鈦及鈦合金為航太、化學及能源工業的骨幹材料，也是極具潛力的生醫材料。這是由於鈦材料具備高比強度、優異的機械性質、抗腐蝕性、良好電磁遮蔽性、不具磁性及抗低溫的特性。然而，製造鈦合金零組件所需的高成本卻限制了鈦材料的應用。因此，發展淨形(net shape)或近淨形(near net shape)技術，用以提高鈦材料或組件的使用。目前精密鑄造是發展最完全，也是使用最廣的淨形技術。鈦鑄造的另一個特色是鈦的鑄件在各方面的性質可以與鍛造件匹敵，因此，鈦鑄造技術的發展有其必要性。

　　鑄造技術需要克服的是各種成因的鑄造缺陷，評估鑄造成功與否的指標即是鑄造性。在諸多影響鑄造性的因素中，本研究選擇以固定製程來控制外秉因素及鑄模、設備等因素，從而瞭解添加微量元素對鑄造性的影響。本研究並且探討添加微量元素對結構、組織或機械性質的影響，俾使所選擇的添加元素對商業用純鈦或鈦合金的製程和性質有整體的助益。

　　結果顯示，微量的合金元素添加到商業用純鈦時，部分元素造成鈦的鑄造性增加，部分元素則使鈦的鑄造性減低，視添加的元素種類而定。鑄造性的總體趨勢與表面能有關，特別是針狀模更明顯。在添加量的影響方面則以凝固機構（樹枝狀凝固）為主導。因此，添加元素對鑄造性表現的影響和差異是由自由能降低與樹枝狀凝固的競爭、妥協的結果。微量添加X元素(1wt%)對Ti-7.5Mo合金鑄造性的改善(34%)比添加X元素於商業用純鈦明顯。在Ti-6Al-4V的鑄造性方面，添加1wt%的X元素可以大幅增加Ti-6Al-4V合金的鑄造性達30%。商業或準商業鈦合金添加1wt%X元素大幅改善了所有測試的鈦合金的鑄造性，其中以Ti-7.5Mo-Xfe系統的鑄造性改善最為明顯（分別為79%、82%和115%）。

　　商業用純鈦和所有添加元素的鈦合金鑄造於銅鑄模或石墨模後都有著相同的晶體結構(hcp)。鑄造後的商業用純鈦具有板狀的α’相，當添加合金元素時，所有的鈦合金都具有類似的細微針狀麻田散結構，而且隨著添加量的增加，針狀組織變得更細。Ti-7.5Mo合金為斜方晶α”相。添加1wt%或3wt%的X元素，對Ti-7.5Mo合金的相沒有明顯的影響；添加5wt%的X元素，發現一定量的β相在鑄造過程的快速冷卻中被保留下來。Ti-7.5Mo合金呈現細羽毛狀組織。添加1wt%的X元素，合金仍有相似的組織；添加3wt%和5wt%的X元素，出現β相的晶界。鑄造的Ti-6Al-4V合金為α’相，添加X元素沒有改變Ti-6Al-4V合金的相結構。不論是否添加X元素，Ti-6Al-4V合金的顯微組織都是細針狀的組織。

　　所有鈦合金的微硬度值比商業用純鈦高很多，而且Ti-Mo系列合金明顯地有特別高的微硬度值。添加X元素對Ti-7.5Mo合金的效應比較不明顯。Ti-6Al-4V合金的微硬度值隨X元素含量的增加逐漸增加。所有鈦合金系統的彎曲強度值皆大於商業用純鈦，Ti-Mo合金系統是所有合金系統中彎曲強度水準最高的。添加合金元素均使鈦的彈性回復角增加，增加幅度約在60~80%， Ti-Mo系統是所有合金系統中彈性回復角最高的（增幅約在150-280%）。添加X元素對Ti-7.5Mo合金對彎曲強度的影響比較不明顯，趨勢類似於微硬度值的結果。Ti-6Al-4V合金的彎曲強度隨著X元素含量增加而上升，彎曲彈性模數也是隨著X元素含量增加而上升。增加X元素添加量(>3wt%)會使Ti-6Al-4V合金的延展性降低。

　　進行腐蝕性測試的合金系統(Ti-X、Ti-Sn、Ti-Mo和Ti-Nb)都有優異抗腐蝕性。

　　X元素是本研究選用的添加元素中對商業用純鈦的鑄造性改善最明顯的元素，微量添加(1wt%或以下)於Ti-7.5Mo合金、Ti-6Al-4V合金或其他商業或準商業用的鈦合金時，對鑄造性的改善更為顯著。綜合而言，微量添加X元素於鈦或鈦合金在工業應用上，具有實用的價值，並是可立即應用的。

　　Due to their high strength-to-weight ratios, excellent mechanical properties and corrosion resistance, titanium and titanium alloys have become one of the backbone materials for many aerospace, energy, sports, marine and chemical applications. Titanium is also finding growing acceptance today for uses medical equipment and implant material, dental (crown and bridge, framework, dental implant, etc.) and orthopedic applications due to its excellent biocompatibility.

　　Titanium castings are unlike castings of many other metals in that they are equal, or nearly equal, in strength to their wrought counterparts. Fracture toughness and crack-propagation resistance may equal or exceed those of corresponding wrought materials.

　　The relatively high costs of titanium/titanium alloy products, however, could largely limit their use. This high cost is a result of the high costs of raw material as well as fabrication, particularly the machining process. For example, the raw material of titanium may cost from 3 to 10 times as much as steel or aluminum, and the machining costs for titanium can easily be 10 times as much as that for aluminum. Because of the high machining costs of titanium, a great deal of effort has been devoted to the study of precision casting of titanium.

　　Nevertheless, titanium is inherently difficult to cast due to its high melting temperature, strong affinity with gases such as oxygen, hydrogen and nitrogen, as well as its high reactivity with most investment materials. These difficulties can more or less affect the castability and mechanical properties of Ti and Ti alloy castings.
The present study is an attempt to improve the castability of titanium through addition of a series of alloying elements into titanium. The effect of the alloying additions on such mechanical properties as bending strength and modulus was also investigated. Along this direction, the research continued to study the effect of bismuth addition (up to 5 wt%) on castability, structure and mechanical properties of c.p. Ti, the most popularly-used Ti-6Al-4V alloy and Ti-7.5Mo alloy.

　　The results are listed as follows:
1. A minor amount (1 wt%) of alloying element could increase or decrease the castability of titanium, depending on the kind of element added. Among all alloying elements, silver, molybdenum, hafnium, tantalum, tin, lead and X element significantly enhanced the castability of titanium cast in plate copper mold. Additions of silver and X element increased the castability value by 32% and 35%, respectively. When graphite mold was used, the improvement in castability was less significant for most added elements, yet X-doped titanium still exhibited a castability value higher than that of c.p. Ti by 24%.
2. All 17 doped titanium had the same crystal structure (hcp) as that of c.p. Ti, with the only exception of iron-doped titanium, in which a slight amount of phase was retained. Optical microscopy showed that all doped titanium had a fine acicular martensitic type structure. Cast c.p. Ti had a coarser, lath type α’ phase morphology. Addition of X resulted in a finer, acicular morphology.
3. Alloying additions largely increased microhardness of titanium. Molybdenum and manganese-doped titanium had the highest microhardness values (242 and 237 HV, respectively). All other doped titanium had microhardness values in the range of 190-220 HV.
4. Alloying additions significantly increased bending strength of titanium. The largest bending strength was found in molybdenum-doped titanium (1574 MPa), that was roughly double that of c.p. Ti (760 MPa). The bending modulus of titanium, however, was not much affected by alloying additions.
5. With addition of 1wt% alloy element, X and Mo were most effective in enhancing the castability of titanium cast in graphite mold with needle-shaped cavities. Ta, Nb, Zr, Sn and Ag only increased castability slightly. With more alloy elements added, except Hf, the castability values more or less decreased.
6. Addition of up to 5wt% alloy element did not changed XRD pattern of c.p. Ti, indicating that doped X was present primarily in the form of solid solution. Except Ti-Mo system, all Ti alloys with a fine acicular morphology had the same crystal structure (hcp) as that of c.p. Ti with a typical lath type morphology. When 3 wt% or more Mo was added, a finer orthorhombic α" phase was formed. Cross-sectional microscopy and microhardness measurement showed that the acicular structure of Ti alloys was finer near surface, but there was no significant reaction layer or alpha case formed on the surface of c.p. Ti or any Ti alloy.
7. The microhardness and bending strength values of Ti alloys were all higher than those of c.p. Ti. Among all alloys, Ti-Mo system exhibited the highest hardness and strength level. For a certain alloy, the bending strength did not necessarily increase with its alloy content. Except Ti-5Zr and Ti-Mo alloys, the bending moduli of most alloy systems were not much different from that of c.p. Ti. The decreased modulus with Mo content is consistent with XRD and morphological results.
8. All alloys showed an excellent resistance to corrosion in Hanks’ solution at 37°C. Breakdown did not occur to any material even at potentials as high as 2400 mV (SCE).
9. Addition of 1 wt% X increased the cast lengths of Ti-7.5Mo alloy by 34%, respectively. Higher X contents caused castability to decline.
10. Addition of 5wt% X caused some β phase to be retained from high temperature during casting.
11. Cast Ti-7.5Mo alloy exhibited a fine feather-like morphology. Addition of 3wt% or more X revealed high temperature β phase grain boundaries.
12. The X element effect on microhardness of Ti-7.5Mo was less significant.
13. Additions of 1 and 3wt% X increased the bending strength of c.p. Ti by 17 and 50%, respectively. Addition of 5wt% X did not further increase the strength level. The bismuth effect on bending strength of Ti-7.5Mo was less significant.
14. Ti-7.5Mo had a much lower modulus than c.p. Ti. Addition of X did not affect much on modulus.
15. The addition of 1 wt% X in Ti-6Al-4V alloy not only largely enhanced castability, the strength of the alloy was also improved. However, when larger amounts of bismuth were added, the ductility of the alloy decreased. From a practical point of view, the 1X-doped alloy has demonstrated its potential for practical application, especially for casting manufacturing process.
As a summary, X element demonstrated a particularly high potential in enhancing the castability of titanium among all the dopants.

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