時效處理對Ti-15V-3Cr-3Al-3Sn鈦合金機械性質之影響研究

Abstract

本研究係針對Ti-15V-3Cr-3Al-3Sn合金(Ti-15-3合金)，進行一段(317~648℃/ 8 h)及二段(317~648℃/ 8 h + 426℃/24 h)時效處理，探討時效處理對於顯微組織與機械性質之影響。機械性質測試項目包括：一般拉伸、缺口拉伸、J積分與疲勞裂縫成長試驗。時效處理促使Ti-15-3合金於β相基地析出α相，而產生硬化效應。實驗結果顯示：317℃/8 h一段時效處理試片(A317)之析出硬化不明顯，需經371℃/8 h (A371試片)處理，方可觀察到硬化現象。A426試片可獲一段時效條件中之最大硬度(Hv 428)，超過482℃則產生過時效，硬度會隨時效溫度提高而逐漸下降。一段低溫時效者，再經426℃/ 24 h時效處理，會使一段析出物成長，並可達尖峰硬度約Hv 460。D648試片(648℃/8 h + 426℃/24 h)在二段時效過程中，會再析出細微α相，相較於A648試片有額外硬化效應。另一方面，D538與D593二段時效試片，分別與相對應之A538與A593一段時效試片相較，硬度差異並不明顯，唯可觀察到析出物粗大化現象。 無論經一段或二段時效，硬化效應較高之試片，其拉伸性質都具有高抗拉強度及低缺口拉伸強度，且延展性較差，斷面則呈現脆性破裂。高溫過時效者，除D648試片外，其餘試片均有較佳之延伸率與缺口強度比(Notch strength ratio)，斷面可觀察到窩穴組織及晶界滑移現象。二段時效試片之抗拉強度較相對應之一段時效者為高，但延伸率與缺口強度比則較低。二段時效試片之J積分值(J-integral value，破壞韌性指標)，除D648試片外，均隨第一段時效溫度上升而增高，影響J積分值之主要原因為試片延展性；延伸率較佳者，具有較高之J積分值。此外，D648試片有細小之α相析出，導致脆性增加，故其J積分值降低。另一方面，疲勞試驗結果亦顯示，疲勞裂縫成長速率與J積分值有相關性；破壞韌性愈佳者，其裂縫成長速率則有愈慢之趨勢。

This study investigated one-step (317~648℃/8 h) and two-step (317~648℃/8 h + 426℃/24 h) aging treatments to the Ti-15V-3Cr-3Al-3Sn alloy (Ti-15-3 alloy). The effects of aging treatment on the microstructure and mechanical properties were also evaluated. The mechanical tests involved the tensile, notch tensile, J-integral and fatigue crack growth tests. For the Ti-15-3 alloy, the α phase precipitated from the β matrix and resulted in hardening during the aging treatment. Experimental results indicated that the hardening effect was insignificant for the 317℃/8 h (A317 specimen) treatment. Precipitation hardening could be observed for the one-step aging treatment at 371℃ (A371 specimen) or higher than that. In one-step aged specimens, the highest hardness could be obtained for the A426 specimen with a value of approximately Hv 428. Over-aging occurred when the temperature was higher than 482℃ for one-step aged specimens, and the hardness gradually decreased as the aging temperature increased. The second aging treatment (426℃/24 h) increased the hardness and the size of α precipitates for the specimens which were previously aged at temperatures equal to or below 426℃ in the one-step aging treatment. For higher one-step aging temperatures, e.g., 648℃, the second aging treatment caused more precipitation of fine α, leading to further hardening of the D648 specimen (648℃/8 h + 426℃/24 h) relative to the A648 specimen. On the other hand, the D538 and D593 (two-step aged) specimens exhibited similar hardness and coarser α precipitates compared to the A538 and A593 (one-step aged) specimens, respectively. In general, the specimen with a pronounced effect of age hardening had high tensile strength and low notched tensile strength, regardless of the aging conditions. Furthermore, the ductility was also relatively poor, and brittle fractures were observed for such specimens. The specimens in the over-aged conditions showed high elongation and notch strength ratio, except the D648 specimen. Dimple and grain boundary sliding were often observed on the fracture surfaces of the over-aged specimens. Compared to the one-step aged specimens, the two-step aged specimens had lower notched tensile strength and elongation. Other than the D648 specimen, the J-integral value increased as the first aging temperature increased of the two-step aged specimen. The ductility was also found to be the main factor to affect the J-integral value, i.e., the higher elongation, the higher J-integral value. The formation of fine α precipitates during the second stage aging of the D648 specimen led to increased brittleness and decreased fracture toughness. On the other hand, the results of fatigue crack growth tests also revealed that those specimens with higher fracture toughness exhibited slower crack growth rates.