T.A. Baser

Analysis of crystallization behavior of Fe 48 Cr 15 Mo 14 Y 2 C 15 B 6 bulk metallic glass by synchrotron radiation

The amorphous-to-crystalline transformation behavior of Fe 48 Cr 15 Mo 14 Y 2 C 15 B 6 bulk metallic glasses was first investigated by high-temperature differential scanning calorimetry. Three events were detected with onset temperatures at 922, 975, and 1036 K, respectively. In situ synchrotron radiation x-ray diffraction patterns were collected during continuous heating and analyzed with the Rietveld approach. To describe simultaneously the amorphous fraction and crystallization products as a function of temperature, a paracrystalline structure-factor model was developed. It was included for quantitative evaluation of the amorphous phase, together with the structure factor of Cr 23 C 6 - and Fe 3 Mo 3 C-type phases observed during crystallization. Volume fractions of phases as well as lattice parameters, average lattice disorder, and crystallite size of the crystallized phases have been followed as a function of temperature. In this way, the structure evolution and thermal events have been closely inspected and accounted for by a crystallization mechanism.

Fracture Behaviour of Brazed Soft Martensitic Stainless Steel Joints under Cyclic Loading

The fracture behaviour of brazed joints of the soft martensitic stainless steel X3CrNiMo13-4 under cyclic loading is investigated. The fatigue crack propagation curves (da/dN-ΔK) were derived for different load ratios R. The fatigue crack threshold values ΔKth were estimated to be 9 MPa m0.5, 7 MPa m0.5, 6 MPa m0.5 and 4 MPa m0.5 for the R values of 0.1, 0.3, 0.5 and 0.7, respectively. In addition, crack growth curves were derived for different constant loads ΔF. The Paris exponent, n, was estimated for the different R values and found to be very high compared to homogeneous materials. The work was completed with microstructural and fractographic investigation by scanning electron microscope (SEM).

Bulk Metallic Glasses

Rapid quenching techniques have been successfully applied since long time for the preparation of metallic glasses in ribbon form. Only in the recent years, the research activity addressed towards the synthesis of bulk metallic glasses (BMG), in form of ingots with a few millimetres in thickness. These materials can be obtained by casting techniques only for selected alloy compositions, characterised by a particularly high glass-forming tendency. Bulk amorphous alloys are characterised by a low modulus of elasticity and high yielding stress. The usual idea is that amorphous alloys undergo work softening and that deformation is concentrated in shear bands, which might be subjected to geometrical constraints, resulting in a substantial increase in hardness and wear resistance. The mechanical properties can be further improved by crystallisation. In fact, shear bands movement can be contrasted by incorporating a second phase in the material, which may be produced directly by controlled crystallisation. Soft magnetic properties have been obtained in Fe-based systems and they are strongly related to small variations in the microstructure, ranging from a fully amorphous phase to nanocrystalline phases with different crystal size. The high thermal stability of bulk metallic glasses makes possible the compression and shaping processes in the temperature range between glass transition and crystallisation. Aim of this paper is to present recent results on glass formation and properties of bulk metallic glasses with various compositions. Examples will be reported on Zr, Fe, Mg and Pd-based materials, focussing on mechanical and magnetic properties.