Tin addition to improve the oxidation behaviour of Nb ss /Nb 5 Si 3 based in-situ composite

. This work focuses on the effect of tin additions (2, 5 and 8%) to the MASC alloy (Nb-25Ti-8Hf-2Cr-2Al-18Si) on the microstructure and the oxidation behaviour at 815°C in air. The alloys are mainly constituted of a niobium solid solution plus the ( α, β, γ ) Nb 5 Si 3 silicides. For the higher Sn additions (5 and 8%), a fourth phase is evidenced: it is enriched in Sn and has a crystal structure close to Nb 3 Sn. The oxidation resistance of these alloys is clearly improved by tin additions: the oxygen inward diffusion is hindered and consequently the fragmentation of the silicides is avoided. Cracks in silicides are no longer observed for the MASC containing 8%Sn. This effect is not attributed to a better efficiency of the oxide scales but rather to the reduction of the niobium solid solution fraction with tin additions.


Introduction
High temperature applications require new categories of structural materials with high toughness and hot corrosion resistance in order to increase the operating temperatures.To date only nickel based superalloys are employed for such types of applications.Their operating temperature is limited to 1150°C because Ni-based superalloys lose their mechanical properties above this temperature.Presently, the improvement of such systems is very limited because of the vicinity of their melting points.For higher temperature applications, metallic systems based on refractory metals are considered as candidates for replacing or complementing nickel-based alloys.Niobium alloys have received great attention because of their high mechanical properties and their lower densities (8.58) among the other refractory systems (Ni 8.91, Mo 10.2, W 19.35).Furthermore niobium exhibits a ductile to brittle transition at lower temperatures than molybdenum.Since the last ten years, niobium alloys are considered as a real potential material allowing the use of the refractory alloys in high temperature applications.High mechanical properties have been achieved with the developments of Nb ss /Nb 5 Si 3 based in-situ composites during the STREP European project ULTMAT (2004ULTMAT ( -2008)).However these alloys suffer from the pest catastrophic oxidation between 600 and 900°C which results in a rapid fragmentation of the alloys.Like the well-known MoSi 2 compound, the strengthening of Nb 5 Si 3 phase is subjected to the pest phenomenon in this range of temperatures.Two works [1,2] report on the beneficial effects of tin additions in niobium in-situ composites resulting in delaying the pest phenomena.However the role of tin is not established.For this study, niobium in-situ composites (Nb-25Ti-8Hf-2Cr-2Al-18Si = MASC alloy) are prepared by arc melting with tin additions of 2, 5 and 8at.%.The as-obtained microstructures are studied by metallographic analysis in order to determine the influence of tin content on the phase equilibria as well as on the evolution of phase compositions.Oxidation tests are carried out by thermo-gravimetric measurements at 815°C, temperature for which the alloys are very sensitive to the pest.

Experimental methods
Sample preparations.High purity powders (purity>99.9%) of Al, Cr and Sn were blended in an agate mortar and compacted under pressure at room temperature.The resulting pill was placed on a water cooled copper mould to be arc melted.The sample preparations were performed under inert argon atmosphere.Then small pieces of all the other Nb, Ti, Hf and Si elements were added to the Al-Cr-Sn sample to reach the desired compositions (Table 1).After three re-meltings, samples were finally cast to obtain a cylindrical ingot.The as-cast samples were then heat treated under vacuum for five days at 1200°C in order to assess the thermodynamic phase equilibrium.The ingots were then cut in disc-like shapes with 1 cm diameter and 1.5 mm thick.Cu = 1.5418Å radiation associated with a fast X'cellerator detector to identify the main constitutive phases of the samples and their oxidation products.The crystal structure of each compound was identified by matching the characteristic XRD peaks against JCPDS data.The metallographic observations were performed by scanning electron microscopy (SEM) using a Philips XL30 equipped with an energy dispersive X-ray spectrometer (EDX).The chemical compositions were determined using a Cameca SX100 microprobe by Wavelength Dispersive Spectrometry.Pure Nb, Ti, Hf, Cr, Al, Si, Sn and Cr 2 O 3 for oxygen were used as standards for the quantitative analyses.The voltage and the current of beam were 15kV and 10nA, respectively.Under this voltage the spatial resolution was about 1-1.5 µm 3 .Thus, the EPMA punctual analyses were performed for phases of diameter higher than 5µm.Oxidation tests.Before the oxidation treatment the samples were polished up to 2400 grid on SiC polishing paper, then cleaned ultrasonically in ethanol and dried.The oxidation tests were conducted using a SETARAM TAG 1750 symmetric thermobalance at 815°C in air.

Results and discussion
Microstructure characterisation of MASC-x%Sn.After the sample preparations and heat treatment processes, the samples were characterised by XRD, SEM and the compositions of the constitutive phases were assessed by EPMA.The XRD pattern of MASC-5%Sn is given as an example in Figure 1 and the identified phases for the three alloys are presented in Table 2.

576
Euro Superalloys 2010 Table 2: Phase detected on each sample by XRD(X present, -absent) Results of SEM and EPMA for the three alloys are given in Figure 2.    with the γ-Ti 5 Si 3 hexagonal structure (P 6 3 /mcm -prototype Mn 5 Si 3 ) containing more than 10%Hf and also a higher titanium content than niobium.Geng et al. [3] reported previously that the γ-type phase is stabilized by hafnium.It should be noted that no Laves phase is observed in our case contrary to what was observed in [2].Small particles of HfO 2 are also identified in all samples.
For the MASC-5%Sn and MASC-8%Sn, the three phases observed for MASC-2%Sn are identified but for (Nb,Ti Advanced Materials Research Vol. 278 577 Nb 5 Si 3 is not evidenced and only the α-Nb 5 Si 3 is observed.Moreover a fourth Sn-rich phase is evidenced in MASC-5% and -8%.The phase has a cubic crystal structure (P m3n -prototype Cr 3 Si) and its lattice parameters are close to those of Nb 3 Sn.According to the SEM micrographs, the higher the initial tin concentration, the higher is the volume fraction of the Sn-rich phase.In addition, this phase appears to gradually substitute the niobium solid solution.Consequently, the thermodynamic phase equilibrium evolves from (Nb,Ti) ss + β-(Nb,Ti -First, EPMA data (Figure 2) shows that tin additions modify the solubility of the other elements within the (Nb,Ti) ss .The addition of Tin is associated with an increase of the Hf, Cr and Si concentrations in the niobium solid solution.The alloying additions of the niobium solid solution decreases the solubility of tin (3.7 at.% ) in bcc niobium as compared to 6at% in pure Nb at 1200 o C according to the Nb-Sn binary [4].Beyond 3.7 at.% the Sn-rich phase (≈Nb 3 Sn) is observed.Furthermore, the Ti and Al concentrations in (Nb,Ti) ss are not affected by the tin additions.
-The Hf and Ti concentrations of the (Nb,Ti) 5 Si 3 decrease and in turn, increase in the Nb solid solution.In parallel, Sn (up to 2%) replaces silicon in the Nb solid solution ".
-The composition of the Hf-rich (Ti,Nb) 5 Si 3 compound does not seem to be affected at all by tin additions.However, this phase was impossible to analyse in the case of MASC-8%Al because its size is too small for accurate analyses.Oxidation behaviour at 815°C of MASC-x%Sn.The oxidation behaviour of the three MASC alloys containing Sn was followed by thermogravimetric measurements for 100 hours at 815°C.The results obtained are given in Figure 3.For the three samples, S m / ∆ increases continuously with time and the shapes of the curves are not"stepwise" as observed by Geng et al. [2].Mass gains remain low after 100 hours of exposure at 815°C and the total mass change decreases from 10.7 to 3.49 mg/cm² for MASC-2% and 8%, respectively.These values are in the same magnitude as those previously reported in [2].with k p = 1.5x10 -10 g 2 .cm - .s - for the first fifty hours of treatment, showing the limitation of the oxidation rate by diffusion in the solid state.This is followed by a linear kinetics with the associated linear constant k l equals to 2.99x10 -8 g.cm -2 .s - .If the tin content is increased, the oxidation kinetic slows down.MASC-5%Sn exhibits two oxidation steps similar to the sample containing 2%Sn: a first one which is parabolic for the first sixty hours (k p =6.56x10 -11 g 2 .cm - .s - ); a second one which is linear until the end of the experiment (K l =1.11x10 -8 g.cm -2 .s - ).

578
Euro Superalloys 2010 Finally, the MASC-8%Sn with the lower mass gain, also exhibits a parabolic kinetics throughout the 100hr of the oxidation test.The corresponding parabolic constant k p is equal to 4.4x10 -11 g 2 .cm -4 .s - .
The increase of Sn content has consequently a positive effect on the mass changes in air at 815°C by reducing the oxidation rate.Furthermore, the linear and then rapid oxidation step is avoided by adding 8% tin, at least during 100h of treatment.
Microstructure characterisation of oxidised MASC-x%Sn.The XRD analyses of corrosion products formed on oxidised samples mainly reveal the presence of TiNb 2 O 7 and also of TiO 2 and Nb 2 O 5 in lower amounts.The characteristic peaks of TiO 2 rutile are identified on XRD patterns but they can also be attributed to CrNbO 4 which crystallises according to the same crystal structure.The nature of the formed oxides is not modified by the tin additions.The oxides developed on the sample are well adhered and no spallation is observed.The composition of the oxide scale determined by EPMA is also close to that of TiNb 2 O 7 .Figure 4 presents the SEM cross-section of the oxidised sample in back scattered electron mode.According to the oxidation results, the tin additions do not modify the nature of the oxides formed on the alloys surface but improve the overall oxidation resistance.As such, the beneficial effect of Sn has to be explained for assessing the oxidation mechanism.For MASC alloys [5] oxidized at 800°C, the proposed oxidation mechanism indicates that the poor resistance (notably regarding pest phenomenon) of Nb/Nb 5 Si 3 alloys is mainly due to the poor oxidation resistance of the niobium solid solution.Indeed, oxygen dissolves easily in the niobium

Enriched tin phase
Advanced Materials Research Vol. 278 579 solid solution and leads to a high volume expansion of the Nb ss phase.This induces a high stress level on the brittle niobium silicides nearby which fragment (and then pest) to relax the stresses.Thus, one of the key points to limit the rapid oxidation of niobium alloys is to reduce the niobium solid solution fraction in order to decrease the overall penetration of oxygen inside the alloy which is permitted by the high solubility of oxygen in niobium solid solution [6].The present results obviously show that tin additions higher than 3.7% lead to the decrease of the niobium solid solution fraction and lead to the formation of the Nb 3 Sn-type phase.The latter dissolves oxygen in a lesser degree than (Nb,Ti) ss and therefore tends to limit the oxygen inward lattice diffusion.Moreover, it allows the formation of a Sn-rich phase below the oxide scale which 'plays the role' of a diffusion barrier for oxygen during the oxidation test.As a consequence, almost no internal oxidation is observed for the MASC-8%Sn and no fragmentation of silicides occurs.
Based on these promising results obtained on tin additions to Nb-Nb 5 Si 3 at 815°C, future work will be focused on the study of the oxidation behaviour at high temperature (1200°C) in order to verify the persistence of the Sn beneficial effect.

Conclusions
The MASC alloys microstructure changes to a large degree with tin additions above 3.7at% that corresponds to the solubility limit of tin in the niobium solid solution.Beyond this value a Sn-rich phase with a crystal structure close to Nb 3 Sn is observed.This phase substitutes the niobium solid solution and impacts dramatically the oxidation resistance of the alloy.The oxygen inward diffusion is hindered and consequently the fragmentation of the silicides is avoided.Cracks in silicides are no longer observed for the MASC containing 8%Sn.
Figure 1: XRD pattern of MASC-5% after sample preparation and heat treatment.