鈦基內造強化複合材料製程及其機械性質之研究

Abstract

本研究的目的主要是以燃燒輔助合成的製程生成具內造強化材之鈦基複合材料，並研究其靜態與動態的機械性質。藉由結合放熱反應和真空電弧重熔的燃燒輔助合成法已成功地製作了TiC/Ti及TiB/Ti兩種複材，將其以不同程度的熱旋鍛後，再以影像分析探究此熱機處理對其微觀組織的影響。除了測試其扯伸與壓縮性質之外，並且 (1) 於扭伸時同步以電子顯微鏡觀察及 (2) 做試驗後破壞表面的定量分析來描述此複材從298到873K的破壞行為。潛變與疲勞裂縫成長試驗是依據ASTM的標準規範進行測試。此研究的結果經歸納如下： 此製程製作出具5至20%體積分率內造強化材的TiC/Ti及TiB/Ti複材，藉由高解析穿透電子顯微鏡觀察和破壞行為分析可以得知其優越的強度與延性是歸因於其強化材具有潔淨的界面與良好的鍵結強度。由金相觀察及定量分析結果顯示內造強化材顆粒尺寸隨著其體積分率的增加而變大，這是由於兩個系統的絕熱溫度會隨著其體積分率的增加而增高所致。根據對TiC/Ti系統熱機處理效應的研究結果顯示，經熱旋鍛後的顆粒間距 (λ) 與熱旋鍛比 (Ｒ) 之間存在一個線性的關係為 λ(μm)＝18.57－0.21Ｒ (Ｒ≧4) 因為在熱機處理期間碳化鈦顆粒被細化且其顆粒間距縮小，此材料的室溫及高溫強度都因此而明顯地強化。由室溫時強度與顆粒間距倒數的正比關係指出此複材的強化是與Orowan機制有關。基於拉伸時電子顯微鏡的同步觀察結果得知，TiC/Ti在室溫時拉伸破壞的過程包括強化材自基地突起、小顆粒破裂、大顆粒破裂及大裂縫的連結粗化導致最終破壞等階段；且破壞機制隨溫度升高而改變，由室溫的顆粒破裂轉換為高溫的顆粒問孔隙主導破壞，其轉換溫度為645Ｋ。於測試條件下，此複材的潛變機制被證實與基材純鈦一致，是格子擴散控制下的差排爬升。經過強化材體積分率及其尺寸大小效應的分析結果，對於內造材強化鈦基複合材料的潛變關係式可以表示 於應力比Ｒ＝0.1及相同外加強度應力因子範圍的條件下，複材在723K下的疲勞裂縫成長速率比在298K時大約低了一個數量級，這與基材金屬的疲勞裂縫成長速率隨溫度升高而劣化加速的趨勢相反。由裂縫閉合效應修正後所有疲勞裂縫成長速率重疊的一致性來看，微裂縫引起的裂縫閉合效應可以歸結為高溫時複材之抗疲勞裂縫成長能力強化的原因。

The main objective of this dissertation is to study the processing and mechanical behavior of titanium matrix in-situ composites produced by combustion- assisted synthesis. Both systems of TiC/Ti and TiB/Ti were successfully fabricated by combustion-assisted synthesis which consisted of exothermic reactions and vacuum arc remelting. The effects of thermomechanical process (TMP) by hot-swaging on microstructure were explored. In addition to evaluation of tensile and compressive properties, fracture behavior was characterized by in-situ SEM observations and quantitative analyses of fracture surface from 298 K to 1022 K. Creep and fatigue crack propagation tests were carried out according to ASTM specification. The results in this study were summarized as follows： Both TiC/Ti and TiB/Ti composite systems with 5 to 20 vol. % in-situ reinforcements demonstrated a superior strength and ductility. This was attributed to the clean interface and good bonding strength of reinforcements as revealed by HRTEM observations and analyses of fracture behavior. The particle size of reinforcement increased with increasing volume fraction of reinforcement since the adiabatic temperature, the raised temperature for products due to heat of chemical reaction under adiabatic condition, increased with increasing volume fraction of reinforcement for both TiC/Ti and TiB/Ti systems. A linear relationship between interparticle distance (X) and reduction ratio of hot-swaging (R) for 10% TiC/Ti was established and expressed by λ(μm)＝18.57-0.21R (R≧4) Due to the refinement of TiC and the decrease of interparticle distance by TMP, tensile strength (σ) at both ambient and elevated temperatures can be significantly enhanced. The relation of σ in proportional to l/λ indicated that the strengthening mechanism of titanium matrix in-situ reinforced composite at room temperature followed the Orowan mechanism. Based on the in-silu SEM observations, the tensile fracture process at room temperature for TiC/Ti composite consisted few stages, including reinforcement protrusion, cracking in small particles, cracking in larger particles and coalescence of large cracks. There is an obvious transition temperature at 645 K for fracture mechanism being changed from particle cracking to interparticle voiding. The creep mechanism of present titanium matrix in-situ composites, which was proved to be diffusion controlled dislocation climb, was identical to that of Ti matrix. All creep data of pure Ti and composites were merged together, which had the true stress exponent of 4.2 after compensating effects of the modulus, threshold stress and particle size. A constitution equation of creep for present Ti-based in-situ composites was proposed as The fatigue crack growth rate (FCGR) of titanium matrix in-situ composite at 723 K was approximately one order lower than that at 298 K for same applied stress intensity range under same stress ratio of R=O.1, showing a disparity tendency in comparison with matrix metal. All data of FCGR merged very well after crack closure correction. The difference in fatigue crack growth behavior at room and elevated temperatures was interpreted by crack closure. The mechanism of microcrack-induced crack closure was proposed to account for improvement of fatigue crack growth resistance at elevated temperatures.