Refractory high entropy alloys: CrMoNbTiVWZr and AlxCryNbMoTiVzZry(x = 0,0.6;y = 0.3,z = 0,0.6)

Refractory High Entropy Alloys: CrMoNbTiVWZr and

Al

Cr

NbMoTiV

Zr

Abstract

Four potential High Entropy Alloys, CrMoNbTiVWZr, Cr0.3NbMoTiZr0.3, Cr0.3NbMoTiV0.6Zr0.3 and Al0.6Cr0.3

NbMoTiV0.6Zr0.3 have been designed using the indications provided by atomic radius and electronegativity

mismatch parameters. Alloys were synthesized by arc melting and characterized in the as-cast state and after heat-treatment by X-ray Diffraction and Scanning Electron Microscope experiments. In the as-cast state all alloys have dendritic microstructure, although with varied shapes of dendrites, accompanied by a cubic Laves phase and an hcp phase in the region between the dendrites. After annealing at 1350 °C for 3h it is shown that only non-equimolar compositions are constituted of bcc high entropy phase complying with the requirements for defining them as HEAs, with a small amount of secondary phases which help in providing high hardness.

Keywords: High Entropy Alloys; XRD diffraction; solid solution; Hardness; alloy design; microstructure. Highlights:

 New High Entropy Alloys designed:

CrMoNbTiVWZr and Al

Cr

NbMoTiV

Zr

 Method to tailor composition while retaining solid solution phases verified.  Microstructure homogenization of cast alloy obtained with proper heat treatment.

Introduction

High Entropy Alloys (HEAs) containing refractory elements sparked interest since their first appearance in 2011 [1-17] because of the finding [1] of improved mechanical properties in comparison to superalloys at temperatures higher than 800 °C in equimolar NbMoTaW and VNbMoTaW. Recently, non-equimolar compositions were devised to meet the threshold of ideal entropy of mixing, Sideal

mix>1.5 R, as an operative definition of HEAs [18-20].

The search of new formulations requires a method to identify the appropriate amount of elements to be mixed in the alloy. In a previous study [14] new High Entropy Alloys were successfully designed employing a procedure based on Hume-Rothery rules [15, 16] relying on the radius and electronegativity mismatch ( and respectively, with threshold values: < 4.99+0.358*< 6.5), number of valence electrons per atom (VEC) and of itinerant electrons per atom (e/a).

Here, the method is used to identify new refractory HEAs. A septenary equimolar composition containing bcc refractory elements only, CrMoNbTiVWZr, was selected at first. The radius and electronegativity mismatch for this alloy fall outside the range for the formation of solid solutions, hence a complex microstructure was expected. Experiments confirmed the accuracy of the prediction and indicated that Zr and Cr promote the formation of secondary phases; moreover, Cr has positive enthalpy of mixing with W. Therefore, aiming at a bcc multicomponent solid solution the non-equimolar composition Cr0.3NbMoTiZr0.3 was designed, leaving the Mo, Nb and Ti ratio equal to one and tuning the content of Zr and Cr to have radius and electronegativity mismatch falling in the solid solution range (Tab.1). Then, a suitable amount of

V and Al was added to Cr0.3NbMoTiZr0.3 toretain the supposed bcc solid solution in the compositions Al0.6Cr0.3 NbMoTiV0.6Zr0.3 and Cr0.3NbMoTiV0.6Zr0.3. After casting, HEAs require an annealing treatment to obtain a homogenous microstructure [11] and, possibly, induce the formation of additional phases or short range clustering [12;17] to strengthen the alloy, at the expense of reducing the ductility [7;12;17;19]. Therefore, all alloys were characterized in the as-cast and annealed state.

Material and Methods

Alloys were synthesized by arc melting and studied by X-ray Diffraction (XRD, Cu K), Scanning Electron Microscopy (SEM) in both Secondary Imaging (SI) and Back Scattered Electrons (BSE) modes, and Energy Dispersive Spectrometry (EDS). High-Temperature Differential Scanning Calorimetry (HT-DSC) and annealing were performed in helium flux and with specimens protected by a tantalum foil. Annealing was performed at 1350 °C (around 0.7 the theoretical melting temperature obtained with the rule of mixtures average of the elemental melting points, Tm) for 3h after a cycle in HT-DSC. Samples were etched in a solution of

hydrofluoric acid, nitric acid and distilled water (HF:HNO3:H2O= 1: 1 : 8) [8] for 70 s for microscopy. One of

the alloys (Co0.3NbMoTiV0.6Zr0.3) was selected to be synthesised in larger quantity (a 1kg ingot) to observe the effect of processing scale. High purity elements were weighed and, if in a suitable form, cold compacted prior to melting. Alloying was done on a hemispherical water cooled copper hearth using a 75 KW RP-75T plasma torch. The furnace was evacuated and backfilled with Ar several times. Solidification took place in the Ar atmosphere, before the ingot was turned and re-melted as before. This process was repeated a minimum of four times to improve homogeneity. After alloying, the alloys were cast into bars. This was done either by crushing the ingots to roughly 1 cm3 _{pieces, which were melted under Ar in a levitation}

melter, and poured into pre-heated moulds, creating bars of roughly 20 mm diameter and 150 mm length, or by induction skull melting ingots and casting into moulds made using the lost wax process to produce bars of 20 mm diameter and 100 mm length. Casting was done under an Ar atmosphere following the Derville process into pre-heated moulds by an automatic tilt/pouring mechanism A specimen was then heat treated at 1100°C for 48 hours under inert atmosphere. Samples for microstructural investigation and mechanical testing were machined and tested in the as cast state.

The lattice parameter of the bcc solid solutions was obtained using the cos cot∙ method.

Vickers Hardness was determined on polished specimens using a micro-hardness tester with load of 300 gF for 15 s. Compression tests were performed at RT on cylindrical specimens of 5mm diameter and 10 mm height, using a Zwick-Roell test frame with compression platens at a crosshead speed of 0.5 mm/minute. Results and Discussion

The microstructure of as-cast CrNbMoTiVWZr is made of a mixture of phases: SEM in BSE mode reveals bright primary islands rich in heavy elements in a light grey matrix embedding a darker phase (Fig.1a and Table 1 in Supplementary Information). The XRD pattern (Fig.2a) shows intense reflections belonging to a bcc solid solution, recognized as the brighter phase, a cubic Laves phase isostructural with Cr2Zr, and a hcp phase. The primary phase does not display a typical dendritic microstructure but is bulky with irregular protrusions, only seldom developing dendrite arms. This and also in view of previous findings of demixed microstructures in some HEA formulations [21] suggests the occurrence of a demixing event in the melt prior to solidification. The enthalpy of mixing of liquid Cr and W, determined with the Miedema model [22], is positive, supporting this hypothesis as in previous cases of liquid phase separation in HEAs [23;24].

Two consecutive scans in HT-DSC from 300 °C to 1550 °C at a rate of 10° C/min did not reveal any appreciable signal, indicating the absence of any phase transformation. However, it would not be expected

for heat effects due to diffusional mixing of elements or phases to be detected because of the low enthalpy of mixing. After annealing the alloy contains three phases identified by combining EDS analyses (Fig.3a) and XRD (Fig. 4) as bcc solid solution, cubic Laves, richer in Cr and Zr, and hcp, rich in Zr. It is noted the matrix is still not homogeneous in contrast indicating the alloy is not yet fully equilibrated (Fig.3a).

EDS analyses reveals the content of Cr and Zr in the bcc solid solution is 7-8 at. %, that Cr and Zr are predominantly contained in the Laves phase and Zr in the hcp phase. Actually, the atomic radius difference and electronegativity of mixing for the pair of Cr and Zr have the largest and smallest values among all seven elements, respectively. Fixing the Cr and Zr content at the amount found in the bcc solid solution, and dropping the W to reduce the chance of liquid demixing, a quinary non-equimolar alloy of composition Cr0.3NbMoTiZr0.3 was designed. Its as-cast microstructure (Fig.1b) is constituted of bright dendrites

surrounded by an inter-dendrite region made of two phases of different average atomic number. The dendrites have an inner brighter trunk and an external grey-hue zone richer in Ti, Zr and Cr (Table 2 in Supplementary Information). Some dendrites appear to originate from a rounded primary particle evocative again of prior liquid demixing. Dark particles rich in Zr (about 60 at. %) are located randomly between the dendrites (Fig.1b, Table 2 in the Supplementary Information). Coupling the information from SEM-EDS and XRD (Fig.2), the dendrites are identified as the bcc solid solution. Some minor reflections are compatible with the cubic Laves phase isostructural with Cr2Zr, whereas a Zr-rich phase cannot be identified.

After annealing the microstructure is made of a grey phase with secondary few darker regions and very thin dark grey lath particles (Fig.3b) which are better seen after etching when the other secondary phase is removed. Wavy contrast in the matrix indicates incomplete homogenization of the specimen. EDS performed before etching (Tab.2) indicates that the composition of the grey phase is close to the nominal one, while the darker phase is Ti and Zr rich (Table 2 in Supplementary Information). The volume fraction of the secondary phase was estimated at about 4 % using the Fiji Image-J software, in agreement with the mass balance computed using the EDS data in Table 2 in Supplementary Information. XRD patterns (Fig.4) indicate the main bright phase is the bcc solid solution while the low intensity reflections are compatible with a hcp solid solution rich in Zr and the hexagonal Cr2Zr-type Laves phase (Fig.4), dark grey and lathy particles in Fig.3b, respectively.

Further aiming at a bcc solid solution, the Cr0.3NbMoTiV0.6Zr0.3 alloy was designed adding 14.3 at. % V, left

apart in the previous formulation, i.e. an amount complying with the threshold in radius and electronegativity mismatch [14]. The results do not differ substantially from those shown for Cr0.3NbMoTiZr0.3 in terms of phases and microstructures in both as-cast annealed states. The as-cast alloy

(Fig.1c) consists of bright dendrites, confirmed by XRD (Fig.2) to be the bcc solid solution, and two inter-dendritic phases of which one is the cubic Laves Cr2Zr-type phase (Fig.2). After annealing the microstructure is composed of the grey matrix with some particles, appearing black after etching ( Fig.3c). The volume fraction of the secondary phase is reduced with respect to the previous case to about 3 % in accordance with the mass balance using EDS data in Table 3 in the Supplementary Information.

The microstructure of the sample produced in larger quantity (1 Kg) is shown in Figure 5 in the as cast state (a) and after heat treatment at 1100 °C for 48 hours (b). In comparison with the microstructure of the laboratory-scale as cast sample (Fig 1b) the volume fraction of the primary phase is lower. After heat treatment the alloy is mainly composed of a dark matrix with lighter spots rich in Mo and Nb. From EDS (Table 4 in the Supplementary Information) analysis the primary phase in the as cast sample and the matrix in the heat treated one have similar composition.

Employing the same methodology for element selection, 12.5 at. % Al was added to design the alloy

Al0.6Cr0.3NbMoTiV0.6Zr0.3. In the as-cast state it retains the microstructure of the previous alloys, as found by

XRD (Fig.2), SEM (Fig.1d) and EDS analyses (Table 5 in Supplementary Information): bcc dendrites rich in high melting point metals (Mo, Nb) and Ti, and two grey phases in the inter-dendrite zone: a hcp solid

solution and a Laves intermetallic compound. After annealing the microstructure is made of grains and a secondary phase appearing in both intergranular regions and inside the grains (Fig.3d). From SEM (Fig.2d) and XRD (Fig.4) the grains belong to the bcc solid solution of composition close to the nominal one. The particles inside the grains and in the intergranular region show similar EDS composition (Table 5 in Supplementary Information) and give the same reflection in the XRD pattern compatible with a Laves hexagonal phase, whose volume fraction is 7 %.

An influence of the composition on the tendency of certain elements to segregate at the grain boundary can be appreciated after annealing. The addition of Al in fact enhances the formation, during evidenced grain growth, of a secondary phase at the grain boundary after annealing at an homologous temperature of about 0.7 Tm. At the same time precipitation of the same secondary phase occur within the grain matrix. That the segregation of refractory elements occur during annealing at high temperature is in accordance with what has been revealed in TiNiW [26] thin film where Monte Carlo simulations confirm that W grain boundary segregation increases with temperature. The presence of Al can hence effect the enthalpy of segregation Hseg or the stability of the secondary phases (Hmix) or both of them [25] in Al0.6Cr0.3NbMoTiV0.6Zr0.3 alloy. Application of model, based on Hseg andHmix [25,27], able to discriminate grain boundary segregation can be further used to tuned HEAs microstructure with proper chemical addition.

The bcc reflections in XRD patterns of the non-equimolar compositions after annealing are broad. The pattern could be fitted in Rietveld analyses using two bcc phases of close lattice parameter (around 1% difference). This is due to the concentration gradients in the alloy matrices. It is, however, recognized that the alloys contain a unique bcc phase which has been referred to throughout the paper. Their average lattice constants are reported in Table 2. Vegard’s law seems to apply to these refractory HEA phases. This has been verified by using the lattice constant of bcc elements and the hypothetical bcc lattice constant of Al obtained from those of Al-V bcc solid solution [28]. Zr and Ti bcc lattice parameters at room temperature were calculated from atomic volumes of hcp phases [12].

Having verified that the solid solutions in Cr0.3NbMoTiZr0.3, Cr0.3NbMoTiV0.6Zr0.3 and Al0.6Cr0.3 NbMoTiV0.6Zr0.3

are ideal, their configurational entropy, Sconf, after annealing are 1.50R, 1.68R and 1.70R, respectively, (Tab. 2) showing that all phases comply with the definition of HEAs.

The Vickers hardness of all alloys is given in Tab.1. All values are substantial both in the as-cast and annealed states, the value of the hardness in the as cast state and after heat treatment are comparable, possibly because of the combined effect of the HEA solid solution and secondary phases. The Cr0.3NbMoTiV0.6Zr0.3 was chosen for compression testing which resulted in brittle behaviour with yield strength of 1619 MPa. This datum obtained for the as cast specimen, compares well with compression yield strength found in alloys of similar microstructure composed of bcc and Laves phase [2] and scales as expected from Tabor law with the hardness. Although strain measurement of the sample was not available in this case, examination of the post-test (failed) sample indicated an estimate of 1.6% plastic strain to failure. Hardness testing of samples before and after heat treatment showed little change in hardness (506 HV before, 494 HV after). Conclusions

The differences in atomic radius (and electronegativity () have been used as parameters to develop non-equimolar High Entropy Alloys based on refractory metals. The method for selecting elements has been verified, first with the septenary alloy CrMoNbTiVWZr which resulted in a multi-phased material, as predicted. The strategy for designing new alloys continued by considering the solubility of elements in the main bcc matrix of CrMoNbTiVWZr and the synthesis of a quinary alloy compositions (Cr0.3NbMoTiZr0.3) meeting the threshold values for the parameters andoutlined in [14] for a bcc multicomponent

solid solution, especially after annealing treatment, and progressed with the addition of a suitable content of a sixth and seventh element (V for Cr0.3NbMoTiV0.6Zr0.3 and Al for Al0.6Cr0.3NbMoTiV0.6Zr0.3) according to the

same parameter requirements. The alloys are mainly composed of bcc multicomponent solid solution together with minor quantities of Laves and hcp phases.

The experimental results highlight the occurrence of refractory high entropy phases in all annealed alloys. The minority Laves phase and hcp solid solution can be considered of use for dispersion hardening of the materials.

Acknowledgments

This work has been performed in the frame of the EU-7FP Accelerated Metallurgy Project (ACCMET, contract NMP4-LA-2011-263206).

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Figure 1

Scanning Electron Microscope images obtained in Back Scattered Electron Mode (BSE) of (a)

CrMoNbTiVWZr, (b) Cr0.3NbMoTiZr0.3, (c) Cr0.3NbMoTiV0.6Zr0.3, (d) Al0.6Cr0.3 NbMoTiV0.6Zr0.3 in the as-cast state.

Figure 2

X-ray Diffraction patterns of CrMoNbTiVWZr, Cr0.3NbMoTiZr0.3, Cr0.3NbMoTiV0.6Zr0.3 and Al0.6Cr0.3

Figure 3

Scanning Electron Microscope images in Back Scattered Mode of (a) CrMoNbTiVWZr, (b) Cr0.3NbMoTiZr0.3, (c)

Cr0.3NbMoTiV0.6Zr0.3, (d) Al0.6Cr0.3 NbMoTiV0.6Zr0.3 after heat-treatment (II cycle in HT-DSC from RT to 1550 °C

for the equimolar CrMoNbTiVWZr alloy and I cycle in HT-DSC from RT to 1550 °C followed by an annealing treatment at 1350 °C for 3h). In the case of (b) Cr0.3NbMoTiZr0.3, (c) Cr0.3NbMoTiV0.6Zr0.3, (d) Al0.6Cr0.3

Figure 4

X-ray Diffraction patterns of CrMoNbTiVWZr, Cr0.3NbMoTiZr0.3, Cr0.3NbMoTiV0.6Zr0.3 and Al0.6Cr0.3

NbMoTiV0.6Zr0.3 after heat-treatment (from RT to 1550 °C followed by an annealing treatment at 1350 °C for

3h for non-equimolar alloy and II cycles in HT-DSC for the equimolar one). In b enlargement of portion of the pattern at low angles are reported in logarithmic scale used to highlight reflections belonging to secondary phases.

Figure 5.

Scanning Electron Microscope images obtained in Back Scattered Electron Mode (BSE) of Cr0.3NbMoTiV0.6Zr0.3 sample produced in larger quantities (1 Kg). As cast state (a) and after heat treatment at

1100 ° C for 48 hours.

Table 1

Selection parameters calculated for the alloy synthesized in this work computed from the properties of elements: atomic radius mismatch Allen electronegativitymismatchnumber of valence electron per atomVEC), number of itinerant electron per atom (e/a), theoretical melting temperature Tm, Vickers Hardness in the as cast state and after annealing.

Alloy   VEC VEC Tm (°C)





 

AS CAST HV0,3 AFTER ANNELING CrMoNbTiVWZr 6,59 6,82 5,143 1,571 2531 1,93 727±18 677±18 Cr0.3 NbMoTiZr0.3 4,93 5,54 5 1,361 2461 1,50 501±31 531±26 Cr0.3 NbMoTiV0.6Zr0.3 5,19 5,62 5 1,452 2422 1,70 518±15 504±29 Al0.6Cr0.3 NbMoTiV0.6Zr0.3 4,85 6,42 4,75 1,646 2236 1,83 612±13 587±31

Table 2

The nominal composition (at %) of the alloys prepared in this work which is confirmed within the scatter by general EDS measurements. The EDS results for composition of the multicomponent bcc solid solution obtained after annealing the synthesized samples. The configurational entropy of the multicomponent bcc solid solution,



The experimental and calculated lattice parameter of the bcc solid solution after annealing.

ELEMENTS Ti V Cr Zr Nb Mo W/Al S_{ }conf _{Lattice par.}Calcul.

(BCC) Exper Lattice par. (BCC) CrMoNbTiVWZr NOMINAL C. 14,3 14,3 14,3 14,3 14,3 14,3 14,3 (W) -MATRIX AFTER ANNELING 20 ± 1 14,2 ± 0,3 7,7 ± 0,2 7 ± 1 19,0 ± 0,3 16 ± 1 16.0 ± 0,4 (W) 1,88 R Cr0.3 NbMoTiZr0.3 NOMINAL C. 27,8 - 8,3 8,3 27,8 27,8 -MATRIX AFTER ANNELING 27 ± 1 - 9,0 ± 0,4 10 ± 1 30 ± 1 24 ± 1 - 1,504 R 3,24 ± 0.02 3,232± 0,004; Cr0.3 NbMoTiV0.6Zr0.3 NOMINAL C. 23,8 14,3 7,1 7,1 23,8 23,8 -MATRIX AFTER ANNELING 23 ± 1 14 ± 1 7 ± 1 8 ± 1 26 ± 1 22 ± 1 - 1,684 R 3,21±0.03 3,220±0.002; Al0.6Cr0.3 NbMoTiV0.6Zr0.3 NOMINAL C. 20,8 12,5 6,3 6,3 20,8 20,8 12,5 (Al) MATRIX AFTER ANNELING 22 ± 1 14 ± 1 6 ± 1 5 ± 1 23 ± 1 22 ± 2 8 ± 1 (Al) 1,800 R 3,19±0.02 3,186±0.004; SUPPLEMENTARY INFORMATION

TABLE 1 IN THE SUPPLEMENTARY INFORMATION.

NOMINAL

COMPOSITION GENERAL EDS St. dev. Element

BRIGHT PHASE St. dev. LIGHT GREY PHASE St. dev. DARK GREY PHASE St. dev. 14,3 15 1 Ti 11 2 10 2 29 6 14,3 15 2 V 14 3 17 2 12 2 14,3 12 2 Cr 7 2 24 2 11 6 14,3 17 1 Zr 2 3 27 3 32 2 14,3 14 1 Nb 16 3 10 2 11 2 14,3 12 2 Mo 16 3 8 2 3 3 14,3 15 2 W 34 9 4 2 1 1

AFTER II CYCLES IN HT-DSC

Element MATRIX St. dev. LIGHT GREY St. dev. DARK

PHASE St. dev. Ti 20,2 0,5 8 1 11 1 V 14,2 0,3 13 1 3 3 Cr 7,7 0,2 16 1 3 4 Zr 7 1 30 2 70 14 Nb 19,0 0,3 7 1 - - Mo 16 1 13 1 11 1 W 16,0 0,4 12 1 2 2

TABLE 2 IN THE SUPPLEMENTARY INFORMATION. EDS analysis of the Cr

as-cast and after I cycle in HT-DSC from RT to 1550 °C followed by an annealing treatment at 1350 °C for 3h.

Element Nominal

compositions General EDS AS-CAST

INNER PART OF DENDRITE GREY ZONE BETWEN DENDRITE AND INTERDRITE INTERDENTRIC GREY PHASE INTERDENTRIC DARK PHASE LARGE GREY PHASE

At% At % St. dev. Elements At % St. dev. At % St. dev. At % St. dev. At % St. dev. At % St. dev.

Ti 27,8 27 1 Ti 22 3 30 1 21 1 30 2 21 2 Cr 8,3 9 1 Cr 5 2 8,7 0,6 27 2 13 4 2 1 Zr 8,3 10 2 Zr 5 2 13 4 30,1 0,7 33 2 62 1 Nb 27,8 28 1 Nb 34 3 26 4 11,9 0,8 15 1 8 1 Mo 27,8 26 1 Mo 34 4 22 2 10 2 9 3 7 1 ANNEALED

Light grey Dark grey Element At% St.

dev. At% St. dev.

Ti 27 1 ₁₈ 7

Cr 9,0 0,4 ₀

-Zr 10 1 ₇₀ 8

Nb 30 1 ₀

TABLE 3 IN THE SUPPLEMENTARY INFORMATION EDS analysis of the Cr

as-cast and after I cycle in HT-DSC from RT to 1550 °C followed by an annealing treatment at 1350 °C for 3h.

NOMINAL COMPOSITION

GENERAL

EDS St. dev. Element

INNER PART OF DENDRITE St. dev. GREY ZONE BETWEN DENDRITE AND INTERDRITE INTERDENTRIC

GREY PHASE St. dev.

INTERDENTRIC DARK PHASE St. dev.

23,8 23 2 Ti 24 3 26 16 1 33 3 14,3 15 1 V 12 1 17 17 1 10 2 7,1 8 1 Cr 6 2 9 20 2 9 2 7,1 9 1 Zr 4 2 10 28,7 0,5 30,2 0,5 23,8 26 1 Nb 28 4 23 9,7 0,6 13,2 0,3 23,8 19 2 Mo 26 4 17 8 1 5 1 ANNEALED Element MATRIX St, dev. DARK

PHASE St, dev. Ti 23 1 16 2 V 14 1 6 2 Cr 7 1 - -Zr 8 1 78 4 Nb 26 1 - -Mo 22 1 -

-TABLE 4 IN THE SUPPLEMENTARY INFORMATION EDS analysis of the Cr

produced in larger quantity (1 Kg) in the as-cast and after an heat treatment at 1100 °C for 48 hours under inert atmosphere. NOMINAL COMPOSITION GENERAL EDS St. dev. Element _DARK MATRIX St. dev. LIGHT PHASE St. dev. BIPHASIC REGION St. dev. 23,8 21.3 0.2 Ti 28 2 17.7 0.5 37.7 0.6 14,3 14.3 0.1 V 15.0 0.8 12.5 0.5 3.5 0.3 7,1 6.9 0.3 Cr 7.5 0.5 3.7 0.2 1.4 0.3 7,1 8.7 0.4 Zr 8 2 2.1 0,3 44 1 23,8 24.4 0.3 Nb 23.6 0.5 29.8 0,3 11.3 0,6 23,8 24.3 0.3 Mo 18 2 34.2 0.9 1.8 0.3 ANNEALED Element MATRIX St, dev. LIGHT SPOT SECONDARY PHASE St, dev. Ti 24 1 17.5 0.3 8.2 0.2 V 15.2 0.2 12.1 0.3 17.1 0.2 Cr 6.6 0.6 3.8 0.2 19.0 0.3 Zr 6 1 2.5 0.3 29.0 0.1 Nb 25.6 0.7 29.9 0.3 10.9 0.1 Mo 23 1 34.4 0.6 15.7 0.5

TABLE 5 IN THE SUPPLEMENTARY INFORMATION EDS analysis of the

NOMINAL COMPOSITION GENERAL EDS St. dev. Element INNER PART OF DENDRITE St. dev. INTERDENTRIC GREY PHASE St. dev. INTERDENTRIC DARK PHASE 12,5 10,0 0,5 Al 7 1 19 1 21 2 20,8 24 1 Ti 22 1 21 3 12 2 12,5 11,1 0,4 V 11 1 7 1 9 1 6,3 6,6 0,4 Cr 5 1 8 1 11 1 6,3 7 1 Zr 2 2 30 6 28 2 20,8 23 1 Nb 28 3 9 4 10 1 20,8 19 1 Mo 25 2 6 2 9 2 ANNEALED Element MATRIX St. dev. phase inside grains St. dev. Intergranular phase St. dev. Al 8 1 13 1 17 1 Ti 22 1 22 1 15 4 V 14 1 11 1 10 1 Cr 6 1 9 1 13 3 Zr 5 1 21 4 29 6 Nb 23 1 12 2 6 2 Mo 22 2 12,4 0,9 10 3