František Hnilica

Problems of Fatigue Crack Growth in Strongly Anisotropic Al-Alloys

An Al 4114 (8090 type) alloy, a material to be used in aircraft struc tu es, is manufactured by extrusion resulting in a strongly oriented anisotropic microstructure. Fatigue crack growth (FCG) data and mechanisms affecting the FCG proces s ar an important knowledge for an assessment of reliability and safety of structur es containing crack-like defects. As a basis, FCG characteristics in the directions of extrusion and the perpendicular one are needed. The FCG properties and mechanisms in the Al 4114 alloy w ere investigated using CT specimens with side notches to prevent cracking in inappropriat e directions caused by the anisotropy. Unlike FCG in the direction perpendicular to extrusion, when dependencies typical for Paris region of stable growth were ascertained, FC G in the extrusion direction was connected with regions of acceleration and retardation resulting in s i nificant scatter. Crack closure measurements and fractographical analyses were performe d to explain the irregularities and FCG mechanisms.

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.

The effect of annealing temperature on the properties of powder metallurgy processed Ti-35Nb-2Zr-0.5O alloy

Ti-35Nb-2Zr-0.5O (wt%) alloy was prepared via a powder metallurgy process (cold isostatic pressing of blended elemental powders and subsequent sintering) with the primary aim of using it as a material for bio-applications. Sintered specimens were swaged and subsequently the influence of annealing temperature on the mechanical and structural properties was studied. Specimens were annealed at 800, 850, 900, 950, and 1000°C for 0.5h and water quenched. Significant changes in microstructure (i.e. precipitate dissolution or grain coarsening) were observed in relation to increasing annealing temperature. In correlation with those changes, the mechanical properties were also studied. The ultimate tensile strength increased from 925MPa (specimen annealed at 800°C) to 990MPa (900°C). Also the elongation increased from ~ 13% (800°C) to more than 20% (900, 950, and 1000°C).

The Effect of Solution Treatment Temperature on Plastic Deformation and Fracture Mechanisms of Beta-Titanium Alloy

The beta-titanium alloys are used mainly in bioapplications for artificial joints and other implants. They posses interesting properties such as, high corrosion resistance, low Young’s modulus, good plasticity or superelasticity etc. In this work the effect of solution treatment temperature on deformation and fracture properties has been studied. The alloy Ti-35Nb-2Zr was processed via powder metallurgy process (cold isostatic pressing, sintering and subsequent swaging). Swaged alloy was annealed at 800, 850, 900, 950 and 1000 °C. Tensile tests have been performed on such heat treated specimens and the fracture surface has been studied in correlation with microstructure. With increasing annealing temperature both tensile strength (from 925 MPa to 990 MPa) and elongation (from 13 to 25 %) increased where the maximum values were obtained for 900 °C annealed specimens and subsequently slight decrease has been observed. The simultaneous increase of strength and elongation was attached to change of deformation mechanisms which was described by studying fracture surfaces and microstructure of deformed (tensile tested) specimens.