When some people think about advanced materials, in most cases, aerospace and/or millitary industry came to their minds. Taking as an example the airplanes, these are propeled by the turbojet. This is an airbreathing jet engine. Usually it is a big and noisy internal combustion engines that aspirates a lot of air cm3. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor, a combustion chamber, and a turbine (that drives the compressor). The compressed air from the compressor is heated by the fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Hence the turbojet engine has large dimensions and is capable to produce high rotational speed. In addition its turbine blades must be lightweight. The titanium (Ti) is a kind of material capable to operate under high thermal stress and centrifugal loads. Not surprisingly, 80% of Ti is used by aerospace industry.
The problem with Ti alloys is the rapid oxidation and its oxide layer which only remais protective until certain temperature (T). The max operation T of the Ti is considerably lower than its melting point, 400 and 1670 oC respectively. This operation T is suggested due to the problem with the mechanical properties decreasing with temperature (graph page 5). In continuous operation with T above 400 C the resistance to oxidation decreases and the material begins to detach oxidized parts because these became brittle. The TiO2 (titanium oxide) is brittle, has a white coloured aspect and it is formed because the oxidation resistance of Ti alloys at high T is poor. However, at room temperature (RT) the Ti alloys behaves very well. Initially it is fast corroded then it passivates in TiO2 layer, a passivating surface. At high T this one does no work, because the diffusion coefficient of oxygem inside TiO2 became extremely large and it is no longer protecting the material.
The Ti has an alotropic structure as hexagonal close packed (HCP) and body-centered cubic (BCC). At RT it has the alpha HCP structure, or alpha-Ti, but it became BCC one at a temperature beta-transus, which is 890 oC. This suggests that at high T, the Ti became less tolerant to plastic deformation. The reason is that BCC structure has lower slip planes for the dislocations occur. Despite this fact its prism is slightly smaller than theoretical values, c/a = 1.587 and th = 1.633. This in addition to the basal, pyramidal and prismatic planes provide means to dislocation occur at RT. Hence Ti is plastically formable at this condition. Other effect is being anisotropic which is the distinct mechanical properties along different axis, for instance c and a axes. The anisotropy became more influent as Ti is plastically deformed.
Therefore the elastic modulus (E) of the Ti variates relative to the gamma angle which is the stress direction angle. It goes from 145 GPa, with stresses along the hexagonal prism axis, to 100 GPa which is obtained with stresses along perpendicular direction relative the hexagonal prism axis. This variation also means that the elastic and plastic Ti behaviour also changes. In general applications the Ti alloy has polycristaline structure which usually exhibits E about 110 GPa. The main Ti parameters are:
- Melting point: 1670 °C;
- Density: 4.51 g/cm3;
- Max use temperature: T < 400 °C.
The components commonly made from Ti alloys are valves, valve springs, retainers, connecting-rods, suspension springs, exhausts, mufflers and silencers. In general, the automotive industry uses Ti alloy to components which require low inertia, good properties at high temperature and reduction of the unsprung mass. It is also used in turbochargers due to the great oxidation resistance at high T and its lightweight. In this case the main objective is to avoid the turbo lag. Hence they are used in components with cyclic and alternative movements under high temperature enviroments.
It is clear that the due to the denser atomic packing along the c-axis (<0001> direction) a maximum value of the Young’s modulus occurs in this direction, which shows a gradual decrease with increasing angle made with the c-axis. It is also apparent that this variation can account for differences of up to 40GPa, which significantly alters the mechanical response of the alloy. However, these changes do not only affect elastic deformation of these alloys, but also the plastic deformation, in terms of the orientation of slip planes with relatively low critical resolved shear.
The Ti alloy can originate solid solutions by adding alloying elements as H, B, C and O which form interstitials. These elements has very low atomic radius. The alloying elements which have higher atomic radius form substitutional solid solutions. These are less effective as interstititials solid solution. The main alloying element for this is the oxygen (O) which results in effects similar to carbon (C) in steels. Hence the amount of O in Ti alloys increases the yield strenght (YS) but at cost of the toughness. For instance a grade 4 Ti alloy has a great static mechanical properties (YS, UTS, E), but definitely poor dynamic mechanic ones (impact resistance). The use of O is justified by its low cost, but it is difficult to control its amount during the manufacturing process of these alloys.
In general Ti alloys have a composition of 99.0-99.5% of Ti and rest is of alloying elements as iron (Fe), nitrogen (N), O and C. The O is the most important alloying element since it is responsible for the YS improvement. Just 0.1% of O is enough to increase YS in 120 MPa. In addition all other alloying elements are evaluated in an equation of O equivalent:
Oeq = O% + 2N% + 2/3C%
As can be seen, the amount of N doubles the O effect in the Ti alloy composition while C has a lower impact. This can improve the material strenght, but, as already mentioned, at cost of toughness and strain. However one way to avoid Ti alloy brittlement is perfoming a remelting process under vacuum condition to obtain an even smaller interstitials. This is called extra low interstitials (ELI) conditions and is required for Ti applications with toughness is a must. A summary of Ti alloys features is given below:
“pure” Ti: 99.5-99.0%Ti
• main alloying gelements: Fe, C, O, N (interstitials)
• it can be considered a phase where the O content determines the
%O equivalent = %O + 2%N + 0.67%C
– Each 0.1%O equivalent makes UTS grow of approx. 120 MPa
– However, this happens at the cost of toughness
• If a high toughness is required, then the ELI (Extra-Low
Interstitials) versions should be preferred
The grade 1 Ti alloy is the purest between the first grades. Hence this has the best ductility, impact and corrosion resistance relative to the others. Its application in automotive industry is restricted to components which do not require a high strength, instead, a good corrosion resitance and in applications where formability is a must. It is usually supplied in plates and tubes. The industrial area examples are Chemical processing, Desalination, Architecture, Medical industry Marine industry, Automotive parts, Airframe structure.
The grade 2 Ti alloy is the most used of this kind of alloy. It has a higher amount of O relative to grade 1, but exhibits a similar corrosion resistance, formability and ductility to this one. Due to the higher O amount, the tensile strength (TS) and YS higher are higher than the grade 1 but also possess good weldability. The grade 2 Ti alloy is supplied in form of sheets and bars and it is also used in the following applications: Architecture, Power generation, Medical industry, Exhaust pipe shrouds, Airframe skin, Desalination, Chemical processing.
The grade 3 Ti alloy is the least used, because the higher O amount makes this material very strong and with all the consequenses of oxidation. In other words, low corrosion resistance and impact resistance. The formability is similar to grade 1 and 2. This also limitates the application of this alloy to aerospace, chemical, medical industries and marine.
The grade 4 Ti alloy is the strongest between the first four grades. It has an excelent formability, ductility and corrosion resistance. In addition it also exhibits good weldability. The impact resistance is still low which concentrates its application in medical field. In addition, Airframe components, Cryogenic vessels, Heat exchangers Condensor tubing, Surgical hardware.
The alpha and beta Ti alloys are made by addition of their stabilizer elements which are aluminum (Al) and Vanadium (V), respectively. In the alpha case as the Al% increases, the alpha existence also increases. In the beta case, as the V% increases, the beta existence is extended. However in elements which follow the eutectoid phase diagram, they are considered as beta stabilizers. This is assumed because under convenctional cooling rates the beta stabilizers do not follow the graph anymore. Those elements are substitutional one, thus when they became solid the process do not follow the line, it extrapolate it. The movement of their atoms in Ti is given by diffusion and this in solid state occurs at a low rate. Hence by convenctional cooling rates these elements do not follow the phase diagram, instead they do another path. This behaviour is different from steels. In this case the alloying element is C and it is interstitial. The alloying elements which are beta stabilizers are substitutional and these are bigger than interstitials one. For alpha phase, the stabilizers are Al and thin (Sn). For beta stabilizers, the elements are molibdenum (Mo), V and tungsten (W). Finally, for beta-eutectoid stabilizers the alloying elements are manganese (Mn), copper (Cu), hydrogen (H), Fe, nickel (Ni) and cobalt (Co).
The alpha-Ti alloys is take advantage of the number of slip planes. It can suffer a plastic deformation of 90% without breaking. According to Von Misses theory it is required 5 independent slip systems to provide a similar plastic deformation. Although this kind of Ti alloy has only 4, the fifth occur by twins as in twinning induced plasticity steels (TWIP).
The alpha-stabilized Ti alloy has the Al as the main alloying element due to its low density. In general this alloy can not be heat treated, because the alpha solid solution is obtained at room temperature. However the solution strengthning can be performed. This alloy provides good weldability, formability, corrosion resistance and thermal stability at high T. As the Al is the main alloying element, the others are quantified relative an equivalent Al amount. This is given by the following formula:
Aleq = Al% + 1/3 Sn% + 1/6 Zr% + 10 (O% + 2 N% + C%)
As can be seen, the Al equivalence is also influenced by O percentage. The oxidation consumes new material by Ti grabbing O. When the O diffuses inside the Ti it creates an alpha case, which is a thin layer of alpha-Ti surrounding Ti. This occur because O it is an interstitial inside Ti and also an alpha-stabilizer. In case of grade 5 Al alloy the alpha case would be alpha+beta-case. For this reason it is required to control the O amount, because it dramatically chance the Ti mechanical properties.
The alpha stabilized Ti alloys have three variations which are: full alpha phase, near alpha phase, which has 2% of beta phase, and the age-hardenable, which has another alloying element. In this case, 2.5% of Cu which forms the TiCu3. In general these three alloys provides good formability and weldability, oxidation and creep resistance.
The alpha stabilized Ti alloys are usually used in two kinds of operation, one is until 480 oC and the other is for short periods at 600 oC. These qualify the alpha stabilized Ti alloy to be used in cryogenic vessels, aircraft parts and tanks. In the second case these can be applied in jet engines, missiles and exhausts systems. In this last one can be mentioned the exhaust systems from Le Mans winner Audi car, which Audi deliberately used Ti for its exhaust sytems. At finish, major part of the system was detached during race due to the oxidation at temperatures about 600 oC.
The Ti oxide layer is good as Al oxide layer, but this one is barrier against the diffusion of O at very high T. So the Al components melt before it can be obtained by O passing through the alumina layer. In Ti at 500 oC it has the oxide formation, but this one do not protect against O inlet. At this condition it passes through the oxide and goes directly to Ti, which oxides and start to grow. The specific volume of the oxide is different from Ti ones. Hence occurs the peeling bedworth corrosion and this oxide detaches from the surface exposing the naked Ti to oxidation. Therefore Ti oxide layer is not protective at high T. Bellow a summary of the alpha-stabilized Ti alloys:
- Solid solubility up to 8% at RT
- Solid solution stregnthening
- Usually not heat treatable
- Lower density
- Good thermal stability and ressitance to oxidation at high T.
The beta-stabilized Ti alloys are major composed by beta phase which is provided by proper alloying elements. These are Mo, V, W, Ta and Si. Hence Ti-beta alloys can undergo heat treatment. This process begins with a solution strenghtening which causes alpha – beta transformation. Alpha is stable at RT while beta is stable above beta-transus T. A quenching process occurs at high cooling rate which avoid beta -> alpha transformation. The aging process is the heat treatment last step. This results in partial transformation of the metastable beta phase in fine alpha grains. Therefore a high strength at cost of some degree of ductility. To improve the burn resistance at high T, the addition of chromium (Cr) can be made. Hence this makes possible to improve the working T up to 510 oC but at cost of weight due to Cr density. Owing to these charateristics, beta-stabilized Ti alloys are usually used by aerospace industry as in aircraft engines parts.
α/β – titanium alloys
The alpha-beta Ti alloys are the most used. They are peculiar in the way which the heat treatment control influence the formation of alpha and beta phase. Under heat treatment the material solidifies as beta alloy and upon cooling some of it generate some alpha. If the solution, quenching and the re-heating of this alloy are performed, it is possible to control the way to form the alpha phase. In general alpha-beta Ti alloys have 4-6% of beta stabilizer alloying elements. They are able to be heat treated. This occur by a solution strengthning, which causes alpha – beta transformation, quenching, which rapidly cools the material to avoid beta – alpha transformation. At the end, the aging process usually transforms some amount of metastable beta phase in fine grains of alpha pase. The alpha-beta Ti alloys exhibit a very high tensile strength, good formability, good corrosion resistance and good creep resistance at T between 300 and 425 oC (570ºF – 800ºF). The main application of this alloy are the aerospace industries in parts as turbine blades and discs, hydraulic tubings, cryogenic parts, rocket motor cases and also marine components. The alpha-beta Ti alloy is the Ti6Al4V, a grade 5 Ti alloy.
The microstructure of alpha-beta alloys is fundamental for fatigue resistance and other mechanical properties. The heat treatment can drastically change these characteristics. For instance, the Ti6Al4V can be slowly cooled or annealed at 700 oC (below beta-transus) to obtain different microstructes. For the first case, it is obtained a Widmasttaten structure which is a needle-like ones with 120o orientation between each other. In the annealed case, the microstructure became alpha and beta grains. Below it has some examples of this kind of Ti alloy.
α+β alloys with transition elements:
- Ti Mn 7 (σT = 900 MPa, ε = 10%)
- Ti Mn 8 (σT = 1000 MPa, ε = 15%)
- Ti Cr 3 Fe 1.5 (σT = 1000 MPa, ε = 12%)
- Ti Cr 2 Fe 2 Mo 2 (σT = 1050 MPa, ε = 12%)
- Ti Mn 3 Fe 1 Cr 1 Mo 1 V 1 (σT = 900 MPa, ε = 20%)
α+β alloys containing Al+ transition elements:
- Ti Al 1.5 Mn 3 (σT = 1000 MPa, ε = 17%)
- Ti Al 4, Mn 4 (σT =1040 MPa, ε = 15%)
- Ti Al 5, Cr 3, Fe 1 (σT =1060 MPa, ε = 8%)
- Ti Al 3, Cr 5 (σT =1080 MPa, ε = 15%)
It is possible to form martensite in Ti alloys, its HCP structure alpha prime. However the martensite of Ti is no strenghtening as ones of steel. Instead it is a highly distorted latice (c/a < 1) which increases the UTS in 70 Mpa. It is difficult to find it in quenched Ti. The higher the allowing element concentration, the lower the martensite transformation start temperature (Ms).
As a consequence of rapid cooling, Martensite a’ (HCP) can be frmed It has a highly distorted lattice, where c/a < 1. The mechanical properties are much lower than steel martensite: a quenched TI has an increased UTS of only 70 MPa.
There are three process of the heat treatment which generates different results on Ti alloys. The annealing, the quenching and the aging. The first is slow cooling which the strenght increases as the beta phase decreases. In this case there is no martensite concentration. The alloying elements are in high amount. The quenching is the fast cooling of the composition after solution treatment. It can result in different characteristics of the material depending on when the quenching began. If it is performed at after Ms, the result in strength increasing is very gentle. The quenching done between the martensite transformation finishing temperature (Mf) and Ms only exhibits a decrease in strength. The best quenching is done below Mf or at RT. In this case an increase in strength is obtained similar one done after Ms, in the presence of precipites formation. The best process is the aging. When it is done after quenching, in this case after beta cooling down, there is no time to form alpha nor to start the martensite transformation. Hence it is obtained a lot of metastable beta which is prone to transform in fine alpha grains. Therefore by aging after quenching it is possible to form alpha inside beta matrix which results the best performance in terms of strength because these precipitates are very small. In cases which quenching is performed with beta content between Mf and Ms, some of this beta is transformed in alpha prime (martensite). This is can not be reverted. Hence, after aging, only partially of rest of metastable beta phase is transformed in alpha phase. Therefore, the aging process results in low strength.
Slow cooling: sigma increases as b content increases. The β is different depending whether it is heat treated starting from α+β or β. The maximum strength for quenched Ti occurs when Mf is at RT, while the minimum is in case of Ms at RT (100% β metastable). However, if these compositions are aged, controlled formation of alfa and decomposition of β, the maximum strength can be achieved.
The main problem with Ti alloys heat treatment is beta -> omega transformation. When this occurs the planes where dislocations propagate merge passing from ABCABC planes, typical of BCC structure, to AB’AB’. The effect of the omega-phase is the material embrittlement. The omega phase is favoured when alloying elements as Zr and Hf are used.
The sequence ABCABC typical of BCC (111). The ω is a metastable phase which forms from β in alloys based on titanium, zirconium and hafnium. It is important because its formation generally leads to a deterioration in the mechanical properties. It is not diffusive and it can not be suppressed by rapid cooling.
Heat resistant alloys
The Ti fire is another problem with Ti alloys during machining process, because the ships must be stored in oil or similar. The TiO2 has a high formation entalpy, thus when these start to oxide they generate heat becoming hotter and hotter. At high T the kinect oxidation fastly occur and a at some point this is high enough to reaction occur almost instantaneously. Hence the Ti fire phenomena begin.
The alpha Ti alloy is the best option for high T applications. These range from heat exchangers, chemical reactors, aerospace, biomedical, formula one cars (halo) and some other car components as springs. The main application of Ti in automotive industry is in exhaust system of high performance sports cars. The Ti exhausts are usually made from TiFeSiO and TiAlSiNb. These contain specific elements for exhausts systems, Al and Si. The reason is both at high T forms alumina and silica which creates a thermal protection for these alloys. They are made by foil obtained by cold rolling, which means that it has some anisotropy.
The oxidation of Ti alloys at high T is the main concern. The coating of components made from these helps to avoid oxidation and to detach of these parts on disassembling process. The oxidation of attached part cause their gripment. The coating can be done by oxidation and carburizing together with TiO2 and Ti carbide. The plasma spray coating with Zr oxides is also an option, but the coating provided can be detached due to the different properties (thermal expansion) from the base metal and Zr coating are not as good as diffusion coating.
The term intermetallics is applied to materials which have characteristics from ceramics and metallics. An intermetallic looks like a metal, it has a shine aspect. Howeve it not deforms as one, instead it behaves more like a ceramic material. In metals the deformation occurs by dislocation and by grain boundary sliding. An intermetallic is brittle if not proper modified. There are two types of intermetallic compounds, the stoichiometric and nonstoichiometric ones. The difference between them is that the first one are modified by changing A and B components while the nonstoichiometric is modified through the range between the elements, its composition.
The Ni aluminides (AlNi) are interesting because they have a high melting point (1640 oC) and low density (5.86 g/cm3). There are some intermetallics which are usually used, AlNi, AlNi3 and Ni3Si.
The AlNi3 aluminide is denser than NiAl, with 7.5 g/cm3 and has a lower meting point relative to this one, 1390 oC. The intermetallic Ni3Al can be improved by adding some amount of beta (50 – 100 ppm) at its composition. This results in defects in the structure which makes the material less tough. Another characteristics of Ni3Al intermetallic is its good behaviour at high T. Therefore it is compound that exhibits a high tensile strength between 600 and 900 oC. In addition it forms an alumina layer which protects the material from oxidizing enviroments.
NiAl (nickel aluminide)
The NiAl has a one by one proportion between its components. This intermetallic has the higher melting point and the lowest density relative to the others. Its mechanical properties are near to superalloys, but with advantage of NiAl lighweight. In addition it has good oxidation resistance, high thermal conductivity. It is similar to metal and has low cost of raw materials.
The gamma TiAl is a lightweight aluminide, because of the high Al% it has a low density. The oxidation resistance is also provided by Al compound because its oxidation characteristics. In addition is not reactive when exposed to hydrogen, thus it has low propensity to hydrogen embrittlement. This alumined is also resistant to creep at high temperature. The cubic structure provide planes for the displacement occur. There are two types of Ti aluminides, the monophasic and the biphasic ones. The first is alloyed with Ta, W and Nb and exhibits a high Ti amount, 50 – 58%, and 1 – 2% of Al. The objective is to increase the ductility of this aluminide. The biphasic is characterized by a gama + alpha2 phase. It has a bit lower amount of Ti, 44 – 49% and similar amount of Al, 1 – 3%. However, there are three groups of compounds which can be alloyed.
In general Ti3Al and TiAl have high elastic modulus, more than 120 GPa, oxidation temperature above 700 oC and ductility about 1 – 3% and 10 – 90% at high T. These characteristics qualify these materials to be used in components as turbine blades. The Ti aluminides can also be used as matrix composites. The SiC/Ti3Al+Nb has addition of Nb, to reduce brittleness, and foils of SiC (40% vol.). The result is a component with extremely higher specific strength, even than superalloys. Hence comparing Ti alloys, Ti3Al and TiAl aluminides with superalloys, it can be observed that even though these one are stronger than aluminides, they are also denser. As the aluminides are lighter and exhibit mechanical properties near superalloys ones, they became a better option at specific stress point of view.
The component NiAlTi, which Ti is used as coating, the Ni and Al are mixed and ignited by fireworks. The temperature of this reaction is very high, about 2000 K, and the consequence is the Ni and Al is formed. At liquid state the heat and temperature are so high which they react with Ti creating a layer. This made from NiAlTi. In a scratch test conducted under controled load, 1 and 5 N, it can be observed that as the scratch mark is higher, the resistance against it is lower. However it can also be noted that the Ti layer has a larger scratch mark relative to ones of NiAl. The reason is the higher toughness of this one. The TiNiAl layer exhibits almost none scratch mark, because of its hardness which is higher than the other two layers. This is called ternary layers.
The extrusion is difficult due to the concentration of Ti in specific places, mainly in Russia. In addition the Ti reactivity to O2 makes its manufacturing difficult due to the extensively care with TiO2 and its oxidation. Hence the price is very high due to this and the few suppliers available.
The casting process must be done in vaccum in order to control its high reactivity to oxygen. In addition it requires specific molds due to its reactivity with traditional mold materials. After that the machining process is also difficult due to the high hardness and low thermal conductivity of the Ti alloys, which results in high machining tool wear and T.
When alloyed with other elements the Ti alloys also are prone to exhibit galvanic corrosion.
- Sachdev, A. K., Kulkarni, K., Fang, Z. Z., Yang, R., & Girshov, V. (2012). Titanium for Automotive Applications: Challenges and Opportunities in Materials and Processing. JOM, 64(5), 553–565. doi:10.1007/s11837-012-0310-8.
- Ashby, Michael F. Materials Selection in Mechanical Design. Third ed., Elsevier, 2005.