Iskanje po katalogu digitalne knjižnice

Opis: This work investigates the possibilities for applying a focussed ion beam (FIB) for the metallographic preparation and characterization of Cu-Al-Ni melt-spun ribbons. Two alloys were selected for this reason: CuAl13Ni4 and CuAl15Ni4. The microstructure of the first alloy was fully martensitic and the microstructure of the second consisted of two phases: martensite and ▫$\gamma_2$▫. It was discovered that with FIB-etching the microstructures of both alloys can be clearly revealed on polished cross-sections of the melt-spun ribbons, as well as on their wheel-side and air-side surfaces. However, better results were obtained when the etched surface was smoother, and finer details were visible when using smaller ion currents. In addition, a study was made into the influence of platinum deposition on the quality of 3D-cross sections. It was found that Pt-deposition is necessary when the edge of the trench should be straight and sharp, and the surface of the 3D cross-section smooth. However, in this case, the microstructure of the ribbons free surface cannot be seen.
Ključne besede: focussed ion beam, FIB, metallography, melt-spinning, shape memory alloy, Cu-Al-Ni
Objavljeno: 31.05.2012; Ogledov: 936; Prenosov: 19
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Opis: Cu-Al-Ni shape memory alloys (SMAs) are currently the only available high temperature SMAs, showing a good resistance against functional fatigue. In polycrystalline state, they are very brittle and exhibit, in general, only small reversible deformations. By melt spinning, thin Cu-Al-Ni ribbons can be manufactured directly from the melt. Appropriate casting parameters can ensurea single layer columnar structure with a fibre texture, which significantly increases the maximal reversible strain in longitudinal direction. Cu-Al-Ni ribbons, containing 13, 14 and 15 wt.% Al were cast by free jet melt spinning. Because of the alloys' low thermal conductivity, the cooling rate was surprisingly low - considering the crystal grain size - significantly below 103 K/s. Therefore, wide ribbons having a single layer columnar and (except the ribbons containing 13 wt.% Al) completely martensiticstructure could not be obtained. Regardless the chemical composition, the ribbons have a single layer columnar structure only if the thickness does not exceed approximately 50 m, otherwise the structure consists of at least two layers of equiaxed grains. In as-cast condition, only ribbons containing 13 wt.% Al seem to be completely martensitic. Heat treatments at temperatures up to 900 °C improved the structure of 13 and 14 wt.% Al ribbons. All ribbons exhibit one-way shape memory effect in as-cast condition. Heat-treated ribbons containing 13 wt.% Al exhibited two-way shape memory effect already after one bending and heating cycle.
Ključne besede: metallurgy, shape memory alloys, Cu-Al-Ni alloy, microstructure, melt spinning
Objavljeno: 01.06.2012; Ogledov: 1246; Prenosov: 76
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Opis: In this work we characterized the structure of continuously cast small cross-section rods (O10 mm) of the Ni-based superalloy GMR 235. In the microstructure prevailed dendritic columnar ?-grains with ?'-precipitates. In the interdendritic regions MC-carbide and M3B2-boride were identified. The inverse macrosegregation was very faint, except at the secondary witness marks and natural corrugations. It was found that the alternating drawing mode had much greater influence on microstructure than other casting parameters. Special attention was given to explanation of processes leading to formation of surface marks (primary and secondary witness marks and natural corrugations). Formation of hot tears and appearance of inverse segregation is also discussed.
Ključne besede: Ni-based superalloy, solidification, microstructure, continuous casting, surface marks
Objavljeno: 01.06.2012; Ogledov: 879; Prenosov: 70
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Opis: V svoji diplomski nalogi sem skušal predstaviti težave, ki nastanejo pri procesu tlačnega litja aluminija. V začetnem delu sem predstavil postopek tlačnega litja, njegove prednosti, stroje na katerih se ta proces vrši in najpogostejše zlitine, ki se uporabljajo. V nadaljevanju sem se osredotočil na opis računalniškega programa, ki s pomočjo numerike izvaja simulacije tlačnega litja. Program tudi prikaže napake, ki pri napačni postavitvi oddušnikov, ki so bili postavljeni na podlagi izkustev ali predhodnih simulacij, v ultikih nastanejo. Nato sem opisal namen izdelka in njegovo geometrijo. V zadnjem delu sem navedel rezultate simulacij in jih ovrednotil. Tako je nastala optimalna rešitev postavitve oddušnikov in s tem tudi najmanjša možnost za nastanek poroznosti končnega izdelka. Ker pa je pot od zasnove 3D modela do njegove izdelave zelo dolga in je za njo potrebno večje število ljudi, sem se osredotočil le na svoj del, ki je bila simulacija.
Ključne besede: Tlačno litje, računalniška simulacija, talina, napake na ulitku, aluminijeve zlitine
Objavljeno: 26.11.2012; Ogledov: 2243; Prenosov: 271
Celotno besedilo (2,51 MB)

Opis: Kompoziti s kovinsko osnovo so sestavljeni iz kovinske maatice in utrjevalne faze, ki ima lahko različno morfologijo (kontinuirna ali nekontinuirna vlakna,lamele, delci...). Do sedaj so bile razvite številne tehnologije izdelave teh materialov, med katerimi je še posebej zanimiv postopek tvorbe kompozita in situ. Ta postopek rešuje problem omočljivosti, ne omogoča pa poljubne izbire vrste in variiranja deleža utrjevalne faze v kompozitu v dovolj širokih mejah. V tem prispevku predstavljamo model novega postopka izdelave kovinskih kompozitov, utrjenih z delci , ki rešuje tako problem omočljivosti, kot tudi problem spreminjanja deleža utrjevalne faze v kompozitu.
Ključne besede: kovinski kompoziti, sinteza dvofaznih mešanic
Objavljeno: 10.07.2015; Ogledov: 624; Prenosov: 23
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