１． Introduction

In the specific gravity of the magnesium alloy, it is about 1.76-1.83, and this is the lightest in about about 2/3 of Al, 1/4 of Fe, and a practical metal materials except the special one. In addition, it has the advantage of excelling in specific strength, ratio power, vibration absorption, heat radiation etc.The magnesium alloy makes the best use of such a material characteristic, and the use of electronic equipment such as AV equipment and notebook-sized personal computers and cellular phones etc. to the case is increasing rapidly as a plastic alternative material in recent years. Moreover, use to auto parts has increased in the background of global environmental concerns because of energy conservation and the environmental burden decrease, too. However, it is a situation to which the accumulation of data to a physical chemical performance is insufficient from the history of the structure material demand shallow, and research and development of the manufacturing capacity and the machining and the surface treatment, etc. is delayed, too. Mechanical properties of the material are very sensitive in a microscopic organization. Therefore, the organization control of the material has an important meaning. An important thing is to analyze the organization from various viewpoints, and to understand the formation process from the organization control deeply. For instance, when two aspect organization is formed by extraction, the organization is greatly changed by differences of a nuclear generation places such as in grain and the grain fields. The change in such a form should not only be classified the organization in the form but also is the one, and is considered from a crystallography viewpoint by relate between the parent phase and the extraction aspect the crystallography.Then, a crystallography feature of the β phase of the BCC structure that precipitates from the α parent phase of the HCP structure is shown in this research.

２． Experimental

Magnesium alloy ground metal AZ91D for the die cast was used for the sample. Speciments were first solution-treated at 703K for 57.6ks in vacuum, followed by water quenching to obtain α single phase. Subsequentry isothermal aging treatments were performed in the temperature range from 558K to 623K for various periods to form (α+β) microstructure. Microstructural observation was carried out by SEM(Scanning Electron Microscope).

３． SEM observation result

The SEM photograph after the solution is shown in Fig.1. And the SEM photographs after the prescription are shown in Fig.2 to Fig.8.

Fig.1 after the solution
Fig.2 558K, 1.8ks	Fig.3 558K, 7.2ks	Fig.4 558K, 14.4ks	Fig.5 558K, 28.8ks	Fig.6 558K, 57.6
Fig.7 623K, 14.4ks	Fig.8 623K, 28.8ks

４． Conclusions

　β-Mg17Al12 melts to the parent phase by processing of making to the solution, and the single α organization. A discontinuous precipitation from the grain field and a continuous precipitation in grain are seen as for the organization that heat-treats it by 558K at any hold time. Perlite Nojul that is the feature of a discontinuous precipitation is formed . In heat-treatment temperature 558K, the precipitation of Nojul has already been confirmed by maintaining 1.8ks. Both of grain in which grain in which the precipitation has happened has not happened with the precipitation exist together at this hold time. In the organization that maintains 28.8ks, the precipitation of Nojul has progressed to the whole in grain. Moreover, the whole is covered with Nojul of the start (the first Nojul), and wide the second Nojul at layer or more intervals has progressed in the organization that maintains 57.6ks. As for the organization that heat-treats it by 588K, the precipitation of Nojul has decreased more than the organization that heat-treats it by 558K. Moreover, the SEM photograph after the prescription is processed by 623K in the heat-treatment temperature is shown in Fig. 7 and Fig.8. A discontinuous precipitation from the grain boundary and a continuous precipitation in grain are seen in the organization that heat-treats it by 623K. However, Nojul by a discontinuous precipitation like the organization that heat-treats it with 558K・588K is not seen. In heat-treatment temperature 623K, when the organizations at each hold time are compared, the precipitation in grain is few, and the thing of the organization that maintains 14.4ks that precipitates as generation to the grain field is also comparatively small. The precipitating thing increases by both the precipitation in grain and the grain field precipitation, and in the organization that maintains 28.8ks, grain grows up, and the amount of the precipitation has increased, too. And, a lot of precipitations in the grain boundary are seen in the point that three grain boundaries intersect regardless of hold time. This is corresponding to assumption that the point that three grain boundaries intersect is a prior nuclear generation place of the second phase . It is guessed that there are some orientation relationship on the parent phase and the precipitation phase.