The Al x CoCrCuFeNi alloys with multiprincipal elements (x ϭ the aluminum content in molar ratio, from 0 to 3.0) were synthesized using a well-developed arc-melting and casting method, and their mechanical properties were investigated. These alloys exhibited promising mechanical properties, including excellent elevated-temperature strength and good wear resistance. With the addition of aluminum from x ϭ 0 to 3.0, the hardness of the alloys increased from HV 133 to 655, mainly attributed to the increased portion of strong bcc phase to ductile fcc phase, both of which were strengthened by the solid solution of aluminum atoms and the precipitation of nanophases. The alloys exhibited superior high-temperature strengths up to 800 °C, among which the Al 0.5 CoCrCuFeNi alloy, especially, had enhanced plasticity and a large strain-hardening capacity. Moreover, the wear resistance of these alloys was similar to that of ferrous alloys at the same hardness level, while the alloys with lower hardness exhibited relatively higher resistance because of their large strain-hardening capacity.

The authors studied the effect of vanadium addition on the microstructure and properties of Al 0.5 CoCrCuFeNi high-entropy alloy. The microstructure of Al 0.5 CoCrCuFeNiV x (x 5 0 to 2.0 in molar ratio) alloys was investigated by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction. With little vanadium addition, the alloys are composed of a simple fcc solidsolution structure. As the vanadium content reaches 0.4, a BCC structure appears with spinodal decomposition and envelops the FCC dendrites. From x 5 0.4 to 1.0, the volume fraction of bcc structure phase increases with the vanadium content increase. When x 5 1.0, fcc dendrites become completely replaced by bcc dendrites. Needle-like s-phase forms in bcc spinodal structure and increases from x 5 0.6 to 1.0 but disappears from x 5 1.2 to 2.0. The hardness and wear resistance of the alloys were measured and explained with the evolution of the microstructure. The hardness values of the alloys increase when the vanadium content increases from 0.4 to 1.0 and peak (640 HV) at a vanadium content of 1.0. The wear resistance increases by around 20 pct as the content of vanadium increases from x 5 0.6 to 1.2 and levels off beyond x 5 1.2. The optimal vanadium addition is between x 5 1.0 and 1.2. Compared with the previous investigation of Al 0.5 CoCrCuFeNi alloy, the vanadium addition to the alloy promotes the alloy properties.

High-entropy Al 0:5 CoCrCuFeNiTi x alloys are synthesized using the well-developed arc-melting and casting method. The molar ratio (x) of titanium is varied from 0 to 2.0. The microstructure, hardness and wear resistance of the alloys are investigated. The alloys exhibit simple FCC, BCC, CoCr-like and Ti 2 Ni-like phases. For a small addition of titanium, the alloys form a monolithic FCC solid-solution phase. Two phases of 1 and 2 based on BCC appear at the titanium content of x ¼ 0:4 and the 1 phase becomes ordered at x ¼ 1:4. With the increase of titanium content, copper segregates to the interdendrite region in which nano-precipitates form. A CoCr-like phase forms when x ranges from 0.8 to 1.2. Ti 2 Ni-like phase forms when the titanium content exceeds x ¼ 1:0. The hardness value increases with titanium content. The alloys with lower titanium content exhibit similar wear resistance to Al 0:5 CoCrCuFeNi. The wear resistance is rapidly improved at titanium contents from 0.6 to 1.0, and reaches a maximum at x ¼ 1:0. This is followed by a gradual decrease with further increase of the titanium. The mechanisms behind the strengthening and wear resistance of the alloys are discussed.

Corrugated tubes are widely used in a range of applications for heat transfer enhancement. The spirally corrugated tube has a better heat transfer performance than the smooth tube. In this paper, the heat transfer performance of a hybrid smooth and six-start spirally corrugated tube is studied. With a validated numerical model, the effects of the corrugation part length on the vortex in the downstream smooth tube are studied for a range of high Reynolds numbers, where the existence of the corrugation part can turn out the secondary flow and enhance heat transfer. Meanwhile, it is found that in the smooth part, the fluid flow part with whirling can reach a maximum length, even if the length of the corrugation part continuously increases. Thus a series of critical corrugation lengths can be obtained. This work can reveal the enhanced heat transfer mechanism of the hybrid smooth and spirally corrugated tube and be of interest to researchers in heat transfer issues of corrugated tubes.