Eutectic High‐Entropy Alloys (EHEAs)
Adrianna Lozinko & Sheng Guo
Industrial and Materials Science (IMS) DepartmentChalmers University of Technology, Gothenburg, Sweden
E‐mail: [email protected]
DTU, Aug 28, 2017
outline
A brief introduction to HEAs Some issues with cast HEAs Eutectic HEAs Conclusions
HEAs: new strategy for alloy design
Traditional alloys have only 1 (steels, Al alloys, Cu alloys, etc.) or 2 principal elements (NiAl, FeAl, etc.)
High‐entropy alloys have at least 5 (4?) principalmetallic elements, and have equal or close‐to‐equal compositions
Example, Al‐Co‐Cr‐Cu‐Fe‐Ni systemEquimole: AlCoCrCuFeNi Non‐equimole: AlCo0.5CrCuFe1.5Ni1.2Minor element addition: AlCo0.5CrCuFe1.5Ni1.2B0.1C0.15
why are they called high‐entropy alloys?
1ln
N
i ii
R c c
they have high configuration entropy
It is more convenient to define HEAs by the magnitude of configuration entropy in the hightemperature (ideal or regular solution) state: ∆Smix > 1.5R
(Miracle et al., Entropy, 2014)
what we talk about when we talk about high‐entropy alloys
why high entropy?high‐entropy stabilizes the formation of solid solution phases
△Gmix =△Hmix ‐T△Smix
single phase solid solutionco‐existence of two solid solution phases
(Murty, Yeh and Ranganathon, High
Entropy Alloys, Elsevier, 2014)
in the middle
why bother?high‐entropy alloys open up vast unexplored compositional space
era of high‐entropy alloys?
(Adv.Eng.Mater, 2004)
(Yeh, et al., Mater Chem Phys, 2007)
N=1
N=2
N=3
N=4
N=5
N=6
N=7
Highly concentrated solid solutions
potential of HEAs as structural materials
Very high hardness can be achieved
(after 1000 oc/12h)
(Yeh, et al., Adv Eng Mater, 2004)
AlCoCrFeNiTi0.5
y=2.26GPa
f=3.14GPa
p=23.3%
(Zhou et al., APL, 2007)
Disordered bcc solid solution was reserved after annealing at 1400 oc for 19h
(Senkov, et al., Intermetallics, 2011)
460 MPa@1600 oCbetter than superalloys
(Gludovatz et al., Science, 2014)
CoCrFeMnNi
some “hot areas” in HEAs (my personal perception)
phase selection and phase stability in concentrated multi‐component alloys
strength‐ductility trade offradiation resistancerefractory HEAsnew functionality(superconductivity,thermoelectricity, etc.)
(Li et al., Nature, 2016)
ductile and oxidation resistant refractory high entropy alloys: single bcc solid solution
interested block
some issue with cast HEAs Porosity, particular for large ingots Inhomogeneity/Segregation Conflict between strength/ductility
(Tong et al., Metall Mater Trans A, 2005)
why eutectic alloys? highly stable microstructures that do not revert, or coarsen, easily at elevated temperatures;
high thermodynamic stability and kinetic resistance to thermaldegradation;
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)
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.
(Glicksman, Principle of Solidification, 2011)
eutectic high‐entropy alloysan example: AlCoCrFeNi2.1
~ 2.5 kg of homogenous and almost casting defects free large ingots
(Lu et al., Sci Rep, 2014)
eutectic high‐entropy alloys
soft fcc/ hard NiAl‐like B2 eutetic microstructuremelting temperature ~ 1350 oC (NiAl: 1674 oC) density of ~ 7.4 g/cm3 (NiAl: 6 g/cm3)
(Lu et al., Sci Rep, 2014)
RT ‐70 oC ‐196 oC
(Lu et al., Acta Mater, 2016)
AlCoCrFeNi2.1 after tensile testing to fracture
no phase transformation till fracture
a large number of dislocations seen in the L12phase, but not in the B2 phase
(Lu et al., Acta Mater, 2016)
In‐situ synchrotron x‐ray diffraction
no phase transformation during tensile deformation deformation behavior resembles that of fcc alloys (limited role of B2 ) {200} and {311} planes show lowest elastic modulus and sustain larger lattice
strain
(Lu et al., Acta Mater, 2016)
eutectic high‐entropy alloys: dual‐phase (soft/hard) solid solutions
(Wani et al., Mater Res Lett, 2016)
(Wani et al., Mater Sci Eng A, 2016)
Summary High‐entropy alloys are highly concentrated, multi‐component
alloys (the name is a bit controversial, but a name is just a name) New materials new properties (high‐temperature/cryogenic temperature, etc.) & new applications Structural or functional properties? Eutectic high‐entropy alloys could become interesting
engineering materials, if they can be further optimized
understanding deformation mechanism helps alloy design
Every material
Thanks for your attention!Sheng Guo
Industrial and Materials Science DepartmentChalmers University of Technology
Gothenburg, SwedenE‐mail: [email protected]