Filip Průša

Preparation of nitinol by non-conventional powder metallurgy techniques

The aim of the present paper was to compare the evolution of Ni–Ti intermetallics in two non-conventional production techniques for the synthesis of NiTi shape memory alloy. Short term ultrahigh energy mechanical alloying is proposed to be able to describe the early stages of the milling process, which was not described in the literature previously, and to obtain intermetallics in shorter process durations. The reactive sintering using high heating rate (>300°Cu2009min−u20091) is a process designed to suppress the formation of secondary intermetallics and to reduce the porosity of the product. The same phases formation sequence was determined for both processes. The detrimental Ti2Ni phase forms preferentially, and therefore, its presence cannot be avoided in any of the investigated techniques.

Nanocrystalline Al7075 + 1 wt % Zr Alloy Prepared Using Mechanical Milling and Spark Plasma Sintering

The microstructure, phase composition, and microhardness of both gas-atomized and mechanically milled powders of the Al7075 + 1 wt % Zr alloy were investigated. The gas-atomized powder exhibited a cellular microstructure (grain size of a few µm) with layers of intermetallic phases along the cell boundaries. Mechanical milling (400 revolutions per minute (RPM)/8 h) resulted in a grain size reduction to the nanocrystalline range (20 to 100 nm) along with the dissolution of the intermetallic phases. Milling led to an increase in the powder’s microhardness from 97 to 343 HV. Compacts prepared by spark plasma sintering (SPS) exhibited negligible porosity. The grain size of the originally gas-atomized material was retained, but the continuous layers of intermetallic phases were replaced by individual particles. Recrystallization led to a grain size increase to 365 nm in the SPS compact prepared from the originally milled powder. Small precipitates of the Al3Zr phase were observed in the SPS compacts, and they are believed to be responsible for the retainment of the sub-microcrystalline microstructure during SPS. A more intensive precipitation in this SPS compact can be attributed to a faster diffusion due to a high density of dislocations and grain boundaries in the milled powder.

Structure and Mechanical Properties of Al-Cu-Fe-X Alloys with Excellent Thermal Stability

In this work, the structure and mechanical properties of innovative Al-Cu-Fe based alloys were studied. We focused on preparation and characterization of rapidly solidified and hot extruded Al-Cu-Fe, Al-Cu-Fe-Ni and Al-Cu-Fe-Cr alloys. The content of transition metals affects mechanical properties and structure. For this reason, microstructure, phase composition, hardness and thermal stability have been investigated in this study. The results showed exceptional thermal stability of these alloys and very good values of mechanical properties. Alloying by chromium ensured the highest thermal stability, while nickel addition refined the structure of the consolidated alloy. High thermal stability of all tested alloys was described in context with the transformation of the quasicrystalline phases to other types of intermetallics.

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.

The Influence of Milling and Spark Plasma Sintering on the Microstructure and Properties of the Al7075 Alloy

The compact samples of an Al7075 alloy were prepared by a combination of gas atomization, high energy milling, and spark plasma sintering. The predominantly cellular morphology observed in gas atomized powder particles was completely changed by mechanical milling. The continuous-like intermetallic phases present along intercellular boundaries were destroyed; nevertheless, a small amount of Mg(Zn,Cu,Al)2 phase was observed also in the milled powder. Milling resulted in a severe plastic deformation of the material and led to a reduction of grain size from several µm into the nanocrystalline region. The combination of these microstructural characteristics resulted in abnormally high microhardness values exceeding 300 HV. Consolidation through spark plasma sintering (SPS) resulted in bulk samples with negligible porosity. The heat exposition during SPS led to precipitation of intermetallic phases from the non-equilibrium microstructure of both gas atomized and milled powders. SPS of the milled powder resulted in a recrystallization of the severely deformed structure. An ultra-fine grained structure (grain size close to 500 nm) with grains divided primarily by high-angle boundaries was formed. A simultaneous release of stored deformation energy and an increase in the grain size caused a drop of microhardness to values close to 150 HV. This value was retained even after annealing at 425 °C.

MICROSTRUCTURE AND THERMAL STABILITY OF Al-Fe-X ALLOYS

In this work, Al-11Fe, Al-7Fe-4Ni and Al-7Fe-4Cr (in wt. %) alloys were prepared by combination of casting and hot extrusion. Microstructures of as-cast alloys were composed of aluminium matrix with large and coarse intermetallics such as Al 13 Fe 4 , Al 13 Cr 2 and Al 5 Cr. Subsequently, as-cast alloys were rapidly solidified by melt-spinning technique which led to the supersaturation of solid solution alloying elements. These rapidly solidified ribbons were milled and compacted by hot-extrusion method. Hot-extrusion caused that microstructures of all alloys were fine with uniform dispersed particles. Moreover, long-term thermal stability was tested at temperature 300 °C for as-cast and hot-extruded alloys and chromium was found to be the most suitable element for alloying to improve thermal stability.

Al-Fe Chips Processed by High-Energy Ball Milling and Spark Plasma Sintering

Typically, conventional casting technologies are employed to manufacture aluminium alloys from scrap, but during recycling iron accumulates and increases in content. Increased iron content in such alloys reduces their mechanical properties. Because powder metallurgy is able to prepare materials with a very fine microstructure, we investigated its use for the preparation of aluminium alloys with a high iron content and the required mechanical properties. We prepared an Al-Fe17 (wt. %) binary alloy using combination of mechanical working (MW), high-energy ball milling (HEBM) and spark plasma sintering (SPS). The thus-prepared samples were analyzed (XRD, XRF, SEM-EDS, compression stress-strain test) and compared to the commercially-available alloy Al-Si12-Cu1-Mg1-Ni1, which is thermally stable. While the MW followed by SPS sample showed improved plastic deformation, the combination of MW, HEBM and SPS led to the absence of plastic deformation at room temperature. However, the MW+HEBM+SPS had much higher strength (579 MPa) and possessed similar thermal stability as the commercial Al-Si12-Cu1-Mg1-Ni1.

Highly Thermally Stable Light-Weight Al Based Alloys Prepared by Centrifugal Atomization and Powder Compaction

Combination of centrifugal melt spraying and hot die-forging of a rapidly solidified semi-product was presented as a promising and inexpensive method for processing of aluminium based alloys of unconventional chemical compositions, e.g., those containing high concentrations of thermally stabilizing transition metals. In our study, the use of this processing method is illustrated for the Al–23Si–8Fe–5Mn (wt. %) alloy. Structure was examined by LM, SEM, EDS and XRD. Mechanical properties were determined by hardness and compressive tests. Thermal stability was assessed by measuring the hardness development during long-term annealing, elevated temperature compressive tests and creep tests. The research showed that the investigated alloy exhibits excellent thermal stability as compared with commercial thermally stable aluminium alloys currently used in automotive and aerospace industry.

Structure and Properties of Magnesium-Based Hydrogen Storage Alloys

Hydrogen is the promising pollutant-free fuel of the near future. For various hydrogen applications, suitable storage systems have to be developed. One of the safe ways is the reversible storage of hydrogen in the form of light metal (lithium or magnesium) hydrides. MgH2 magnesium hydride shows very high storage capacity (approx. 7 wt. %), but its problem is high thermodynamic stability. Therefore, high temperature (over 400°C) is necessary for MgH2 to decompose producing hydrogen. The solution of this problem can be the utilization of the complex magnesium hydrides containing nickel, copper or other transition metals. In this work, the microstructure and hydrogen storage properties of the various magnesium alloys (Mg-Ni, Mg-Zn, Mg-Cu and Mg-Cu-Al) are described. The aim was to find suitable hydrogen storage system with good storage capacity and sufficient rate of formation and decomposition of hydrides. Microstructure, chemical and phase composition of the alloys were determined by the light and scanning electron microscopy, EDS and XRD. Hydrogen saturation was carried out by cathodic polarization in the alkaline solution. Hydrogen content in the material was estimated by XRD from the shift of the diffraction lines of present phases.