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Investigation on Microstructural Evolution and Mechanical Property Relationship of a New Beta Ti-2Al-9.2Mo-2Fe Alloy

李成林  
【摘要】:Titanium alloy has widely been used as structural components in the aerospace field in the past decades due to their high specific strength at intermediate temperatures. Since the1970s, excellent corrosion resistance of titanium has been discovered. Therefore its usage in the marine and petrochemical industries is sharply increased. Benefiting by their excellent combined properties, titanium alloys are increasingly attractive for usage in a variety of applications such as automobile, biomedical devices, sporting goods and leisure products, etc. However, because of its relatively high price compared to conventional structural materials it is basically used only when it is the only choice. Especially it is limited in civilian areas such as automobiles. Automotive applications have been a prime target for titanium and its alloys for the last two decades. Alloy development work to allow maximum use of recycled units and less expensive raw materials has been one of the major activities performed by titanium manufactures. TIMETAL LCB (Ti-6.8Mo-4.5Fe-1.5Al) alloy is a good example of an alloy designed for automotive applications, especially for the springs. The key to its success is to take advantage of low cost ferro-moly master alloy additions to reduce formulation cost. However, as a spring material it should be of low modulus as well as low density. Although the TIMETAL LCB alloy has low cost and high strength level, its high elastic modulus (about90GPa) is not good enough for spring application. On the other hand, high content of Fe addition (4.5%) is readily to form segregation during melting and cannot be eliminated during subsequent thermo processing or heat treatment. Because of that, it is not perfect for using in the spring application. Thus, new alloys are expected to be designed and developed based on the TIMETAL LCB alloy to meet the needs (low cost, low modulus and high strength) of automotive spring. Hence, the objective of this study is primarily to develop beta titanium alloys with low elastic modulus and high strength level for automobile spring usage. The first target is to realize cost reduction for automotive usage by means of adding low cost alloying elements. Thus low-price Fe-Mo master alloy is considered as alloying additions. Based on Ti-Mo-Fe system, the second aim is to realize a low modulus alloy design. Finally, the goal is to obtain alloy strengthening to meet the desired demands of the spring application according to proper thermo-mechanical processing and heat treatment. The focus of the present study was directed at designing a new beta Ti-Al-Mo-Fe alloy with low modulus and high strength for an automotive spring material on the basis of the Bo-Md method, and then investigating the microstructural evolution during hot deformation and heat treatment, and microstructure-property relationship for the newly designed Ti-AI-Mo-Fe alloy. Primary investigations were conducted to develop a Ti-Al-Mo-Fe alloy with low elastic modulus and high strength. Alloy design and modulus prediction were performed with the Bo-Md method based on the DV-Xα molecular orbital method.42alloy compositions were considered as candidate alloys under the molybdenum equivalency (Moeq) will be15that equal to the Ti-15Mo alloy. After property prediction among42alloy compositions with the method, eight compositions (1%Fe and2%Fe contained) were selected as secondary alloys to be optimized. Laboratory scaled ingots of the eight compositions are melted by ISM (Induction Skull Melting) for further experimental property evaluation and optimization. The results showed that the elastic modulus decreases with increasing the Al content and decreasing the Bo and Md values. The elastic modulus of the Ti-Al-Mo-Fe alloy system could be reduced to55~80GPa, which was much lower than common commercial β titanium alloys. The Ti-2Al-9.2Mo-2Fe alloy was optimized among the secondary8compositions according to property evaluation (low modulus and high strength). The alloy had an elastic modulus of65GPa and ultimate strength above800MPa in solution treated condition. Subsequently, the alloy was considered as a target material for the subsequent study. It has been proved that the Bo-Md method is effective to design a beta titanium alloy with low elastic modulus and high strength. As a new designed alloy, the microstructural evolution and flow behavior during hot deformation were firstly studied to find an appropriate hot work processing for the alloy. By using the hot compression test, the samples were hot compressed at temperatures ranging from750℃to1000℃, covering the α+β and β phase region. Then, a study was performed to understand the β grain growth behavior of the alloy during solution treatment with different holding temperature and time. The grain growth exhibits no remarkable rise as time increases (grain size about50μm) at low temperatures below900℃. It also means that there is no significant grain coarsening when solution treated at temperatures lower than900℃within4hours, which insures the grain size to be a level of about50μm. Thus, here it provides a basic guidance on solution treatment of the Ti-2Al-9.2Mo-2Fe alloy in the future. Similar studies were conducted to understand the effect of cooling rate from the β phase region on the beta phase decomposition of the alloy. The microstructure and phase transformation of the alloy were investigated when the alloy was solution treated at temperatures ranging from820℃to1000℃followed by water quench, oil quench, air cooling, step quench and furnace cooling. The athermal ω phases are found in all the temperature range in conditions of water, oil quench and air cooling by TEM observation. It means that the alloy exhibited low quench sensitivity. Although the detailed microstructural characteristics with the cooling rates are not known, the athermal ω phases are said to providing nucleation sites for the α precipitations during subsequent aging treatment and thus leading to uniform distribution in the beta matrix. Finally in order to find out the effect of microstructural characteristics on the mechanical properties of the alloy, a variety of thermal experiments were taken up. The main objective of solution treatment and aging was to find optimal heat treatments to obtain attractive property combination, such as low elastic modulus and high strength with good ductility. The results showed that the alloy in solution condition has quite low elastic modulus about60~80GPa and excellent ductility (20~43%of elongation) with a moderate strength level (800~1000MPa). Therefore, the next objective was to find out proper aging treatments to strengthening the alloy while insuring the low elastic modulus. Then, the alloy in both the α/β and β solution condition were aged at400℃to600℃for2hours, to find the optimal aging condition. Meanwhile, the microstructure-property relationship was discussed to reveal the effect of secondary precipitations on the mechanical properties. The results showed that, although the isothermal ω phase was less common in beta alloys when treated in the α/β-ST and aged at lower temperatures. It was detected in the alloy with the α/β-ST and aged at low temperatures (400℃~450℃). The ω phase contributed to very high strength levels (1600MPa of ultimate strength), with low ductility (2.5~4.5%of elongation) or even brittle fracture during the elastic stage of tensile test. It showed a poor balance of strength and ductility in the aged alloy with ω phases. Therefore, the improvement of the ductility needs more detailed investigations. The a phase with1~3μm obtained at high aging temperatures leads to attractive combinations of strength and ductility (1200~1400MPa of ultimate strength with7.5-12.5%of elongation) with elastic modulus controlled to be below100GPa. It can be assumed that the alloy may be a usable structural material.


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