| Summary: | 碩士 === 中原大學 === 機械工程研究所 === 92 === Abstract
The MAR-M432 superalloys employed in this study were prepared by vacuum induction melting and followed by precision casting into test bars. Both coarse and fine grain size structure test bars were obtained by means of controlling the pouring parameters and hot isostatic pressing (HIP) treatment and heat treatment. Thus, their effects on microstructure and mechanical properties of the MAR-M432 superalloys were investigated.
From this experiment, it was found that an average size of 100μfine grain（ASTM 3.5）equiax microstructure can be obtained under 20℃superheat pouring temperature. It is further found that the effective micro-shrinkage porosity, which is often observed in the fine grain microstructure, can be minimized from 0.38% to 0.14% using 1140℃/20Ksi/4hrs HIP treatment. Consequently, the mechanical properties of the test bars with HIP treatment are greatly improved. For examples, the associated ultimate tensile strength (UTS) and elongation values were found to increase 6.8% and 69.9% at room temperature, 4.3% and 29.1% at 8000F, and 2.6% and 33.3% at 14000F.
It is also found that increasing the temperature of solid solution treatment can spheroidize the γ' precipitates and make them even finer. These effects can also reflect on the mechanical properties of the MAR-M432 superalloys. The room temperature UTS for a standard 1090℃heat treatment is 1240MPa; however, for the heat treatment temperature of 1140℃, which is the best heat treatment condition in this experiment, there is a 5.6% increment in the ultimate tensile strength, that is 1310MPa. Similar results can also be observed in the test bar of 1170℃heat treatment temperature, in which there is a 4.0% increment in the UTS compared to standard temperature 1090℃heat treatment. Tensile test at higher temperature also confirms this effect. It is found that, at 8000F, both the test bars heat treated at 1140℃ and 1170℃give a 4.1% increase in the tensile strength compared to standard temperature 1090℃heat treatment.
In this study, the drawback found in the MAR-M432 superalloy is its elongation property. Although a large number of micro-shrinkage pores can be eliminated through the HIP treatment, the elongation of all the test bars is still less than 5%, which is even lower than the minimum requirement for aerospace applications. TEM analysis suggests that the main reason for this phenomenon may due to Ni-Co-Cr brittleσphase precipitation. Fracture surface analysis also confirms that cracks initiate from brittleσphase, and propagate along σ phase exhibiting a zigzag type crack lines and extended to all area.
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