Mart Saarna

Processing and properties of bulk ultrafine-grained pure niobium

An ultrafine-grained (UFG) microstructure in electron beam-melted casts of commercial pure niobium (Nb) was produced in an equal-channel angular pressing die with a right angle, without roundings, at 12 passes, and by the Bc route. Additional microstructural improvements were made by hard cyclic viscoplastic deformation and the double-bounded microstructure was formed. The new nanoindentation technique was used to study the pure Nb micro-mechanical properties of the shear bands (SBs). The wear resistance of Nb was studied by a ball-on-plate tribometer with an alumina (Al2O3) counterface ball. The results showed that the nanohardness of pure Nb on boundaries of SBs was ~6 GPa, while inside of SBs, it was only ~3.5 GPa (measured under an indentation load of 10 mN). The corresponding elastic modules were ~150 and ~100 GPa, respectively. Such heterogeneity of the micromechanical properties has an influence on the coefficient of friction (COF) and the wear rate. The Nb with an UFG microstructure has an increased COF and a higher specific wear rate as compared to the as-cast sample. The COF depends on the direction of the wear test relative to the SBs’ orientation. As the boundaries of SBs have the highest hardness, compared to areas inside of SBs, the wear track surface has a high roughness, which leads to an increase in the COF of the double-banded ultrafine-rained pure Nb.

Influence of the SPD Processing Features on the Nanostructure and Properties of a Pure Niobium

The structurization of a high purity niobium from double electron-beam melted cast microstructure to fine-grained microstructure was completed by equal-channel angular pressing by the Bc route up to a Von Mieses strain of 13.8. In addition, for the viscoplastic behavior study as well as nanostructure and properties improving the hard cyclic viscoplastic deformation, die forging at room temperature and followed heat treatment with low heating rate were conducted. The nanostructure of processed samples was characterized by transmission electron microscopy and X-ray diffraction testing. This paper focuses on several new trends in the study of improved mechanical and physical properties of pure niobium, to what purpose these materials will be used in industry. The crystallite size, microstrains and dislocation density in severe plastic deformed pure niobium were calculated and electric conduction was measured. The nanocrystalline microstructure with minimal crystallite size down to 62 nm as mean in cross-section of sample was received. By this the dislocation density varies from 5.0 E+10 to 2.0 E+11 cm-2 and was maximal for pure niobium which has minimal electrical conductivity, maximal value of hkl-parameter and maximal relative microstresses. The microhardness was maximal for sample after 12 passes by Bc route and for samples with 8 and 10 passes followed heat treatment at 170 and 350°C. The mechanisms answerable for the electronic conduction were discussed according to the microstructure evolution in the different directions and for different strain levels.

Cyclic Loading of TiCN Coating by Vickers Indentation

Physical Vapour Deposited (PVD) coatings are used in wide range of industrial applications where requirements differ. For example, in cutting applications adhesive-abrasive wear along with high contact stresses prevail and PVD coating with thickness of ~2 μm are used. In forming applications adhesive wear usually dominates and relatively thick PVD coatings (~5 μm) are preferred. For both the applications coatings are subjected to cyclic stress and therefore it is a point of interest to learn the behaviour of PVD coatings with different thickness under cyclic loading. Cracking resistance and fatigue properties of gradient TiCN on hard metal substrate was evaluated by means of the cyclic Vickers indentation method. Hard metal was chosen as a substrate material to avoid pile-up effect and support the hard coating during indentation. The results of the single indentation Vickers test show that secondary radial and circumferential cracks appear in tested coatings already after the first indent. With increasing cycles the cracks grow up to a critical crack length after which the crack length doesn’t increase further. The tested coating thickness has no significant effect on cracking behaviour.

Effect of Hot Dip Galvanizing on the Mechanical Properties of High Strength Steels

Present study focuses on investigating the hot dip galvanizing effect on the mechanical properties of high strength steel. The effect of chemical pre-treatment (hydrogen diffusion) and the effect of hot dip galvanizing temperature on mechanical properties was studied with high strength steel S650MC. Additional tests were made with widely used structural steel S355J2. A batch type hot dip galvanizing process was used and zinc bath temperature was 450 °C and 550 °C. Results of the study show the behaviour of high strength steel during hot dip galvanizing process.

Surface Fatigue of Al-Metal Matrix Composites at Impact Loading

Aluminium 6061 has proven to be a suitable alloy as a basis for producing metal matrix composites (MMC). These MMCs have a low specific weight combined with a relatively good specific stiffness and high specific strength. The hardness and compressive strength of Al composites can be increased by reinforcing bulk material with nano particles. However the ductility of such alloys is relatively low, therefore one of the applications for such light alloys could be wear applications. In many wear conditions such as erosive or abrasive wear at normal impact angles the surface wear resistance plays a significant role. The surface fatigue properties have not been widely studied for such nanoparticle reinforced aluminium composites. The nano-reinforced composite materials were produced by means of high-energy milling (HEM) of nano-sized reinforcement particles together with a metallic matrix powder, followed by hot pressing. By utilizing up to 6 wt% multiwalled carbon nanotubes (MWCNT) as reinforcement the hardness of Al6061 MMC has been increased from 45 HV10 up to 317 HV10, compressive yield strength from 58 MPa up to 660 MPa and indentation modulus from 60 GPa up to 90 GPa compared to hot pressed Al6061. Surface fatigue tests were conducted at impact (dynamic) loading conditions using a hardened steel sphere as indenter. The Wöhler-like curves are plotted to estimate the surface fatigue. The surface fatigue indents were photographed by the aid of light optical microscopy (LOM) and analysed by image analysis software and optical profilometry (OP).

Properties and Performance of Hard Coatings on Tool Steels under Cyclic Indentation

Properties and Performance of Hard Coatings on Tool Steels under Cyclic Indentation In this paper cracking resistance and fatigue properties of five hard coatings - TiN, TiCN, TiAlN, AlTiN and nanocomposite nACo® (nc-AlTiN/a-Si3N4) on tool steels - Weartec™ and Vanadis 6 are evaluated by means of the cyclic Vickers indentation method. The analytical part covers an evaluation of damage evolution of the coated system versus the number of cycles. The effect of mechanical response - Youngs module/Hardness ratio (E/H ratio) on the crack propagation is described in the form of a diagram with various curves, each one associated with a certain number of indentation cycles. The comparative adhesion testing was conducted with the use of the Rockwell C technique. It was found that the type of cracks formed in the coated systems under cyclic loading is dependant on the E/H ratio values. The data obtained enable the cracking and fatigue resistance of the coated system to be compared and an optimal coating for metal forming tools applications to be selected.