|Abstract: ||摘要 本論文主要是對β相鈦合金Ti-15V-3Cr-3Al-3Sn (BCC結構，以下簡稱Ti-15-3)經熱氫製程及時效處理後，探究其顯微結構與機械性質提昇之關連性。Ti-15-3合金先經由不同參數之熱氫製程處理之後，藉由機械性質的變化，探究最佳熱氫製程參數用以進行表面改質，再於不同溫度下進行時效處理。結果顯示，Ti-15-3合金在壓力150 Psi，溫度350oC，持溫4小時之氣態充氫後，經760oC，5小時氫化熱處理接著進行780oC脫氫處理1.5小時後，由於合金表面之晶粒細化而可得到最高之硬度(~499 Hv)與缺口拉伸強度(~1047 MPa)，而且其在不同環境溫度下之缺口拉伸強度均優於未經熱氫製程之母材。再者，經熱氫製程處理後之Ti-15-3合金，再經426oC時效處理後可以得到最高的硬度值(~506 Hv)，其優於僅經426oC時效處理之硬度值。相反地，其由於細小針狀的析出物並列在晶界處以及交織在晶粒內而導致穿晶劈裂與撕裂痕而呈現較差之室溫缺口拉伸強度。然而在450oC環境溫度下，由於溫度效應消弭了缺口脆性而使之具有高的缺口拉伸強度。其亦被注意到經熱氫處理而後時效的Ti-15-3試樣之硬度值雖隨著時效溫度升高而遞減，但衰退的程度明顯小於僅經時效處理的結果，即使經600oC時效後，其硬度值仍相當高(~475 Hv)。熱氫製程產生的晶粒細化效果在時效處理期間僅稍微衰退。此外，當Ti-15-3合金先進行熱氫製程再做540oC時效處理，在室溫到300oC環境溫度之間其呈現相當高的缺口強度，此由於粗的惠德曼析出物有助於晶粒內的滑移及阻礙裂縫的蔓延。因此藉由熱氫製程而後時效的方式，Ti-15-3合金可同時具有高硬度及強度。 關鍵字：Ti-15V-3Cr-3Al-3Sn、熱氫製程、時效處理、缺口拉伸強度。|
Abstract The research mainly aims to study the relationship between micro-structure and mechanical behaviors enhancement of phase tatanium alloys, Ti-15V-3Cr-3Al-3Sn (Ti-15-3), after undergoing thermohydrogen processing (THP) and then aging treatment. Ti-15-3 alloys were first carried out thermohydrogen treatment tested at distinct processings to evaluate the optimal conditions for surface modification. Subsequently, the aged treatment following optimal THP was performed at various temperatures ranging from 426 to 600oC for 8 hours. As a result, the Ti-15-3 specimen after thermohydrogen treatment hydrogenated at 350oC for 4 hours in 150 Psi of pure hydrogen and then heated to 760oC, hold for 5 hours in 1×10-5 torr vacuum, followed by dehydrogenation at 780oC for 1.5 hours, could achieve the peak hardness (~499 Hv) and notched tensile strength (NTS, ~1047 MPa) due to grain refinement around surface. Also, the NTSs of Ti-15-3 after optimal THP are all superior to the as-received specimen at elevated temperature. Moreover, the post THP specimen aged at 426oC attained the peak hardness (~506 Hv), which is better than the hardness only undergone aging treatment at 426oC. On the contrary, it shows the inferior NTS tested at room temperature (RT) owing to fine acicular precipates juxtaposed at the prior boundary and interlocked within the matrix resulting in transgranular cleavage and tear-ridge. However, under 450oC ambient temperature, it appears high NTS due to temperature effect diminishing notched brittleness. It is also noted that the hardnesses of Ti-15-3 specimen undergone THP and then aged treatment were declined with aging temperature increased, but the declined variation was not as apparent as the specimen only undergone aging treatment, even then aged at 600oC, it was still considerably high (~475 Hv). The influence of THP resulting in grain refinement was just degenerated slightly during aged treatment. Additionally, when Ti-15-3 alloys were first carried out THP and then aged treatment at 540oC, it exhibited relatively high NTS at the ambient temperature ranging from RT to 300oC due to coarse Widermanstatten precipitates benefical to slip in the grain interior and resist crack propagation. Consequently, by mean of THP and then aging treatment, Ti-15-3 alloys could possess simultaneously high hardness and strength. Keywords：Ti-15V-3Cr-3Al-3Sn, microstructure, thermohydrogen processing, aged treatment, notched tensile strength.