| High-entropy alloys?HEAs?are widely concerned by scholars because of their unique composition design concept,simple phase structure and excellent mechanical properties,corrosion resistance,oxidation resistance et al.,combined with the potential applications in practical production.Refractory high-entropy alloys?RHEAs?are mainly composed of refractory elements such as Mo,W,Cr,Ta et al.,and have high strength and softening resistance in high temperature applications.In this study,we designed the HfCrVWMox?x=0,0.2,0.6,1?and HfCrNbTaM?M=Al,Mo,Ti?RHEAs containing HfCr.The influence of milling time on microstructure and morphology evolution for RHEA powders fabricated by mechanical alloying?MA?has been studied.The bulk RHEAs with high relative density,metallic luster,and excellent performances have been successfully acquired by spark plasma sintering?SPS?technique.In this subject,the phase compositions and morphologies of as-milled powders and as-sintered bulk RHEAs were characterized by X-ray diffraction,scanning electron microscope and transmission electron microscope.As well as,we also used the hydrostatic balance,Vickers hardness tester,MTS universal tester,thermogravimetric analyzer and electrochemical workstation to analyze the properties including the density,micro-hardness,mechanical properties,oxidation resistance and corrosion resistance of bulk RHEAs.The effects of composition adjustment on the structure and properties of RHEAs were also discussed.The main conclusions are as follows:1.The milling products of HfCrVW?Mo0?,HfCrVWMo0.2(Mo0.2)and HfCrVWMo0.6(Mo0.6)RHEA powders are a single W-type body centered cubic?BCC1?solid solution phase.The phase structure of Mo1 RHEA is converted to Mo-type BCC2.When the Mo content is increased to a certain amount,the alloying rate of Mo is the slowest due to its higher melting point and a more positive mixture of Mo with other elements,and it is easy to act as a solvent in the crystal lattice,thereby inducing Mo-type BCC.The sintered products of Mo0 RHEA are W-type BCC1,FCC and BCC3 solid solution phases and a small amount of compounds,and the samples contained Mo element only include FCC and BCC3 solid solution phases and a small amount of compounds.As well as,the composition distribution of Mo1 RHEA is more uniform.The relative density of the sintered samples reach over 97.0%,and the micro-hardness of samples reach 1050.00 HV or more,especially for Mo0 RHEA?1184.00HV?.The uniform distribution of nano-scale grains?about 100 nm?in the matrix makes the maximum compressive strength??b?of Mo0.6.6 RHEA reach 499.69 MPa.The potentiodynamic polarization curve test shows that the sintered samples exhibite the anodic activation behavior and a broad passivation platform.Mo1 has the most positive corrosion potential(Ecorr=-1.09V),the smallest corrosion current(icorr=1.42×10-66 A/mm2)and a wide passivation region??E=1.99 V?,which is characterized by excellent corrosion resistance.Mo0 RHEA exhibits the excellent pitting resistance,because of the most stable passivation platform and the widest?E?2.57 V?.The addition of Mo reduces the contents of WO3 in the passivation film,which increases the corrosion pits on the surface of the sample,thereby impairing the pitting resistance of the system.?2.The milling product of HfCrNbTa RHEA powders is a single Ta-type BCC phase.The particle size is refined to 1?m when milling time reaches 100 h,and the composition is more uniform.The sintered product is a solid solution phase of FCC and a small amount of compound,and the bulk has the compact structure?95.1%?and no obvious defects.Due to the uniform distribution of the Ta-rich and Nb-rich hard precipitates,the overall average hardness of sintered bulk reaches 1285.35 HV and the?b is 962.35 MPa.The uniformly distributed Laves phase nanoparticles?<100 nm?precipitated from the matrix also have obvious strengthening effects.The oxidation weight gain??M?of the tested alloy is 18.8mg/cm2after being kept at 1000°C for 120 min.The potentiodynamic polarization curve test shows that the icorrorr of HfCrNbTa RHEA is 7.81×10-66 A/mm2,the Ecorrorr is-1.31 V,and the?E is2.38 V.Compared with the quaternary Mo0 RHEA,it showed good comprehensive corrosion resistance.3.The milling products of HfCrNbTaAl?X-Al?,HfCrNbTaMo?X-Mo?,HfCrNbTaTi?X-Ti?RHEA powders are still Ta-type BCC phase.Combined with XRD,DSC and TEM,we easy to get that X-Al and X-Mo RHEAs also contain a small amount of amorphous phase.For the sintered bulks,the addition of Al leads to the precipitation of more solid solution phases.The products of alloys after the addition of Mo and Ti are relatively simple,and consist of a solid solution phase and a small amount of compound.The relative densities of all bulks reach more than 96.0%,and X-Al RHEA has the highest micro-hardness,which indicates that it can contribute to the improvement of hardness value under the strengthening of more solid solution phases.X-Mo RHEA with fine nanoparticles?about 50 nm?precipitated from the matrix has the highest?b?887.65 MPa?because of the fine-grained strengthening.Compared with HfCrNbTa,the?M of X-Mo RHEA is smaller.The initial weight gain of X-Al RHEA is larger,but it gradually becomes flat,indicating that it is easy to form a dense oxide film and can maintain stability.While X-Ti RHEA has the largest?M,that is,the oxidation resistance is the worst.The results of the potentiodynamic polarization curve show that the X-Ti RHEA has the smallest icorr?4.64×10-66 A/mm2?and the widest?E?2.65 V?compared to the other two sintered products,showing the best overall corrosion resistance. |
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