Metals2015, 5(3), 1148-1162; doi:10.3390/met5031148 (registering DOI) - published 30 June 2015 Show/Hide Abstract
Abstract: The present article focuses on the influence of machining on the fatigue life of a titanium alloy: Ti6Al4V. An experimental design was adopted in order to highlight the effects of machining parameters on surface integrity while generating very different surfaces with a view to subsequent fatigue testing (four point bending tests). Firstly, the impact of machining parameters on surface integrity was demonstrated. Then, the influence of surface integrity on fatigue lifetime was observed: no influence of the geometric and metallurgical parameters was observed. However, the mechanical parameter (e.g., residual stress) seemed to have a preponderant influence. To conclude, a machining plan of procedure was proposed to significantly improve the fatigue lifetime as compared with a reference industrial plan of procedure.
Metals2015, 5(3), 1136-1147; doi:10.3390/met5031136 (registering DOI) - published 30 June 2015 Show/Hide Abstract
Abstract: Green nanocompositesbased on renewable plant oils and polyhedral oligomeric silsesquioxanes (POSS) have been developed. An acid-catalyzed curing of epoxidized plant oils with oxirane-containing POSS derivatives produced transparent nanocomposite coatings with high gloss surface, in which the organic and inorganic components were linked via covalent bonds. The hardness and mechanical strength were improved by the incorporation of the POSS unit into the organic polymer matrix. Nanostructural analyses of the nanocomposites showed the formation of homogeneous structures at the micrometer scale. On the other hand, such improvements of the coating and mechanical properties were not observed in the composite without covalent bonds between the plant oil-based polymer and POSS unit. The study demonstrates the correlation between the nanostructure of composites and macroscopic properties.
Metals2015, 5(3), 1127-1135; doi:10.3390/met5031127 - published 29 June 2015 Show/Hide Abstract
Abstract: The glass-forming ability (GFA) and the magnetic properties of the [(Fe0.5Co0.5)0.75B0.20Si0.05]96Nb4−xYx bulk metallic glasses (BMGs) have been studied. The partial replacement of Nb by Y improves the thermal stability of the glass against crystallization. The saturation mass magnetization (σs) exhibits a maximum around 2 at. % Y, and the value of σs of the alloy with 2 at. % Y is 6.5% larger than that of the Y-free alloy. The coercivity shows a tendency to decrease with increasing Y content. These results indicate that the partial replacement of Nb by Y in the Fe–Co–B–Si–Nb BMGs is useful to simultaneous achievement of high GFA, high σs, and good soft magnetic properties.
Metals2015, 5(2), 1112-1126; doi:10.3390/met5021112 - published 22 June 2015 Show/Hide Abstract
Abstract: In this study, Al-17%Si alloys were dual modified by fine-grained structural materials (FSM) according to structural heredity. Microstructure and thermal analyses were undertaken to study the modification effect of the FSM master alloy on primary and eutectic Si. Primary Si is refined to a smaller size and eutectic Si is modified from needle-like to fibrous shape after FSM master alloy addition. The optimal content of FSM master alloy is 20% and the holding time is 15min. Finer FSM master alloy leads to finer Al-17%Si alloy microstructure and more area percentage of α-Al. DSC analyses results show that FSM master alloy can raise the precipitation temperatures of primary and eutectic Si, meanwhile it can reduce the latent heat of Si solidification process.
Metals2015, 5(2), 1073-1111; doi:10.3390/met5021073 - published 19 June 2015 Show/Hide Abstract
Abstract: Relaxation phenomena in glasses are a subject of utmost interest, as they are deeply connected with their structure and dynamics. From a theoretical point of view, mechanical relaxation allows one to get insight into the different atomic-scale processes taking place in the glassy state. Focusing on their possible applications, relaxation behavior influences the mechanical properties of metallic glasses. This paper reviews the present knowledge on mechanical relaxation of metallic glasses. The features of primary and secondary relaxations are reviewed. Experimental data in the time and frequency domain is presented, as well as the different models used to describe the measured relaxation spectra. Extended attention is paid to dynamic mechanical analysis, as it is the most important technique allowing one to access the mechanical relaxation behavior. Finally, the relevance of the relaxation behavior in the mechanical properties of metallic glasses is discussed.
Metals2015, 5(2), 1061-1072; doi:10.3390/met5021061 - published 16 June 2015 Show/Hide Abstract
Abstract: Precursor effects of indium melting have been investigated by means of Mechanical Spectroscopy (MS) and High Temperature X-ray Diffraction (HT-XRD). MS tests evidenced a sharp drop of dynamic modulus in the temperature range between 418 K and 429 K (melting point). At 429 K, HT-XRD showed partial grain re-orientation, peak profile broadening, in particular in the lower part, and peak shift towards lower angles. Experimental results are consistent with density increase of self-interstitials and vacancies in the crystal lattice before melting. Self-interstitials and vacancies play a synergic role in the solid–liquid (S-L) transformation. The increase of self-interstitials over a temperature range of about 10 K before melting has the effect of weakening interatomic bonds (modulus drop) that favors the successive vacancy formation. Finally, the huge increase of vacancy concentration above 428 K leads to the collapse of crystal lattice (melting).