Stjepan Kožuh

Comparison of the corrosion behavior and surface morphology of NiTi alloy and stainless steels in sodium chloride solution

The corrosion behavior of NiTi alloy and stainless steels (AISI 316L and X2CrNiMoN22-5- 3) in 0.9% sodium chloride (0.154 mol/l) solution was investigated using open circuit potential measurements, potentiodynamic polarization and electrochemical impedance spectroscopy measurements. Microstructural analyses before and after electrochemical tests were performed with the scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). The lowest corrosion current density has NiTi alloy and the extent of the passive range increased in the order AISI 316L stainless steel < NiTi alloy < X2CrNiMoN22-5-3 duplex stainless steel. The oxide film formed on all samples has a double-layer structure consisting of a barrier- type inner layer and a porous outer layer. Oxide films formed on the surface of steels mainly contains iron oxides and chromium oxide, while the surface film of the NiTi alloy mainly contains TiO2 oxide.

Microstructural characterization of Cu82.3Al8.3Mn9.4 shape memory alloy after rolling

In this paper, the microstructure of Cu82.3Al8.3Mn9.4 (in wt. %) shape memory alloy after hot and cold rolling was investigated. The Cu82.3Al8.3Mn9.4 alloy was produced by a vertical continuous casting method in the form a cylinder rod of 8 mm in diameter. After the casting, hot and cold rolling was performed. By hot rolling a strip with a thickness of 1.75 mm was obtained, while by cold rolling a strip with a thickness of 1.02 mm was produced. After the rolling process, heat treatment was performed. Heat treatment was carried out by solution annealing at 900 °C held for 30 minutes and water quenched immediately after heating. The microstructure characterization of the investigated alloy was carried out by optical microscopy (OM), scanning electron microscopy (SEM) equipped with a device for energy dispersive spectroscopy (EDS). Phase transformation temperatures and fusion enthalpies were determined by differential scanning calorimetry (DSC) method. The homogenous martensite microstructure was confirmed by OM and SEM micrographs after casting. During rolling the two-phase microstructure occurred. Results of DSC analysis showed martensite start (M s ), martensite finish (M f ), austenite start (A s ) and austenite finish (A f ) temperatures.

The Effect of Long-Term Annealing at Elevated Temperature on Microstructure and Hardness of Heat-Resistant Steel

The stainless steel group AISI 310 is very often used for application in high-temperature conditions. This study investigated the effect of long-term annealing on microstructure and hardness of AISI 310S (24.3% Cr-19.3% Ni 1.8% Mn 0.21% Mo 0.67% Si, wt.%) heat-resistant steel. Microstructural changes and hardness distribution were analyzed after isothermal annealing at 800 °C in long-term exposure (from 1 to 1740 hours). Microstructure and fracture surface mode were analyzed using scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). It was found that after annealing the sigma phase particles were precipitated at grain boundaries and within austenite grains. The chromium content in the sigma phase increased with the prolongation of annealing time. The morphology of sigma phase was changed from fine precipitates at lower annealing times to coarser particles at higher annealing times. Also, presence of sigma phase was confirmed by transmission electron microscopy (TEM). At annealing temperature of 800 °C the hardness began to increase after 16 hours. With longer times of annealing the fracture surfaces were changed. This change is associated with a change of content and the morphology of sigma phase.

Shape Memory Alloys (Part I): Significant Properties

U ovom radu naveden je pregled termomehanickih svojstava slitina s prisjetljivosti oblika i opca obilježja martenzitne transformacije. Slitine s prisjetljivosti oblika pripadaju skupini relativno „novih“ metalnih materijala, te se od drugih slitina (npr. dentalnih slitina) razlikuju po pseudoelasticnosti i efektu prisjetljivosti oblika. Pseudoelasticnost je povezana s nastankom martenzita transformacijom austenita. Austenitno – martenzitna transformacija može se inducirati mehanickim (naprezanjem) ili toplinskim (zagrijavanje i hlađenje) metodama. Tijekom martenzitne transformacije ne odvijaju se procesi difuzije, nego dolazi do neelasticne deformacije kristalne resetke. Austenitno – martenzitnu transformaciju karakteriziraju temperature pocetka te zavrsetka austenitne i martenzitne transformacije (As i Af, Ms i Mf). Također su detaljno objasnjeni jednosmjerni, dvosmjerni i visestruki efekt prisjetljivosti oblika, te je uoceno da je jednosmjerni efekt prisjetljivosti oblika jos uvijek najcesce koristeni efekt.

Slitine s prisjetljivosti oblika (II. dio): podjela, proizvodnja i primjena

Slitine s prisjetljivosti oblika funkcionalni su “pametni” materijali s jedinstvenim svojstvom prisjetljivosti oblika. Prisjetljivost oblika karak