Phase Formation And Strengthening Mechanisms Of Nano/ultra-fine Grained CoNiFeAlTi System High Entropy Alloys And Their Composites

Multi-principal-component high entropy alloys is commonly referred to as high entropy alloys(HEAs), which were loosely defined as a new class of alloy consisting of at least four principal-elements with 5-35 at.% concentrations for each element. Solid-solution phases are usually formed in HEAs and this has been attributed to a high entropy of mixing originating from multi-principal elments that can suppress formation of intermetallics or complex phases. Moreover, published reports show that HEAs can exhibit high strength and hardness, exceptional high-temperature strength, excellent corrosion resistance, high thermal stability excellent magnetic properties and mechanical performance at cryogenic temperatures, etc. Therefore, HEAs are attractive candidate structural and functional materials for a wide range of potential applications. Howerver, design rules, the actual state of high entropy of mixing, phase formation mechanisms, phase transformation rules and strengthening mechanisms, etc., remain poorly understood.In the present work, nano/ultra-fine grained CoNi FeAlTi system HEAs and their composites were fabricated by mechanical alloying(MA) of pure elemental powders followed by consolidation via spark plasma sintering(SPS). In the CoNiFeAlTi system HEAs, alloying behavior, phase composition, microstructure, mechanical properties, phase formation mechanisms and strengthening mechanisms were studied in detail, as well as the effect of entropy of mixing. An FCC structured single-phase Co25Ni25Fe25Al7.5Cu17.5 HEA was designed successfully, and accordingly its phase formation mechanisms and strengthening mechanisms were investigated deeply. In addition, four 10 vol.%TiC/CoNi FeAl0.4Ti0.6 HEA composites were prepared by four different methods respectively, i.e., TiC reinforcement was introdued into the Co NiFeAl0.4Ti0.6 matrix by adding TiC directly or using Ti+C(graphite) powders via reaction, and the CoNi FeAl0.4Ti0.6 matrix powders were produced by MA of elemental powders or using the 49 h alloyed powders. Subsequently, microstucture, phase formation mechanisms and strengthening mechanisms of the four 10 vol.%TiC/CoNiFe Al0.4Ti0.6 HEA composites were investigated deeply. Through in-depth investigation and analysis, the main conclusions are as follows:(1) Following MA, the Co NiFeAl0.4Ti0.6 HEA was composed of a primary BCC phase, a small amount of FCC phase and a trace amount of amorphous phase, in the mean time, a large number of dislocations and stacking faults, as well as some FCC structured twins were observed. Following SPS, bulk ultra-fine grained(UFG) CoNiFeAl0.4Ti0.6 HEA consisted of a primary ordered FCC(L12) phase and some ordered BCC(B2) phase. In addition, the L12 phase have a pretty high density of hierarchical intragranular nanoscale precipitates, and the primary precipitates are L12 structured(Ni,Co)3-(Ti,Al)-based intermetallic phase. The orientation relationship of the matrix of the L12 phase, the primary precipitates and the secondary precipitates is “Cubic-Cubic-Cubic”, in addition, a small amount of nanoscale twins were observed in partial matrix of the L12 phase. The bulk UFG CoNiFeAl0.4Ti0.6 HEA exhibits an utral-high strength and a utral-high haredness which is attributed to solid-solution strengthening, grain boundary strengthening, twin boundary strengthening, dislocation strengthening and precipitation strengthening. The yield strength, compressive strength, strain-to-failure and Vickers hardness are 2613 MPa, 3104 MPa, 7% and 761 HV, respectively. Note that the yeild strength of 2613 MPa is the highest value than that of previously reported HEAs.(2) Five alloys, CoNi Fe, CoNiFe Al0.4, Co NiFeAl0.4Ti0.6, Co NiFeAl0.4Ti0.6Cr0.5 and CoNi FeAl0.4Ti0.6Cr0.5Cu0.5 were designed in the order of increasing values of entropy of mixing. During the MA process, entropy of mixing have insignificant influence on the alloying sequence for the constituent elements. Following SPS, bulk CoNiFe medium-entropy alloy(MEA) exhibited a single FCC solid-solution phase with few WC contaminants, and bulk CoNi FeAl0.4 HEA was composed of a primary FCC solid-solution phase and a small amount of BCC solid-solution phase. Bulk CoNiFe Al0.4Ti0.6Cr0.5 and CoNi FeAl0.4Ti0.6Cr0.5Cu0.5 HEAs both contained two different FCC phases possessing similar lattice parameters with different chemical compositions. It is obvious that in the above-mentioned HEAs or MEAs, phase composition, phase formation mechanisms and crystal structure are not governed by the entropy of mixing, in contrast, they are dominated by atomic-size difference, enthalpy of mixing of different atom pairs, the intrinsic nature of constituent elements and mutual solubility in most binary atom-pairs of the constituent elements, etc.(3) Single-phase HEAs can not be attained by increasing the entropy of mixing in a multi-principal-component alloy system, in other words, a multi-principal-component alloy can hardly reach a state of high entropy of mixing. An FCC structured single-phase Co25Ni25Fe25Al7.5Cu17.5 HEA that can attain truly high entropy of mixing was successfully desgined and prepared. The single-phase feature of the Co25Ni25Fe25Al7.5Cu17.5 HEA principally resulted from remarkably high mutual solubility in most binary atom-pairs of the constituent elements. Following SPS, bulk Co25Ni25Fe25Al7.5Cu17.5 HEA exhibited an average grain diameter of 95 nm. The actual entropy of mixing is high in the Co25Ni25Fe25Al7.5Cu17.5 HEA and accordingly the sluggish-diffusion effect is significant. Hence, a high thermal stability and nanocrystalline(NC) grians are achieved in the Co25Ni25Fe25Al7.5Cu17.5 HEA. The bulk NC Co25Ni25Fe25Al7.5Cu17.5 HEA exhibits a compressive yield strength of 1795 MPa with a hardness of 454 Hv, which is dramatically higher than the yield strength of most previously reported FCC structured HEAs. Quantitative calculations of the respective contributions from each strengthening mechanism demonstrate that grain boundary strengthening and dislocation strengthening are principally responsible for the measured ultra-high strength of the bulk NC Co25Ni25Fe25Al7.5Cu17.5 HEA.(4) Adding TiC directly or through in situ reaction of Ti and C into the CoNi FeAl0.4Ti0.6 HEA matrix, on one hand, it can increase the thermal stability of the CoNiFeAl0.4Ti0.6 HEA, leading to significant grain refinement in the four 10 vol.%TiC/CoNi FeAl0.4Ti0.6 HEA composites in comparison with the HEA matrix, and therefore the four bulk HEA composites display nano/ultra-fine grains. On the other hand, the TiC reinforcement does not affect evidently on the phase formation rules and mechanisms of the HEA matrix. Howerver, the in situ composites exhibit finer grains when the CoNi FeAl0.4Ti0.6 HEA matrix was produed by the same method. Differences between the HEA matrix and the four HEA composites are primarily governed by grain boundary strengthening, and are slightly governed by dispersion strengthening.