PSI - Issue 23

Ayan Ray et al. / Procedia Structural Integrity 23 (2019) 299–304 Author name / Structural Integrity Procedia 00 (2019) 000 – 000 5 where,  1 and  2 are the stresses corresponding to the imposed strain rates ̇ 1 and ̇ 2 respectively at a fixed strain and  is the Taylor factor for isotropic polycrystalline material. The magnitudes of V * and   were calculated using the procedure as described earlier (Kumari and Ray, 2016), these procedures are not included for the sake of brevity. Typical raw data of strain rate change test and the method of estimating stress change are shown in Fig. 4(a) and (b). 303

(c)

Fig. 4: (a) Typical raw data of strain rate change test, (b) method of estimating stress change (   ) during the SRC test, and (c) TEM photograph revealing super-dislocations in a band as well as planar dislocations in deformed HEA-F.

4. Discussion

The microstructures of HEA-F and HEA-B (Fig.1) both exhibit some precipitates on the matrix phases; Al-Ni rich fcc precipitates on Fe-Cr fcc matrix is found in HEA-F while HEA- B shows Ni-Al rich plates (bcc) with Fe-Cr rich inter-plates (bcc) originated due to spinoidal decomposition of its cast dendritic structure. The detailed characteristics of the phases like their lattice parameters, crystal structures and nature of ordering are available in the existing literature (Roy et al., 2014, Manzoni et al., 2013, Dey, 2003). The hardness, yield strength and fracture toughness of the alloys are summarized in Table 1 together with their activation volumes for plastic flow. Fracture toughness of the materials is determined using two types of specimens. The fracture toughness values of HEA-B by both the approaches are similar as these correspond to plane strain condition. But for HEA-F, the estimated fracture toughness values are of the same order with some difference. This difference arises because of the different constraints at the crack tip in the two types of specimens under plane stress condition. The measurement using CVRNB specimen is considered relatively more reliable because of the notch configuration. The magnitudes of the activation volume estimated by SR using two approaches of analyses and that by SRC tests are in the similar ranges. The activation volume for the HEA-F alloy could be determined at several strain levels but that for HEA-B could be done only at two strain levels because of the short range of plastic strain range available for the latter alloy. An overview of the estimated properties indicate that hardness and strength of HEA-B is considerably superior compared to HEA-F, but fracture toughness and minimum activation volume for plastic flow of HEA-F is several times that of HEA-B.

Table 1 Mechanical properties and thermal activation of the investigated high entropy alloys

Properties

HEA-F

HEA-B

147.2 ± 72.4

375 ± 37.6 1260 + 30 5.8 + 0.2 5.4 + 0.2 121 - 302 118 - 301

Hardness (HV 0.02 ) Yield strength (MPa)

392 ±1.8

Fracture Toughness (MPam 1/2 ) SENB specimen Fracture Toughness (MPam 1/2 ) CVRNB specimen Activation Volume (b 3 ) Feltham’s approach (SR) Activation Volume (b 3 ) Conrad’s approach (SR)

26.6 + 1.4 42.7 + 1.2 320 - 434 324 - 438

Activation Volume (b 3 ) Strain rate change test (SRC) 329 - 410

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