Petr Haušild

Comparison of Isolated Indentation and Grid Indentation Methods for HVOF Sprayed Cermets

This paper compares the results of two approaches of instrumented indentation for characterization of mechanical properties of HVOF coatings. Three types of HVOF sprayed coatings (Cr3C2-NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were investigated by the means of isolated nanoindentation and grid indentation methods. The results of the isolated indentation revealed hardness and elastic modulus of the individual phases in very good agreement with the corresponding bulk material. The grid indentation method, based on statistical evaluation of a large number of indentations, was influenced by the carbide-matrix interface, which gave rise to a third peak apart from the two peaks corresponding to the carbides and metallic matrix. As a consequence, the bimodal Gaussian fit was insufficient and a trimodal fit had to be used. The results extracted from low load grid nanoindentations were quite close to the results of isolated indentations whereas higher load grid nanoindentation revealed overall properties of the coating.

Fracture Behaviour of Fe3Al and FeAl Type Iron Aluminides

Fracture behaviour of two intermetallic alloys based on FeAl and Fe3Al was studied. On the alloys Fe-40Al-1C (at%) and Fe-29.5Al-2.3Cr-0.63Zr-0.2C (at%) (FA06Z), a basic characterization, the fracture toughness tests and fractographic analysis were carried out. Tensile tests and fracture toughness tests were performed at 20, 200, 400 and 600°C. The fracture toughness values range from 26 MPa.m1/2 at 20°C to 42 MPa.m1/2 at 400°C. In addition, Jintegral dependence on a obtained by potential method was measured. The fractographic analysis showed that samples fractured at 20, 200 and 400°C in the tensile or fracture toughness tests exhibit transgranular cleavage fracture, while at 600°C the ductile dimple fracture predominates.

TEM phase analysis of NiTi shape memory alloy prepared by self-propagating high-temperature synthesis

Abstract The effect of annealing a Ni–48at.%Ti alloy prepared by self-propagation high-temperature synthesis (SHS) by means of transmission electron microscopy (TEM) was studied. The alloy in the as-sintered condition shows the presence of B2 NiTi matrix, relatively large particles (2–100 μm) of cubic NiTi2 phase and very fine densely distributed particles of rhombohedral Ni4Ti3 phase. The phase fractions were determined by the Rietveld refinement of neutron diffraction. Twelve-hour annealing at 1000 °C leads to coarsening of Ni4Ti3 particles. Slow furnace cooling from the annealing temperature results in a transformation of the rhombohedral Ni4Ti3 phase to a metastable Ni3Ti2 phase with cubic symmetry. Its particles, coherent with the B2 matrix, have the form of thin oval platelets with lenticular cross-section. The orientation relation is either cube-to-cube or [115]B2 || [111]P and (552)B2 || (121)P. An additional isothermal annealing of the water cooled samples at 720 °C for 3 and 10 h was carried out. According to the scanning and transmission electron microscopy observation of the sample annealed for 3 h, the cubic Ni3Ti2 phase transforms to the high-temperature tetragonal Ni3Ti2–H variant (space group I4/mmm).

Fatigue Crack Growth in Bodies with Thermally Sprayed Coating

Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the localization of crack initiation sites and the dynamics of crack propagation are studied. The resonance bending fatigue test was employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical thermal barrier coating consisting of yttria stabilized zirconia and NiCoCrAlY layers. The strain distribution on the coating surface was evaluated by the Digital Image Correlation method through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e., the changes of material properties, crack initiation, and crack propagation, were identified. The tested coatings strongly influenced the first two phases, and the influence on the crack propagation was less significant. In general, the presented crack detection method can be used as a sensitive nondestructive testing method well suited for coated parts.

Characterization of Anisotropy in Hardness and Indentation Modulus by Nanoindentation

Abstract This paper provides a useful guide how to characterize material anisotropy by nanoindentation. Hardness and indentation modulus of austenitic stainless steel (grade A304) were characterized by instrumented indentation at the grain scale (at low indentation load and depth of penetration). We applied the grid indentation method on an area containing several grains with different crystallographic orientation which was simultaneously characterized by electron back-scatter diffraction. Hardness and indentation modulus dependencies on crystallographic orientation were then evaluated and compared with single crystal Young’s modulus and finite element simulations.

High-temperature mechanical properties of Fe-40 at.% Al based intermetallic alloys with C or Ti addition

Abstract Tensile and creep properties of vacuum-induction-melted and cast Fe-40Al-1C and Fe-40Al-1Ti (at.%) were studied. The ingots were hot-rolled at 1200 °C to plates of 12.5 mm thickness using a stainless steel sheath and cooled in air. The alloys showed microstructures with coarse elongated grains having diameters up to 500 and 300 m in the direction of rolling, respectively. Tensile tests were carried out at temperatures 20, 400, 600, 700, and 800 °C. The creep tests were performed under constant load of 100 MPa and 150 MPa at temperatures 550, 600, 650, and 670 °C. Fracture surfaces of tensile specimens tested up to 700 °C exhibited mainly intergranular decohesion. With increasing temperature, the proportion of ductile dimpled fracture increased, and at 800 °C the fracture surfaces of both alloys were practically completely covered by ductile dimples. In comparison to tensile test specimens, fracture surfaces of creep specimens showed an increased fraction of intergranular ductile fracture.

Characterisation of Al-Cu and Al-Mg-Si Free-Cutting Alloys

Free cutting alloys of Al-Cu (AA2011 and AA2111B) in T6 temper and Al-Mg-Si system (AA6023 and AA6262) in tempers T6 and T8 were subjected to Charpy U - notch impact testing at the temperatures ranging from 20°C to 350°C. The microstructure of the materials was characterized by light metallography, fracture surfaces were observed using scanning electron microscope (SEM). The alloys showed a significant decrease in the impact energy KU at temperatures ~125°C (AA2011, AA2111B), ~170°C (AA6023), and ~250°C (AA6262), respectively. This decrease of KU was caused by melting of disperse phases containing low-melting point metals (Pb, Sn, Bi), which was confirmed by differential scanning calorimetry. Additional annealing of the AA6262-T8 alloy for 2h at 400°C followed by slow cooling led to the transformation of Pb + Bi particles accompanied by the shift of the melting temperature from ~250 to ~310 °C. Higher temperature solution annealing of the AA6023 alloy for 30 min at 540°C (as a replacement of common 30 min at 520°C) resulted in a partial transformation of Sn + Bi particles accompanied by melting point shift from ~170 to ~200°C. Chemical composition of the corresponding phases was monitored by energy dispersive X-ray spectroscopy in SEM.

Microstructure and mechanical properties of hot rolled Fe–40 at-%Al intermetallic alloys with Zr and B addition

Abstract The microstructure and mechanical properties of hot rolled Fe–40 at-%Al based intermetallic alloys, with 0·1 at-%Zr and different additions of B (varying from 0·01 to 0·1 at-%), are characterised. The additions of Zr and B improve tensile properties at room and elevated temperatures. Increasing B content is also associated with a number of other effects. First, the fracture mode changes from intergranular decohesion to cleavage, which correlates with significant increases in the fracture toughness. Second, there is a certain stabilisation of dislocations arranged in parallel systems of slip bands, as shown by transmission electron microscopy. Numerous complex stacking faults on {100} planes are also observed in the alloy with the highest B content. Third, B is found to modify the formation of second phase particles; such particles (coarse and fine) are analysed by energy dispersive X-ray spectroscopy and electron energy loss spectrometry to obtain compositional information.

Phase Transformation in Austenitic Steel Induced by Plastic Deformation

Kinetics of deformation induced martensitic transformation in metastable austenitic AISI 301 steel was characterized by several techniques including classical light metallography, X-ray diffraction, neutron diffraction and electron back scattered diffraction. In order to characterize the martensitic transformation, several specimens were tensile pre-deformed to 5%, 10% and 20% of plastic deformation and compared with non-deformed state. During straining, the volume fraction of α’-martensite rapidly prevails over the volume fraction of original austenite and reach the value circa 70%.

Effect of Actual Indenter Shape on the Results of Spherical Nanoindentation

Actual shape of the diamond spherical indenter of nominal radius 20 μm was investigated in this study. 3D reconstruction was performed by atomic force microscope and by the method of stereopair using SEM images of the tip taken under several different angles. The results were compared with the shape obtained indirectly by the calibration performed on specimens with known Young’s modulus. It was found that lower effective values of tip radius for the small penetration depths are caused by the irregular geometry of contact between indenter and specimen surface. With increasing penetration depth the radius increased to the theoretical values and it decreased again for high penetration depths. The stress-strain curves were determined using corrected effective indenter radius.