Microstructure And Mechanical Properties Of Mo-Nb-Hf-Zr-Ti Refractory High-entropy Alloy

High entropy alloys break the design concept of the traditional alloys that are based on one or two principle elements and contain at least five principal elements. High entropy alloys form simple phase constitutes and possess excellent comprehessive properties due to the four effects of thermodynamically high mixing entropy, dynamically sluggish diffusion, structurally serious lattice distortion and behavioral cocktail effects. To explore advanced high temperature structural materials, the alloy system Mo-Nb-Hf-Zr-Ti refractory high entropy alloys was prepared by arc melting in this paper based on the concept of high entropy alloys. And the microstructure and mechanical properties are also investigated with adding metal alloying element of Al or Cr and adding nonmetal alloying element of Si, B or C.Firstly, the phase diagram of Mo Nb Hf Zr Ti alloy was calculated using Thermo-calc software with TCNI-5 database for predicting phase constitutes. The microstructure, phase stability, mechanical properties, hot deformation behavior of equiatomic Mo Nb Hf Zr Ti alloy and the effect of each composing element in this alloy on microstructure and mechanical properties are invesitigated. The study shows that the result of calculated phase diagram is consistent with the experimental result. Mo Nb Hf Zr Ti alloy is composed of single body-centered cubic(BCC) solid solution phase.The DSC and homogenizaed results verify that this solid solution phase has high structural stability and there is no phase transformation below 1450 oC. The compressive yield strength of Mo Nb Hf Zr Ti alloy in the cast and homogenized states is 1719 MPa and 1575 MPa at room temperature, respectively. The facture mechanism belongs to the brittle fracture. At ε?=0.1 s-1 and T=1200 oC, the yield still can reach more 750 Mpa. The stress-strain curves have the typically dynamic recrystallization characteristics above deformation temperature of 900 oC. The discontinuous dynamic recrystallization(DDRX) and continuous dynamic recrystallization(CDRX) occur simultaneously during hot deformation and the DDRX is the dominant nucleation mechanism. The increase of deformation temperature and the strain rate weaken the nucleation mechanism of CDRX. This alloy still remains the single bcc solid solution phase in which each composing element has the concentration between 11.11% and 27.27% while the concentration increase or decrease of each composing element leads to the decrease of hardness and yield strength and the improvement of the plasticity due to the weakened solution strengthening. For the non-equiatomic Mo-Nb-Hf-Zr-Ti alloys, the strength increases with the increase of Mo content and with the decrease of Hf, Zr or Ti content. The variation of Nb content has no obvious effect on the strength.Mo0.5Nb Hf0.5Zr Ti alloy was selected as the based alloy in consideration of strength, plasticity, density and cost. The investigation on microstructure and mechanical properties of Mo0.5Nb Hf0.5Zr Ti alloy with adding Al or Cr shows that the dominant phase is the same BCC1 phase as that of Mo0.5Nb Hf0.5Zr Ti alloy and this phase contains part of Al or Cr atoms. The addition of Al leads to the formation of the other BCC2 phase which has the similar lattice parameter to the BCC1 phase. For Mo0.5Nb Hf0.5Zr Ti Alx alloys, the BCC1 phase is riched with Mo and Nb elements while the BCC2 phase is riched with Zr and Hf elements. The addition of Cr leads to the formation of Mg Cu2-typed Laves phase that is(Nb, Hf, Zr, Ti)-(Mo, Cr)2. More BCC2 phase or more Laves phase precipitates in Mo0.5Nb Hf0.5Zr Ti Al0.3(Al0.3) alloy or Mo0.5Nb Hf0.5Zr Ti Cr0.3(Cr0.3) alloy after homogenization treatment. The strength of Mo0.5Nb Hf0.5Zr Ti alloy increases and the plasticity is improved with a small amount of Al or Cr. The strength has no obvious increase while the plasticity decreases with the increase of Al content. The increase of Cr content leads to the increase of the strength and the decrease of the plasticity due to more formation of the Laves pahse.The microstructure and mechanical properties of Mo0.5Nb Hf0.5Zr Ti alloy were investigated after adding B, Si or C. The results show that the based phase of Mo0.5Nb Hf0.5Zr Ti alloy is still BCC solid solution phase. There are boride phase of MB2 formed with adding B(M is Mo, Nb, Hf, Zr and Ti). The silicide phase of M5Si3(M is Mo, Nb, Hf, Zr and Ti) forms as Si is added. The carbide phase MC(M is Nb, Hf, Zr and Ti) forms with adding C. There are more refractory metal element if this refractory metal element has the stronger binding energy with Si, B, or C. The strength inceases and the plasticity is improved when the concentration of Si, B or C is x≤0.3. The improvement of the plasticity is attributed to the refinement of the microstructure and the weakness of the solid solution strengthening of the BBC1 phase. With more addition of Si or B elements, the strength increases while the plasticity decreases.Mo-Nb-Hf-Zr-Ti alloy is still composed of one solid solution phase as the matrix phase after adding many kinds of elements. The formation of new phase and its content is closely related with the binding energy between the adding elements and the composing elements of the Mo-Nb-Hf-Zr-Ti matrix alloy. For example, the main new phase of Mo Nb1.5Hf0.3Zr0.5Ti1.5Al0.2Cr0.2C0.05B0.05Si0.05 alloy is MC carbide phase though the adding content of Si, C and B is the same, which is because C element has the strongest binding energy with Mo, Nb, Hf, Zr and Ti compared to Si or B elements.