Istraživanje efekta ojačavanja žarenjem kod sinterovanih i livenih legura sistema bakar-zlato
Study of anneal hardening effect in sintered and cast alloys of copper-gold system
Докторанд
Marković, Ivana I.Ментор
Nestorović, SvetlanaЧланови комисије
Marković, DesimirRajčić-Vujasinović, Mirjana
Stevanović, Jasmina
Ćosović, Vladan
Метаподаци
Приказ свих података о дисертацијиСажетак
Istrаţivаnjа 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 termome...haničkoj 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.
The 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 trea...tment 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 alloys