Fig. 1 Changes in strength and hardness at various solution treatment temperatures
@Our understanding is that during high-temperature treatment that involves eutectic melting, the macroscopic properties are defined by the overlap of negative effects, such as the creation and growth of pores caused by eutectic melting and the coarsening of the intermetallic compound phase, and positive effects, such as the homogenization of material properties and increased age hardenability (Papers 1,2,3
).Our aim is to realize an extremely high temperature solution treatment that is highly reliable by clarifying these positive and negative effects both quantitatively and visually in 3D and 4D with 3D and 4D imaging techniques such as K-edge subtraction imaging.
Fig. 2 Histogram showing the concentration distribution of copper in the alpha phase for several solution treatment temperatures and time conditions.
@By using solution time as a parameter, the relationship between the tensile strength and hardness of Al-6Si-4Cu and the solution temperature is shown in Fig. 1(Papers 2,3
).The sample subjected to a solution treatment at 807 K for 7.2 ks showed the highest tensile strength. The tensile strength was over 12% greater and there was about a 1/5 reduction in treatment time compared with a sample subjected to the T6 treatment condition under the Japanese Industrial Standards (JIS). The macroscopic hardness was generally consistent with this finding, but the hardness of the alpha phase differed greatly and the difference was greatest for samples treated at 807 K. This indicated a severe degradation in the strength of the eutectic phase. As shown in Figure 2, the age hardenability improved when the solution treatment was performed at higher than the ternary eutectic temperature, the copper concentration distribution converged at around 1.8 ks at 807 K, and the copper concentration also increased gradually from that point and beyond(Paper 3
).This was, as shown in Figure 3, because the high copper concentration region was disappearing rapidly from the final solidification zone owing to the high temperature (Paper 3
).However, it is interesting to note that due to eutectic melting there was also a simultaneous negative effect caused by the loss of homogeneity in the parent phase.
Fig. 3 Changes in the area with a copper concentration above 10% at 807 K. The arrow indicates the area with a copper concentration above 30%.
Fig. 4 Distribution within a phase containing both pores (blue) and copper (gray) at 807 K.
@Meanwhile, as shown in Figure 4, pores formed during the solution treatment existed adjacent to a locally melted area and grew at a high temperature(Paper 3
). If the solution treatment was applied for a much longer time and/or at a much higher temperature thus exceeding the conditions under which the positive effect of the improved age hardenability surpassed the effect of increased pores and eutectic melting, the negative effect gradually became dominant and preferential fracturing, especially in the area of eutectic melting, defined the deterioration of macroscopic properties. Figure 5 shows the relationship between fractures and the area of eutectic melting(Paper 3
Fig. 5 Association between a fractured surface and areas with more than 10% copper concentration shown after the tensile fracture of a material subjected to 807 K for 10.8 ks.