Abstract
High-entropy alloys (HEAs) with multiple-principal elements seriously challenge the conventional alloy-design and strengthening theories, and thus prompt extensive attention because of their interesting properties and the related scientific importance. However, the understanding of their complex phase stability and sluggish diffusion effects, as well as their correlation with mechanical properties, remains unclear and needs further investigation. A variety of criteria has been proposed from empirical parameters to evaluate the phase selection in cast HEAs, but none of them has worked well when referring to predicting the equilibrium phase accurately and effectively for guiding alloy composition design. In this thesis, a computational-aided software called thermo-Calc was employed to guide the alloy design, and alloying effects on phase evolution and mechanical properties of HEAs were systematically investigated.Phase selection and stability are vitally important for HEAs, but the understanding of this phenomenon is very limited at the present time. The ability to predict the phase formation from the fundamental properties of constituent elements would greatly benefit our capability for alloy design. Here, the phase competition and stability of several HEAs were studied, and effects of alloying additions and processing conditions on phase formation in these alloys were discussed. Alloying with chemically incompatible elements having a large difference in either the atomic size or enthalpy of mixing with constituting components in HEAs, e.g., Cu and Al in the FeCoNiCr alloy system, inevitably induced phase separation and stimulated formation of duplex solid solution phases and even intermetallic compounds. The as-cast phase of the FeCoNiCrMn HEA was extremely stable at high temperatures due to the good chemical compatibility among constituent components, but in the FeCoNiCrAl and (FeCoNiCrAl)99Si1 HEAs with incompatible elements of Al and Si, pretreatment and annealing processes could induce phase transitions and formation of new phases, indicating that the as-cast solid solution phases were destabilized by quenched-in chemical segregation, resulting from additions of the dissimilar elements.
Recent studies indicated that HEAs possess unusual structural and interesting mechanical performance, for example, face-centered cubic (fcc)-type HEAs exhibit outstanding ductility and fracture toughness even down to the liquid nitrogen temperature. However, a fcc HEA matrix alone is insufficiently strong for engineering applications, and other strengthening mechanisms are needed to be incorporated. The alloy design principle generally suggests the suppression of ‘brittle’ intermetallic compound formation which seriously embrittles structural materials during plastic deformation. To against this common alloy design principle, we are purposely added those brittle but hard particles in the ductile fcc CoCrFeNi HEA to strengthen it through a computational-aided alloy design approach.
It was found that additions of Nb into the CoCrFeNi HEA changed the original phase constitution and led to the formation of an ordered Nb-enriched Laves phase in the fcc matrix. After the precipitation of the Nb-enriched Laves phase, the strength was drastically increased. Among these alloys, the CoCrFeNiNb0.155 alloy containing a volume fraction of 9.3% Laves phase exhibited attractive tensile properties with the yield strength, fracture strength and plastic strain reaching 321 MPa, 744 MPa and 21.3%, respectively. This particular alloy with a strong solid-solution strengthening and precipitation hardening resulted from the Nb additions can serve as a good candidate for engineering applications at varied temperatures. Moreover, it was found that the alloying of Mo into the CoCrFeNi HEA catalyzed precipitation of brittle and hard σ and μ intermetallic phases in the fcc matrix; surprisingly we have revealed that such precipitation tremendously strengthened the CoCrFeNiMo0.3 alloy without causing serious embrittlement. This particle-strengthened alloy exhibited a tensile strength as high as 1.2 GPa and a tensile elongation of ~19% at ambient temperature. Our major efforts have thus been devoted to characterize the structures and properties of these intermetallic particles in this alloy. A large number of σ and μ intermetallic phases with different morphologies were formed in the Mo0.3 alloy as detected by transmission electron microscopy (TEM) analyses. Nanoindentor measurements reveal that these particles have a hardness at least higher than 8 GPa. However, the large precipitates formed upon solidification, possessing a mixture of the fcc phase and σ phase, were toughened by the ductile fcc phase. While those particles precipitated out from the supersaturated fcc matrix are relatively fine because of the slow diffusion process in HEAs, and they are very effective in strengthening the alloy.
The study of mechanical behavior of the (CoCrFeNi) matrix reveals an extremely high work-hardening exponent of 0.75, which promotes a uniform deformation and suppresses microcracks propagation associated with these particles. All of these contribute to a high fracture strength and decent ductility of the Mo0.3 alloy hardened by the σ and μ particles. Our study presents a very first successful demonstration of using complex hard and brittle intermetallic particles to manipulate the mechanical properties of a fcc-type (CoCrNiFe) HEA with Mo additions, and the gained findings are important not only for the understanding of the strengthening mechanism of these emerging metallic materials, but also for the future development of high-performance HEAs for engineering applications.
Furthermore, previous characterization of the lattice distortion in HEAs was based on the diffraction intensity decrease of alloys prepared by casting. However, cast alloys usually have serious preferred orientation effects, which could seriously affect the X-ray diffraction distributions of individual crystal planes. Here, powders X-ray diffraction patterns of the CoCrFeNiMox (x=0~0.23) alloy system (alloy powders were prepared as the collaboration between City U and Central South University in Changsha, China.) without preferred orientation effects were utilized and the effects of large atom Mo additions on lattice distortion of the CoCrFeNi alloy were characterized carefully. The measurement of diffraction line broadening confirmed that the lattice in HEAs is highly distorted and this distortion also contributes to solid solution hardening.
| Date of Award | 30 Aug 2016 |
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| Original language | English |
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| Supervisor | Chain Tsuan LIU (Supervisor) |