4 results
Search Results
Now showing 1 - 4 of 4
- ItemProcessing metallic glasses by selective laser melting(Amsterdam [u.a.] : Elsevier, 2013) Pauly, S.; Löber, L.; Petters, R.; Stoica, M.; Scudino, S.; Kühn, U.; Eckert, J.Metallic glasses and their descendants, the so-called bulk metallic glasses (BMGs), can be regarded as frozen liquids with a high resistance to crystallization. The lack of a conventional structure turns them into a material exhibiting near-theoretical strength, low Young's modulus and large elasticity. These unique mechanical properties can be only obtained when the metallic melts are rapidly cooled to bypass the nucleation and growth of crystals. Most of the commonly known and used processing routes, such as casting, melt spinning or gas atomization, have intrinsic limitations regarding the complexity and dimensions of the geometries. Here, it is shown that selective laser melting (SLM), which is usually used to process conventional metallic alloys and polymers, can be applied to implement complex geometries and components from an Fe-base metallic glass. This approach is in principle viable for a large variety of metallic alloys and paves the way for the novel synthesis of materials and the development of parts with advanced functional and structural properties without limitations in size and intricacy.
- ItemMicrostructure and elastic deformation behavior of β-type Ti-29Nb-13Ta-4.6Zr with promising mechanical properties for stent applications(Amsterdam : Elsevier B.V., 2019) Plaine, A.H.; Silva, M.R.D.; Bolfarini, C.In this paper, an attempt was made to combine theoretical composition design and thermo-mechanical treatments to produce a metastable β-type titanium alloy with mechanical compatibility for self-expandable stent applications. Metastable β-type Ti-29Nb-13Ta-4.6â»Zr (wt.%) thin-wires with an elastic modulus of 46â»GPa and a yield strength of 920â»MPa were successfully fabricated by cold rolling and low temperature aging. This combination of high yield strength and comparatively low elastic modulus resulted in enhanced elastic recoverable strain of 1.9%, which is much higher than that of the conventional metallic stent materials. The microstructure responsible for the much sought-after mechanical properties was observed to be mainly consisted of a homogeneous distribution of nanometer-sized α-precipitates in a β-phase matrix obtained via a spinodal decomposition of the pre-existed α″-martensite phase through α″â»→â»α″ leanâ»+â»α″ richâ»→â»αâ»+â»β. The α-precipitates increase the strength of the material by hindering the motion of dislocations (spinodal hardening) while the β-matrix with relatively low content of β-stabilizers gives rise to the observed low elastic modulus. More broadly, these findings could be extended to developing advanced metastable β-type titanium alloys for implant and other engineering applications.
- ItemAb initio based study of finite-temperature structural, elastic and thermodynamic properties of FeTi(Amsterdam [u.a.] : Elsevier, 2014) Zhu, L.-F.; Friák, M.; Udyansky, A.; Ma, D.; Schlieter, A.; Kühn, U.; Eckert, J.; Neugebauer, J.We employ density functional theory (DFT) to calculate pressure dependences of selected thermodynamic, structural and elastic properties as well as electronic structure characteristics of equiatomic B2 FeTi. We predict ground-state single-crystalline Young's modulus and its two-dimensional counterpart, the area modulus, together with homogenized polycrystalline elastic parameters. Regarding the electronic structure of FeTi, we analyze the band structure and electronic density of states. Employing (i) an analytical dynamical matrix parametrized in terms of elastic constants and lattice parameters in combination with (ii) the quasiharmonic approximation we then obtained free energies, the thermal expansion coefficient, heat capacities at constant pressure and volume, as well as isothermal bulk moduli at finite temperatures. Experimental measurements of thermal expansion coefficient complement our theoretical investigation and confirm our theoretical predictions. It is worth mentioning that, as often detected in other intermetallics, some materials properties of FeTi strongly differ from the average of the corresponding values found in elemental Fe and Ti. These findings can have important implications for future materials design of new intermetallic materials.
- ItemThe influence of plasma treatment on the elasticity of the in situ oxidized gradient layer in PDMS: towards crack-free wrinkling(London : Royal Soc. of Chemistry, 2018) Glatz, Bernhard Alexander; Fery, AndreasControlled surface wrinkling is widely applied for structuring surfaces in the micro- and nano-range. The formation of cracks in the wrinkling process is however limiting applications, and developing approaches towards crack-free wrinkles is therefore vital. To understand crack-formation, we systematically characterized the thickness and mechanics of thin layers formed by O2-plasma-oxidation of polydimethyl siloxane (PDMS) as a function of plasma power and pressure using Atomic Force Microscopy Quantitative Nano-mechanical Mapping (AFM-QNM). We found a nearly constant layer thickness with simultaneously changing Young's moduli for both power and pressure screenings. We determined the respective crack densities, revealing conditions for crack-free wrinkling. Thus we could identify correlations between the intensity of plasma treatment and the cracking behavior. The primary cause for crack-suppression is a continuous elasticity gradient starting within the soft bulk PDMS, and rising up to several hundred MPa at the oxidized layer's surface. With mechanical simulations via the Finite Elements Method (FEM) we were able to demonstrate a noticeable difference in maximal stress intensity σmax between a comparable, but theoretical single layer and a gradient interface. A threshold in tensile stress of σcrit = 14 MPa distinguishes between intact and cracked layers.