Strain Rate Induced Crystallization in Bulk Metallic Glass-Forming Liquid

We report on the solidification of Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass under various strain rates. Using a copper mold casting technique with a low strain rate during solidification, this alloy is capable of forming glassy rods of at least 5 mm in diameter. Surprisingly, when the liquid alloy is splat cooled at much higher cooling rates and large strain rates, the solidified alloy is no longer fully amorphous. Our finding suggests that the large strain rate during splat cooling induces crystallization. The pronounced difference in crystallization behavior cannot be explained by the previously observed strain rate effect on viscosity alone. A strain rate induced phase separation process is suggested as one of the explanations for this crystallization behavior. The strain-rate-dependent critical cooling rate must be considered in order to assess the intrinsic glass forming ability of metallic liquid

Bulk metallic glass formation in binary Cu-rich alloy series – Cu100−xZrx (x=34, 36, 38.2, 40 at.%) and mechanical properties of bulk Cu64Zr36 glass

The compositional dependence of a glass-forming ability (GFA) was systematically studied in a binary alloy series Cu100−xZrx (x=34, 36, 38.2, 40 at.%) by the copper mold casting method. Our results show the critical casting thickness jumps from below 0.5 mm to above 2 mm when x changes from 34 to 36 while further increase in x reduces the critical casting thickness. The best glass former Cu64Zr36 does not correspond to either the largest undercooled liquid region (ΔT=Tx1−Tg, where Tg is the glass transition temperature, and Tx1 is the onset temperature of the first crystallization event upon heating) or the highest reduced glass transition temperature (Trg=Tg/Tl, where Tl is the liquidus temperature). Properties of bulk amorphous Cu64Zr36 were measured, yielding a Tg ~ 787 K, Trg ~ 0.64, ΔT ~ 46 K, Hv (Vicker's Hardness) ~ 742 kg/mm^2, Young's Modulus ~ 92.3 GPa, compressive fracture strength ~ 2 GPa and compressive strain before failure ~ 2.2%

Microstructural and Diffusion Analysis of Au-Sn Diffusion Couple Layer Undergoing Heat Treatment at Near Eutectic Temperatures

Diffusion couples of pure gold and pure tin were created by mechanical cold rolling method. The couples were isothermally treated at temperatures slightly above and below the eutectic temperature near tin-rich region of the equilibrium phase diagram. Differences in the diffusion behaviors were observed as a function of treatment temperatures below (473 K) and above (498 K) the eutectic temperature. At the boundary, it was found that first solid state inter-diffusion was initiated which resulted in local compositional change and solid-state formation of intermetallic compounds. As the composition shifts away towards mixing, the growth of the intermetallic phases was monitored as a function of temperature and time. At temperature above the eutectic, there may be a liquid fraction as the interface isothermally melted. The kinetic involves dissolution of Au atoms into locallized tin-rich liquid. At below eutectic temperature, the formation and growth kinetic of phases follows a solid state diffusion mechanism. By investigation the exponent n values in the growth equation l = k(t/t0)n, the values were found to be in between 0.62 - 0.77 which implies that the kinetics of IMC formations experiment are controlled by both diffusion and intermetallic reaction. The bonding temperature was found to be faster and more reliable at bonding temperature slightly above the eutectic

Study of Microstructure and Mechanical Properties of Commercially Pure Sn and Sn-4%Bi Alloys Fabricated by Permanent Mold Gravity Casting and Forging

The influences of 4 wt% bismuth addition and room temperature strain on microstructure and mechanical properties in tin alloys were investigated in this study. Commercially pure tin and Sn-4%Bi alloys were fabricated by permanent mold gravity casting. The samples were then subjected to forging process at room temperature. As-cast microstructures were compared with 0.25 and 0.5 strained samples. Differential Scanning Calorimetry (DSC) was used to confirm the effect of bismuth on undercooling. The recrystallization and grain growth processes were confirmed by grain size distribution and misorientation study using Electron Backscattered Diffraction (EBSD). Furthermore, position and morphology of the bismuth precipitates were investigated by using Field Emission Scanning Electron Microscope (FESEM). X-ray Photoelectron Spectroscopy (XPS) revealed that tin oxide was the main species found on the surface of these alloys. There was no evidence of bismuth oxide on the surface. Furthermore, the Hall-Petch hardness approximation analysis revealed that there were other influences, which increased the hardness beyond the grain refinement effect

Nickel - Induced Oral Pemphigus Vulgaris -Like Lesions

U literaturi je zabilježen samo jedan slučaj oralnog pemfigusa za koji se kao uzrok navodi nikal. U ovom prikazu opisali smo deskvamativni gingivitis kod 49-godišnjeg muškarca. Lezija se nalazila u prednjoj regiji mandibule koja je bila u kontaktu s keramičkim krunicama i mostovima. Osim tog oštećenja pronađene su i opsežne ulceracije u području lijeve i desne obrazne sluznice. Nakon godine dana liječenja lezije se nisu povukle. Uklanjanjem krunica i mostova te jakim topikalnim kortikosteroidima postignut je zadovoljavajući rezultat. Dentalna legura ispitivala se nakon toga metalurgijskim tehnikama. Rezultati su pokazali da je njezin glavni sastojak nikal. Patohistološki test i test imunofluorescencije potvrdili su dijagnozu pemphigus vulgaris. To nas je navelo na zaključak da pacijent boluje od lezija nalik na pemphigus vulgaris izazvanih niklom.So far, only a single case of nickel-induced pemphigus has been reported in the literature. We present a case of a 49-year-old male who had experienced a desquamative gingivitis on the anterior mandibular region which was in contact with porcelain crowns and bridges and severe ulcerations on the right and left buccal mucosa. The lesions did not respond to any medications for a year. After removal of those crowns and bridges with the treatment of potent topical steroids, the lesions responded dramatically. The dental alloy used as the core of crowns and bridges was further investigated using metallurgy techniques. The results showed that the dental alloy mainly contained nickel. Histopathologic and direct immunofluorescence evaluations confirmed a diagnosis of pemphigus vulgaris. We concluded that the patient had experienced nickel-induced pemphigus vulgaris-like lesions on the oral mucosa

Gold based bulk metallic glass

Gold-based bulk metallic glass alloys based on Au-Cu-Si are introduced. The alloys exhibit a gold content comparable to 18-karat gold. They show very low liquidus temperature, large supercooled liquid region, and good processibility. The maximum casting thickness exceeds 5 mm in the best glassformer. Au49Ag5.5Pd2.3Cu26.9Si16.3 has a liquidus temperature of 644 K, a glass transition temperature of 401 K, and a supercooled liquid region of 58 K. The Vickers hardness of the alloys in this system is similar to 350 Hv, twice that of conventional 18-karat crystalline gold alloys. This combination of properties makes the alloys attractive for many applications including electronic, medical, dental, surface coating, and jewelry

Laser-based Additive Manufacturing of Bulk Metallic Glasses: Recent Advances and Future Perspectives for Biomedical Applications

Bulk metallic glasses (BMGs) are non-crystalline class of advanced materials and have found potential applications in the biomedical field. Although there are numerous conventional manufacturing approaches for processing BMGs, the most commonly used like copper-mould casting have some limitations. It is not easy to manage and control the critical cooling rate, especially when the fabrication of complex BMG geometries is involved. Other limitations of these techniques include the size constraints, non-flexibility, and the tooling and accessories are costly. The emergence of additive manufacturing (AM) has opened another promising manufacturing route for processing BMGs. AM processes, particularly laser powder-bed fusion (PBF-LB/M) builds parts layer-by-layer and successively fused the powder-melted feedstocks using prescribed computer-controlled laser scanner system, thereby forming a BMGs part upon sufficiently rapid cooling to ensure the glass forming-ability. PBF-LB/M overcomes the limitations of the pre-existing BMGs processing techniques by not only improving the part size, but also produces exceptionally complex structures and patient-specific implants. This review article aims to summarise and discuss the mechanism of BMGs formation through PBF-LB/M for biomedical applications and to highlight the current scientific and technological challenges as well as the future research perspectives towards overcoming the pore-mediated microcracks, partial crystallisation, brittleness and BMG size constraint

Laser-inherent porosity defects in additively manufactured Ti-6Al-4V implant: Formation, distribution, and effect on fatigue performance

Porosity defects are inherently present in Ti–6Al–4V (Ti6-4) parts produced using additive manufacturing (AM) methods like laser powder-bed fusion (LPBF). This work aims to investigate different laser-inherent porosity defects at various LPBF parameter settings and assess their impact on the fatigue behaviour of Ti6-4 implants processed by LPBF. The presence of LPBF-inherent porosity defects with different shapes and sizes was established using microstructural examination and X-ray micro-CT analysis. These mostly comprise lack-of-fusion porosity (LoFP), gas-entrapped porosity (GeP), and pores-induced microcracks. Volumetric porosity defects were seen to range from 1.9 × 104 to 9.52 × 105 μm3. The L-1 specimen exhibited the lowest defect, while the L-6 specimen displayed the largest number of defects. While LoFP defects predominate in L-6, there was a notable presence of GeP defects in the specimens processed using the factory default condition (L-D). Upon examination of the majority of specimens, GeP and LoFP coalesced to form clusters, leading to the formation of pores-induced microcracks. This ultimately leads to a decrease in fatigue performance. By maintaining the power at the default setting and increasing the scan speed by 8% of the default value, a specimen (L-1) with minimal porosity defects and superior fatigue performance is achieved. L-6 exhibits defects with significant dimensions and irregular form. Consequently, it displays inferior fatigue characteristics