The fracture behaviour of an ultrafine grained WC-Co hard metal was investigated in tensile and bending tests using different specimen sizes and test arrangements in order to study the size effect on the tensile strength, by varying the effectively tested volume over a range of roughly 10 orders of magnitude. Mechanical testing of centimetre sized specimens was performed by means of tensile tests using an hour glass shaped specimen. Millimetre sized specimens were tested in four point and three point bending test set-ups. Micrometre sized specimens, rectangular beams produced via focused ion beam milling, were loades in situ in a scanning electron microscope utilizing a piezo-electrically controlled cube corner micro-indenter. The resulting fracture surfaces were examined in order to identify crack origins. The main result of the present work is that strength values are found to increase from about 2500 to about 6000 MPa when the size of the effectively loaded volume isvaried from about 100 to about 10[sub]{-8} mm[sub]3. This kind of behaviour is typical for brittle materials in which strength is defect controlled and can be explained by a size effect according to Weibull theory. In the case of the micrometre sized specimens no defects were found on the fracture surfaces. Estimations of critical defect sizes in these specimens based on linear elastic fracture mechanics give values in the order of magnitude of the submicron sized tungsten carbide particles. It is therefore expected that the high strength values found in these specimens are close to the inherent material strength.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 855722The evolution of the passive films on 2205 duplex stainless steel (2205 DSS) and AISI 316L stainless steel in artificial saliva, and with the addition of fluoride, was studied using electrochemical impedance spectroscopy (EIS) and potentiodynamic measurements. The extent of the passive range increased for the 2205 DSS compared to the AISI 316L in both solutions. The formation of the passive film was studied by EIS at the open-circuit potential (OCP). The passive layers were studied at the OCP by X-ray photoelectron spectroscopy (XPS). The passive films on both materials predominantly contained Cr-oxides, whereas the Fe species were markedly depleted.
F.10 Improvements to an existing technological process or technology
COBISS.SI-ID: 24275239Residual stresses are present in engineering components as an unintended consequence of manufacturing processes, but they are also deliberately introduced to beneficial effect during surface engineering procedures. Plasma nitriding is a process of particular importance for forming tools and dies, giving significant advantages in wear, and fatigue resistance through the generation of near-surface compressive residual stresses. A precise knowledge of the level and distribution of residual stresses that exist in engineering components is necessary for an accurate design and prediction of a component’s fatigue resistance. However, measurement of residual stresses is not always possible, which is especially true for forming tools. Therefore, other methods for residual stress evaluation and prediction are required. Results of this investigation show that residual stress level and depth in plasma nitrided tool steel increase with nitriding time and temperature. On the other hand, experimental data indicates that the residual stress distribution in plasma nitrided tool steels can be determined on the basis of microhardness depth distribution. A minimum in the microhardness depth profile derived over depth corresponds to the location of the compressive residual stress maximum. Furthermore, the residual stress level can be extrapolated by using reference residual stress data and genetic programming. In this way the residual stress level and distribution can be obtained even for components where measurement is not possible.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 1795835