Powder route processing of Nb-silicide based alloys

For reasons of performance and fuel efficiency, aerospace engines of the future are expected to be able to operate at temperatures that are ever closer to and ultimately in excess of the melting temperatures of nickel-based superalloys. Consequently, alloys based on the refractory metal niobium are being investigated as alternatives to nickel-based superalloys. Alloys based on niobium with the addition of silicon show promising high temperature creep and room temperature toughness properties. They are also less dense than nickel-based superalloys. This suggests that they could, with development, meet the needs of future ultra-high temperature aerospace engine applications. To date, most of the published research has concentrated on cast and heat treated alloys. Powder route processing offers the potential for the manufacture of near net shape components but has attracted less attention in the open literature. This is probably due to the sluggish kinetics in Nb-Si based alloys, the sensitivity of Nb to contamination by interstitials and the availability of powders. This presentation will concentrate on powder route processing of model Nb-silicide alloys using elemental and pre-alloyed powders and plasma spark sintering or additive layer manufacturing. The microstructures produced by each processing method will be compared to ascertain the influence of processing route on microstructure. Emphasis will be given to phase selection and phase transformations and the contamination of the microstructure by interstitials during processing

Phase equilibria in the Nb-Si-Ge phase diagram

Niobium silicide-based in-situ composites have the potential to supersede nickel-based superalloys due to their excellent high temperature mechanical properties and low density. A thermodynamic database is being developed using the CALPHAD method to aid in alloy development. The addition of small amounts of germanium into these systems is of particular interest as it can significantly improve oxidation resistance. For example, germanium is reported to benefit high temperature oxidation resistance of coatings used on refractory silicide alloys by the formation of a glassy GeO2.SiO2 phase which fills cracks and is impermeable to further oxygen penetration. The effect of germanium on the phases formed in bulk niobium silicide-based in-situ composites is not particularly well understood, and limited data exists in the literature. To understand the effect of germanium on alloys, a thermodynamic description of the ternary Nb-Si-Ge phase diagram has been developed using the Calphad method. To support thermodynamic modelling samples were produced along the Nb5Ge3-Nb5Si3 pseudo binary and assessed using XRD. Experimental results show that germanium stabilises the high temperature Nb5Si3 (W5Si3 prototype) to low temperatures. The thermodynamic assessment will be presented and compared to experimental data from the current work and the literature

Phase equilibria in the Nb-rich region of Al-Nb-Sn at 900 and 1200 °C

The Al-Nb-Sn phase diagram was studied experimentally in the Nb-rich region to provide important phase equilibria information for alloy design of Nb-silicide based materials for aero engine applications. Three alloys were produced: Nb-17Al-17Sn, Nb-33Al-13Sn and Nb-16Al-20Sn (at.%). As-cast and heat-treated alloys (900 and 1200 °C) were analysed using XRD (X-ray diffraction) and SEM/EDS (scanning electron microscopy/ electron dispersive x-ray spectroscopy). Tin showed a high solubility in Nb2Al, reaching up to 21 at.% in the Sn-rich areas, substituting for Al atoms. Tin and Al also substituted for each other in the A15 phases (Nb3Al and Nb3Sn). Tin showed limited solubility in NbAl3, not exceeding 3.6 at.% as it substituted Al atoms. The solubility of Al in NbSn2 varied from 4.8 to 6.8 at.%. A ternary phase, Nb5Sn2Al with the tI32 W5Si3 crystal structure, was found to be stable. This phase was observed in the 900 °C heat-treated samples, but not in the 1200 °C heated samples

The role of Sn in the oxidation of Nb silicide based alloys

Aero-engine materials used in critical components must have acceptable oxidation behaviour at the temperatures of service. Niobium silicide based alloys have the potential to replace Ni based superalloys in future aero-engines owing to their lower densities, higher melting points and balance of properties. Niobium silicide based alloys must have inherent oxidation resistance to survive in case of coating failure . Great advances have been made towards improving the oxidation behaviour of developmental Nb silicide based alloys. Tin was reported [1, 2] to improve oxidation and subsequent research [3, 4] confirmed that Sn as an alloying addition contributes towards suppressing pest oxidation and is an essential alloying addition for suppressing the spallation of scale at high temperatures [5]. Evidence for the enrichment of the microstructure below the alloy/scale interface with Sn was provided for the first time by the group [3, 4]. “How Sn manages to deliver” better oxidation behaviour in Nb silicide based alloys was not understood. In the last “beyond the Ni superalloys” ECI conference some preliminary results were presented in a poster addressing this point. This presentation will be based on the results of recently completed systematic experimental and modelling research of model alloys and ternary systems to show how Sn affects microstructure and oxidation behaviour. Particular emphasis will be given to the links between phase stability, volume fraction and distributions of key intermetallic phases in the microstructure of Sn containing Nb silicide based alloys and their oxidation at 800 and 1200 oC. The role of Sn for the microstructure at the alloy/scale interface will be discussed

On the microstructure and properties of Nb-12Ti-18Si-6Ta-5Al-5Cr-2.5W-1Hf (at.%) silicide-based alloys with Ge and Sn additions

The microstructures and properties of the alloys JZ3 (Nb-12.4Ti-17.7Si-6Ta-2.7W-3.7Sn-4.8Ge-1Hf-4.7Al-5.2Cr) and JZ3+(Nb-12.4Ti-19.7Si-5.7Ta-2.3W-5.7Sn-4.9Ge-0.8Hf-4.6Al-5.2Cr) were studied. The densities of both alloys were lower than the densities of Ni-based superalloys and many of the refractory metal complex concentrated alloys (RCCAs) studied to date. Both alloys had Si macrosegregation and the same phases in their as cast and heat treated microstructures, namely βNb5Si3, αNb5Si3, A15-Nb3X (X = Al, Ge, Si, Sn), C14-Cr2Nb and solid solution. W-rich solid solutions were stable in both alloys. At 800 °C only the alloy JZ3 did not show pest oxidation, and at 1200 °C a thin and well adhering scale formed only on JZ3+. The alloy JZ3+ followed parabolic oxidation with rate constant one order of magnitude higher than the single crystal Ni-superalloy CMSX-4 for the first 14 h of oxidation. The oxidation of both alloys was superior to that of RCCAs. Both alloys were predicted to have better creep at the creep goal condition compared with the superalloy CMSX-4. Calculated Si macrosegregation, solid solution volume fractions, chemical compositions of solid solution and Nb5Si3, weight changes in isothermal oxidation at 800 and 1200 °C using the alloy design methodology NICE agreed well with the experimental results

Microstructures and isothermal oxidation of the alumina scale forming Nb1.45Si2.7Ti2.25Al3.25Hf0.35 and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 alloys

Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb1.45Si2.7Ti2.25Al3.25Hf0.35 (MG5) and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 (MG6) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are αAl2O3 scale formers. A (Al/Si)alloy versus Nb/(Ti + Hf)alloy map, which can be considered to be a map for Multi-Principle Element or Complex Concentrated Nb-Ti-Si-Al-Hf alloys, and a [Nb/(Ti + Hf)]Nb5Si3 versus [Nb/(Ti + Hf)]alloy map were constructed making use of the alloy design methodology NICE and data from a previously studied alloy, and were used to select the alloys MG5 and MG6 that were expected (i) not to pest, (ii) to form αAl2O3 scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb5Si3 and (v) to have microstructures consisting of hexagonal Nb5Si3, Ti5Si3, Ti5Si4, TiSi silicides, and tri-aluminides and Al rich TiAl. Both alloys met the requirements (i) to (v). The alumina scale was able to self-heal at 1200 °C. Liquation in the alloy MG6 at 1200 °C was linked with the formation of a eutectic like structure and the TiAl aluminide in the cast alloy. Key to the oxidation of the alloys was the formation (i) of “composite” silicide grains in which the γNb5Si3 core was surrounded by the Ti5Si4 and TiSi silicides, and (ii) of tri-aluminides with high Al/Si ratio, particularly at 1200 °C and very low Nb/Ti ratio forming in-between the “composite” silicide grains. Both alloys met the “standard definition” of high entropy alloys (HEAs). Compared with HEAs with bcc solid solution and intermetallics, the VEC values of both the alloys were outside the range of reported values. The parameters VEC, Δχ and δ of Nb-Ti-Si-Al-Hf coating alloys and non-pesting Nb-silicide based alloys were compared and trends were established. Selection of coating alloys with possible “layered” structures was discussed and alloy compositions were proposed

Ab initio study of ternary W5Si3 type TM5Sn2X compounds (TM = Nb, Ti and X = Al, Si)

The adhesion of the scale formed on Nb-silicide based alloys at 1473 K improves when Al and Sn are in synergy with Si and Ti. This improvement is observed when there is segregation of Sn in the microstructure below the alloy/scale interface and a layer rich in intermetallics that include TM5Sn2X compounds is formed at the interface. Data for the ternary compounds is scarce. In this paper elastic and thermodynamic properties of the Nb5Sn2Al, Ti5Sn2Si, Ti5Sn2Al and Nb5Sn2Si compounds were studied using the first-principles, pseudopotential plane-wave method based on density functional theory. The enthalpy of formation of the ternary intermetallics was calculated using the quasi-harmonic approximation. The calculations suggest that the Nb5Sn2Si is the stiffest; that the Nb5Sn2Al and Ti5Sn2Si are the most and less ductile phases respectively; and that Nb significantly increases the bulk, shear and elastic moduli of the ternary compound compared with Ti

Corrosion of aluminium metal in OPC- and CAC-based cement matrices

Corrosion of aluminium metal in ordinary Portland cement (OPC) based pastes produces hydrogen gas and expansive reaction products causing problems for the encapsulation of aluminium containing nuclear wastes. Although corrosion of aluminium in cements has been long known, the extent of aluminium corrosion in the cement matrices and effects of such reaction on the cement phases are not well established. The present study investigates the corrosion reaction of aluminium in OPC, OPC-blast furnace slag (BFS) and calcium aluminate cement (CAC) based systems. The total amount of aluminium able to corrode in an OPC and 4:1 BFS:OPC system was determined, and the correlation between the amount of calcium hydroxide in the system and the reaction of aluminium obtained. It was also shown that a CAC-based system could offer a potential matrix to incorporate aluminium metal with a further reduction of pH by introduction of phosphate, producing a calcium phosphate cement

Effect of Zn- and Ca-oxides on the structure and chemical durability of simulant alkali borosilicate glasses for immobilisation of UK high level wastes

Compositional modification of United Kingdom high level nuclear waste (HLW) glasses was investigated with the aim of understanding the impact of adopting a ZnO/CaO modified base glass on the vitrified product phase assemblage, glass structure, processing characteristics and dissolution kinetics. Crystalline spinel phases were identified in the vitrified products derived from the Na2O/Li2O and the ZnO/CaO modified base glass compositions; the volume fraction of the spinel crystallites increased with increasing waste loading from 15 to 20 wt%. The spinel composition was influenced by the base glass components; in the vitrified product obtained with the ZnO/CaO modified base glass, the spinel phase contained a greater proportion of Zn, with a nominal composition of (Zn0.60Ni0.20Mg0.20)(Cr1.37Fe0.63)O4. The addition of ZnO and CaO to the base glass was also found to significantly alter the glass structure, with changes identified in both borate and silicate glass networks using Raman spectroscopy. In particular, these glasses were characterised by a significantly higher Q3 species, which we attribute to Si–O–Zn linkages; addition of ZnO and CaO to the glass composition therefore enhanced glass network polymerisation. The increase in network polymerisation, and the presence of spinel crystallites, were found to increase the glass viscosity of the ZnO/CaO modified base glass; however, the viscosities were within the accepted range for nuclear waste glass processing. The ZnO/CaO modified glass compositions were observed to be significantly more durable than the Na2O/Li2O base glass up to 28 days, due to a combination of the enhanced network polymerisation and the formation of Ca/Si containing alteration layers

Short communication : the dissolution of UK simulant vitrified high-level-waste in groundwater solutions

Dissolution of a simulant UK nuclear waste glass containing Mg, Ca and Zn was investigated over 35 d at 50 °C in water and simulant groundwater solutions. The dissolution rates were influenced subtly by the groundwater composition, following the trend, from least to most durable: clay > water > granite ≈ saline. Solutions were rapidly silica saturated but boron dissolution rates continued to increase. This is hypothesised to be due to the formation of secondary Mg-silicate precipitates, preventing the formation of a passivating silica gel layer and allowing glass dissolution to proceed at close to the maximum rate