Eutectic High Entropy Alloys (EHEAs)

Transcription

1 Eutectic High Entropy Alloys (EHEAs) Sheng Guo Materials and Manufacturing Technology Department Chalmers University of Technology, Gothenburg, Sweden E mail: sheng.guo@chalmers.se C MAC Days 2014, Zagreb

2 Outline A brief introduction to HEAs Phase selection in cast HEAs Some issues with cast HEAs Eutectic HEAs: An example Conclusions

3 Introduction: High Entropy Alloys (Yeh, et al., Mater Chem Phys, 2007) N=7 N=6 N=5 N=4 (Adv.Eng.Mater, 2004) N=3 N=2 N=1 Highly concentrated solid solutions

4 Potential of HEAs as structural materials AlCoCrFeNiTi 0.5 y =2.26GPa f =3.14GPa p =23.3% (Yeh, et al., Adv Eng Mater, 2004) (after 1000 o c/12h) 460 MPa@1600 o C better than superalloys (Zhou et al., APL, 2007) Very high hardness can be achieved (Senkov, et al., Intermetallics, 2011) Disordered bcc solid solution was reserved after annealing at 1400 o cfor 19h

5 High entropy effect enhances the formation of solution phases Possible competing states (elemental phases, compounds, solid solutions) G mix = H mix T S mix Solid solution phases having the highest mixing entropy thus become highly competitive and more stable especially at high T

6 Q1:Solid solution or amorphous phase? (Nature, 1993) S R c ln c N mix i i i 1 when N elements are mixing in equiatomic ratio (c 1 =c 2 = =c N ), the mixing entropy reaches the maximum: S Rln N mix Based on the confusion principle and high entropy points of view, we can easily understand that random solid solutions tend to be stable in HEAs. But why not form a glassy (amorphous) phase then?

7 High entropy bulk metallic glasses (Ma et al., Mater Trans, 2002) (1.5mm) (Takeuchi et al., Intermetallics, 2011) (Gao et al., J Non-Crys. Solids, 2011)

8 Intermetallic compounds can certainly form in equiatomic multi component alloys For example: XRD patterns of the CoCrCuFeNiTi x samples (x = 0, 0.5, 0.8, and 1) (Wang et al., Intermetallics, 2007) (Yang et al., Mater Chem Phys, 2007) So, can we predict the phase selection (solid solution, amorphous phase and intermetallic compound) in equiatomic multi component alloys?

9 A1: Statistical analyses of phase selection in HEAs 2-parameter map Solid solution phases form when is small, and H mix is either slightly positive or insignificantly negative; Amorphous phases form when is large, and H mix is noticeably negative; In the intermediate conditions (in terms of and H mix ), intermetallic compounds compete with tboth amorphous phases & solid solution phases. (Guo et al., Prog Nat Sci: Mater Int, 2011; Guo et al., Intermetallics, 2013)

10 Q2: fcc or bcc solid solution? (Yeh, et al., Mater Chem Phys, 2007) N=7 N=6 N=5 N=4 N=3 N=2 N=1 bcc bcc fcc+bcc fcc+bcc fcc+bcc fcc+bcc fcc+bcc fcc+bcc fcc+bcc fcc+bcc fcc fcc fcc Al x CoCrCuFeNi x=3 x=0 (Tong et al., Metall Mater A, 2005)

11 Q2: fcc or bcc solid solution? Why is that?!

12 A2: Valence Electron Concentration is the key AlCo 0.5 CrCuFeNi; AlCoCr 0.5 CuFeNi AlCoCrCu 0.5 FeNi; AlCoCrCuFe 0.5 Ni AlCoCrCuFeNi 0.5 ; AlCo x CrCu 0.5 FeNi AlCo x CrCu 0.5 FeNi; AlCo x CrCu 0.5 FeNi AlCoCr x Cu 0.5 FeNi; AlCoCrCu 0.5 Fe x Ni bcc bcc+fcc fcc Valence electron concentration AlCoCrCu 0.5 FeNi x ; AlCoCrCu 0.5 FeNi x CrCuFeMnNi; CoCrFeMnNi Al x CrCuFeMnNi; Al x CrCuFeMnNi Al 0.8 CrCu 1.5 FeMnNi; Al 0.8 CrCuFe 1.5 MnNi Al 0.8 CrCuFeMn 1.5 Ni; MoNbTaW MoNbTaVW; AlB x MnNiTi Al x C 0.2 CuFeMnNi (Guo et al., JAP, 2011) A higher VEC favors the formation of fcc solid solutions, while a smaller VEC tends to stabilize the bcc solid solutions A mixture of fcc and bcc solid solutions forms at intermediate VEC

13 Some issue with cast HEAs Porosity, particular for large ingots Inhomogeneity/Segregation Conflict between strength/ductility (Tong et al., Metall Mater Trans A, 2005)

14 Why Eutectic Alloys? highly stable microstructures that do not revert, or coarsen, easily at elevated temperatures; high thermodynamic stability and kinetic resistance to thermal degradation; development of low energy lamellar and rod form boundary structures; high strengths and creep resistance because their microstructures act as natural in situ composite materials; better castability (less porosity) better compositional homogeneity (less segregation) (Glicksman, Principle of Solidification, 2011) Inspirations: Eutectics with high melting points have formed the basis for a number of interesting candidate high temperature alloys for application to the high temperature components of gas turbine engines.

15 Eutectic High Entropy Alloys An example: AlCoCrFeNi 2.1 ~ 2.5 kg of homogenous and almost casting defects free large ingots

16 Eutectic High Entropy Alloys soft fcc/ hard NiAl like B2 eutetic microstructure melting temperature ~ 1350 o C (NiAl: 1674 o C) density of ~ 7.4 g/cm 3 (NiAl: 6 g/cm 3 )

17 Eutectic High Entropy Alloys a Engineering stress-strain True stress-strain b o C 700 o C Stress/MPa True stess/mpa Strain/% True strain/% balanced tensile fracture strength and ductility, for large ingots the decent mechanical properties can be maintained to 700 o C strong work hardening behavior

18 Eutectic High Entropy Alloys a NiAl <001> b True stress/mpa , non-eheas UTS, non-eheas 0.2, EHEA True stress/mpa UTS, EHEA Elogation to failure/% Temperature/ o C overall fracture strength/tensile ductility better than NiAl/Cr(Co) eutectic alloys a large space to improve at higher temperatures though, with a compromise with density

19 Eutectic High Entropy Alloys after 8% cold rolling mechanical properties can be further tuned by thermomechanical treatments

20 Conclusions Entropy alone can not stabilize the solid solutions in multi principal element alloys; By using empirical physical metallurgy principles, formation and even type of solid solutions can be reasonably controlled; Eutectic high entropy alloys might be a promising alloying strategy to develop new class of high temperature alloys.