PSI - Issue 23

Ayan Ray et al. / Procedia Structural Integrity 23 (2019) 299–304 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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were carried out between strain rates of 10 -3 s -1 and 10 -2 s -1 also at different stress levels. The details of these tests are available in an earlier report by some of the current authors (Kumari and Ray, 2016).

3. Results

The detected constituents in the microstructure of HEA-F are Al-Ni rich fcc precipitates on a Fe-Cr fcc matrix (Fig. 1a) while that of HEA-B (Fig. 1b) comprises of bcc Al-Ni plates with bcc Fe-Cr inter-plates as the matrix. X ray diffraction analyses of HEA-F indicated peaks from fcc phases primarily corresponding to fcc Ni-Al rich phase with L1 2 structure. HEA-B also exhibits some Cu-rich fcc precipitates. Further details about the phases of these alloys are available in some earlier reports (Manzoni et al., 2013, Roy et al., 2014).

(b)

(a)

Fig. 1: (a) Microstructure of HEA-F showing Al-Ni rich fcc precipitates in Fe-Cr rich fcc matrix, and (b) platelets of ordered bcc Ni-Al rich phase of B2 structure with inter-plates of disordered bcc Fe-Cr rich phase in HEA-B.

The average micro-hardness values (HV 0.02 ) of the alloys estimated from 25 readings are found to be 147.2±72.4 (HEA- F) and 375±37.6 (HEA -B); the higher hardness value of HEA-B is due to the nano-sized BCC precipitates present on the BCC matrix (Fig. 1b). The higher standard deviation associated with the micro-hardness values for both the alloys, originates from the fact that indentations could not be taken on the individual constituents because of their finer sizes. The compressive flow curves for HEA-F and HEA-B were generated till fracture. The average compressive yield strength of HEA-F and HEA-B are found to be 0.4 GPa and 1.23 GPa respectively. The compressive stress strain curve of HEA-F illustrated higher fracture strain compared to that of HEA-B; the average compressive ductility for HEA-F is found to be 92% compared to 2.1% for HEA-B. Fracture toughness values for the two alloys were determined using single edge notched bend (SENB) specimens as well as using chevron notched rectangular bar (CVRNB) specimens as mentioned in section 2. The conditional fracture toughness ( K Q ) values for SENB specimens were estimated following ASTM standard E-399-09 as: = . . 3⁄2 f( ⁄ ) (1) where, B , W and S are specimen thickness, width and span length respectively, a is crack length, P Q is the conditional value of load; and f( a / W ) is the geometry factor. The exact dimensions of each test specimens were first recorded. After the tests, the nature of the load-load line displacement plots for HEA-F is assessed to be of type-I and that of HEA-B is of type-III as per the standard. Following the standard, the conditional load ( P Q ) values were estimated followed by calculation of the magnitudes of K Q . Considering the K Q values and the corresponding yield strength (  ys ) of the alloys, the critical thickness values for plane strain condition B=2.5 ( K Q /  ys ) 2 were evaluated for both types of alloys. The results indicate that the estimated values for HEA-B can be referred as K Ic but for HEA-F, the values should be termed as K Q only. The estimated results from SENB specimens for both the alloys are shown in Fig. 2(a) and (b). The fracture toughness values for CVNRB specimens were estimated using the relation (Shang-Xian, 1984): = √ (α 0 , α 1 ) (2) where P max is the maximum load applied on the specimen when a crack grows to a critical length a c , B and W are specimen dimensions, Y c ( α 0, α 1 ) is the minimum value of the compliance function, α 0 (= a 0 /W ) and α 1 (= a 1 /W ) are