Search

• Ti x VNbMoTa refractory high-entropy alloys with ultra-fine grains were fabricated by MA and SPS. • The mechanical alloying time of the milling powders with high Ti content was extended by the severe agglomeration. • The fluctuation of yield strengths is attributed to the changes of strengthening effect. Ti x VNbMoTa refractory high-entropy alloys (RHEAs) were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) based on classic VNbMoTaW RHEA. The effect of Ti content on the microstructures and mechanical properties of the Ti x VNbMoTa RHEAs were systematically investigated. The results showed that the milling powders with more Ti content were agglomerated severely in the early stage of the MA process, leading to the extension of mechanical alloying time. The mechanically alloyed powders in all groups exhibited single body-centered-cubic (BCC) phase after the MA process. The sintered alloys were consisted of ultra-fine matrix and precipitation phase. The grain sizes of the matrix and precipitation phases, as well as the volume fraction of the precipitation phases were increased with the increase of Ti content. The MA and SPS processes and the replacement of Ti not only greatly reduce the densities, but also significantly improved the ductility, strength and specific strength of the alloys. The yield strengths were decreased first and then increased with the increase of Ti content in the Ti x VNbMoTa RHEAs, which is mainly attributed to the weakened effect of grain boundary strengthening, and the enhanced effect of the substitution solid solution strengthening and the interstitial solid solution strengthening. The rapidly increased sizes and volume fractions of the precipitated phases in Ti1.5 and Ti2 destroyed the continuity of matrix and the coordination of deformation, resulting in the decrease of ductility. [ABSTRACT FROM AUTHOR]

The hot deformation behaviors of an ultrafine-grained MoNbTaTiV refractory high entropy alloy (RHEA) fabricated by powder metallurgy (P/M) were investigated using isothermal compression tests in the deformation temperature range of 1100 °C ~ 1300 °C and the strain rate range of 0.0005 s−1–0.5 s−1. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques were used to analyze the effect of the deformation temperatures and the strain rates on the flow stress behaviors and microstructural evolution during hot deformation. The results showed that the flow stresses exhibit typical dynamic recrystallization (DRX) characteristics, but show work hardening characteristics at 1100 °C and 0.5 s−1 deformation conditions and at 1200 °C and 0.5 s−1 deformation conditions. With increasing deformation temperature and decreasing strain rate, the maximum compressive stresses were dramatically decreased, and the average grain sizes also slowly increased. The limited growth of grains was due to the sluggish diffusion effect of high entropy alloys (HEAs) and the pinning effect of the ultrafine precipitated phases. The dominant DRX process gradually changed from discontinuous dynamic recrystallization (DDRX) to continuous dynamic recrystallization (CDRX) with increasing deformation temperature and decreasing strain rate. Both of these DRX processes were promoted by the ultrafine precipitated phases. The dominant deformation mechanism at low deformation temperature and high strain rate was grain deformation. Grain boundary gliding became the dominant deformation mechanism at higher deformation temperature and lower strain rate, but it was inhibited to some extent by grain growth and the ultrafine precipitated phases distributed at grain boundaries. [ABSTRACT FROM AUTHOR]

This work focused on the microstructure, morphology and thermal stability of a MoNbTaTiV refractory high-entropy alloy synthesized by mechanical alloying. The mechanically alloyed powder with a uniform chemical distribution and an average grain size of less than 15 nm was obtained after 40 h milling process. With a refinement of the grain size and an increase of the lattice strain, the milled powder transformed from large and inhomogeneous to refined and homogeneous. The mechanically alloyed powder exhibited excellent thermal stability at 1200 °C. • MoNbTaTiV refractory high-entropy alloy was synthesized by mechanical alloying. • The alloyed powder presented a uniform chemical distribution and an average grain size of less than 15 nm. • The alloyed powder exhibited refinement and homogeneous size distribution. • The alloyed powder maintained bcc phase and nanoscale grain size at 1200 °C. [ABSTRACT FROM AUTHOR]