PSI - Issue 50

A.S. Smirnov et al. / Procedia Structural Integrity 50 (2023) 266–274 A.S. Smirnov, A.V. Konovalov,V.S. Kanakin and I.A. Spirina / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction

The mechanical properties of alloys are determined not only by their chemical composition, but also by their internal constitution including grain and subgrain microstructure, phase composition, porosity, dislocation density, etc. (Watanabe, 2011; Cardarelli, 2008; Doherty et. al., 1997). One of the ways of forming the required microstructure providing the necessary mechanical properties of the product is thermomechanical processing, which consists in mechanical action on the workpiece under certain temperature conditions (Tikhonova et. al., 2018; Lin et. al., 2016; Kimura and Inoue, 2020; Zupanc et. al., 2018). As a rule, thermomechanical treatment is carried out at elevated temperatures, when alloy restructuring actively occurs. During plastic deformation at high temperatures, competing processes occur in alloys, namely hardening through increasing dislocation density and phase transformations and softening through dynamic recrystallization, recovery, and damage (Smirnov et. al., 2014; Rollet et. al., 2004; Smirnov et. al., 2018; Yang et. al., 2022; Smirnov et. al., 2013). Moreover, the interaction of these processes is unsteady and depends on the deformation history. For example, after the end of deformation at the same temperature, strain, and strain rate, different microstructures are formed in the specimens if they are deformed according to patterns that differ in time. Thus, mathematical models describing structure formation in alloys depending on the thermomechanical parameters of deformation cannot be written in the form of a function whose arguments are only temperature, strain, and strain rate. As a result, it is correct to establish interrelation of the rheological behavior, the forming microstructure, and the thermomechanical parameters of deformation on the basis of physically grounded multilevel models (Su et. al., 2019; Karhausen and Roters, 2002; Konovalov, 2008; Smirnov et. al., 2015; Gourdet and Montheillet, 2003), as well as using computational models, including those based on cellular automata (Wang et. al., 2016; Svyetlichnyy et. al., 2015; Lin and Chen, 2011; Chen et. al., 2021; Chen et. al., 2020) and molecular dynamics (Barret et. al., 2017). The goal of this study is to interrelate the rheological behavior of the V95 alloy (the Russian analogue of the 7075 alloy), dynamic recrystallization, and strain and strain rate parameters at a temperature of 400 °C. 2. Mathematical model, material, and research methodology The V95 alloy (similar to the 7075 aluminum alloy in the chemical composition) was used as a material for the study, which had the following chemical composition, wt%: 89.4 Al, 5.96 Zn, 2.06 Mg, 1.74 Cu, 0.32 Mn, 0.25 Fe, 0.14 Cr, 0.1 Si, 0.03 Ti. The chemical composition was determined by means of a Spectromaxx LMF04 analyzer. The alloy under study was heterogeneously vacuum-annealed at a temperature of 470 °C for 24 hours and then slowly furnace-cooled. The annealed V95 alloy billets were lathed in order to obtain cylindrical specimens with a diameter of 6 mm and a height of 12 mm. These specimens were compressed between flat dies at a temperature of 400 °C with strain rates ranging from 0.01 to 5 s − 1 . A graphite-containing lubricant was used to reduce the friction coefficient. The specimens were compressed in a plastometric testing machine designed at the Institute of Engineering Science, UB RAS. In order to fix the specimen microstructure formed during deformation, the specimens were water-cooled immediately after the end of deformation. After high-temperature compression, the specimens were cut along the symmetry axis. Mechanical grinding and polishing were carried out thereafter by means of a Struers LaboPol-5 machine to smooth the thin section surface. A diamond abrasive disc was used for grinding. Diamond suspensions with fraction sizes of 9, 3, and 1 µm were successively used for polishing. The specimens were then ion-polished using a Technoorg-Linda SEMPrep2 SC 2100 ion-polishing system. Ion polishing was performed for 30 min at an accelerating voltage of 10 kV with the angle of s pecimen inclination to the ion beam equal to 6°. Then, the surface was cleaned from possible redeposition of atoms at an angle of inclination of 5°, an accelerating voltage of 4 kV, and a polishing time of 5 minutes. The microstructure of the V95 alloy specimens before and after deformation was studied and analyzed from data obtained by electron backscattered diffraction (EBSD). These studies were performed by means of a Vega II Tescan electron scanning microscope with an Oxford HKL Nordlys F+ EBSD analysis accessory. The scan step during the EBSD analysis was equal to 300 nm with a beam diameter of 300 nm. The grain misorientation was supposed to exceed 15°. When constructing EBSD images, it was supposed that there should be at least three points with a certain crystallographic direction per grain.