| Abstract | Detaljnim proučavanjem literature nisu u potpunosti utvrđeni mehanizmi utjecaja
toplinske obrade (kaljenje 850 – 920 °C i popuštanje 150 – 300 °C) na svojstva
(mikrostruktura, tvrdoća, istezljivost, obradivost) kontinuirano lijevane CuAlNi slitine, stoga
postoji opravdani razlog za provođenje ovog istraživanja.
U radu je detaljno analizirana promjena mikrostrukture i mehaničkih svojstava
CuAlNi slitine s prisjetljivošću oblika (kontinuirano lijevana šipka ϕ 8 mm) toplinskom
obradom pri različitim temperaturama. Toplinska obrada je obuhvaćala postupak kaljenja na
temperaturama 850 °C, 885 °C i 920 °C u trajanju od 60 minuta te naglog hlađenja u vodi
sobne temperature. Nakon postupka kaljenja provedeno je popuštanje na 150 °C, 200 °C, 250
°C i 300 °C u trajanju od 60 minuta te hlađenje u vodi.
Ispitivanje mikrostrukture je provedeno optičkim, pretražnim i transmisijskim
elektronskim mikroskopom te rendgenskom faznom analizom. Utvrđeno je da je martenzitna
mikrostruktura prisutna u svim uzorcima (lijevano, kaljeno i popušteno stanje). Rendgenskom
faznom analizom utvrđeno je postojanje dvije vrste martenzita u mikrostrukturi nakon
kaljenja i popuštanja (β1' i γ1'), što znači da temperatura i postupak toplinske obrade značajno
utječe na razvoj mikrostrukturnih konstituenata slitine, koji imaju važan utjecaj na
funkcionalna svojstva slitine. Analizom prijelomnih površina utvrđeno je postojanje
transkristalnog prijeloma u uzorcima nakon lijevanja, kaljenja i popuštanja na 150 °C.
Međutim, gotovo potpuni interkristalni tip prijeloma zamijećen je u uzorcima popuštenim na
300 °C.
Ispitivanjem mehaničkih svojstava utvrđeno je da najveći iznos vlačne čvrstoće ima
uzorak kaljen 850 °C/60'/H2O (498,6 MPa), a najnižu vrijednost vlačne čvrstoće nakon
kaljenja ima uzorak kaljen 885 °C/60'/H2O (367,5 MPa). Uočeno je da granica razvlačenja s
porastom temperature kaljenja opada. Vrijednost istezanja je niska za sve ispitane uzorke (< 5
%). Najveća vrijednost istezanja izmjerena je za uzorak slitine nakon lijevanja (4,75 %).
Postupak kaljenja nepovoljno utječe na istezanje slitine, gdje je uočeno smanjenje vrijednosti
istezanja (3,55 %, 2,72 % i 2,95 %) u odnosu na lijevano stanje. Istezanje nakon popuštanja
opada s porastom temperature popuštanja, s iznimkom uzorka kaljenog 920 °C/60’/H2O i
popuštenog 200 °C/60’/H2O.
S obzirom na uvjete primjene CuAlNi slitine, provedena su detaljna elektrokemijska
ispitivanja u 0,9% NaCl, pri različitim pH vrijednostima te različitoj koncentraciji kloridnih
iona, kako bi se pratio utjecaj promjena u okolini (otopini) na ponašanje CuAlNi slitine.
Zamijećeno je da dolazi do pada gustoće korozijske struje na uzorcima kaljenim na 850 °C i
920 °C u odnosu na lijevano stanje. Iznimka je uzorak kaljen na 885 °C kod kojeg dolazi do
porasta što upućuje na intenzivnije otapanje slitine. Također, negativan utjecaj uočen je
smanjenjem pH i povećanjem koncentarcije kloridnih iona na korozijsko ponašanje CuAlNi
slitine s prisjetljivošću oblika. Brzina korozije s porastom temperature kaljenja opada,
iznimka je uzorak kaljen na 885 °C. Sa smanjenjem pH otopine uočen je porast brzine
korozije, a porastom koncentracije kloridnih iona zamijećen je porast brzne korozije osobito u
0,9 % i 1,5 % NaCl. |
| Abstract (english) | Through a detailed study of literature it is noticed that the mechanisms of influence of
heat treatment (solution annealing at 850 – 920 °C and tempering at 150 – 300 °C) on
properties (microstructure, hardness, ductility, formability) in continuously cast CuAlNi alloy
are not fully determined, and therefore exists a justified reason for the following research.
In this work, a detailed analysis of changes in microstructure and mechanical
properties of the CuAlNi shape memory alloy (continuously cast bar of 8 mm in diameter)
with heat treatment at different temperatures will be carried out. The heat treatment processes
included solution annealing at temperatures of 850 °C, 885 °C and 920 °C for 60 minutes and
rapidly cooling in room temperature water. After the annealing process, tempering is
performed at 150 °C, 200 °C, 250 °C and 300 °C for 60 minutes, followed by cooling in
water.
Microstructural analysis was performed by optical, scanning and transmission electron
microscopy, and X-ray diffraction analysis. It was found that the as-cast sample and samples
after quenching and tempering had fully martensitic microstructure. X-ray diffraction analysis
showed the existence of two types of martensite in the microstructure after solution annealing
and tempering (β1' and γ1'), which means that the temperature of the heat treatment has
significant influence on the development of alloys’ microstructural constituents, which have
an important influence on the functional properties of the alloy. The analysis of the fracture
surfaces, the existence of transgranular type of fracture in the samples after casting, quenching
and tempering at 150 ° C was noticed. However, almost complete intergranular type of
fracture can be observed in the samples tempered at 300 ° C.
During the study of the mechanical properties it was found that the largest value of
tensile strength has the sample solution annealed at 850 °C/60'/H2O (498.6 MPa), and the
lowest tensile strength has the sample solution annealed at 885 °C/60'/H2O (367.5 MPa). It
was observed that the yield strength with the increasing of tempering temperature decreases.
Elongation has very low values for all tested samples (below 5%). The maximum value of
elongation was measured for the sample of the alloy after casting (4.75%). Solution annealing
process has no favourable influence on alloys’ elongation, where there is visible decrease of
the elongation (3.55%, 2.72% and 2.95%) compared to the value obtained in as-cast
condition. Elongation after tempering decreases as the tempering temperature decreases, with
the exception of the sample solution annealed at 920 °C/60'/H2O and tempered 200
°C/60'/H2O.
Considering to a CuAlNi alloys application conditions a detailed electrochemical tests
in 0.9% NaCl were performed. Electrochemical tests were performed at different solution pH
and different concentrations of chloride ions, in order to observe the influence of
environmental changes (solution) on the behaviour of the CuAlNi alloy. It was noticed that
corrosion current density decreases on the samples solution annealed at 850 °C and 920 °C
compared to the as-cast state sample. The exception is the sample annealed at 885 °C in
which there is an increase of corrosion current density indicating a more intensive dissolution
of the alloy. Also, the negative impact on the corrosion behaviour of CuAlNi shape memory
alloy was observed by decreasing the pH of the solution and increasing the concentration of
chloride ions in the solution. Corrosion rate decreases with the increase of solution annealing
temperature; the exception is sample solution annealed at 885 °C. Corrosion rate increas es
with decreasing pH of the solution and increasing the concentration of chloride ions,
especially in 0.9 and 1.5% NaCl solution. |
