Structural analysis and mechanical properties of amorphous steel composites

In the first part of this research, we developed a differential scanning calorimetry method for calculating the initial crystallinity, change of crystallinity and crystallinity percentage of amorphous metal alloys as a function of temperature. Using thermodynamic enthalpies of amorphous, crystalline and partially devitrified specimens, our methodology can determine crystallinity percentages as low as a few percent. This technique also eliminates the need for expensive in situ accessories, such as those required in electron microscopy. In the second part, we report the effect of crystallinity percentage on the compressive deformation response of partially devitrified in situ Fe-based bulk metallic glass matrix composites (SAM25-Dev), consisting of an amorphous matrix and evenly dispersed nanocrystals (BCC iron, carbides, and borides) of sizes smaller than 20 nm. The SAM25-Dev specimens were obtained by consolidating SAM25 powders using the spark plasma sintering technique. The crystallinity percentage of the sintered samples varied between 20% and 61% depending on the sintering temperature. Macroscopically, bulk samples tested under compression exhibited an ~8% elastic deformation, which is significantly higher than elastic strains of other Fe-based bulk metallic glass composites, and strengths of ~2 GPa at fracture. Indentation toughness of the samples was below 1.6 MPa•m0.5, which categorizes the material as a brittle composite. Compression tests on micro-pillars resulted in specimen failure from the propagation of one principle shear band. Here, yield strength was corrected for the effect of taper angle of the micro-pillars and values above 5 GPa were obtained, making the material an ultra-hard composite. Such a high strength is attributed to the extremely small Poisson’s ratio (< 0.1) and higher glass transition temperature (883 K) of this material Both the compressive strength and fracture toughness of the specimens were negligibly dependent on the crystallinity percentage, implying that the radii of the plastic zones at the crack tips are smaller than the distance between nanocrystals on the specimens. Moreover, we find that SAM25-Dev shows a very high sample size dependency of strength (above 200%), which indicates a significant defect sensitivity. As a result, the residual porosity despite being minimal, strongly deteriorates the strength of the materials