Materials Selection of Optimized Titanium Alloys for Aircraft Applications

The aim of the present work was to explore the correlation between the physical metallurgy of titanium alloys and its main attributes to select optimized materials for structural aircraft applications in the landing gear beam. The Ashby's method was employed as the materials selection strategy to consolidate and evaluate the data collected from the current literature in a comprehensive and consistent analysis. Landing gear beam materials are mainly β and near-β alloys. Considering the need for high specific strength and fatigue resistance, the best candidate among them was Ti-3.5Al-5Mo-6V-3Cr2Sn-0.5Fe alloy. Finally, a brief discussion of additional aspects related to alloy design, microstructural features and their influence on the mechanical properties is presented.


Introduction
The global market for the aircraft industry presents a strong increasing trend.Recently, the Airbus Company forecast a growing need for new airplanes until 2035, representing an investment of over 5 trillion dollars 1 .In this expanding scenario, several aviation programs put forth requests for lowering fuel consumption, CO 2 and NO x emissions during aircraft operation 2 , thus, weight reduction is a key issue for aircraft manufacturers.Titanium alloys are used in several aircraft components such as landing gears, engine parts, springs, flap tracks, tubes for pneumatic systems and fuselage parts [3][4][5] .This widespread applicability derives from an impressive set of favorable attributes such as high strength-to-weight ratio, high oxidation resistance, fracture toughness, corrosion resistance, fatigue strength and creep resistance [6][7][8] .Titanium alloys can be divided into three different classes of alloys, designated as α, α+β and β alloys.Characteristics such as creep resistance, weldability, elastic modulus, and toughness are affected by the microstructural features of each class [9][10][11] .
The physical metallurgy of titanium alloys has been explored to enhance specific properties for a variety of engineering applications.Some of the main properties of titanium alloys for structural aircraft components are fatigue strength, impact strength, Young's modulus and hardness 12 .These properties can be tailored according to the alloy composition and microstructure control that can be achieved by proper heat treatments.Different phases and different morphologies for the same phase can be produced depending on the thermal processing of the titanium alloy.This subject will be further explored in section 2.
Materials selection methods can be used to guide the development of aircraft titanium alloys with optimal mechanical properties.However, as a consequence of the different classes of titanium alloys, the task of selecting the best candidate for a particular application is challengingdiverse properties have to be simultaneously addressed.A variety of materials selection strategies can be found to help the designer in the choice of optimum candidates for an engineering application 13 .Ashby et al. 14 provided a thorough assessment of materials selection strategies based on quantitative analysis, questionnaire (expertise-capture) and inductive reasoning based on previous experience.Recently, multi-criteria decision making (MCDM) methods have drawn much attention solving engineering problems where multiple attributes must be dealt with simultaneously by the designer [15][16][17][18] .Chatterjee et al. 19 have shown the ability of two MCDM methods, VIKOR and ELECTRE II in successfully identify the best candidates for flywheels and sailing-boat masts.Chakraborty and Chatterjee 20 employed other MCDM methods (TOPSIS and PROMETHEE) to select materials for cryogenic storage tank materials.However, in spite of the suitability of MCDM methods for selection materials when multiple objectives must be met, Ashby's approach 14,21 is regarded as a robust methodology to screen and rank the best candidates for an engineering application in a simple and direct manner 22,23 .
Some authors have assessed different titanium alloys for aircraft applications.Huda and Edi 24 reported a review on the materials of structures and engines for supersonic aircrafts.They elaborated a survey of design requirements to guide the materials selection for supersonic aircrafts.The information is very useful, but do not provide a direct assessment of the physical metallurgy aspects of titanium alloys that would allow for the development of optimized alloys for structural applications.Inagaki et al. 25 provided an overview of the main characteristics of different titanium alloys employed in the aerospace industry.Technical information of the main contemporary aircraft titanium alloys is presented in a direct way, correlating the alloy attributes with its application.However, a materials selection procedure to meet the core design criteria is not presented.Santos et al. 26 provided a materials selection analysis for the interiors of executive airplanes, giving a deep understanding of the design requirements in such application.Notwithstanding, information about aircraft structural applications of titanium alloys is not provided.In this respect, other authors gave a valuable contribution towards the understanding of the role of titanium alloys in the aircraft industry, exposing their main strengths and weaknesses.Boyer 27 evaluated the potential applications of α, α+β and β titanium in the aerospace industry, describing the main properties of several specific alloys.Peters et al. 28 highlighted applications of titanium alloys in the airframe, engine, helicopters and space components.Similar purpose can be found in the report by Veiga et al. 29 who stressed out the applications of titanium alloys in the biomedical, automotive and aircraft fields, giving emphasis to the Ti-6Al-4V alloy.Henriques 30 reviewed the production routes for titanium alloys used in the aerospace industry.More recently, Cotton et al. 31 gave a deep analysis of the potential applications of β alloys in the aircraft industry.Useful information about the microstructure, processing and properties correlation of several beta alloys is provided.
In spite of the high-quality reports available in the literature, there is a lack of papers dedicated to the materials selection of titanium alloys for aircraft applications, exploring the correlation between performance and mechanical metallurgy in a comprehensive way.In the light of this scenario, the aim of the present work was to explore the compromise between materials attributes and mechanical metallurgy to select titanium alloys for structural aircraft applications using Ashby's approach.The paper is divided into two parts.An initial section is devoted to describing the different types of titanium alloys used in the aircraft industry.This section will serve as a basis for the materials selection procedure, giving the reader a few materials science essential concepts.The second part applies the concepts described in the first one as a support to the materials selection analysis for a landing gear beam.

Physical metallurgy aspects
The allotropic transformation temperature in Ti known as β-transus is placed at about 882 ºC.Alloying elements considered α-stabilizers (Al, O, N, etc) tend to increment the β-transus temperature, stabilizing α phase at higher temperatures.On the other hand, the addition of β-stabilizers (Nb, Ta, Mo, V, Cu, Co, Cr, Ni) may reduce it, allowing β-phase to form at temperatures lower than 882 ºC 32 .
In Ti alloys, the β/α interfaces block the dislocation motion, hence, the mechanical strength of any β+α microstructure derive from the size, morphology, and distribution of the α phase 33 .The formation of α from β usually starts at the β-phase grain boundaries, giving rise to many colonies of Widmanstätten α phase laths 34 .In addition to the β→α transformation, many other solid state transformations can be observed in Ti alloys 32 as the formation of ω phase which has deleterious effects on mechanical properties, reducing the alloy toughness 35,36 .Since α phase also has a hexagonal crystal structure, it is stiffer and has lesser slip systems than the β phase (bcc), thus, it has a limited conformability 37 .To further improve the mechanical properties of Ti alloys, Ti-6Al-4V was developed with the combined addition of α (Al) and β (V) stabilizers.The duplex microstructure with both α and β phases present at the room temperature (RT) allows Ti-6Al-4V to achieve yield strength of approximately 1000 MPa.

Selected alloys
In order to support the materials selection process developed in the next section, Table 1 brings a summary of many aircraft titanium alloys' properties at a given microstructure.This information was collected from a thorough and extensive literature review.The density values were obtained by calculating the theoretical density of the

Fundamentals of Ashby's method
A detailed description of Ashby's method for materials selection can be found can be found in Ashby, 2004 14 and Ashby, 2010 21 .The procedure is comprised of four different steps: translation, screening, ranking, and documentation.Firstly, in the translation step, the design requirements are defined, identifying the material's function, the constraints or minimum attributes that must be respected so that the material can properly perform its function, the objectives of the selection process, seeking for an attribute that must be maximized or minimized and the variables that the designer is free to choose.Next, the candidates that do not meet the constraints are screened from the selection process.In the third step, the surviving candidates are ranked according to the objectives.Finally, the documentation step can give further insights about the suitability of the candidates after ranking.
The methodology described in the previous paragraph can be applied in a more comprehensive way if the material function is clearly defined.Consequently, in order to give the reader an accurate view of the problem, the materials selection procedure will be applied for the landing gear beam.

Materials selection for the landing gear beam a) Translation step
Landing gears are critical components of an aircraft structure, being responsible for supporting the airplane during landing, take-off, parking, and taxiing 49 .The landing gear beam connects the wing to the landing gear, carrying bending loads and acting as a shock absorber 50 .Figure 1 displays part of the landing gear structure of a Boeing 757 airplane, showing the landing gear beam at the upper left.This highly stressed component must be carefully designed to withstand mechanical loads since most aircraft failures are related to landing gears 52 .High specific strength and fatigue resistance are vital properties for the landing gear beam.Titanium alloys meet this profile and, therefore, have dominated this application over the last years 53,54 .Based on this scenario the design requirements for the landing gear beam can be defined as shown in Table 2.The only constraint is that the landing gear beam must be manufactured using titanium alloys.The objectives were set as to maximize specific strength and fatigue limit simultaneously.Component size and choice of material are the free variables.However, the lack of design data for β alloys is considered as a major obstacle for expanding their market share in the aircraft industry 31 .Fatigue properties have been highlighted as important design parameters with insufficient published data in the current literature 55,56 .Hence, we report the yield strength of the alloys in the ordinates.It is an acceptable way of evaluating the data considering the fatigue limit and yield strength of metallic alloys are directly correlated 21 .Nevertheless, one should bear in mind that this approach is an approximation and does not replace or eliminate the need for actual fatigue data.The complexity of titanium alloy microstructures can give rise to different fatigue behaviors for the same particular alloy, depending on its thermomechanical processing history 57 .Moreover, the scattering of high cycle fatigue life for titanium alloys has been reported to be between 2-3 orders of magnitude 58 .Even similar microstructures can give rise to different fatigue properties 59 .As recently pointed by Campanelli et al. 60 , the presence of interstitial hydrogen can be responsible for this behavior.They observed the formation of hydrides in the metastable β Ti-5Al-5Mo-5V-3Cr-0.5Fe (Ti-5553) alloy with a hydrogen concentration between 30 and 100 ppmw that led to a variation of up to 100 MPa for the fatigue limit.Furthermore, in addition to the yield strength, the fatigue limit of a material is affected by the type of crack initiation.Microstructures with similar values of yield strength can present different fatigue properties, depending on fatigue crack initiation mode.Microstructures that favor subsurface crack initiation provided higher fatigue limit than those where surface crack initiation dominates.According to Shi et al. 40 , the fatigue limit of the Ti-5Al-5Mo-5V-1Cr-1Fe alloy (Ti 55511) increased for the microstructure with limited volume fraction of coarse α-phase due to the high subsurface crack initiation ratio.In spite of this variation, the authors emphasize that the microstructures with high yield strength led to high fatigue limit.In this respect, due to the lack of fatigue data in the current literature, we adopted the yield strength to rank the candidates for the landing gear beam.aging of Ti-15-3 alloy, consisting of a pre-aging for 24 h at 250 °C and a final aging for 8 h at 500 °C.They observed that the duplex heat-treatment provided superior high cycle fatigue properties to the alloy with respect to the conventional one-step aging treatment (10h at 500 °C).The improved performance was ascribed to microstructural changes during the duplex treatment, mainly due to the higher number of precipitates per unit volume and a smaller fraction of α phase located at grain boundaries.In this case, grain boundary α nucleation and precipitation free zones are related to the onset of internal fatigue crack initiation 63 .
Ti-3Al-8V-6Cr-4Zr-4Mo alloy (Ti-beta C) has applications in the aircraft industry due to a suitable combination of strength and plasticity 64 .High cycle fatigue and fracture toughness can be tailored depending on prior cold work, followed by proper solution annealing and aging conditions that lead to α precipitation, thus forming a gradient microstructurei.e. a significant hardness increase in near-surface regions without modifying the hardness of the bulk 44 .Ti-10-2-3 was employed in the landing gear structure of Boeing 777 in the 1990's and also by Airbus in the A340-500/600 airplanes 16 .It is mainly used as forged components owing to its excellent ductility 65 .The strength level depends on the thermomechanical processing route by promoting the formation of different microstructures, especially related to the α-phase morphology 66 .
The documentation step gives irrefutable evidence for the suitability of the design requirements defined in Table 2 to allow the successful selection of Ti alloys for landing gear

Function
Landing gear beam  2 were set as the chart axes.There are 53 possible candidates which represent the whole set of titanium alloys in the software database.The best trade-off between both objectives is met by beta titanium alloys.The ultimate performance is presented by Ti-15-3, Ti-3Al-8V-6Cr-4Zr-4Mo, and Ti-10-2-3.

Constraints
Following the Ashby methodology, additional information prospected for the selected candidates was considered in order to document the material's choice.Ti-15V-3Cr-3Sn-3Al (Ti15-3) is a prominent β alloy with consolidated aircraft applications owing to its high strength-to-weight ratio and cold formability, making it attractive to replace the conventional Ti-6Al-4V in several components 61 .The tensile and fatigue properties are strongly affected by the alloy microstructure.Thus, the performance presented in Fig. 2 can be further optimized by proper heat treatments.Santhosh et al. 62 , for instance, studied the effect of a duplex beams.In order to expand this scenario, though, further data about 18 newly developed titanium alloys have been collected from the literature and a new chart was plotted (Figure 3), drawing a comparison between the alloys shown in Table 1 with the Ti-15-3, Ti-3Al-8V-6Cr-4Zr-4Mo and Ti-10-2-3 alloys (Fig. 2).This chart does not display the fatigue limit of the alloys since this property was not yet reported for most of the alloys shown in Tab. 1.
which can be beneficial to the mechanical properties.At lower temperatures, stripe-shaped TiB precipitates (referred as whiskers) are commonly formed.They vary from 10-100 microns in size and act pinning the grain and phase boundaries, improving the overall mechanical strength.On the other hand, the presence of B-rich particles can result in loss of ductility, depending on their size 4 .According to Seok et al. 70 , the addition of B to Ti-6Al-4V did not alter the α phase volumetric fraction, compared to the base alloy, even though it caused an expressive reduction of the α-β colonies size, thus reducing the planar slip length.That was identified as a relevant strengthening mechanism among β+α alloys with small B additions, alongside with the composite effect (i.e. the presence of reinforcing precipitates such as TiB).Analogously, it seems B can increase the critical stress for slip deformation in Ti-based shape memory alloys, enhancing their superelastic behavior 71 .Further studies suggest that the incoherent TiB-B interfaces can assist the α-phase precipitation, leading to more fine and dispersed alpha phase precipitates 68,72 , and that the co-addition with C is useful, reducing B aggregations on grain boundaries 73 .Additionally, as boron addition affects the alloy microstructure, it is likely to influence the corrosion behavior of structural aircraft titanium alloys.This effect is hardly investigated in the literature.Notwithstanding, there are reports showing that boron addition decreased the corrosion resistance of the biomedical Ti-35Nb-7.2Zr-5.7Taβ alloy in a simulated physiological solution 74 .This effect was ascribed to the formation of fine TiB precipitates that lead to a galvanic effect at the interface with the beta matrix.Ravi et al. 75 have found that pitting corrosion was facilitated when the boron content was above 0.01 wt.% for the Ti-6Al-4V alloy when exposed to phosphate buffered solution (PBS) at 37°C.Below this content the resistance to localized corrosion was increased.The authors hypothesized that the beneficial effect of the very low boron additions could be due to the fact that, at such low contents, boron remains in solid solution and do not form precipitates responsible for galvanic effects.Davis et al. 76 , in turn, did not find any evidence of detrimental effects of boron additions to the corrosion resistance of Ti-6Al-4V in Hanks' solution.There is no consensus yet in the literature about the effect of boron addition on the corrosion behavior of titanium alloys but it should not be disregarded when developing new materials for specific applications.
The Ti-15V-3Sn-3Al-3Cr alloy reported by Guo et al. 48is also much more attractive than its standard counterpart whose data were taken from the CES Edupack 2009 software.Such optimized behavior is derived from the heat treatments steps to which the alloy was subjected, promoting the formation of nanosized β grains and α precipitates.
By analyzing the data displayed in Figs. 2 and 3 it is evident that the performance of well-established aircraft alloys can be optimized by tailoring their chemical composition and microstructure.In this respect, limiting factors for As can be promptly seen in Fig. 3, from the additional 18 alloys whose data are plotted in the chart, there are 13 with better performance than the three standard alloys selected at the first moment.The best performances were for the following materials:  4 .The attention paid to the influence of B on Ti alloys has been increasing during the last years.Despite the limited solid state solubility of B in both β and α phases, its impact on the microstructure and therefore on the mechanical properties of Ti alloys has been reported by several authors.During solidification, small additions of B (0.05 to 0.5 wt.%) offered an important strategy to control the prior β grain size among Ti alloys 67,68 .Also, B presence leads to a weak β-phase texture development 69 Materials Selection of Optimized Titanium Alloys for Aircraft Applications achieving the ultimate response to the mechanical loads to which the landing gear beam is subjected depend on proper combinations of alloying elements and favorable microstructural features.The formation of hard nanosized α precipitates within the β matrix plays a central role in this scenario.Despite our efforts to rank the materials based on their specific yield strength, the experimental assessment of fatigue properties of recently developed Ti alloys, especially the ones highlighted in this work, seems to be of paramount importance to the aerospace industry.

Conclusions
In this work, we explored the relationship between the physical metallurgy and performance of structural aircraft titanium alloys, aiming to select the best candidates for the landing gear beam.The materials selection procedure was based on the Ashby method.Two objectives were simultaneously evaluated, to maximize the specific strength and the fatigue strength.Due to the lack of fatigue data for newly developed titanium alloys in the current literature, we used the yield strength as the performance metric instead of the fatigue limit, since these properties are positively correlated.Titanium metastable-β alloys provided the best balance between these attributes, being the ultimate performance that of the Ti-3.5Al-5Mo-6V-3Cr-2Sn-0.5Fealloy.It is important to emphasize, though, that the alloy performance can be optimized by proper microstructural design.The formation of hard nanosized α precipitates within the β matrix increases the fatigue strength of β alloys.This type of microstructure can be considered an adequate guideline when developing new alloys for the landing gear beam.However, designers should be aware that actual fatigue data of new β titanium alloys are still needed to drive the titanium industry in wider markets.The complexity of titanium's physical metallurgy can give rise to specific structural responses depending on the volume fraction and morphologies of β, α and ω phases, as well as the concentration of interstitial elements.In the near future, with new fatigue data, Figure 3 could be remodeled in order to drive an even more accurate selection process.

Figure 1 .
Figure 1.Titanium forging parts in the 757 landing gear structure.The landing gear beam is at the upper left 51 .
Titanium alloy; section shape specified Objectives To maximize specific strength; to maximize fatigue strength Free variables Component size; choice of material b) Screening, ranking and documentation Using the database of the CES Edupack software (version 2009) the chart shown in Fig. 2 was plotted.The two objectives displayed in Table

Figure 2 .
Figure 2. Material property chart for titanium landing gear beam, plotting fatigue strength at 10 7 cycles versus specific strength.Colors: Titanium alloys are in olive, alpha-beta alloys are in navy, beta alloys are in cyan, commercially pure titanium alloys are in purple, near-alpha alloys are in yellow and near-beta alloys are in black.

Figure 3 .
Figure 3. Chart plotting the yield strength versus specific strength of titanium alloys shown in Tab. 1.The data from the CES Edupack 2009 software for the Ti-15V-3Cr-3Sn-3Al, Ti-3Al-8V-6Cr-4Zr-4Mo and Ti-10V-2Fe-3Al alloys (Fig. 2) are also shown for comparison.Each circle represents a titanium alloy at a specific condition.The numbers in square brackets represent the references from which the data were taken, which are also displayed in Tab. 1.

Table 1 .
Microstructure and engineering properties of aircraft titanium alloys.Heat treatment details abbreviated*.Materials Research alloy based on its chemical composition.It is noteworthy that the vast majority of the references do not report the alloy's fatigue limit, thus revealing a lack of information regarding the fatigue behavior of aircraft titanium alloys in the current literature.

Table 2 .
Design requirements for the landing gear beam.