynamic recrystallization of α phase occurs predominantly. It is concluded that critical strain needed for occurrence of dynamic recrystallization is decreased by increasing fraction of the α phase at the same deformation temperature, i.e., by decreasing Fe content. Furthermore, by increasing strain rate grain size of the recrystallized α is decreased.