Nadendla Hari Babu

We report a simple, scalable route to produce ultrahigh-strength magnesium (Mg) via solidification of a colloidal solution containing nanoscale niobium carbide (NbC) particles suspended in liquid magnesium (Mg(l)). A single-atom-level investigation reveals that NbC exhibits spontaneous wetting with molten Mg, driven by the formation of an ordered layer of Mg atoms strongly bonded to the carbon atoms on the NbC {001} surface. This creates Mg-coated NbC (Mg@NbC) particles in liquid Mg and is referred to as Mg(l)-Mg@NbC nanocolloid. This unique and spontaneous wetting behaviour enables uniform nanoparticle dispersion in the molten Mg without external fields, and in the solidified Mg matrix without the need for thermomechanical processing. The resulting NbC dispersoids act as coherent, hard reinforcement phases, significantly strengthening the Mg matrix. As a result, the Mg-NbC material exhibits ultrahigh tensile strength and stiffness, surpassing those of all previously reported Mg alloy systems.

Bu çalışmada in-situ yöntemi ile Al matriste NbB2 partikül takviyeli kompozitler üretilmiştir. Farklı oranlarda partikül takviyesinin ve üretim süresinin kompozitler üzerindeki etkileri incelenmiştir. Oluşan partiküller, boyutları ve dağılımları incelenmiş, tane boyutu ve sertliğe etkisi belirlenmiştir. Yapılan çalışmalar neticesinde partikül oluşumunda karıştırma süresinin kullanılan alaşım oranı gibi önemli olduğu belirlenmiş ve en iyi değerler 1,5 saat karıştırma sonucu oluşan %3,75 NbB2 ilaveli kompozitte elde edilmiştir. Bu kompozitte tane boyutu 112 μm seviyelerine inmiştir ve sertlik 306,4 MPa olmuştur.

Alumina-to-alumina brazed joints were formed using 96.0 and 99.7 wt.% Al₂O₃ ceramics using 150 µm thick Ticusil^® (68.8Ag-26.7Cu-4.7 wt.% Ti) braze preforms. Brazing was conducted in a vacuum of 1 × 10⁻⁵ mbar at 850 °C for 10 minutes. Joint strengths were evaluated using four-point bend testing and were compared to the monolithic flexural strengths of standard alumina test bars according to ASTM C1161-13. Brazed joints made using 96.0 wt.% Al₂O₃ consistently outperformed brazed joints made using 99.7 wt.% Al₂O₃, despite similarities in both the flexural strengths of the standard alumina test bars and the microstructures of brazed joints. Secondary phase interaction led to the formation of Ti₅Si₃ reaction products at locations where the triple pocket grain boundaries of the 96.0 wt.% Al₂O₃ surface intersected the Ti-rich reaction layers. It is proposed that due to this interaction, brazed joints made using 96.0 wt.% Al₂O₃, which were relatively cost-effective to produce, achieved higher strengths than brazed joints made using 99.7 wt.% Al₂O₃.

High iron impurity affects the castability and the tensile properties of the recycled Al-Si alloys due to the presence of the Fe containing intermetallic β-Al₉Fe₂Si₂ phase. To date only Mn addition is known to transform the β-Al₉Fe₂Si₂ phase in the Al-Si-Fe system. However, for the first time, as reported here, it is shown that β-phase transforms to the ω-Al₇Cu₂Fe phase in the presence of Cu, after solutionization at 793 K. The ω-phase decomposes below 673 K resulting into the formation of θ-Al₂Cu phase. However, the present thermodynamic description of the Al-Si-Fe-Cu system needs finer tuning to accurately predict the stability of the ω-phase in these alloys. In the present study, an attempt was made to enhance the strength of Al-6wt%Si-2wt%Fe model recycled cast alloy with different amount of Cu addition. Microstructural and XRD analysis were carried out in detail to show the influence of Cu and the stability range of the ω-phase. Tensile properties and micro-hardness values are also reported for both as-cast and solutionized alloys with different amount of Cu without and with ageing treatment at 473 K. The increase in strength due to addition of Cu, in Fe-rich Al-Si alloys is promising from the alloy recyclability point of view.

The study reports phase and microstructural evolution in MgB ₂ bulk superconductors fabricated by an infiltration and growth ( IG ) process. Three distinct stages, (1) intermediate boride formation, (2) bulk liquid Mg infiltration, and (3) MgB ₂ layer formation, were identified in IG process after detailed examination of series of samples prepared with varied heating conditions. The intermediate phase Mg ₂ B ₂₅ , isomorphous to β‐boron, was detected prior to MgB ₂ phase formation in stage (1). Due to volume expansion involved in stage 1, cracks formed in the β‐boron particles and propagated radially inwards during stage 3. The growing MgB ₂ particles sintered simultaneously with formation of grain boundaries during the process, as evidenced by the measured hardness and critical current density in these samples. From our observations, we estimate the total time needed for complete transformation to MgB ₂ .