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The CoNiCr x (x = 1, 1.4, 1.8, and 2) medium-entropy alloys were synthesized by vacuum arc-melting method to systematically investigate the roles of Cr on microstructures and mechanical properties. The results of microstructural characteristics indicated that both CoNiCr and CoNiCr 1.4 alloys have a single face-centered cubic (FCC) structure, while CoNiCr 1.8 and CoNiCr 2 alloys consist of FCC plus body-centered cubic (BCC) hypoeutectic and eutectic. The formation of FCC and BCC phases could be probably evaluated using the VEC criteria. The tested mechanical properties demonstrated that CoNiCr 1.8 alloy exhibited the best comprehensive properties at room and liquid nitrogen temperatures (RT and LNT), whereas the yield strength and hardness of CoNiCr 2 alloy were higher than those of other alloys, which may be mainly attributed to the second-phase strengthening and the fine and lamellar microstructure. The yield strength of CoNiCr 2 alloy at RT and LNT was 3.8 and 1.9 times (514 MPa and 825 MPa) that of CoNiCr alloy. In addition, the precipitation of dense and ordered FCC (L1 2) phase could primarily contribute to the enhance of yield strength of CoNiCr 1.4 alloy. • Cr addition facilitates the formation of L1 2 nano-particles and BCC/B2 phase. • The CoNiCr 1.8 hypoeutectic medium-entropy demonstrates an excellent strength and ductility synergy at liquid nitrogen temperature. • A CoNiCr 2 lamellar eutectic medium-entropy with FCC/L1 2 and BCC/B2 is designed. • L-C lockers dominate the deformation of lamellar eutectic MEAs under cryogenic condition. [ABSTRACT FROM AUTHOR]

In this study, the nacre-like alumina ceramics, with introduction of zirconia, are fabricated by freeze casting followed by hot pressing. The experimental results show that zirconia particles are uniformly distributed between the layered alumina 'brick' in the form of 'mortar'. Due to the incorporation of zirconia with proper particle size, both flexural strength and fracture toughness of the nacre-like alumina are greatly improved, much stronger and tougher than pure alumina. In regard to the sample containing 13.3 wt% zirconia, the flexural strength reaches 522 MPa, ∼47% increase in comparison with the monolith. Particularly, when zirconia content is 10 wt%, the fracture toughness is increased by ∼66%, from 6.1 MPa m1/2 to 10.1 MPa m1/2. Microstructure observation and fracture behavior analysis reveal that the coupling effects of bioinspired nacre-like structure and stress-induced transformation toughening are responsible for the enhanced mechanical properties. The results show that it is an effective method to increase the mechanical properties of layered alumina ceramics by introducing proper interlayer phase. [ABSTRACT FROM AUTHOR]

A novel κʹ-strengthened FCC-based Al 0.5 CoFeNiC 0.1 high entropy superalloy (HESA) with minor B2 phase was prepared by arc melting. The FCC/κʹ dual-phase microstructure with thermal stability is similar to γ/γʹ microstructure of Ni-based superalloys. Surprisedly, the large volume fraction (nearly 50%), high density and nano-scale coherent κʹ particles with uniform distribution are easily accessible only by direct solidification, which gives rise to superior compressive yield strengths at both room and elevated temperatures, reaching 1130 MPa at 25 °C and 830 MPa at 700 °C, respectively. The sustained high-temperature yield strength is contributed to both the antiphase boundaries (APBs) and stacking faults (SFs) shearing κʹ particles. Further, optics and electron microscopy analyses revealed the changes (transition point about at 0.6 T m , where T m is melting temperature) of the dominant deformation and failure mechanisms, dependent on the temperature. The dominant deformation mode is planar slip and SFs at relatively low temperatures, while the cross slip and dislocation dissociation above 800 °C (0.6 T m), coupled with the diffusion-controlled recovery mechanisms. The failure mechanism shows failure mode from shearing fracture to thermal softening. Our κʹ-strengthened FCC-based superalloy provides a route for elevated-temperature applications. [ABSTRACT FROM AUTHOR]

The microstructural evolution and mechanical properties of 3 wt% TiB 2p reinforced 2024Al (TiB 2p /2024Al) composite stimulated by T6 heat treatment are reported in this work. Morphological results showed that the initial TiB 2p /2024Al composite fabricated by laser-directed energy deposition (LDED) consists of fine equiaxed grains with well dispersed TiB 2 particles, in which nanoscale Al 2 Cu(Mg) precipitates are dispersed in the Al matrix. After the T6 heat treatment, ultrafine Al 2 CuMg precipitates and few AlCuMnFe (T) dispersoids were observed in the matrix and grain boundaries. The sample which was solution treated at 495 °C temperature showed a higher compression yield strength of about 371.3 MPa and hardness of about 145HV, whilst the sample treated at 505 °C solution temperature (ST) had a relatively low compression yield strength of 289.8 MPa and hardness of about 128 HV. The reason is deduced to originate from the existence of negative factors such as the formation of microvoids. Quantitative analysis unveiled that grain refinement and Orowan strengthening should contribute to the high strength of the annealed TiB 2p /2024Al composite. • The microstructural evolution and mechanical properties of TiB 2p /2024Al stimulated by T6 heat treatment are performed. • The mechanical measurements reveal that the ST 495 °C specimen has the highest compressive yield strength. • The Al 2 CuMg nano-precipitates plays an important role in the improvement of mechanical properties for the composites. • Grain refinement and Orowan strengthening contribute to the high strength of the annealed TiB 2p /2024Al composite. [ABSTRACT FROM AUTHOR]

Microstructure–property relationships of pressureless-sintered multi-walled carbon nanotube (MWCNT)/alumina composites have been investigated using four types of MWCNTs having different mechanical characteristics as well as almost the same diameter and length; the influences of grain size, location of MWCNTs, fracture patterns, dispersion states of MWCNTs, and crack bridging characteristics of MWCNTs on the composites׳ mechanical properties have been explored. It has been found that the improvement in bending strength and fracture toughness observed for all types of the composites was primarily attributable to the grain refining effect by adding the MWCNTs and to the energy-dissipation by a MWCNT debonding and pullout, respectively. The number and size of clustered MWCNTs were increased in response to the increase in the MWCNT content, which resulted in the decrease in mechanical properties. The pressureless-sintered composites made with MWCNTs having larger load-bearing ability led to ~25% and ~45% improvement in bending strength (742.6±13.1 MPa) and fracture toughness (5.83±0.19 MPa m 1/2 ), respectively, than a MWCNT-free alumina. [ABSTRACT FROM AUTHOR]