氧化鋯接合金屬高溫反應機構及其微觀分析

Abstract

本研究以兩種不同的中間層接合氧化鋯和金屬(鈦或不鏽鋼)，中間層分別為Ti/Ni/Ti三明治金屬箔或商用Ticusil，接合的溫度為900°C，以掃描式電子顯微鏡(SEM/EDS)、及穿透式電子顯微鏡(TEM/EDS)鑑定界面微觀結構，探討彼此間的差異性，並利用兩元或三元相圖，進一步闡述氧化鋯和金屬界面微觀結構的演化(Microstructural Evolution)及接合機構(Bonding Mechanisms)。

氧化鋯與鈦金屬以Ag68.8Cu26.7Ti4.5焊料(Ticusil)接合後，中間層在900°C會分割成兩種液相(一個為富銅-鈦相;一個富銀-銅相)並與基材反應。經900°C/0.1hr銲接，靠近氧化鋯的界面發現有兩層反應層(TiCu與TiO)，其中TiO是由氧化鋯被還原所產生。另外，靠近鈦側界面有三層(Ti2Cu/TiCu/ Ti3Cu4)反應層。Ti3C4是在潤濕金屬鈦表面產生，而樹枝狀或塊狀TiCu4則在凝固過程產生。最後，冷卻下來的中間層為銀的固溶體和TiCu4相。經900°C/1hr銲接，靠近鈦側有一連續鈦-銅反應層(Ti2Cu/TiCu/Ti3Cu4/ TiCu4)形成。中間層的銅被大量消耗，促使液相線(liquidus)溫度提高且改變中間層的組成由銀-銅共晶偏向銀的單相區。同時，被鈦還原出來的氧擴散進入中間層而產生三層反應層(Ti3Cu3O/Ti2O/TiO)。此外，Ti3Cu3O與Ti2O相生成是起因於相互擴散與化學反應。最後冷卻過程中發現Cu2O析出在銀的固溶體中。經900°C/6hr銲接，靠近鈦的反應層變為兩層(Ti2Cu/TiCu)，其中TiCu取代Ti3Cu4與TiCu4。冷卻後，經亞共析轉變在鈦側內部發現瘦長a-Ti(proeutectoid a-Ti)與層狀(a-Ti + Ti2Cu)結構。中間層則發現有Ti3Cu3O和Cu2O和銀的固溶體。最後，介面的擴散路徑在銀-銅-鈦及鈦-銅-氧三元相圖標示出來。

氧化鋯與不鏽鋼316L以Ti-Ni-Ti multilayer中間層在在900°C/1hr退火條件下接合，藉此分成三個反應區(Zone I, II, and III)以及兩個殘留的鈦箔來被討論。在不鏽鋼/鈦側(Zone I)界面有四層反應層(sigma phase/TiFe2/TiFe + b-Ti/Ti2Fe)，代表鈦與不鏽鋼的鐵反應。值得注意是sigma phase和分散於TiFe層中b-Ti是在冷卻過程出現。在鈦/鎳(Zone II)界面有三層(Ti2Ni/TiNi + Ti2Ni/TiNi3)反應層。Kirkendall voids出現在TiNi3/Ni的界面處是因為鎳擴散到鈦箔中。受限Ti在TiNi中飽和度關係，當溫度下降時會有第二相(Ti2Ni)析出。鈦箔/氧化鋯(Zone III)界面只觀察到一層反應層(TiO)，此層有效阻擋Zr原子擴散進入鈦箔中。最後，不鏽鋼/鈦與鈦/鎳之介面的擴散路徑在鈦-鐵及鈦-鎳兩元相圖標示出來。

近一步觀察兩側鈦箔，其溶質原子(鐵、鉻、鎳、氧)不同導致殘留鈦箔其微觀結構差異甚大。在不鏽鋼與鎳箔間的鈦箔，微觀結構為針狀a-Ti析出在b-Ti母材中，其方位關係為[0001]a-Ti//[01-1]b-Ti 與 (02-20)a-Ti//(211)b-Ti 。另外在b-Ti母材中也發現奈米等級的omega phase析出物，其方位關係為[1-10]b-Ti//[1-210]omega 與 (111)b-Ti//(0001)omega。這是因為不鏽鋼中的鐵、鉻、鎳固溶導致殘留鈦箔成份上改變。此外，利用立體模型來模擬b-Ti如何變形至omega phase。在鎳箔與氧化鋯間的鈦箔，其微觀分析b-Ti母材中有初析Ti2Ni(proeutectoid Ti2Ni)和層狀結構(a-Ti + Ti2Ni)，而生成原因是在冷卻過程中進行過共析反應;部份b-Ti則是因為固溶大量的Ni觸發Ti2Ni析出且附近區域為缺鎳區，在冷卻過程中變成a-Ti。此外，被鈦還原出來的氧原子有效抑制omega phase產生。

Zirconia-to-metal (commercially pure titanium or 316L stainless steel) joints were bonded with various interlayers isothermally at 900°C in an argon atmosphere. The distinct reaction layers between titanium and metal were investigated using analytical scanning electron microscopy (SEM) and analytical transmission electron microscopy (TEM) attached with an energy-dispersive spectrometer (EDS). Furthermore, the microstructural evolution and bonding mechanisms of the ZrO2/metal system were elucidated by the aid of binary or ternary phase diagrams.

The joints of ZrO2 and Ti were fabricated by an Ag68.8Cu26.7Ti4.5 interlayer at 900°C for various brazing periods. An interlayer would be separated into two liquids at 900°C (one is Cu-Ti rich liquid; the other is Ag-Cu rich liquid) and each liquid reacted with preferred substrate. After brazing at 900°C/0.1 h, two reaction layers (TiCu/TiO) existed at the Ticusil/ZrO2 interface, while the TiO layer resulted from the reduction of ZrO2 by dissolved Ti. In addition, three reaction layers, including Ti2Cu, TiCu, and Ti3Cu4, were observed at the Ti/Ticusil interface. The formation of Ti3Cu4 occurred during reactive wetting on the Ti surface, while that of dendritic or clumpy TiCu4 took plae during solidifactions in the residual interlayer. Finally, the residual interlayer was comprised of Ag solid solution and the TiCu4 phase after cooling. After brazing at 900°C/1 h, a sequence of Ti-Cu reaction layers (Ti2Cu/TiCu/Ti3Cu4/TiCu4) formed at the Ti/Ticusil interface. Thus, the consumption of Cu from the molten interlayer during brazing resulted in the increase of liquidus (>900°C) and the compositional deviations of interlayer from the Ag-Cu eutectic towards the Ag-rich region. Meanwhile, the reduced oxygen from ZrO2 by Ti diffusing into the interlayer produced three reaction layers such as Ti3Cu3O, Ti2O and TiO. Additionally, the formations of Ti3Cu3O and Ti2O resulted from the interdiffusion and chemical reactions. Finally, fine Cu2O precipitates existed along with Ag solid solution in the residual interlayer after cooling. After brazing at 900°C/6 h, only two reaction layers (Ti2Cu/TiCu) left at the Ti/Ticusil interface. The TiCu phase grew at the sacrifice of Ti3Cu4 and TiCu4 phases. The elongated a-Ti (proeutectoid a-Ti) and a lamellar structure of a-Ti + Ti2Cu were found due to the hypoeutectoid transformation on the Ti side. The residual interlayer consisted of Cu2O, Ti3Cu3O and Ag solid solution after cooling. Finally, the diffusion paths for the Ti/Ticusil and Ticusil/ZrO2 interface could be plotted through Ag-Cu-Ti and Ti-Cu-O ternary phase diagrams, respectively.

The joints between ZrO2 and stainless steel 316L were fabricated by a Ti-Ni-Ti multilayer after annealing at 900°C/1 h, whereby three interfacial zones (Zone I, II, and III) and two residual Ti foils were discussed. Four reaction layers, comprised of sigma phase/TiFe2/TiFe + b-Ti/Ti2Fe, existed in the zone I at the stainless steel 316L/Ti interface, indicated that Ti reacted with Fe from the stainless steel 316L. It was noted that the sigma phase and dispersed b-Ti (in TiFe) were precipitated during cooling. Three reaction layers, including Ti2Ni/TiNi + Ti2Ni/TiNi3, existed at the Ti/Ni interface (zone II). Kirkendall voids took place at the TiNi3/Ni interface because Ni diffused out into Ti foil. The secondary phase, Ti2Ni, would be precipitated from the TiNi matrix due to the solubility of Ti in TiNi with descending temperature. Only one reaction layer of TiO occurred at the Ti/ZrO2 interface (zone III), indicated that TiO acted as a diffusion barrier phase for Zr diffusion into the Ti foil. Finally, the diffusion paths for the interfaces such as 316L/Ti or Ti/Ni could be plotted through Ti-Fe and Ti-Ni phase diagrams, respectively.

Two residual Ti foils displayed distinct microstructures because varying solid solutions (Fe, Cr, Ni, and O) dissolved into two residual foils. The residual Ti foil between stainless steel 316L and Ni consisted of acicular a-Ti in the b-Ti matrix and their orientation relationships followed as bellow:[0001]a-Ti//[01-1]b-Ti and (02-20)a-Ti//(211)b-Ti. In addition, the nanoscale omega phase was alos observed during cooling in the b-Ti with orientation relationships [1-10]b-Ti//[1-210]omega and (111)b-Ti// (0001)omega . This is because the dissolutions of Fe, Cr and Ni from the stainless steel 316L lead to a compositional shift of the residual Ti. Moreover, the formation mechanism of beta → omega was elucidated using a model from the crystallographic viewpoint. The other residual Ti foil between Ni and ZrO2 consisted of proeutectoid Ti2Ni (p-Ti2Ni), granular a-Ti with dispersive Ti2Ni precipitates, and eutectoid colony with the lamellar structure (a-Ti + Ti2Ni). It was believed that hypereutectoid reaction led to the formations of p-Ti2Ni and lamellar structure of a-Ti + Ti2Ni. Meanwhile, the b-Ti → a-Ti was triggered following the precipitation of Ti2Ni during cooling. The oxygen solid solution from the reduction of ZrO2 by dissolved Ti could effectively suppress the formation of the omega phase.