NbC reinforced FeMnCoCr high entropy alloy with excellent mechanical properties and wear resistance

In this paper, a NbC reinforced FeMnCoCr dual-phase high entropy alloy was used. After hot rolling, cold rolling and annealing, partial recrystallization sample and fully recrystallized fine-grained sample were obtained. The annealed samples are a single FCC phase, with micron-sized NbC particles of different shapes including columnar, granular and polygonal are distributed in the matrix. The yield strength of the partial recrystallization sample is 708 MPa, which was 1.4 times of the yield strength of the fine-grained sample, and the tensile elongation is 39.4%. The main reasons for the high yield strength of partially recrystallized samples is dislocation strengthening. The recrystallization volume fraction of partially recrystallized samples is 66.2%, and the un-recrystallization grains contain high dislocation density, and the increment of dislocation strengthening is 144 MPa. The work hardening rate of partially recrystallized samples is lower than that of fine-grained samples. Due to the high dislocation density in the partially recrystallized samples, there are fewer dislocations that can be activated and propagated during the tensile process. The high yield strength and the existence of a large number of micron-sized NbC make the NbC reinforced FeMnCoCr high entropy alloy have good wear resistance.


Introduction
High entropy alloy is a new kind of alloy composed of multi-principal elements [1,2], which provide a new way to find high performance structural materials.In particular, the high entropy alloy with FCC structure can also obtain good combination of strength and ductility at low temperature [3,4], the high entropy alloy with FCC structure faces a common problem: low yield strength.With the in-depth study of high entropy alloys, in addition to the control of alloy composition, it will also learn from the TRIP effect in steel phase transformation induced plasticity steel (TRIP) to improve the strength and plasticity of the alloy.Li et al. [5] successfully prepared dual-phase TRIP high entropy alloy, and obtained good mechanical properties.The commonly used methods to improve the strength and toughness of materials include fine grain strengthening, solid solution strengthening, second phase strengthening, phase transformation induced plasticity steel (TRIP) and twin induced plastic deformation (TWIP) effects.For dual-phase TRIP high-entropy alloy, the mechanical properties can be further improved by fine grain strengthening [6,7].The addition of interstitial atoms C and N can play multiple strengthening roles, including solution strengthening, precipitation strengthening and fine grain strengthening, and can also regulate the stacking fault energy (SFE) of the alloy and the stability of the matrix [8,9].In high entropy alloys, intermetallic compounds are commonly used as precipitation strengthening phases, such as (Ni, Co)3Ti [10] and L12-Ni3(Ti,Al) [11] phases.In addition, TiC [12] and NbC [13] precipitates can also be used for precipitation strengthening.The wear resistance of low alloy high strength steel can be improved by adding a lot of TiC particles [14], and the same effect can be played in high entropy alloy [15].
In this paper, NbC was selected as the reinforcement phase and NbC reinforced FeMnCoCr high entropy alloy was designed.Because the density and material of NbC were similar, the smelting difficulty was reduced.Partially recrystallized sample was obtained through rolling and annealing, and good comprehensive mechanical properties could be obtained.

Experimental procedure
The nominal composition of NbC reinforced FeMnCoCr high entropy alloy was Fe48Mn30Co10Cr10NbC (at %).The NbC reinforced FeMnCoCr high entropy alloy billet was obtained by induction heating furnace smelting.After hot forging with the section size of 100 mm × 100 mm at 1200 o C, the alloy was hot rolled at 1100 o C via several passes to a 4 mm thick billet.Finally, the hot rolled alloy billet was further cold-rolled to a 2 mm plate with several passes and then annealed at 800 o C for 3 and 60 min followed by water quenching.The polished samples were used to observe particles using a Zeiss Ultra-55 scanning electron microscopy (SEM) operating at 15 kV.Microstructure and phase compositions of the alloys after annealing were characterized using a Zeiss Crossbeam550 focused ion beam scanning electron microscope (FIB-SEM) equipped with an Oxford symmetry electron backscatter diffraction (EBSD) detector.The operating voltage was 20 kV and the step size was 50 nm.The specimens for EBSD mapping were first mechanically polished to obtain mirror-like flat surfaces and then electrochemically polished to remove the strain layer.Subsequently, the EBSD results were analyzed by AZtecCrystal software.Dog-bone shaped tensile specimens were prepared with the gauge length, thickness and width of 25, 1.6 and 12.5 mm, respectively.The tensile specimens were tested at least three times with a speed of 1 mm/min on the tensile testing machine equipped with video extensometer.The morphology of fracture after tensile testing was observed by SEM.

Results and discussion
The morphology and distribution of particles can exhibit more clearly in the polished samples, and the morphology of particles in NbC reinforced FeMnCoCr high entropy alloy after annealed at 800 °C is shown in figure .1.The morphologies of particles include columnar, granular and polygonal shapes.Meanwhile it can be observed that the columnar and polygonal shapes particles are easy broken after rolling, but the granular shape particles are not easy broken.The results of EBSD indicate that there are a large number of low-angle grain boundaries in the sample annealing at 800 °C for 3 min, and the number fraction is about 0.32, meanwhile there are also a few twin boundaries about 0.20.With the increase of annealing time to 60 min, the fraction of lowangle grain boundaries decreases rapidly to about 0.05, and the fraction of twin boundaries increase to 0.35.The equivalent circle diameter can be obtained according to the area enclosed by high-angle grain boundaries.The mean grain size of the sample annealing at 800 °C for 3 and 60 min is 1.17 and 1.51 μm, respectively.Figure 2a shows a large number of low-angle grain boundaries in the sample annealing at 800 °C for 3 min, which indicates that recrystallization does not occur completely in this sample; however, about 20% annealing twin boundaries can be observed.After annealing at 800 °C for 60 min, the low-angle grain boundaries decrease rapidly, and a large number of annealing twin boundaries appear, about 35 %.It can be seen from the phase analysis that after annealing at 800 °C for 3 and 60 min, a single FCC phase is observed, and the presence of NbC particles is also observed (figure 3a1 and b1).In addition, no obvious annealing recrystallization texture is observed.The volume fraction of recrystallized grain can be measured by EBSD analysis according to the value of grain orientation spread (GOS).Grains with GOS less than 1° are generally considered as recrystallization grains [16].The distribution of grain orientation spread (GOS) map of sample is show in figure 4, and the volume fraction of recrystallized grain is 66.2% and 91.2% in samples annealing at 800 °C for 3 and 60 min， respectively.This indicates that the sample annealing at 800 °C for 3 min will have higher dislocation density.The density of geometrically necessary dislocations can also be measured by EBSD analysis.One method is according to the value of kernel average misorientation (KAM) combined with the corresponding formula [17,18].It is worth mentioning that, the density of geometrically necessary dislocations can be calculated directly by AZtecCrystal software with the 1st nearest neighbor correlation.The dislocation density in the nonrecrystallization grains of the sample annealing at 800 °C for 3 min is 14.68 × 10 14 /m 2 , while the    High yield strength (YS) of 708 MPa and ultimate tensile strength (UTS) of 957 MPa are observed for the partially recrystallized sample, meanwhile a satisfactory total elongation is obtained about 40%.With the increase of annealing time to 60 min, the YS, UTS and total elongation are 483 MPa, 909 MPa and 53%, respectively, (figure 5a).The YS of the partially recrystallized sample is 1.47 times that of fine-grained samples.Furthermore, we can find that the difference of UTS between the two samples is small.Combine with the analysis of dislocation density, with the increase of annealing time, the dislocation density decreases, resulting in the decrease of yield strength.With the decrease of dislocation density, the work hardening capacity increase, this is consistent with the results in figure 5b.The work hardening rate of partially recrystallized samples is lower than that of fine-grained samples.Due to the high dislocation density in the partially recrystallized samples, there are fewer dislocations that can be activated during the tensile process.At the same time, the sample has smaller grain size, which will also reduce the work hardening rate.

Discussion
The contribution of the GNDs to the YS can be calculated by Taylor hardening law [19]: where, f is the volume fraction of non-recrystallization grain; M is the Taylor factor which is determined based on electron backscatter diffraction (EBSD) mapping, MPa; α is a constant (0.2) and G is the shear modulus which is determined to be 80.89 GPa for this HEA as measured by impulse excitation technique.
The contribution of grain refinement to the yield strength can be calculated by the Hall-Petch relationship [20]: where, k is the strengthening coefficient of grain refinement (assumed as ~573 MPa•μm 1/2 ) and d is the average size of grain, μm.
Based on the data of fine-grained sample, the increment of dislocation strengthening of partially recrystallized sample is 144 MPa, and the increment of fine grain strengthening is 70 MPa, which indicates that the increment of yield strength is mainly provided by dislocation strengthening.

Conclusion
After rolling and annealing, NbC reinforced FeMnCoCr high entropy alloy with partial recrystallization structure is obtained, and the yield strength was 708 MPa, which was 1.4 times of the yield strength of the fine-grained sample, while still maintaining a good elongation of 39.4%.The increment of yield strength is mainly provided by dislocation strengthening following the Taylor hardening law.

References
[1] Yeh J W, Chen S K, Lin S J, Gan J Y, Chin T S, Shun T T, Tsau C H and Chang S Y 2004 Adv.

Figure 2 .
Figure 2. EBSD results of NbC reinforced high entropy alloy annealed at 800 °C.(a) 3 min; (b) 60 min; (1) Band contrast and grain boundary map; (2) The distribution of misorientation angle.The red and blue lines in (1) are related to high-angle grain boundaries and low-angle grain boundaries.
the non-recrystallization grains of the sample annealing at 800 °C for 60 min is 3.08 × 10 14 /m 2 .High density dislocation provides higher dislocation strengthening increment for sample.

Figure 5 .
Figure 5. (a) Engineering stress-strain curves of the specimens after annealing; (b) Strain hardening curves of the specimens after annealing.