錸元素添加對鎳基Mar-M247超合金顯微組織及機械性能之影響

Abstract

本論文旨在探討錸元素添加對鎳基Mar-M247超合金顯微組織及機械性能之影響。實驗結果顯示Mar-M247超合金中添加錸元素含量為0、1、3、5 wt.%時，其晶粒尺寸分別為90、70、60、50μm。隨著錸含量之增加，一次□′相被細化及立方體化會越為顯著。在□晶粒內之長條狀MC碳化物中的鉭及鈦含量，會隨著合金中錸含量的增加而逐漸減少。當添加3 wt.%錸元素時，晶粒內長條狀MC碳化物會分解成不連續狀M23C6碳化物，且少量拓撲密堆(TCP)有害相會開始在晶粒內形成。當錸含量增加至5 wt.%時，會因錸及鎢等元素嚴重的偏析，增加顯微組織之不穩定性，導致大量針狀P相在晶粒內形成。 室溫至中溫(760℃)拉伸測試結果顯示，添加3 wt.%錸可顯著提昇抗拉及降伏強度。760℃/724MPa潛變測試結果顯示，潛變性質主要由穩態潛變(潛變第二階段)所控制。與未添加錸元素合金相較，含3 wt.%錸之合金其穩態潛變速率下降約31%，而潛變壽命延長約63%。微硬度測試結果顯示，錸的添加具有提高γ/γ′基地強度的效果，其中又以含3 wt.%錸合金之γ/γ′基地強度最強。含3 wt.%錸之Mar-M247超合金其拉伸及潛變性能的提昇是因為錸具有細化晶粒及提昇γ/γ′基地強度的效果。然而，添加5 wt.%錸，會因晶粒內大量針狀P相的形成，造成拉伸及潛變性能的下降。破斷面分析結果顯示，Mar-M247超合金中錸元素含量為0–3 wt.%時，其拉伸及潛變之破裂模式是沿晶破裂；而錸含量為5 wt.%時，其拉伸及潛變之破裂模式是裂縫沿著晶界及針狀P相成長。 982℃拉伸及982℃/200MPa潛變測試結果顯示，拉伸性能及潛變壽命會隨著錸在Mar-M247超合金中含量的增加而提昇，至3 wt.%錸含量時達到最大值。針對晶界碳化物形貌統計分析結果顯示，隨著錸含量增加，晶界碳化物的尺寸會變的越小而其數量會變的越多，而此晶界碳化物的演變，有助於晶界強度的強化。對拉伸性能而言，拉伸強度提昇之原因是錸具有增加晶界細小碳化物數量及提昇γ基地強度的效果。而γ基地強度提昇的原因是錸具有固溶強化γ基地的效果。就潛變性能而言，與未添加錸之合金相較，含3 wt.%錸之合金其潛變壽命延長約2–3倍。研究發現添加1–3 wt.%錸時，會因：(1)立方體一次γ′相(最佳化形貎γ′相)數量的增加、(2)促進γ′竹筏狀的發展及(3)增加γ基地強度等三項因素，降低合金之穩態潛變速率，延後潛變由穩態潛變階段進入加速(第三)潛變階段之時間。另晶界碳化物的細化及數量增加，會阻止裂縫沿著晶界成長，延長加速潛變階段的時間。綜上因素，促使含1–3 wt.%錸之合金其潛變壽命延長。而添加5 wt.%過量的錸，會因P相的形成造成拉伸及潛變性能的下降。 經由本論文之研究及探討證實，顯示添加3 wt.%錸是能維持Mar-M247細晶超合金顯微組織穩定和能提昇中高溫機械性能之最佳添加量。

The effects of rhenium (Re) addition on microstructures and mechanical properties of Ni-base Mar-M247 superalloy were systematically investigated. The experimental results indicated that the grain size of the Mar-M247 superalloy with 0, 1, 3, and 5 wt.% Re content was 90, 70, 60, and 50 μm, respectively. The primary γ′ phase became finer and more cuboidal as Re content increased. The concentrations of Ta and Ti in strip-like MC carbides existing within the γ□grains decreased as Re increased. The addition of 3 wt.% Re caused MC carbides within the grain to decompose into discontinuous M23C6 carbides and initiated the formation of a deleterious topological closed-packed (TCP) phase within the grain interior. The addition of 5 wt.% Re further promoted phase instabilities that led to the precipitation of large amounts of needle-like P phase within the grain interior, attributable to Re and W segregation. The results of tensile tests carried out at temperatures ranging from room temperature to 760°C showed that the addition of 3 wt.% Re significantly improved the ultimate tensile strength and yield strength. Creep test performed under the condition of 760°C/724MPa showed that the creep properties were primarily dominated by steady-state creep. The steady-state creep rate and creep life of alloy with 3 wt.% Re was respectively reduced by 31% and prolonged by 63%, as compared to those of the alloy without Re. Microhardness tests showed that addition of Re could enhance the strength of the γ/γ′ matrix, and the strongest γ/γ′ matrix could be obtained in the alloy with 3 wt.% of Re addition. The improvement in tensile and creep properties was associated with a decrease in grain size and an increase in the strength of γ/γ′ matrix caused by Re addition. However, by increasing the Re addition to 5 wt.%, degradations in both of the tensile and creep properties were obtained, presumably being due to the formation of needle-like P phase within the grain interior. Fracture analysis demonstrated that during tensile and creep tests, cracks initiated and propagated along grain boundary in 0–3 wt.% Re-containing alloys; however, in the alloy containing 5 wt.% Re, cracks initiated and propagated along both grain boundary and the P phase. The results of 982□C tensile and 982□C/200MPa creep tests showed that both of the tensile properties and creep life increased with increasing the Re content up to a maximum at 3 wt.%. Quantitative statistical analysis showed a decrease in the size of grain boundary carbides and an increase in the number of grain boundary carbides as the Re content was increased. This grain boundary carbide evolution is conducive to the promotion of grain boundary strength. The tensile strength increased with increasing the number of fine grain boundary carbides and the strength of γ□matrix. The improvement of the γ matrix strength was attributed to the solution strengthening effect resulted from the Re addition. The creep life of the alloy with 3 wt.% Re was extended by 2–3 times more than that of the Re-free alloy. The addition of 1–3 wt.% Re reduced steady-state creep rates and postponed the onset of the acceleration stage in three ways: (1) by increasing the amount of primary cuboidal□γ′ phase; (2) by increasing the development of□γ′ raft; and (3) by increasing the strength of γ□matrix. An increase in the duration to the accelerated creep stage was caused by both carbide refinement and an increase in the number of grain boundary carbides prevented cracks from propagating along the grain boundary. The above mentioned factors resulted in a prolongation of creep life. However, addition of excessive amounts of Re, such as 5 wt.% Re, causes deterioration of the tensile and creep properties due to the formation of P phase. Consequently, the present study has critically identified that 3 wt.% of Re addition is the optimum concentration for obtaining fine-grain Mar-M247 superalloy with superior microstructural stability and much enhanced mechanical properties at moderate and elevated temperatures.