Al-%4.5 Cu-%(x) Li-%0.5 Mg-%0.5 Ag-%0.15 Zr Alaşımlarının Oksidasyon Özellikleri

Abstract

Yüksek mukavemetleri ve düşük yoğunlukları nedeni ile son yıllarda üzerinde yoğun çalışmalar yapılan alüminyum-lityum esaslı alaşımların çökelme karakteristikleri, mekanik özellikleri ve oksidasyon özellikleri hakkında genel bilgiler verilmiştir. Ağırlıkça %1.42'den %2.64'e kadar lityum içeren Al-%4.5Cu-%(X)Li-%0.5Mg- %0.5Ag-%0.15Zr alaşımları 508 °C ta 1 saat 20 dakika süre ile çözeltiye alma ısıl işlemine tabi tutulup mikroyapı analizleri yapılmıştır. Çözeltiye alma işlemi sonrası alaşımlarda yeniden kristalleşme olayı görülmemiştir. Yapıda çözünmeyen kaba partiküllerin miktarının da az olduğu görülmüştür. Çözeltiye alma ısıl işlemi sonrası 180 °C'ta 6 saat süre ile alaşımlara yapay yaşlandırma işlemine uygulandıktan sonra çekme deneyleri yapılmıştır. Bu deneylerin sonucunda alaşımdaki lityum miktarının artışı ile alaşımın akma ve kopma mukavemetlerinde bir düşüş, % uzama miktarlarında ise bir artışın olduğu tespit edilmiştir. Ağırlıkça %1.42, %1.56, %1.71, %1.93 ve %2.64 lityum içeren alaşımlar çeşitli firın ortamlarında (açık atmosfer, %20 O2+%80 N2 gaz karışımı, %40 O2+%60 N2 gaz karışımı ve %10 O2+%90 N2 gaz karışımı), çeşitli sıcaklıklarda (350, 450 ve 500 °C) ve sürelerde (0.5, 1, 2, 4 ve 8 saat) oksidasyon işlemlerine tabi tutulmuştur. Tüm ortamlar için sıcaklık veya sürenin artışı ile oluşan oksit ürünlerinin artışına bağlı olarak numunelerde meydana gelen ağırlı artışı parabolik bir şekilde artmaktadır. 350 °C'ta yapılan deneylerde ağırlık artışının en fazla %40 O2+%60 N2 gaz karışımının kullanıldığı fırın atmosferinde gerçekleştiği görülmüştür. 450 ve 500 °C'ta is ağırlık artışı en fazla atmosfere açık deneylerin yapıldığı şartlarda gerçekleşmektedir.

The successful development of new generation transport vehicles which can travel at higher speeds, for longer ranges, withstand greater payload capacity, provide better fuel economy and have improved landing capabilities, requires reliance on the use of more efficient engines such as supersonic breathing engine, improved airframe design, and use of high performance materials. While improvements in engine performance and aircraft design have been realized, it is currently believed that design with available commercial materials alone will not meet the demands for a significant improvement in structural efficiency the newer generation aerospace vehicles such as advanced tactical fighter and the national aerospace plane. The critical need for structural materials to be both cost-effective and provide an optimum level of performance coupled with an increased emphasis on efficiency and reliability have engendered considerable widespread interest in the development of new aluminum alloys. These alloys potentially provide excellent combinations of reduced density, high strength, good fracture toughness, resistance to exfoliation corrosion, resistance to stress corrosion cracking, improved thermal stability and better stiffness. The family of lithium containing aluminum alloys has in recent years received much attention for use in weight-critical and stiffness-critical structures for military, space and commercial applications because they offer the promise of low density, improved specific strength and high stiffhess-to-weight ratio over the other commercial 2XXX and 7XXX series aluminum alloys and carbon fiber composites. Lithium additions to aluminum give the greatest reduction in density and increase in elastic modulus per wt% of any known alloying element. Lithium one of the few elements with substantial solubility in solid aluminum ( 4.2wt.% in a binary aluminum-lithium alloy ). The potential for aluminum alloy density reduction through lithium additions is evident by comparing its atomic weight (6.94) with that of aluminum (26.94). Each 1% increment of lithium addition to an aluminum alloy up to 4 wt.% lithium decreases the density by about 3% and increases the elastic modulus by 6%. In principle, weight saving in aircraft structural parts could reach 15% or possibly as high as 19%. Secondly, compared to fiber reinforced composite materials, the lithium containing aluminum alloys can be easily fabricated and assembled using existing facilities and consequently, they possess lower manufacturing costs. Besides, the lightweight aluminum-lithium alloys would be available in various product forms and sizes commensurate with currently available commercial aluminum alloys, thereby, allowing manufacturers of aircraft and aerospace structures to make use of standard manufacturing methods. Lastly, their potential long term use in large quantities has resulted in an emphasis on the development of extremely high standards of both equality and reproducibility. These are important factors that would help avoid the large expenditures of money required by other technologies associated with the manufacture of fiber-reinforced composite materials and rapidly solidified materials. The general requirement for precipitation strengthening of supersaturated solid solutions involves the formation of finely dispersed precipitates during aging heat treatments which may include either natural aging or artificial aging. The aging must be accomplished not only below the equilibrium solvüs temperature, but below a metastable miscibility gap called the Guinier-Preston (GP) zone solvüs line. The supersaturation of vacancies allows diffusion, and thus zone formation, to occur much faster than expected from equilibrium diffusion coefficient. In the precipitation process, the saturated solid solution first develops solute clusters, which then become involved in the formation of non-equilibrium precipitates. Like other age-hardened aluminum alloys aluminum-lithium alloys achieve precipitation strengthening by thermal aging after a solution heat treatment. The precipitate structure is sensitive to a number of processing variables, including, but not limited to, the quenching rate following the solution heat treatment, the degree of cold deformation prior to aging, and the aging time and temperature. Minor alloying elements can also have a significant effect on the aging process by changing the interface energy of the precipitate, by increasing the vacancy concentration, and/or by raising the critical temperature for homogeneous precipitation. Like some other age-hardened 2XXX aluminum alloys, aluminum-lithium-base alloys also gain increased strength and toughness from deformation prior to aging. This unusual phenomenon has given rise to a number of thermomechanical processing steps for aluminum-lithium alloys aimed at optimizing mechanical properties after artificial aging. The age hardening of aluminum-lithium alloys involves the continuous precipitation of 8 '(Aİ3Lİ) from a supersaturated solid solution. The aluminum and lithium in the 8 '(AI3Lİ) precipitates are positioned at specific locations. The eight shared corner sites are occupied by lithium and the six shared faces are occupied by aluminum. This gives rise to the aluminum-lithium composition of 8 '(Al3Li) precipitates. The geometrical similarity between the lattice ot the precipitates and the face centered cubic lattices of the solid solution facilitates the observed cube/cube orientation. The lattice parameters of the precipitate are also closely matched to those of the matrix. Consequently, the microstructure of an aluminum-lithium alloy solution heat treated and aged for short times below the 8 ' solvüs is characterized by a homogeneous distribution of coherent, spherical 8 '(Al3Li) precipitates. Aluminum-lithium-base alloys are microstructurally unique. They differ from most of the aluminum alloys in that once the major strengthening precipitate (81) is homogeneously precipitated, it remains coherent even after extensive aging. In addition extensive aging at high temperatures (>190°C) can result in the precipitation of icosahedral grain-boundry precipitates with five-fold symmetry. Although the quasi-crystalline structure and the composition of this grain boundry precipitates are not yet exactly known, it has been suggested that both the precipitates and the precipitate free zones (PFZs) near the grain boundaries might play a major role in the fracture process. Development of commercially available aluminum-lithium base alloy was started by adding lithium to aluminum-copper, aluminum-magnesium and aluminum-copper- magnesium alloys. These alloys were chosen to superimpose the precipitation hardening characteristics of aluminum-copper, aluminum-magnesium and aluminum- copper-magnesium base precipitates to the hardening of lithium containing precipitates. The types of precipitates observed in such a complex system can be primarily divided into two categories. The first type are those that are stable equilibrium phases, their presence being determined by the alloy composition and the heat treatment condition. These include binary phases such as Al2Cu, AlLi, Al3Zr and ternary precipitates like Tı(Al2CuLi), T2(Al6CuLi3), Tb(A17LİCu4), R (Al5Li3Cu) and S(Al2CuMg). A large quantity of these primary phases, present from the cast and homogenized condition dissolve during the solutionizing treatment and are retained in supersaturated solid solution upon quenching, providing the driving force for precipitation of metastable phases upon subsequent aging. To maximize the alloy's capabilities in terms of its mechanical properties, it is critical to optimize the solution treatment temperature and time. Certain alloy compositions dictate the presence of excess primary phases even after optimal heat treatment and in these cases some primary phases are carried through the entire thermal processing schedule. Whereas these primary phases, depending on the size, shape, and volume fraction can be deleterious to mechanical properties, especially toughness, they may enhance weldabilty. Since the challenge of strengthening aluminum-lithium alloys with coherent lithium rich phases, such as AI3Lİ, which do not increase the density has been met with limited success, additional strengthening has been achieved by the co-precipitation of other binary and ternary phases. The addition of various amounts of copper and magnesium to lithium containing aluminum alloys has been shown to be effective in strengthening. These elements modify the precipitation sequence either by altering the solubility of the principal alloying elements, or by forming copper rich and magnesium rich phases, and co-precipitating with the 8 '(AI3Lİ) precipitates. The elements also combine with lithium and precipitate as phases that exist in the ternary and quaternary systems. The low ductility and toughness of binary aluminum-lithium alloys can be traced, at least in part, to the inhomogeneous nature of their slip, resulting from coherent particle hardening of spherical 8 '(AI3Lİ) precipitates. The presence of equilibriums (AlLi) precipitates at grain boundaries can also cause PFZs, which can induce further strain localization and promote intergranular failure. Consequently, for the development of commercial alloys, slip has been homogenized by introducing dispersoids (manganese, zirconium) and semicoherent/incoherent precipitates, such as Ti(Al2CuLi), 0' (Al2Cu)or S (Al2LiMg), through copper or magnesium additions. Concurrent developments in thermomechanical processing have optimized aluminum-lithium microstructures for the best combinations of strength and toughness. The resulting material tends to be highly textured where zirconium additions are used to inhibit recrystallization. Texture increases the variability of properties with orientation. Most current aluminum alloys can be readily heat treated in air because they tend to form protective oxide coatings. However the aluminum-lithium alloys oxidize at a far greater rate, and apparently will continue to oxidize until all of the lithium in the material has been exhausted. Aluminum-lithium base alloys oxidize more rapidly than do non-lithium containing alloys. Aluminum-lithium base alloys oxidize more rapidly than their lithium free counterparts by more than an order of magnitude. This tendency arises because of the reactivity of lithium and the nonprotective nature of the lithium containing oxides. In the aluminum-lithium base alloys lithium oxidizes preferentially. This behavior is expected, as the oxidation potential of lithium, 3,045 V, is the highest of all metals, and its diffusivity is among the greatest of the common alloying elements used in aluminum alloys. Al-%4.5Cu-%(X)Li-%0.5Mg-%0.5Ag-%0.15Zr alloys with lithium content from wtl.42% to 2.64% were used in this study. The iron and silicon contents were 0.125 wt pet and 0.08 wt pet respectively. Alloys were cast in the laboratory by melting induction furnace under an argon atmosphere. The alloys were homogenized at 450°C for 16 hour. The homogenized alloys were extruded at 370°C, 20:1 reduction ratio, an 0.3 cm/s ram speed. After that the samples which have different composition, were prepared for metallographic examination, tensile testing and oxidation study. The samples which were used in metallographic examination and tensile testing, were solutionized at 508 °C for 1 hour 20 min. in enclosed air in a resistance furnace. Metallographic examination analysis were used to identify the phases present in the solutionized alloys. After solutionizing each samples was prepared for metallographic examination using standard preparation techniques and etching with Keller's reagent and examined firs via light microscopy. After solutionizing cylindrical tensile samples aged at 180 °C for 6 hours and quenched at room temperature. Then cylindrical tensile samples with gage dimensions of 5.4 mm diameter and 22 mm length were machined. The gage sections were polished to 800 grit. Tensile tests were carried out at room temperature in laboratory air. Samples tested on an Instron universal testing machine at a strain rate 0.5 mm/min. The ultimate tensile strength, 0.2% proof strength and percent elongation value obtained. Eight samples were tested and their average values are reported in this study. The cylindrical samples (2.5cm dia. X 0.5cm length) surfaces were polished with 1200 grit. The polished specimens were oxidized. The rate of oxidation of aluminum-lithium based alloys has been studied using weight gain measurements. In order to compare our lithium loss result to weight gain experiments, several measurements were made. The topography of the oxidized coupons was studied using scanning electron microscopy. Figure 7.1 shows a typical tree-dimensional optical micrograph of as-solutionized alloy. The microstructure is composed of unrecrystallized grains which are elongated in the extrusion direction. The volume fraction of coarse undissolved constituent particles is minimal. The variation of mechanical properties with lithium contenting the aged at 180 °C for 6 hours conditions were given in table 7.2. As we see in table7.2.ultimate tensile strength and yield strength decrease with increasing in lithium content and percentage elongation increase with increasing in lithium content. Typical results showing the increase in weight due to oxide formation on samples are presented in figure 7.2, figure 7.3, figure 7.4, figure 7.5, figure 7.6.for different environment, temperature and time condation. As we see in figures weight-gain increase with increasing temperature and time. At 350 °C mass increasing the order %40 O2+%60 N2 gas mixture, %20 O2+%80 N2 gas mixture, moist air, %10 O2+%90 N2 gas mixture. However at 450 and 500 °C mass increasing the order moist air, %40 O2+%60 N2 gas mixture, %20 O2+%80 N2 gas mixture, %10 O2+%90 N2 gas mixture.