Enhanced Strength and Ductility in a Friction Stir Processing Engineered Dual Phase High Entropy Alloy Page: 3
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Increasing strain
Pre-deformation f.c.c. h.c.p. Post-deformation
d 1400a
-a 350 RPM
1200
1000 650 RPM
l60 AH
400
S 0 ~. .% 10 20 3 4 $0
True Strain (%)
tepeaur a n niil tri rteo"1-3sh,(bd ESDm p hwn ~ .3 n hcp -hs rcin
40 " 50 "60 70I80 90
20 ()
LIf.c.c.fl h.c.p. Post-deformation XRD D f.c.c.fl h.c.p.
Figure 2. (a) True stress-true strain curves for the as-homogenized and FSP samples deformed at room
temperature at an initial strain rate of 10-3 -1 (b-d) EBSD maps showing f c.c. -y- and h.c.p. 8--phase fractions
prior to tensile deformation and, (e-h) EBSD maps and corresponding XRD patterns showing f.c.c. 3- and h.c.p.
E-phase fractions after tensile deformation. AH: as-homogenized; RPM: rotations per minute.
includes the conversion of f.c.c. a-phase to h.c.p. 8-phase under deformation, and thus a higher starting fraction
of f.c.c. a-phase should lead to a higher level of TRIP effect, provided that the thermodynamic stability of the f.c.c.
--phase is similar in both cases. Therefore, FSP provided an expedient route for obtaining fine-grained TRIP
HEAs with higher f.c.c. a-phase fraction, as compared to conventional thermomechanical processing, which
required multiple processing steps.
Stress-strain behavior. Figure 2a-g present the true stress-true strain curves for the 350 and 650
RPM treated samples, along with the EBSD phase maps before and after tensile deformation, as compared to
the as-homogenized reference material. Since the material in as-homogenized state had coarser grains and a
higher 8-phase content (Fig. 2b), it also exhibited lower strain hardening response, reaching a maximum true
stress of 800 MPa with uniform elongation of 35%. The as-homogenized material experienced a limited TRIP
effect, as indicated by the similar color codes in the EBSD phase maps (Fig. 2b,e), and the XRD data obtained
before and after tensile deformation (Fig. 2h). The FSP treatment led to significant improvement in the tensile
mechanical properties of the alloy, i.e. maximum true stress values of 1400 MPa and 1200 MPa at uniform elon-
gations of almost 45% and 42% were obtained for the samples with tool rotational rates of 350 and 650 RPM,
respectively (Fig. 2a). This enhanced combination of strength and ductility was partly attributed to -90% f.c.c.
1-phase, as shown in Fig. 2c,d prior to deformation, and the associated high TRIP-related strain hardening during
deformation.
The change in color from green to red shown by the EBSD phase maps (Figs 2c,d,f,g) and majority h.c.p.
8-phase peaks shown by XRD (Fig. 2h) after failure of the FSP tensile samples confirmed the deformation-induced
phase transformation from f.c.c. 7- into the h.c.p. 8-phase. Tensile deformation resulted in transformation
of -90% of the starting a-phase to almost 79% and 75% h.c.p. 8-phase for the 350 and 650 RPM conditions,
respectively.
The values of true ultimate stress and ductility for the 350 RPM treated sample appear similar to the highest
values reported by Li et al.3'4 after cold-rolling and 3 min annealing. Moreover, the properties of the single phase
equi-atomic Fe20Mn20Co2OCr20Ni20 alloy appeared to be significantly inferior to those observed for the FSP alloys
having similar grain sizes". The work hardening response of the 350 RPM treated and as-homogenized refer-
ence material after tensile deformation confirmed that FSP produced a pronounced increase in work hardening.
Regarding the yield strength (YS) values of the alloy, significant improvement of the 350 RPM treated sampleled to a value of 298 MPa compared to a value of 198 MPa for the as-homogenized sample. The 650 RPM treated
sample showed -200 MPa increase in YS (Fig. 2a) compared to the as-homogenized condition. These improvedS C REPORT. 7: 16167 DOI:10.1038/s41598-017-16509-9
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Nene, Saurabh S.; Liu, Kaimiao; Frank, Michael; Mishra, Rajiv; Brennan, R. E.; Cho, K. C. et al. Enhanced Strength and Ductility in a Friction Stir Processing Engineered Dual Phase High Entropy Alloy, article, November 23, 2017; London, United Kingdom. (https://digital.library.unt.edu/ark:/67531/metadc1049692/m1/3/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Engineering.