Shigenori Hori

A possible mechanism of phase transformation of Al3Hf from Ll2 to DO22 during aging in a rapidly solidified Al-3Hf-0.3Si alloy

Abstract The aging process of the rapidly solidified solid solution of Al-3 wt% Hf-0.3 wt% Si has been investigated by electron microscopy. The following precipitation stages of an Al3Hf phase have been obtained: metastable Ll2 transforms to a new H-phase and finally to a DO22 type superstructure. The H-phase was found to have the space group Pmmm [Fig. 4(a)]. The transformation from one phase to the other can be explained by a periodic shear. Thus, a shear of 1 2 [110](110) Ll 2 on every second plane leads to the H phase which transforms to DO22 by a shear of 1 2 [010](101) H on every second plane.

Strain rate dependence of internal and effective stresses in superplastic deformation

Abstract Flow stresses at a wide range of the strain rate have been analyzed by evaluation of an internal stress and an effective stress in the flow stress for the superplastic Zn-Al eutectoid. A sigmoidal relationship between the maximum stress on the stress-strain curve and the strain rate has been observed in the double logarithmic plot, so that deformation behavior is devided into three regions I, II (superplastic region) and III, with an increase in the strain rate. It has been found also that the rate of the internal stress σi to the flow stress σ, increased with a decrease in the strain rate ϵ. The rate, σ i σ was unity in region I. The strain rate sensitivity exponent of the internal stress σi or the effective stress σe(= σ − σi), mσi or mσe, (σ = Kϵm, K: constant) was evaluated to be 0.2 or 0.8 in region II, respectively. The activation energy, the grain-size exponent of the flow stresses and the contribution of grain boundary sliding to the total strain have been estimated systematically for each deformation region. It has been considered that the internal stress is a back stress to dislocation motion, while the effective stress induces the dislocation movement which results in the grain boundary sliding or in the dislocation creep in region II or III, respectively. Basing on results obtained, the deformation mechanism in each region and the transition between neighboring regions will be discussed.

MICROSTRUCTURES OF RAPIDLY SOLIDIFIED Al-Ti ALLOYS CONTAINING TITANIUM UP TO 40% AND ITS THERMAL STABILITY

Microstructures of Al-Ti alloys containing Ti up to 40% solidified at various solidification rates from 10 −1 to 10 7 K/s were investigated. The microstructures of these alloys are classified into three types in accordance with the Ti content and the solidification rate:(1) Al-Ti enforced solid solution. (2) Al-Ti enforced solid solution + petal-like structure containing metastable Ll 2 type Al 3 Ti. (3) Al-Ti enforced solid solution + equilibrium DO 22 type Al 3 Ti. Thermal decomposition of these structures at 723 and 823K were examined.

Multiplication process of twin bounderies in feathery crystal.

Feathery crystals formed in Al-5 mass%Mg alloys unidirectionally solidified were microscopically observed at intervals of about ten micronmeters for examining multiplication of twin boundaries. Two types of site for multiplication of feathery crystals are found. Type I is that a new twin boundary originates at a site separating from preexisting twin boundaries. And type II is that a new twin crystal originates at a site on the subgrain boundary generated in either individual of a preexisting twin crystal. These types of twin multiplication have particular features of the variations of length and spacing of twin boundaries.

Tensile Properties at Very Low Temperature in Aluminum Magnesium Alloys

ABSTRACT Tensile strength, ductility, serrated deformation, work-hardening, cracking within the large intermetallic particles and intergranular cracking during straining in the temperature range 4 to 70 K have been mainly investigated. The experiments were performed with polycrystals of 5083–0 aluminum alloy, Al-5.3% Mg alloy and pure aluminum (99.99%). A helium gas flow cryostat and cooling system were used for tension test at the various temperature. Decrease in ductility and cessation of increase in the tensile strength with decreasing temperature were observed in the temperature range 4 to 20 K for 5083–0 aluminum alloy and 7 to 35 K for Al-5.3% Mg alloy. Non uniform deformation corresponding to the serrated flows which were accompanied by heat generations is a main cause for the decrease in ductility, and cracking of the intermetallic particles in 5083–0 aluminum alloy and intergranular cracking in Al-5.3% Mg alloy might be possible causes.

PHASE DECOMPOSITION PROCESS OF A RAPIDLY SOLIDIFIED Al-3 wt% Hf-0.3 wt% Si ALLOY DURING AGING

The phase decomposition sequence of a solid solutioned Al-3wt%Hf-0.3wt%Si alloy during aging at 723 K has been investigated mainly by an electron diffraction technique. As a result, it is suggested that a possible structure of the H phase, which was found as a new transition phase of Al 3 Hf from the Ll 2 (metastable) to the DO 22 (equilibrium) phases, is considered to be P mmm in space groups, as shown in Fig. 2 in the text. On the basis of this structure, a reasonable sequence for the decomposition process of this alloy is discussed with proposed mechanisms involving atomic shears in every two planes of 1/2[110](110)Ll 2 and 1/2[010](101) H for the transitions from the the Ll 2 and the H structures to the H and the DO 22 structures, respectively.

Some Torque-Twist Relations on the Low Temperature Dynamic Torsion Test of Low Carbon Steel

The present investigation was carried out for the size effects of test piece and various testing conditions on the torsional fracture behaviour, using a newly developed dynamic torsion machine. Dynamic torque-twist curves were obtained photographically at various temperatures. The maximum rate of straining was 60per second and the range of temperatures from +200°C to -196°C.Yield and fracture strength and plastic strain to fracture were not affected by the variation of gauge length of the test piece, but yield and fracture strength increased with decreasing of the diameter. The size effects on torsional strength were also appeared for hollowed specimens as well as for solid ones. In the static torsion test, farcture type at comparatively low temperature had a ductile manner and the cleavage fracture appeared only at -196°C under the tested conditions. On the other hand, in the dynamic test, the specimens used for the investigation took a ductile fracture, excepting a specimen having a larger grain than the A.S.T.M. No 4.The torque-twist curves in static test or room temperature dynamic test dropped at maximum torque after drop in load. To the contrary, in the low temperature dynamic test, after reaching the maximum stress, torque suddenly decreased with increasing twist angle and then kept a constant value until the beginning of fracture. Ruptured specimen with cleavage type due to the dynamic torsion test had been broken at maximum torque. These strange phenomena in the fracture manners and the torque-twist curves were discussed and assumed to be due to the temperature rise in the tested part connected with the rapid large plastic deformation.

Change of macrostructure and dimention of bent aluminium by successive isothermal annealing

There are many investigations on recrystallization of cold rolled aluminium sheets, but few on recrystallization phenomena of cold bent one. But forming occupies the greater parts in cold forming works and when such a bent material is heated in working or using, the metallurgical defects have frequently appeared. Nevertheless, probably nothing has been reported on the investigations which studies systimatically how to prevent from these accidents. This is a study on the changes of macrostructure and dimension of cold bent aluminium which take place through successive isothermal annealing. The findings are summarized as follows:The appearances of recrystallized grain in the longitudinal section almost correspond with the stress distribution diagram induced from stress-strain curve.Under low temperature annealing, there is a slight change in curvature of bending and the magnitude of shrinking in dimention increases remarkably caused by recrystallization on strain field below the value near 0.10. But when these samples are heated at temperature higher than 480°C, large shrinking in dimension and coarse grain in the longitudinal section take place, to which attention must be paid in practice.