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Investigation of Structural-Phase States and Features of Plastic Deformation of the Austenitic Precipitation-Hardening Co-Ni-Nb Alloy

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Investigation of Structural-Phase States and Features of Plastic Deformation of the Austenitic Precipitation-Hardening Co-Ni-Nb Alloy Article Investigation of Structural-Phase States and Features of Plastic Deformation of the Austenitic Precipitation-Hardening Co-Ni-Nb Alloy Aidyn Tussupzhanov 1,2,3,*, Dosym Yerbolatuly 1, Ludmila I. Kveglis 1,3 and Aleksander Filarowski 2,4,5 ID 1 S. Amanzholov East Kazakhstan State University, Department of Physics and Technology, 30 Gvardeiskoi Divisii Str. 34, 070020 Ust-Kamenogorsk, Kazakhstan; [email protected] (D.Y.); [email protected] (L.I.K.) 2 Faculty of Chemistry, Wroclaw University, F. Joliot-Curie Str. 14, 50-383 Wroclaw, Poland; aleksander.fi[email protected] 3 Polytechnical Institute of Siberian Federal University, Svobodny Ave 79, 660041 Krasnoyarsk, Russia 4 Frank Laboratory of Neutron Physics, JINR, 141-980 Dubna, Russia 5 Department of Physics, Industrial University of Tyumen, 625-000 Tyumen, Russia * Correspondence: [email protected]; Tel.: +7-777-4775878 Received: 6 November 2017; Accepted: 23 December 2017; Published: 30 December 2017 Abstract: This article presents the results of investigation of the influence of holding temperature during the quenching process on the microstructure and superplasticity of the Co-Ni-Nb alloy. Temperature-strain rate intervals of the deformation of the superplasticity effects are stated. The optimal regimes of the preliminary treatment by quenching and rolling as well as the routine of the superplastic deformation of the Co-Ni-Nb alloy are defined. The interval of the temperatures of the precipitation, morphology, composition, type and parameters of the lattice of the secondary phase, which appears after the annealing + rolling (to 90%) Co-Ni-Nb alloy, are determined. Keywords: superplasticity; superplastic deformation; Co-Ni-Nb alloy; microstructure; precipitation; secondary phase; shear transformation zone 1. Introduction The application of the superplasticity (SP) effect in the technology of metal treatment under pressure enables the production of metallic parts of complex configuration in a single operation. This approach reduces energy consumption and cost of the items and increases labor productivity [1–3]. Therefore, of specific interest are high-strength disperse-hardening alloys with the base of cobalt and nickel, which are highly resistant to treatment and become fragile after standard methods of treatment [4]. The paper [5] presents phase diagrams of the triple Co-Ni-Nb systems, where there were performed identification of phases that may form at the micro level, to predict how the microstructure changes upon the heat treatment used. Most effort has been focused on mechanisms of superplasticity [6]. A practical application of the SP of alloys poses important problems to be solved: the features of the precipitation and parameters of particles of the secondary phases, as well as the detection of basic and accommodative mechanisms of superplastic deformation (SPD). The understanding of these phenomena will allow for a deeper insight into the nature of superplasticity of alloys, and form the basis for the development of new methods of treatment for the design of structures with optimal technological properties. So far the literature has presented no data on the influence of holding temperature before quenching on superplastic properties of the Co-Ni-Nb alloy and the contradictory information about the secondary phases, defined in the given alloy. Such types of alloys are employed as a conducting spring in dental lighting devices [7]. Therefore, this suggests the following tasks: Metals 2018, 8, 19; doi:10.3390/met8010019 www.mdpi.com/journal/metals Metals 2018, 8, 19 2 of 11 • The influence of quenching temperature on the structure and superplastic properties of the Co-Ni-Nb alloys; • The study of peculiarities of the formation of fine-grained superplastic structures at the annealing temperature of the rolled samples; • The influence of structure and phase composition on the superplasticity of an alloy; • The definition of optimal deformation regime. 2. Materials and Methods The Co-Ni-Nb alloy samples were prepared in accordance with GOST-9651-84 and had chemical composition: base—Co, wt. 28%-Ni, wt. 5%-Nb, wt. 0.2%-Si. The quenching was performed in room temperature water. The deformation treatment was performed on a standard rolling mill. The thermal treatment of the samples was carried out in a tube-type furnace “SUOL-4” (Tula-Therm, Tula, USSR) in quartz tubes, where vacuum was maintained with a residual pressure less than 10−3 MPa. The superplastic deformation was completed on the 1246R device in vacuum with a residual pressure 10−5 MPa. The study of a microstructure was made on “HITACHI S-3400N” (HITACHI, Japan) in the scanning electron microscope by the regime of secondary electrons under the accelerating tension of 25 kV voltage. The analysis of a phase state of the samples was performed on the X-ray diffractometer “DRON-3” with using Co Kα and Cu Kα X-rays (Burevestnik inc, St. Petersburg, Russia). The elemental analysis was performed by the energy dispersive X-ray fluorescence spectrometer SRV-1 (TechnoAnalit, Ust-Kamenogorsk, Kazahstan). The tube with a molybdenum anode BS-1 (U = 25 kV, I = 30 mA) and the energy resolution semiconductor detector were used (170 eV). The exposure time was 180 s. The Scanning Electron Microscopy (SEM) images were prepared using Hitachi S-3400N equipped with detector EDS Thermo Scientific Ultra Dry operating at 5 kV/10 kV. Microhardness measurements of the samples were performed on a PMT-3 microhardener (GEO-NDT, Moscow, Russia), with an indentor load P = 100 and a holding time of 10 s. As an indenter, a regular tetrahedral diamond pyramid with a vertex angle of 136◦ was used to measure the microhardness, similar to the method for determining the Vickers hardness. 3. Results and Discussion 3.1. Structural-Phase State of Co-Ni-Nb Alloy after the Quenching The metallographic (Figure1a) and X-ray diffraction (Figure2a (2)) studies show that quenching of the Co-Ni-Nb alloy at 1423 K for 10 min creates a one phase state with the formation of homogeneous solid solution Nb in Co-Ni matrix, having face-centered cubic lattice with a parameter a = 0.356 nm. The microstructure of the initial state sample of the Co-Ni-Nb alloy is characterized by equigranular grains and the presence of twins, which are traced in almost every grain (Figure1a,b). The average size of grains determined by the intercept method is about 30 µm (Figure2a). However, according to the data presented in paper [8], which revealed the quenching temperature and the influence of the regime of the consequent ageing on the service properties of the Co-Ni-Nb alloy, the most optimal combination of the durability properties (the relaxation stability and low electric resistance) is reached as a result of quenching at 1223 K (10 min) and ageing at 873 K (5 h). Metals 2018, 8, 19 3 of 11 Metals 2018, 8, 19 3 of 10 FigureFigure 1. 1.Structural-phase Structural-phase state state of of the the Co-Ni-Nb Co-Ni-Nb alloy: alloy: ( left(left side side)) initial initial sample sample microstructure microstructure and and (right(right side side) fragment) fragment microstructure microstructure with with twins twins (black (black arrows). arrows). TheThe data data obtained obtained by by the the methods methods of of optic optic microscopy microscopy (Figure (Figure1b) 1b) and and X-ray X-ray diffraction diffraction analysis analysis γ (Figure(Figure2a) 2a) also also establish establish that that the the Co-Ni-Nb Co-Ni-Nb alloy alloy takes takes the the state state of of only only one one γ-phase-phase in in solid solid solution solution bothboth after after quenching quenching at at 1223 1223 K K (10 (10 min) min) and and after after quenching quenching at aat much a much higher higher temperature temperature than than 1423 1423 K. µ TheK. The structure structure of the of alloy the alloy quenched quenched at 1223 at K1223 features K features grains grains of less of size less (d size= 18 (dm) = 18 and µm) much and fewer much twinsfewer (Figure twins 1(Figureb). 1b). FigureFigure 2. 2.Fragments Fragments of of diffractograms diffractograms of of Co-Ni-Nb Co-Ni-Nb alloy: alloy: ( a(a)) after after quenching quenching at at 1223 1223 K K (10 (10 min) min) ( 1()1) andand at at 1423 1423 K K (10 (10 min) min) (2 ();2); (b ()b after) after annealing annealing at at 773 773 K K (1 (1 h) h ()1 (),1), 973 973 K K (1 (1 h) h ()2 ()2 and) and at at 1173 1173 (1 (1 h) h) (3 ().3). 3.2.3.2. The The Results Results of of the the Studies Studies of of the the Secondary Secondary Phase Phase of of Co-Ni-Al Co-Ni-Al Alloy Alloy AnnealingAnnealing at at 773773 KK (1(1 h)h) ++ rollingrolling (90 (90%)%) the the Co Co-Ni-Nb-Ni-Nb alloy alloy did did not not evoke evoke decomposition decomposition of of γ- γphase-phase of of solid solid solution solution (Figure (Figure 2b).2b). The The X-ray X-ray diffraction diffraction of annealing of annealing at 973 at K 973 of alloy K of shows alloy shows reflexes reflexesof the ofsecondary the secondary phase phase with withthe thehexagonal hexagonal close close-packed-packed (HCP) (HCP) lattice lattice (Figure (Figure 22b).b). TheThe electron electron microscopymicroscopy (Figures (Figures3 and3 and4) 4) and and metallographic metallographic methods methods confirm confirm the the appearance appearance of of the the secondary secondary phasephase of of spherical spherical shape shape after after rolling rolling (90%) (90%) and and annealing annealing of of the the Co-Ni-Nb Co-Ni-Nb alloy. alloy. TheThe analysis analysis of of the the structure structure of of the the surface surface of of the the Co-Ni-Nb Co-Ni-Nb alloy alloy by by the the scanning scanning electron electron microscopymicroscopy makes makes it possibleit possible to state to thatstate particles that particles of the secondaryof the secondary phase at temperingphase at cantempering precipitate can notprecipitate only on thenot edgesonly on of the grains edges (Figure of grains3a) but (Figure also inside3a) but in also their inside volume.
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