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Study of anneal hardening effect in sintered and cast alloys of copper-gold system

dc.contributor.advisorNestorović, Svetlana
dc.contributor.otherMarković, Desimir
dc.contributor.otherRajčić-Vujasinović, Mirjana
dc.contributor.otherStevanović, Jasmina
dc.contributor.otherĆosović, Vladan
dc.creatorMarković, Ivana I.
dc.date.accessioned2016-08-13T10:03:13Z
dc.date.available2016-08-13T10:03:13Z
dc.date.available2020-07-03T09:17:16Z
dc.date.issued2014-04-02
dc.identifier.urihttp://eteze.bg.ac.rs/application/showtheses?thesesId=3551
dc.identifier.urihttp://nardus.mpn.gov.rs/handle/123456789/6251
dc.identifier.urihttps://fedorabg.bg.ac.rs/fedora/get/o:12271/bdef:Content/download
dc.identifier.urihttp://vbs.rs/scripts/cobiss?command=DISPLAY&base=70036&RID=46215183
dc.description.abstractIstrаţivаnjа u okviru ove doktorske disertаcije obuhvаtila su eksperimentаlnа ispitivаnjа sinterovаnih i livenih bakarnih legurа nominalnog sastava Cu-4аt.%Au, kao i sinterovanog i livenog bаkrа radi uporeĎenja poboljšanja svojstava legura. Na sinterovanom i livenom bakru kao i na Cu-Au legurama je sprovedena kompleksna termomehanička obrada po istoj proceduri, koja je u Cu-Au legurama stvorila uslove za pojavu originalnog mehanizma ojačavanja - efekta ojačavanja ţarenjem. Sinterovani bakar i Cu-Au legura su dobijeni posle jednostranog presovanja homogene smeše prahova pritiskom od 360 MPa i sinterovanja na temperaturi od 850oC u trajanju od 1 h u redukcionoj atmosferi vodonika. Dobijanje livenih uzoraka bakra i Cu-Au legure je realizovano preko topljenja i livenja proračunate količine polaznih sirovina, homogenizacionog ţarenja odlivaka na 800oC u trajanju od 24 h i sečenja odlivaka. Tako dobijeni uzorci sinterovanog i livenog bakra i Cu-Au legura su bili podvrgnuti termomehaničkoj obradi po istovetnoj proceduri, koja se sastojala iz više etapa. Izvršene su sledeće etape termomehaničke obrade: predzavršno valjanje na proračunate visine - ţarenje na 500oC u trajanju od 45 minuta i kaljenje u vodi sa ledom - završno valjanje stepenima deformacije od 0 %, 20 %, 40 % i 60 % - izohrono ţarenje na vazduhu na temperaturama iz intervala od 60 oC do 700oC u trajanju od po 30 minuta i izotermalno ţarenje na 250oC u vremenskom intervalu od 1 minut do 100 sati. Sprovedena termomehanička obrada na opisani način je imala za cilj proučаvаnje rаzličitih mehаnizаmа ojаčаvаnjа sa osvrtom na legirаnje, hlаdnu plаstičnu deformаciju ostvarenu predzavršnim i završnim valjanjem, ali je najveća paţnja posvećena istraţivanju efekta ojačavanja ţarenjem, koji se javio kod sinterovanih i livenih Cu-Au legura, no različitog intenziteta. Zа kаrаkterizаciju sinterovanog i livenog bakra i Cu-Au legura u toku sprovedene termomehаničke obrаde je korišćeno više savremenih, standardizovanih, visoko rezolutivnih metoda i analitičkih tehnika. Ispitivanja mehaničkih karakteristika obuhvatila su merenja tvrdoće i mikrotvrdoće po Vikersu primenom standardizovanih metoda merenja. Električna svojstva su praćena merenjem električne provodnosti. Mikrostrukturna analiza je obavljena primenom optičke mikroskopije (LOM) i skenirajuće elektronske mikroskopije sa energetsko-disperzivnom spektrometrijom (SEM-EDS). Koroziona postojanost sinterovanih i livenih Cu-Au legura je ispitivana u alkalnoj sredini, primenom metoda merenja potencijala otvorenog kola i ciklične voltаmetrije. U cilju proučavanja mehanizma ojačavanja ţarenjem u hladno deformisanim sinterovanim i livenim Cu-Au legurama obavljena su sledeća ispitivanja. Rendgenskom difrakcijom (XRD) je izvršena fazna analiza i proračunati su parametri jediničnih ćelija hladno deformisanih sinterovanih i livenih Cu-Au legura pre i posle ţarenja. Termijsko ponašanje hladno deformisanih sinterovanih i livenih Cu-Au legura je praćeno primenom metode diferencijalno-termijske analize (DTA). Za analizu mikrostrukture na nano nivou korišćena je trаnsmisionа elektronskа mikroskopijа (TEM). Legiranje bakra sa 4 at.% zlata, bez menjanja postupka dobijanja, dovelo je do formiranja legura sistema Cu-Au, koje imaju veće vrednosti tvrdoće i mikrotvrdoće, a manje vrednosti električne provodnosti u odnosu na čist bakar. Vrednosti tvrdoće i mikrotvrdoće sinterovanih uzoraka su nešto veće, a vrednosti električne provodnosti nešto manje u odnosu na odgovarajuće vrednosti livenih uzoraka, što je dovedeno u vezu sa homogenijom distribucijom manjih i brojnijih pora u sinterovanim uzorcima u odnosu sa gasnu poroznost u livenim materijalima. Pokazano je da sa porastom stepena deformacije, i kod predzavršnog i kod završnog valjanja, rastu vrednosti tvrdoće, mikrotvrdoće i električne provodnosti usled promena u mikrostrukturi izazvanim hladnom plastičnom deformacijom. Niskotemperaturno izohrono ţarenje završno valjanih uzoraka sinterovanih i livenih Cu-Au legura u intervalu od 60oC do 350oC doprinelo je daljem porastu vrednosti tvrdoće, mikrotvrdoće i električne provodnosti, kao posledice pojave efekta ojačavanja ţarenjem. Poboljšanje osobina tokom ţarenja je ostvareno kroz dva stadijuma, a najbolja kombinacija osobina je ostvarena u drugom stadijumu ojačavanja posle ţarenja na 260oC. Neznatne promene u strukturi su vidljive tokom ţarenja na 260oC, ali EDS analiza je pokazala znatnu promenu u koncentraciji atoma zlata unutar samog zrna. Izotermalnim ţarenjem hladno deformisanih uzoraka sinterovanih i livenih Cu-Au legura na 250oC je dodatno ostvareno ojačavanje preko dva stadijuma – primarnog i sekundarnog ojačavanja. Efekat ojačavanja ţarenjem u Cu-Au legurama je intenzivniji kod sitnozrnije sinterovane legure, u odnosu na krupnozrnu livenu leguru. Uočeno je smanjenje parametra jedinične ćelije i sinterovanih i livenih hladno deformisanih Cu-Au legura posle ţarenja na 260oC, čime je potvrĎeno da je došlo do pojave efekta ojačavanja ţarenjem jer se u toku ţarenja atomi zlata izdvaja iz bakarne osnove. Na DTA krivama dobijenim pri zagrevanju hladno deformisanih sinterovanih i livenih Cu-Au legura uočeno je postojanje tri egzotermna pika i jednog blagog endotermnog pika. Objašnjenje svih reakcija je u skladu sa literaturnim podacima i potvrdama o uporednom uticaju sreĎivanja kratkog dometa i reakcija rastvorenih atoma sa greškama u rešetki na poboljšanje osobina hladno deformisanih čvrstih rastvora u toku ţarenja, a usled odvijanja mehanizma efekta ojačavanja ţarenjem. Na svim TEM mikrofotografijama su registrovane oblasti oblika diska, ali su kod sinterovanih Cu-Au legura u odnosu na livene te oblasti brojnije, gusto rasporeĎene i uniformnije po veličini. Elektrohemijska ispitivanja u rastvoru 0,1 M NaOH su potvrdila da hladno deformisani uzorci sinterovanih i livenih Cu-Au legura posle ţarenja na 260oC (kod kojih se manifestuje najveće poboljšanje mehaničkih osobina na račun pojave efekta ojačavanja ţarenjem) poseduju najpozitivnije vrednosti potencijala otvorenog kola u odnosu na ostala termomehanički obraĎena stanja. Voltamogrami za Cu-Au leguru pokazuju manju korozionu stabilnost u odnosu na čiste metale – bakar i zlato, jer se gustine struja pikova koji potiču od bakra superponiraju sa gustinama struje pikova koji potiču od reakcija na zlatu. Voltamogrami za različita termomehanički obraĎena stanja pokazuju da su završno valjani uzorci sinterovanih i livenih Cu-Au legura koroziono najpostojaniji. Dobijeni rezultati predstavljaju nova saznanja o intenzitetu i mehanizmu efekta ojačavanja ţarenjem u sistemu Cu-Au u zavisnosti od postupka dobijanja (sinterovanje ili klasični postupak topljenja i livenja), i pruţaju informacije o poboljšanju osobina Cu-Au legura u odnosu na čist bakar, koji je dobijen i termomehanički tretiran na isti način kao i legure.sr
dc.description.abstractThe research conducted in this PhD thesis includes the experimental study of sintered (PM) and cast (IM) copper based alloys with nominal composition Cu- 4at.%Au, as well as PM and IM copper in order to compare the improvements of the alloy properties. An identical procedure of thermomechanical treatment was performed on PM and IM copper and Cu-Au alloys, which created the conditions for the occurrence of original mechanism of hardening (anneal hardening effect) in alloys. PM copper and Cu-Au alloys were obtained after one-sided pressing of the homogeneous powder mixture using the pressure of 360 MPa, and sintering at the temperature of 850°C for 1 h in a hydrogen reducing atmosphere. IM samples of copper and Cu-Au alloys were obtained through melting and casting of the calculated amount of the raw materials, as well as ingot homogenization at 800oC for 24 h, and ingot cutting. The obtained samples of PM and IM copper and Cu-Au alloys were subjected to thermomechanical treatment using the identical procedure consisting of several stages. The following stages of thermomechanical treatment were performed: prefinal rolling on a calculated height - annealing at 500°C for 45 minutes and quenching in the ice water - the final rolling with 0 %, 20 %, 40 % and 60 % reductions - isochronal annealing in air in the temperature interval of 60oC to 700°C for a period of 30 minutes, and isothermal annealing at 250°C in a time interval from 1 minute to 100 hours. The above described thermomechanical treatment was used in order to analyze different strengthening mechanisms such as alloying, cold plastic deformation by prefinal and final rolling, and most of all, the anneal hardening effect, occurring with different intensity in PM and IM Cu-Au alloys. For the characterization of the PM and IM copper and Cu-Au alloys during the thermomechanical treatment,many modern, standardized, high resolution methods and analytical techniques were used. Mechanical properties testing included the Vickers hardness and microhardness test using the standardized measurement methods. Electrical properties were observed by measuring the electrical conductivity. Microstructural analysis was performed using the light optical microscopy (LOM) and scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS). Corrosion resistance of PM and IM Cu-Au alloys was studied in alkaline medium, using the method of measuring the open circuit potential and cyclic voltammetry.With the aim to study the mechanism of anneal hardening in the cold deformed PM and IM Cu-Au alloys, the following tests were performed. The phase analysis and calculation of lattice parameter of the cold deformed PM and IM Cu-Au alloys before and after annealing were performed using X-ray diffraction (XRD). The thermal behavior of the cold deformed PM and IM Cu-Au alloy was studied using the differential thermal analysis (DTA) method. Transmission electron microscopy (TEM) was used for the microstructural analysis at the nano size level.Copper alloying with 4 at.% gold without a change in the method of obtaining of the samples has led to the formation of Cu-Au alloys with higher hardness and microhardness values, but lower electrical conductivity values compared to the pure copper. Hardness and microhardness values of the PM samples are slightly higher, while the values of the electrical conductivity are slightly lower than the corresponding values of the IM samples which can be attributed to a more homogenous distribution of the smaller and more numerous pores in the PM samples compared to the gas porosity in IM materials. It is shown that with an increase of reduction (during the prefinal the final rolling), the values of hardness, microhardness and electrical conductivity increase as well due to microstructural changes caused by cold plastic deformation. Low temperature isochronal annealing of finally rolled samples of PM and IM Cu-Au alloys in the temperature range of 60oC to 350°C contributed to a further increase in hardness, microhardness and electrical conductivity values, as a result of anneal hardening effect. The properties improvement during annealing was accomplished in two stages, and the best combination of properties was achieved in the second stage of hardening, after annealing at 260oC. Minor structural changes were evident during annealing at 260°C, but the EDS analysis showed a significant change in the gold concentration within the grain. Two-stage strengthening (primary and secondary) was further achieved through isothermal annealing of cold deformed samples of PM and IM Cu-Au alloys at 250°C. The anneal hardening effect in Cu-Au alloys is more intense in fine-grained PM alloys, than in the coarse-grained IM alloys. The reduction in lattice parameters of PM and IM cold deformed Cu-Au alloys after annealing at 260°C was evident, which confirmed the occurrence of anneal hardening effect as the gold atoms dissociated from the copper matrix during the annealing. Three exothermic peaks and a slight endothermic peak appeared on the DTA curves obtained by heating of cold deformed PM and IM Cu-Au alloys. All reactions were explained according to the literature data confirming the comparative effects of short-range ordering and the reactions of solute atoms with lattice defects on the properties improvement of cold-deformed solid solution during annealing, due to the occurrence of anneal hardening effect. The disc-shaped areas were evident in all of the TEM micrographs, but in the PM Cu-Au alloys the areas were more numerous, higherdensity and more uniform in size than in the IM Cu-Au alloys.Electrochemical study in 0.1 M NaOH confirmed that the cold-deformed samples of PM and IM Cu-Au alloys after annealing at 260°C (the temperature at which the greatest increase in mechanical properties due to anneal hardening effect was achieved) had the most positive values of the open circuit potential compared to the other analyzed thermomechanical conditions. Voltammograms for Cu-Au alloys show lower corrosion stability compared to the pure metals (copper and gold), because the current density peaks originating from copper overlay on the current density peaks originating from the gold atom reactions. Voltammograms for the different thermomechanically treated conditions show that the finally rolled samples of PM and IM Cu-Au alloys are the most corrosion-stable.These results present new information on the intensity and mechanism of anneal hardening effect in the Cu-Au system depending on the obtaining method (powder metallurgy process or the common way by melting and casting), as well as the properties improvements in Cu-Au alloys compared to the pure copper which is obtained and thermomechanically treated on the same way as alloysen
dc.formatapplication/pdf
dc.languagesr
dc.publisherУниверзитет у Београду, Технички факултет, Борsr
dc.relationinfo:eu-repo/grantAgreement/MESTD/Technological Development (TD or TR)/34003/RS//
dc.rightsopenAccessen
dc.sourceУниверзитет у Београдуsr
dc.subjectbakarsr
dc.subjectcopperen
dc.subjectCu-Au alloysen
dc.subjectalloyingen
dc.subjectcold plastic deformationen
dc.subjectanneal hardening effecten
dc.subjectthermomechanical treatmenten
dc.subjectCu-Au leguresr
dc.subjectlegiranjesr
dc.subjecthladna plastična deformacijasr
dc.subjectefekat ojačavanja ţarenjemsr
dc.subjecttermomehanička obradasr
dc.titleIstraživanje efekta ojačavanja žarenjem kod sinterovanih i livenih legura sistema bakar-zlatosr
dc.titleStudy of anneal hardening effect in sintered and cast alloys of copper-gold systemen
dc.typedoctoralThesis
dc.rights.licenseBY-NC
dcterms.abstractНесторовић, Светлана; Ћосовић, Владан; Стевановић, Јасмина; Рајчић-Вујасиновић, Мирјана; Марковић, Десимир; Марковић, Ивана И.; Истраживање ефекта ојачавања жарењем код синтерованих и ливених легура система бакар-злато; Истраживање ефекта ојачавања жарењем код синтерованих и ливених легура система бакар-злато;
dc.identifier.fulltexthttp://nardus.mpn.gov.rs/bitstream/id/16810/Disertacija4185.pdf


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