Size-Dependent Brittle-to-Ductile Transition in Silica Glass Nanofibers
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Abstract:
Silica (SiO2) glass, an essential material in human civilization, possesses excellent formability near its glass-transition temperature (Tg > 1100 °C). However, bulk SiO2 glass is very brittle at room temperature. Here we show a surprising brittle-to-ductile transition of SiO2 glass nanofibers at room temperature as its diameter reduces below 18 nm, accompanied by ultrahigh fracture strength. Large tensile plastic elongation up to 18% can be achieved at low strain rate. The unexpected ductility is due to a free surface affected zone in the nanofibers, with enhanced ionic mobility compared to the bulk that improves ductility by producing more bond-switching events per irreversible bond loss under tensile stress. Our discovery is fundamentally important for understanding the damage tolerance of small-scale amorphous structures.Keywords:
Ductility (Earth science)
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In the formability of sheet metal, distinction is made between the formability at the critical section, the formability of the material, and process efficiency. It is shown that draw-in has an adverse effect on the formability at the critical section, but greatly improves the gross process efficiency, which measures the extent to which the formability of the material is exploited in a particular process. The effect of draw-in on the strain distribution in the workpiece is shown in a triangular coordinate system for strains.
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Regression analysis method are used in the study of Regression analysis and correlation between essential formability and simulated formability of eleven furnace cold rolled sheets, based on amount of experiments. The result showed that there exist a good homologous relationship between essential formability and simulated formability. Some simulated formability related to essential formability strongly, and the coefficient of total correlation reached more than 0.95.
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Press forming of magnesium alloy sheet is conducted at elevated temperatures to improve the press formability due to its low formability at room temperature. At elevated temperatures, magnesium alloy sheet formability is known to be very sensitive to the strain rate. In this paper, warm deep drawing tests of magnesium alloy AZ31 sheet was conducted under double forming velocity as well as single forming velocity to examine the formability change by forming velocity profile. The observed formability improvement by double forming velocity was analyzed by using the finite element analysis.
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Comparing the technical parameters of sheet metal forming with formability indexes,the proposition was put forward to update these parameters and set up their database for various sheet metals by means of real and virtual forming tests in this paper.How to use formability test data to analyze various tecchnical processes,in the general,complex and virtual sheet metal forming were explained,the Shape analyses method was introduced which could calculate forming severity of sheet metal using the formability test data,and therefore the methodological reference is put forward for applying GB/T 15825《Sheet metal formability and test methods 》 in press forming manufacture.
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Abstract An attempt is made to define formability quantitatively. Owing to the complexity of the concept of formability, it is analysed and four aspects of formability are distinguished. They are: the formability of the material at a critical section, the formability of the material in general, process efficiency, and proportional increase in surface area. Distinction is thus made between formability (or ductility) of the material and the extent to which the ductility is exploited in particular processes. A method of quantitative assessment is proposed for each of the four aspects of formability and demonstrated with experimental results.
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