1、FeCoNiCrMn 高熵合金的组织稳定性高熵合金的组织稳定性及变形行为及变形行为吕昭平吕昭平 教授教授北京科技大学北京科技大学新金属材料国家重点实验室新金属材料国家重点实验室Acknowledgements? Students: W. H. Liu, S. Y. Li, H. L. Huang and Z. F. Lei? Collaborators: H. Wang, Y. Wu, and X. J. Liu? National science foundation of China (Nos. 51010001, 51001009, and 51271212)? “111” Progra
2、m (B07003)? Program for Innovative Research Team in UniversityTraditional alloys are mostly based on one primary elementSteels (Fe), Al alloys, Ti alloys, Mg alloys, Supper alloys (Ni)Discovery of high entropy alloysIn 2004, Multicomponent FeCoNiCrMn alloy firstly reported by Cantor B. with a simple
3、 fcc solid-solution structure.Cantor B et al. MSE A, 2004, 375-377: 213-218. Definition of high entropy alloysStill in 2004, the concept of high entropy alloy was firstly introduced by Yeh JWYeh JW et al. Adv Eng Mater, 2004, 6: 299-303. Simple phase formation but complex metallurgical phenomenonCu-
4、Zr binary alloy systemThe Gibbs Phase RuleP=N+1-FWhen N=5, P=6FeCoNiCrMnActually HEAs alloy have a simple solid-solution structure (mainly fcc or bcc) !Features of HEAs severe lattice distortionLattice distortionLattice distortionSolution Solution strengthening effectstrengthening effectZhou, et al.
5、 Appl Phys Lett, 2008, 92: 241917Senkov ON, et al. Intermetallics, 2011,19:698Features of HEAs sluggish diffusion?Constituent elements in the HEA matrix diffuse much slowly over the entire temperature range?The diffusion coefficient of Ni is the smallest among that of all constituentsTsai et al. Act
6、a Mater 2013; 61:4887Features of HEAs high phase stability1400oC19hAs-castAfter compression at 1073 KSenkov et al. Intermetallics 2011; 19: 698-706?Formation of a single bccphase?The bccphase is highly stable up to 1600 oCFeatures of HEAs cock tail effect?the effect indicates thatthe unexpected prop
7、erties can be obtained after mixing many elements, which could not be obtained from any one independent element.Extremely high toughness of a typical fcc HEA ?The fracture toughness of Bernd Gludovatz et al. Science 2014,345:1153FeCoNiCrMn exceeds 210 MPam-1/2Abnormal low-temperature mechanical prop
8、erties?With the decrease of testing temperature, both tensile strength and ductility are increased;?The fracture toughness kept almost unchanged;Promising high-temperature mechanical properties of bcc HEAsSenkov et al. Intermetallics 2011; 19: 698-706?High yield strength at temperatures up to 1600o?
9、CThe strong resistance to high-temperature softening, as compared to the superalloysInteresting physical properties of HEAs: discovery of superconductivity?Ta34Nb33Hf8Zr14Ti11HEA possesses an body-centered cubic structure of lattice parameter a 3.36 ?.?It is a type II superconductor with a transitio
10、n temperature Tc 7.3 KP. Ko?elj et al. PRL 2014 ,113: 107001 Diffusion barrier materials?High phase stability -no interaction with substrates;?Low diffusion kinetics- -high diffusion resistance at elevated temperatures 15The research activities on HEAs at USTB Progress in Materials Science, 2014; 61
11、:1-93Our purposeDesign the FeCoNiCrMn based HEAs for high-temperature applicationsContent?Phase formation and stability ?Grain growth at elevated temperatures?Deformation behavior?Alloying effects (to enhance high-T mechanical performance)1.Phase formation and stability are influenced by not only ch
12、emistry but also processing conditions2.Effects of alloying additions on phase formation, stability and properties are not as simple as expectedPhase formation in the as-cast FeCoNiCrMn?Basically the alloy has a single fcc phase but with a small fraction of unidentified phase (Cr2Mn oxide ?)The FeCo
13、NiCrMn high entropy alloy showed high phase stability (a)(b)(c)?Thesingle fcc phase in the FeCoNiCrMn alloy is stable even after 30 days annealing at 950 oC The FeCoNiCrMn high entropy alloy showed high phase stability (b)(c)?Texture seems changed with the processing conditions?No second phase was f
14、ormed during the rolling/annealing processes Content?Phase formation and stability?Grain growth at elevated temperatures?Deformation behavior?Alloying effectsGrain growth behavior of the FeCoNiCrMn high entropy alloy was studied in detail70% cold rolled850C/1h850C/2h850C/2h925C/1h925C/2h925C/3.5hGra
15、in coarsening exhibited a classical power law behavior in the FeCoNiCrMn alloy?n = 3 and D0 = 1.0 m m?n is larger than 2 which is for Liu et al., Scripta Materialia 2013;68:526the “ideal” grain growth in single-phase pure materialsThe apparent activation energy for grain growthsuggests that sluggish
16、 diffusion indeed occurred?The Q value is much higher than that for AISI 304LN -1stainless steels, which is only about 150 kJ molThe hardness values at different temperatures closely follow the classical Hall Petch relationship ?The softening mainly from grain coarsening?The KHPis larger than 600 Mp
17、a m m-0.5(the upper-bound for fcc metals) , suggesting that grain boundary hardening efficiency is obviously higher Content?Phase formation and stability?Grain growth at elevated temperatures?Deformation behavior?Alloying effectsIn-situ neutron diffraction study of deformation behavior of the FeCoNi
18、CrMn alloy was conductedVULCAN system at Spallation Neutron Scattering, Oak Ridge National Laboratory FCC(111)(200)Relative Intensity, a. u.(331)(311)(222)(220)1.0d spacing, Angstrom1.52.0Wu et al., Appl. Phys. Letts. 2014;104:051910Stress Increase Strong elastic anisotropy was observed during the t
19、ensile loading?The lattice strain change is strongly dependent on the grain orientations?The 200 planes have the lowest elastic modulus while the 331 planes have the highest?Youngs modulus anisotropy (E111/E100) is 1.98, close to that of Ni ( 2.17) but smaller than that of typical fcc steels (3.20)M
20、odulus: GPaDevelopment of a textured structure during the tensile deformation?The peak intensity of the 220 and 200 reflections decreased, while that of the 111 reflections increased?Textural evolution of the current HEA during tension appears to be similar to those of typical FCC metals and alloys,
21、 for example, polycrystalline copper with columnar grainsThe single crystal elastic constants of the current HEA were determined by the Kroners modelThe cubic elastic anisotropy factor?The shear anisotropy was calculated as 2.84, much smaller than that of the ternary FeCrNi alloy (3.77), but close t
22、o that of the pure FCC-Ni (2.51), manifesting that the elastic anisotropy behavior of the current HEA is close to that of its FCC component, i.e., pure Ni.The experimentally determined peak broadening data can reveal the dislocation types during deformation?The slope in the above plot is 2.189, whic
23、h seems like a balancing value for the edge (1.492) and screw (2.298) dislocations, but close to that of the screw dislocation?A representative bright field image with wiggled dislocations in a deformed specimen, suggesting a mixed dislocation characteristicsFlow behavior at different temperatures a
24、nd strain rates were investigated ?The temperature is in between 1023 He et al., Intermetallics 55, 2014; 9-14.and 1123 K?The strain rate ranges from 6.410 10-7to 8.013 10-4s-1Steady-state deformation behavior of FeCoNiCrMnThe Norton Equation?Region II with a high stress exponent at the high strain
25、rates (or stresses) while Region I with a low stress exponent at the low strain rates (or stresses)?The activation energy in two regions are comparable to that for lattice diffusion. For example, Ni (317.5 kJ/mol), Cr (292.9 kJ/mol), and Mn (288.4 kJ/mol)The normalized plot indicates that the data o
26、btained at various temperatures (1023-1123 K) collapse into one single master curve ?In the high strain rate regime, n is 5 and the activation energy is 330 kJ/mol, suggesting a dislocation-climb mechanism and the slowest diffusing species Ni controls the rate process. ?In the low strain rate regime
27、, n is 3 and the activation energy is 280 kJ/mol, suggesting that the mechanism is the dislocation gliding and the deformation rate is controlled by the diffusion of one of the constituent elements which acts as the solute atom.1. Relatively lower steady state flow stress than super alloys2. Oxidati
28、on resistance is low due to highly active element MnContent?Phase formation and stability?Grain growth at elevated temperatures?Deformation behavior?Alloying effectsAl effectsAddition of Al in the (FeCoNiCrMn)100-xAlxalloysinduced a phase transition from fcc to bcc starting at Al8He et al., Acta Mat
29、er. 2014; 62:105.Microstructure observation further confirms the phase transition resulted from the Al addition?Bcc phase becomes dominant in alloy Al13, Al14 and Al16 , and the minor fcc phase mainly lies in the bcc grain boundaries.TEM characterization reveals the detailed phase characteristics Th
30、e cubic-shaped particles are ordered B2 while the inter-precipitate area is disordered A2. fcc: 74.6%.The cubic-shaped precipitates become slightly smaller but more close-packed. fcc: 11% Al are too brittle to be deformed due to formation of ordered BCC structures.Summary Phase formation and stabili
31、ty - high(homogenization, long term aging, rolling)Grain growth at Al effects -high temperatures solution -slowstrengtheningFeCoNiCrMnalloyDeformation behavior -RT: similar to Ni, mix dislocationsHT: high strain rate, dislocation climblow strain rate, dislocation glideHigh-T materials Hardening mech
32、anisms urgently needed in the FeCoNiCr HEA system?Secondary phase hardening resulted from bcc phases (NiAl B2) in FeCoNiCrMn is not very satisfactory.?Based on previous studies in steels and superalloys, L12 phase ( phase -Ni3Al) is a desirable secondary phase for a fcc matrix, both at room and elev
33、ated temperature.金属学报，金属学报，1978,14(3) :227-237金属学报金属学报. 2014, 50(10): 1260-12GH984G合金中球形Ni3Al型 沉淀相颗粒Addition of Al alone cannot effectively promote precipitation of a large density of nanosized particles uniformly dispersed in the matrixShun TT et al. JAC 2009;479:157 160The L12 precipitates was obs
34、erved in the fccAl0.3FeCoNiCr?The tensile strength is only increased slightly after aging due to the fact that the precipitates distribute inhomogeneously. ?More work has been done for precipitation in HEAs.Unsolved scientific problems/ Key research topics?Phase formation and stability(why solid-sol
35、utions & under what conditions, phase transition, enthalpy effects, etc.)?Alloy design and preparation(alloying effects, structures & properties optimization, processes, etc.)?Deformation mechanism and mechanical properties?Physical and chemical properties?Applications: different service temperatures
