- 學位論文
MAR-M432超合金細晶鑄造後熱均壓及熱處理過程對顯微組織及機械性能 影響之研究
Effect of Hot Isostatic Pressing and Heat Treatment on Microstructure and Mechanical Properties of Cast Fine-Grain MAR-M432 Superalloy
摘要
摘 要 本研究係以MAR-M432超合金為素材，以真空精密鑄造爐，利用控制澆鑄參數法，澆鑄成粗晶及細晶試桿後，進行熱均壓處理及熱處理過程，對MAR-M432超合金顯微組織及機械性能影響探討研究。 利用控制澆鑄參數法，控制澆鑄溫度則會具有晶粒細化的結果，傳統的過熱溫度150℃，可得晶粒大小為3mm之柱狀粗晶，降低過熱溫度至20℃，確實可得到ASTM晶粒大小為100μm之細晶。利用1140℃/20ksi/ 4hrs之熱均壓處理條件，可有效降低微縮孔含量由0.38％減少至0.14％，使細晶室溫拉伸之抗拉強度提昇6.8%，延伸率提昇69.9％。427℃拉伸之抗拉強度提昇4.3%，延伸率提昇29.1%，在760℃拉伸之抗拉強度提昇2.6%，延伸率提昇33.3％，由以上之測試結果可知，細晶試桿經過熱均壓處理可有效提昇其拉伸強度及延伸率。 此外提高固溶溫度可細化及球化γ’析出相，在室溫拉伸方面之抗拉強度為1240 MPa，固溶溫度為1140℃時較規範固溶溫度1090℃之抗拉強度提昇5.6％，固溶溫度為1170℃時抗拉強度較規範1090℃之固溶溫度提昇了4.0％。在427℃拉伸亦有同樣之結果，固溶溫度為1140℃及1170℃較規範固溶溫度1090℃，同為提昇了抗拉強度4.1％。且當固溶溫度為1140℃時，室溫的拉伸抗拉強度高達1310 MPa，為最佳之熱處理條件。 由本研究中得知MAR-M432超合金在熱均壓處理後之細晶拉伸試桿測試結果，發現延伸率仍是一大弱點，並非是大量的微縮孔的因素，而使得延伸率全部都低於航空規範5％之要求。經TEM分析研判證實為Ni-Co-Cr之σ脆性相，且由破損分析可知拉伸破斷之起始點，均是由σ相開始破裂，且繼續沿著σ相斷裂，呈之字型的裂紋及擴展型態，其破斷模式為σ相脆性破斷之型態。
並列摘要
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.