PSI - Issue 23

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

304

6

The lower hardness and strength of HEA-F is governed by the ease of <110> {111} type dislocation movement in the Ni – Al rich fcc phase having L1 2 structure. This results in higher fracture strain and higher activation volume. However, TEM examinations of the substructure in HEA-F indicate both individual dislocations and super dislocations in band (Fig. 4c). The mechanism of deformation from the magnitude of activation volume (Table 1) could be inferred as dislocation-dislocation interaction in slip bands possibly assisted by cross slip. The HEA-B, on the other hand, is constituted of predominantly Ni – Al rich ordered bcc phase. In this bcc phase, plastic deformation occurs primarily by a<001> dislocation movement possibly being assisted by movement of a<110> or a<111> dislocations, all these dislocations have non-planar core configuration. The movement of these type of dislocations require large stress (Dey, 2003) and hence the higher hardness and strength of the HEA-B with attendant lower activation volume. In addition, lattice friction (Diao et al., 2017, Laplanche et al., 2018, Zhao et al., 2019) of the dislocations in HEA-B could be higher due to its higher Al-content, which would also hinder the ease of dislocation movement. The similar order of activation volumes at higher strain in HEA-F and HEA-B, however, appears to indicate that the deformation mechanisms in these alloys may not be considerably different. Thus on an overview of the current findings, it can be inferred that searching application potential of HEA as structural materials need systematic investigations of the role of substructure on the evolution of the mechanical properties. 5. Conclusions The major results and their analyses indicate that (a) Al 23 Cr 23 Co 15 Cu 8 Fe 15 Ni 15 (HEA-B) alloy exhibits significantly higher hardness and yield strength associated with inferior fracture toughness compared to Al 8 Cr 17 Co 17 Cu 8 Fe 17 Ni 33 (HEA-F) alloy; the order of the fracture toughness values are found to be complimentary when estimated using two methods. Thermal activation analyses using the results of stress relaxation and strain rate changes demonstrate that the activation volume for plastic flow in the fcc alloy is higher compared to the bcc alloy. The difference in the strength and fracture toughness of the alloys originates from the micro-mechanism of deformation, the fcc alloy depicting lower hindrance to dislocation movement than the ordered bcc alloy, the phenomenon being governed by the intrinsic structural and substructural characteristics of the alloys. References Conrad, H., 1970. The athermal component of the flow stress in crystalline solids, Materials Science Engineering A 6, 265 – 273. Dey, G. K., 2003. Physical metallurgy of nickel aluminides, Sadhana 28, 247-262. Diao, H.Y., Feng, R., Dahmen, K.A., Liaw, P.K., 2017. Fundamental deformation behavior in high-entropy alloys: An overview. Current Opinion in Solid State and Materials Science 21(5), 252-266. Feltham, P., 1961. Stress relaxation in copper and alpha – brasses at low temperatures, The Institute of Metals 89, 1960 – 61. Gludovatz, B., Hohenwarter, A., Catoor, D., Chang, E.H., George, E.P., Ritchie, R.O., 2014. A fracture-resistant high-entropy alloy for cryogenic applications . Science 345(6201), 1153-1158. Huang, C.X. , Hu, W.P., Wang, Q.Y., 2014. Strain-rate sensitivity, activation volume and mobile dislocations exhaustion rate in nanocrystalline Cu – 11.1 at% Al alloy with low stacking fault energy. Material Science Engineering A, 611, 274 – 279. Kumari, M. and Ray, K.K., 2016. Effect of the mode of deformation on activation volume of a material. Materials Science and Engineering: A 650, 335-344 Laplanche, G., Bonneville, J., Varvenne, C., Curtin, W.A., George, E.P., 2018. Thermal activation parameters of plastic flow reveal deformation mechanisms in the CrMnFeCoNi high-entropy alloy. Acta Materialia 143, 257-264. Li, W., Liaw, P.K., Gao, Y., 2018. Fracture resistance of high entropy alloys: a review. Intermetallics 99, 69-83. Li, Z., Zhao, S., Ritchie, R.O., Meyers, M.A., 2019. Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys. Progress in Materials Science 102, 296-345. Manzoni, A., Daoud, H., Mondal, S., Van Smaalen, S., Völkl, R., Glatzel, U. , Wanderka, N., 2013. Investigation of phases in Al 23 Co 15 Cr 23 Cu 8 Fe 15 Ni 16 and Al 8 Co 17 Cr 17 Cu 8 Fe 17 Ni 33 high entropy alloys and comparison with equilibrium phases predicted by Thermo Calc. Journal of Alloys and Compounds 552, 430-436. Ray, K.K., Veerababu, A., Sarkar, R., 2009. Stability analysis and experimental verification for optimum notch configuration in chevron-notched round bar specimens. Fatigue & Fracture of Engineering Materials & Structures 32(6), 531-539. Roy, U., Roy, H., Daoud, H., Glatzel, U., Ray, K.K., 2014. Fracture toughness and fracture micromechanism in a cast AlCoCrCuFeNi high entropy alloy system. Materials Letters 132, 186-189 Sarkar, R., Ghosal, P., Prasad, K.S., Nandy, T.K., Ray, K.K., 2014. An FCC phase in a metastable β -titanium alloy. Philosophical Magazine Letters 94(5), 311-318. Shang-Xian,W. (1984). Fracture toughness determination of bearing steel using chevron-notch three point bend specimen, Engineering Fracture Mechanics 19, 221 – 232. Yeh, J.W., Chen, S.K., Lin, S.J., Gan, J.Y., Chin, T.S., Shun, T.T., Tsau, C.H., Chang, S.Y., 2004. Nanostructured high entropy alloys with multiprincipal elements: Novel alloy design concepts and outcomes. Advanced Engineering Materials 6, 299 – 303.