Alena Michalcová

Mechanism and kinetics of the intermediary phase formation in Ti–Al and Ti–Al–Si systems during reactive sintering

Abstract In this work, chemical reactions and phase transformations during reactive sintering of Ti – Al and Ti – Al – Si materials were investigated by differential thermal analysis. Kinetics and mechanism of the formation of intermediary phases are described on an experimental model system consisting of a bulk titanium sample exposed to molten aluminium or AlSi50 alloy. The results show that the reactive sintering is a reaction-controlled process in both systems. The reaction rate increases significantly with addition of silicon. According to the kinetics study, reactive sintering of TiAl36 and TiAl15Si15 alloys was carried out. TiAl15Si15 alloy was successfully sintered, while TiAl36 material showed extremely high porosity and presence of unreacted components.

Selective aluminum dissolution as a means to observe the microstructure of nanocrystalline intermetallic phases from Al-Fe-Cr-Ti-Ce rapidly solidified alloy.

Rapidly solidified aluminum alloys are promising materials with very fine microstructure. The microscopy observation of these materials is complicated due to overlay of fcc-Al matrix and different intermetallic phases. A possible way to solve this problem is to dissolve the Al matrix. By this process powder formed by single intermetallic phase particles is obtained. In this paper a new aqueous based dissolving agent for Al-based alloy is presented. The influence of oxidation agent (FeCl(3)) concentration on quality of extraction process was studied.

The Structure and Mechanical Properties of High-Strength Bulk Ultrafine-Grained Cobalt Prepared Using High-Energy Ball Milling in Combination with Spark Plasma Sintering

In this study, bulk ultrafine-grained and micro-crystalline cobalt was prepared using a combination of high-energy ball milling and subsequent spark plasma sintering. The average grain sizes of the ultrafine-grained and micro-crystalline materials were 200 nm and 1 μm, respectively. Mechanical properties such as the compressive yield strength, the ultimate compressive strength, the maximum compressive deformation and the Vickers hardness were studied and compared with those of a coarse-grained as-cast cobalt reference sample. The bulk ultrafine-grained sample showed an ultra-high compressive yield strength that was greater than 1 GPa, which is discussed with respect to the preparation technique and a structural investigation.

Microstructure and mechanical properties of the micrograined hypoeutectic Zn–Mg alloy

A biodegradable Zn alloy, Zn–1.6Mg, with the potential medical applications as a promising coating material for steel components was studied in this work. The alloy was prepared by three different procedures: gravity casting, hot extrusion, and a combination of rapid solidification and hot extrusion. The samples prepared were characterized by light microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. Vickers hardness, tensile, and compressive tests were performed to determine the samples’ mechanical properties. Structural examination reveals that the average grain sizes of samples prepared by gravity casting, hot extrusion, and rapid solidification followed by hot extrusion are 35.0, 9.7, and 2.1 μm, respectively. The micrograined sample with the finest grain size exhibits the highest hardness (Hv = 122 MPa), compressive yield strength (382 MPa), tensile yield strength (332 MPa), ultimate tensile strength (370 MPa), and elongation (9%). This sample also demonstrates the lowest work hardening in tension and temporary softening in compression among the prepared samples. The mechanical behavior of the samples is discussed in relation to the structural characteristics, Hall–Petch relationship, and deformation mechanisms in fine-grained hexagonal-close-packed metals.

Effect of Alloying Elements on Microstructure and Properties of Fe-Al and Fe-Al-Si Alloys Produced by Reactive Sintering

Pressureless reactive sintering production of iron aluminides is always connected with high porosity of the product. Previous research showed that silicon reduces the porosity significantly. In this work, the effect of alloying elements (Cu, Ni) on the reactive sintering behaviour and on the porosity of Fe-Al and Fe-Al-Si alloys was studied. Microstructure, phase composition, mechanical and tribological properties were studied as functions of alloy composition.

Influence of Heat-Treatment on Mechanical Properties and Transformation Temperatures of Nitinol

Nearly equi-atomic Ni-Ti alloys (nitinol) show shape memory behavior, superelasticity, high strength, excellent corrosion resistance and biocompatibility, making them of interest for various biomedical applications. In processing, they experience various heat treating steps. The present work illustrates influence of a short-time heat treatments (several minutes) at moderate temperatures (around 500°C) on transformation temperature Af and on mechanical properties.

Structure of Rapidly Solidified Al-Fe-Cr-Ce Alloy

An alloy containing Al – 3wt.% Cr – 3wt.% Fe – 0.8wt. % Ce, was prepared by melt spinning. Structure of obtained ribbons was observed by light, scanning and transmission electron microscopy. It was found out that the structure is very fine. Microhardness of cross sectioned ribbons was also measured. Defects in structure were determined by positron annihilation spectroscopy. The thermal stability of the alloy was observed by comparing rapidly solidified ribbons and ribbons annealed at 400°C and at 500°C for 100 h

High-Strength Ultra-Fine-Grained Hypereutectic Al-Si-Fe-X (X = Cr, Mn) Alloys Prepared by Short-Term Mechanical Alloying and Spark Plasma Sintering

In this work, Al-20Si-10Fe-6Cr and Al-20Si-10Fe-6Mn (wt %) alloys were prepared by a combination of short-term mechanical alloying and spark plasma sintering. The microstructure was composed of homogeneously dispersed intermetallic particles forming composite-like structures. X-ray diffraction analysis and TEM + EDS analysis determined that the α-Al along with α-Al15(Fe,Cr)3Si2 or α-Al15(Fe,Mn)3Si2 phases were present, with dimensions below 130 nm. The highest hardness of 380 ± 7 HV5 was observed for the Al-20Si-10Fe-6Mn alloy, exceeding the hardness of the reference as-cast Al-12Si-1Cu-1 Mg-1Ni alloy (121 ± 2 HV5) by nearly a factor of three. Both of the prepared alloys showed exceptional thermal stability with the hardness remaining almost the same even after 100 h of annealing at 400 °C. Additionally, the compressive strengths of the Al-20Si-10Fe-6Cr and Al-20Si-10Fe-6Mn alloys reached 869 MPa and 887 MPa, respectively, and had virtually the same values of 870 MPa and 865 MPa, respectively, even after 100 h of annealing. More importantly, the alloys showed an increase in ductility at 400 °C, reaching several tens of percent. Thus, both of the investigated alloys showed better mechanical properties, including superior hardness, compressive strength and thermal stability, as compared to the reference Al-10Si-1Cu-1Mg-1Ni alloy, which softened remarkably, reducing its hardness by almost 50% to 63 ± 8 HV5.

A Lifetime of Metallic Nanoparticles in Heat Exchange Liquids

This work is focused on lifetime prediction of metallic nanoparticles in heat exchange nanofluids. Copper, nickel and iron nanoparticles were studied in 40 wt.% aqueous solution of potassium formate and propylene glycol. Materials were observed by means of mass loss exposure tests, linear polarization resistance and potentiodynamic measurements. Potassium formate solution is not suitable bearing liquid for metallic nanoparticles. Propylyne glycol seems promising, however additional corrosion prevention needs to be applied.

Structure of Al-TM-Ce alloy

Introduction Rapid solidification is one of few possible methods for preparing thermally stable aluminium alloys. The structure of such alloys is very fine (with average grain size about 1 μm and even smaller intermetallic particles with size of few hundreds nm [1]). This means, the only suitable method for observing these materials is transmission electron microscopy (TEM). On the other hand this method needs very special samples: thin foils or extremely small pieces of material prepared by selective dissolution of materials matrix. The choice between these two types of samples depends on information, which is wanted to be obtained. For summary view of material it is necessary to use for observing a thin foils but for more detail information about present intermetallic phases it is advantageous to prepare samples by selective dissolution of rapidly solidified ribbons. The aim of this paper was to prove the possibility of selective dissolution reagent utilization for investigation of Al-TM-Ce based alloy.