Vit Janik

Influence of Thermal and Thermo-Mechanical Processing on the Creep Resistance of Mg-10Gd-3Y-0,4Zr Alloy

Alloy Mg-10Gd-3Y-0,4Zr in as-cast, as-extruded, cast-T6 (peak aged) and extruded-T5 (peak aged) state was tensile creep tested at 200, 250 and 300 °C and stress 50, 80 and 120 MPa. Comparison of minimal creep rate shows that alloy Mg-10Gd-3Y-0,4Zr in cast-T6 conditions is characterized by an excellent creep resistance, which is higher than that of commercially available Mg-alloys. Creep resistance of as-cast, as-extruded and extruded-T5 alloy Mg-10Gd-3Y-0,4Zr is lower. Cavity nucleation is heavily affected by the amount of secondary phases on the grain boundaries and also by the initial grain size of the microstructure. After extrusion and in the extruded-T5 conditions creep cavitation was not observed, whereas in the as-cast and cast-T6 conditions creep cavitation occurred on the high fraction of grain boundaries.

Phase composition and morphology development in WE-type alloys modified by high Zn content

Abstract Quasicrystalline icosahedral equilibrium phase was found in Mg – Y – Nd (WE43) alloys with the addition of 12 wt.% and 25 wt.% Zn constituting grain boundary eutectic phases. In response to the isochronal annealing, morphologically similar precipitates of phases with the same structure as those developing during heat treatment of binary Mg-8 wt.% Zn alloy at temperatures higher than 150 °C were observed, namely β (b-centered monoclinic crystal structure a = 2.596 nm, b = 1.428 nm, c = 0.524 nm, = 102.5°) and β (hexagonal MgZn2). The evolution of the resistivity and microhardness during isochronal annealing corresponds to the phase transformations and the morphology changes of the phases observed. Dislocations with the c Burgers vector component generated due to the presence of icosahedral phase are responsible for the improved formability as compared to the WE43 alloy.

Microstructural investigation of the failure mechanisms after creep exposure of Mg-Y-Nd-Zn-Mn alloy

Abstract Tensile creep tests of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn (wt.%) magnesium alloy were performed at constant load. Measurement of grain-boundary sliding by observation of the offset of marker lines was carried out. The role of mutual orientation of neighbor dendrites and possible influence of active slip systems inside dendrites on the cavity nucleation was investigated by electron backscatter diffraction. Experimental results show that creep failure in this alloy takes place by continuous nucleation, growth, and coalescence of cavities on dendritic boundaries. A nucleation mechanism involving stress concentrations due to slip inside the grains is not probable. Additionally, although the total extent of deformation due to grain boundary sliding is small, the contribution of sliding to the origin of required stress concentrations necessary for cavity nucleation cannot be excluded.

Cavitation and grain boundary sliding during creep of Mg-Y-Nd-Zn-Mn alloy

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30–80 MPa. Tensile creep tests were performed at 300 °C up to the final fracture. Several tests at 50 MPa were interrupted after reaching the steady state creep; and another set of creep tests was interrupted after the onset of ternary creep. Fraction of cavitated dendritic boundaries was evaluated using optical microscopy. Measurement of grain boundary sliding by observation of the offset of marker lines was carried out on the surface of the crept specimens after the test interruption by scanning electron microscopy and by confocal laser scanning microscopy. The results show that the dominant creep mechanism in this alloy is dislocation creep with minor contribution of the grain boundary sliding. Creep failure took place by the nucleation, growth and coalescence of creep cavities on the boundaries predominantly oriented perpendicular to the applied stress. Increasing amount of cavitated boundaries with time of creep exposure supports the mechanism of continuous cavity nucleation and growth.

In Situ Characterisation of Austenite/Ferrite Transformation Kinetics and Modelling of Interphase Precipitation Inter-Sheet Spacing in V Microalloyed HSLA Steels

A new generation of low-carbon microalloyed High Strength Low Alloy (HSLA) steels has been developed to utilize a combination of single-phase ferritic microstructures and optimized interphase precipitation to provide high level strength and exceptional formability. The interphase precipitation reaction is a transient process lending itself strongly to take advantage of in-situ characterization techniques. The austenite/ferrite interface kinetics during isothermal transformation at 1003 K is measured using HT-CSLM, the pre-exponential effective mobility constant was found to be mobility 0.822 (m J)/(mole s). The V interphase precipitation is characterised using TEM at isothermal transformation temperatures of 923 and 973 K as having inter-sheet spacing of 22±7 and 32±9 nm respectively. Interphase precipitation inter-sheet-spacing is simulated using a revised Quasi-Ledge model and qualitatively predicts the observed trends observed for inter-sheet spacing. The results of in-situ characterisation and modelling suggest that it is possible to optimize the strengthening potential of the precipitation processes by controlling the thermal processing of microalloyed HSLA.

Effect of Grain Size Distribution on Recrystallisation Kinetics in a Fe-30Ni Model Alloy

The high strength and toughness of microalloyed steels are derived by controlled rolling schedules, using recrystallisation to give grain refinement. A number of relationships (based on the JMAK equations) for the rate of recrystallisation, with a range of Avrami parameter values, have been reported. This paper discusses the role of grain size distribution on the recrystallisation rates and Avrami values for an austenitic model steel (Fe-30 Ni). Cold deformation has been used to provide uniform macroscopic strain distributions (strains of 0.2 and 0.3, which are equivalent to 0.27 and 0.45 strain if deformed at 850°C), followed by recrystallisation during annealing at 850 – 950°C. It is shown that the Avrami parameter is directly related to the grain size distribution, with a lower Avrami exponent being seen for a larger average and wider grain size distribution. In-situ heating in an SEM with EBSD has shown the recrystallisation kinetics to be affected by both nucleation site and local strain heterogeneity.