PSI - Issue 60

B.P. Kashyap et al. / Procedia Structural Integrity 60 (2024) 494–509

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B.P. Kashyap et al. / Structural Integrity Procedia 00 (2023) 000 – 000

Mechanical properties like strength and minimum creep rate accordingly change rapidly with decreasing grain size, but these properties tend to be nearly insensitive to grain size becoming larger than ~100 μm (Leo and Rimoli, 2019; Sherby and Burke, 1968). The increasing proportion of grain boundaries in nanocrystalline materials is even noted to change their role from the typical low temperature activity to that generally happens during high temperature deformation. For example, nanocrystalline materials of ≤ 20 nm are noted to exhibit high temperature deformation behavior even at lower temperatures (Greer et al., 2011). The occurrence of plastic deformation in polycrystalline materials is invariably connected with the deformation of an individual or a group of grains within the entire volume of material subjected to deformation. Here comes the role of grain boundaries, whether they are permeable to the transfer of atoms from one grain to another or resistant enough not to allow this process but cause decohesion between the adjoining grains. These two opposing effects account for whether the material exhibits elongation or fracture, respectively. At elevated temperatures, the former process favorably occurs. Plastic deformation is known to be the sequence of elastic deformation through interatomic bonds stretching to the peak barrier height of potential energy vs. interatomic distance relation, whereas plastic deformation is the consequence of subtle driving force that moves the atoms from the activated state to the next atomic position site and re-establishes bonds between near neighbor atoms.In view of the mass transfer requirement for plastic deformation, it is important to understand the initial microstructures not only in terms of grain size but also grain orientation and grain morphology,and grain boundaries in terms of their structure and properties, and relation with the adjoining grains. During deformation at elevated temperature, plastic deformation is accomplished by one or combination of different processes of mass transfer, which involve diffusion, inter-granular (grain boundary sliding) and intra-granular (dislocation) slip. These processes responsible for deformation also contribute to concurrent microstructure change, both of which guide the structure-property relationship dynamically. Understanding of flow properties and their correlation with microstructure evolution have led to different micro-mechanisms proposed in the literature(Mukherjee, 1971; Sherby and Wadsworth, 1989, Zelin and Mukherjee, 1996). The vast literature on deformation, microstructure evolution and their applications to several conventional and newly emerging alloys have led to widespread views. Therefore, it is felt necessary to revisit the literature and own works published earlier so as to strengthen our understanding of the subject by extending the analysis of deformation and damage behavior of materials in a search for some relationships between the two. For this, the theoretical aspects of deformation and cavitation, along with the existing results in several materials,are summarized first,and then an attempt is made to establish the structure-property relationship in its present form. 2. Theoretical aspects of deformation and damage behavior Flow stress( σ )of a given material is a function of test condition involving strain ( ε ), strainrate ( ̇ ) and temperature ( T ),material properties like stacking fault energy ( γ ) and microstructure ( S ), viz. = ( , ̇ , , ) (1) The effects of strain and strain rate on flow stress is of the form = ̇ (2) Here, n is called the strain hardening exponent, m is the strain rate sensitivity index, and K is a proportionality constant that depends on microstructure and test temperature. At low temperature the magnitude of m turns out to be very low, and as such, the stress is taken to be a function of strain alone, according to the Hollomon relationship(Hollomon, 1945). = (3) Here K L is known as the strength coefficient. However, stress increases with decreasing grain size ( d ) according to the Hall-Petch relationship (Hall, 1951; Petch, 1953), expressed in general form as,