Łukasz Rogal

TEM and HRTEM studies of ball milled 6061 aluminium alloy powder with Zr addition

The effect of mechanical alloying on the microstructure of atomized 6061 aluminium alloy powder and 6061 powder with a zirconium addition was studied in the work. The atomized 6061 aluminium alloy powder and 6061 powder with addition of 2 wt.% Zr were milled in a planetary ball mill and investigated using X‐ray diffraction measurements, conventional and high‐resolution electron microscopy (TEM/HRTEM) and high‐angle annular dark field scanning transmission electron microscopy combined with energy dispersive X‐ray microanalysis. An increase of stresses was observed in milled powders after the refinement of crystallites beyond 100 nm. In the powder with zirconium addition, some part of the Zr atoms diffused in aluminium forming a solid solution containing up to 0.5 wt.% Zr. The remaining was found to form Zr‐rich particles containing up to 88 wt.% Zr and were identified as face centred cubic (fcc) phase with lattice constant a= 0.48 nm. That fcc phase partially transformed into the L12 ordered phase. Eighty‐hour milling brought an increase of microhardness (measured with Vickers method) from about 50 HV (168 MPa) for the initial 6061 powder to about 170 HV (552 MPa). The addition of zirconium had no influence on the microhardness.

Evolution of Globular Microstructure and Rheological Properties of Stellite 21 Alloy after Heating to Semisolid State

Metal alloys can be successfully thixoformed in the partially liquid state if they display non-dendritic near-globular microstructures. The article presents the development of feedstock with such non-dendritic microstructure produced through the solid-state route of strain-induced melt-activated (SIMA) method, for a Stellite™ 21 alloy. Stellite™ alloys are a range of cobalt-chromium alloys designed for wear and corrosion resistance, currently shaped by casting, powder metallurgy or forging processes, but semisolid-state processing offers the possibility of a near-net-shaping method for these alloys. In this work, sprayformed followed by extrusion samples were heated to the temperature range at which the liquid and solid phases coexist in the material and spheroidal shape solid particles in a liquid matrix were obtained as required for semisolid processing. Microstructural investigations were carried out using scanning electron microscopy (SEM) in combination with energy-dispersive spectroscopy (EDS), with a further objective of analyzing the rheological properties of Stellite™ 21 alloy in the semisolid state, providing results to be used for identification of a processing window of temperature and viscosity ranges for thixoforming this alloy.

HRTEM studies of amorphous ZrNiTiCu nanocrystalline composites

Ball milling of easy glass forming Ti25Zr17Ni29Cu29 alloys lead to the formation of an amorphous structure accompanied by a substantial increase of powder microhardness. The powders show clear glass transition effect and a few stage crystallization starting above 500°C. High‐resolution transmission electron microscope technique allowed identifying nanocrystalline inclusions as Cu12NiTi7 within the amorphous powder. The amorphous powders mixed with nanocrystalline iron or silver powders were hot pressed to form composites. A narrow 200 nm broad intermediate single‐phase layer at the amorphous‐phase/iron interface containing all elements present in the composite was identified using transmission electron microscope and high‐angle annular dark field detector techniques. scanning transmission electron microscopy energy dispersive spectroscopy line profile showed gradual change of composition within the intermediate zone. Amorphous phase contains small nanocrystals of size close to 10 nm identified using High‐resolution transmission electron microscope as Cu12NiTi7. Compression tests have shown better plasticity of composites than in the case of pure hot‐pressed amorphous powder; furthermore, high elastic limit of composites and the ultimate compression stress of about 1800 MPa for composites containing 20% Fe and near 700 MPa for those with 20% Ag.

Preparation of Globular Microstructure in H18 Steel for Semi Solid Processing with the Use of Boron Addition

Martensitic stainless steel was chosen for a semi-solid processing due to high mechanical properties and resistance to high temperature surface oxidation. Modified H18 martensitic steel (1.1 % - C, 18.9 % - Cr, 0.1 % - V, 0.7 % - Mo, 0,9 % - Si, 2.0 % - Mn, balance Fe, all in weight %) modified with addition of 0.01 % boron was applied as a feedstock for semi-solid range temperature experiments. The samples were heated up to 1330°C to obtain about 26% of the liquid phase followed by rapid quenching in water. The microstructure of the samples consisted of austenitic globular grains (average grain size 42 μm, about 78 % of volume) surrounded by a eutectic mixture (ferrite, and M7C3, M23C6 carbides as identified by X-ray and electron diffraction). The initial hardness of as-cast sample was 357 HV5 and that after quenching from liquidus solidus range was 422 HV5. The X-ray analysis confirmed the presence of 16% - α-Fe, 80% - γ-Fe and 4% - M7C3 carbides in rapid quenched sample. The EDS analysis of a eutectic mixture was as follows: 6.2 % - C, 31.7 % Cr, 0.1 % - Mn, 0.6 % - Si, 61.2 % Fe. The chemical composition analysis of globular grains confirmed the presence of 2.0% C, 16.3 % - Cr, 1 % - Si, 1.7 % - Mn, 0.5 % - Mo, 78.5% Fe.

Effect of Hot Rolling and Equal-Channel Angular Pressing on Generation of Globular Microstructure in Semi-Solid Mg-3%Zn Alloy

A combination of hot rolling and equal channel angular pressing (ECAP) was explored to generate globular microstructures in the Mg-3%Zn alloy after re-heating to the semisolid state. It was found that the single-step deformation of as-cast alloy via hot rolling at 350°C with a thickness reduction of 50% refined the alloy microstructure by creating deformation bands of the Mg (α) phase with a size of the order of tenths of micrometers. After re-heating to 630 °C, the microstructure transformed into spheroidal morphologies with an average globule size of 82 μm. An additional deformation of the hot-rolled alloy by the ECAP method at 250 °C further refined the alloy microstructure to sub-micrometer grains of lath and equiaxed shapes. After re-heating of this microstructure to 630 °C the average globule size reached 62 μm, which is roughly 25% smaller than that achieved for the hot-rolled precursor. The role of strain-induced melt activation (SIMA) techniques in generation of globular morphologies in Mg-based alloys after partial re-melting is discussed.

Microstructure of ball milled and compacted Co–Ni–Al alloys from the β range

Two powder alloys from the β phase region of compositions Co28.5Ni36.5Al35 and Co35Ni30Al35 were ball milled for 80 h in a high energy ball mill. The formation of amorphous structure was observed after 40 h of milling and further milling did not change their structure. The analytical and high‐resolution transmission electron microscopy (TEM, HREM) examination of powder structure showed that nanoparticles of  L10 phase of size of about 5 nm were present within the amorphous matrix. The vacuum hot pressing of the milled powders under pressure of 400 MPa at 700°C for 12 min resulted in the formation of compacts with density of about 70% of the theoretical one. The additional heat treatment at 1300°C for 6 h followed by water quenching, led to significant improvement of density and induced the martensitic transformation manifested by a broad heat effect. The characteristic temperatures of the transformation were determined using DSC measurements, which revealed only small differences within the examined alloys compositions. TEM structure studies of heat‐treated alloys allowed to identify the structure of an ordered β (B2) phase and L10 martrensite.

Electron beam additive manufacturing with wire – Analysis of the process

The electron beam additive manufacturing process with wire is a part of global trend to find fast and efficient methods for producing complex shapes elements from costly metal alloys such as stainl...

Melt-spinning and semi-solid processing of bainitic steel

A steel with a chemical composition meant to form nanostructured bainite following appropriate heat treatment, was, cooled rapidly from the liquid phase (1550°C) using melt spinning and modified injection-suction methods, as well as from a semi-solid temperature (1430°C) through thixoforming. The hardness of the as-cast melt spun ribbons was ∼960 HV due to a fine martensite–austenite mixture surrounded by three-dimensional skeleton-like primary carbides of length scale 0.2–0.3 µm. The suction-injection cast method led to a similar structure but less hard (780 HV) due to a lower cooling rate. The thixoformed material showed unmelted globular, fine grains and a fine eutectic mixture formed directly from the liquid phase. The variety of processed steel samples were tempered and their microstructures, examined.

HRTEM studies of NiNbZr + Ag amorphous-nanocrystalline composites

Amorphous powder of composition corresponding to Ni60Ti20Zr20 (in at%) was obtained by ball milling in a high‐energy mills starting from pure elements. Formation of the amorphous structure was observed already after 20 h of milling, although complete amorphization occurred after 40 h. The microhardness of powders increased from about 30 HV for pure elements to above 400 HV (1290 MPa) after 40 h of milling. Transmission electron microscopy (TEM) allowed to identify nanocrystalline inclusions of intermetallic phases of size 2–10 nm. Uniaxial hot pressing was performed in vacuum at temperature below the crystallization Tx it is 510°C and pressure of 600 MPa, Mixed amorphous powders and nanocrystalline silver powders were used to form a composite, in which microhardness was near 970 MPa HV and 400 HV for the amorphous phase and nanocrystalline silver, respectively. The compression strength of the composite containing 20 wt% of nanocrystalline Ag powder was equal to 600 MPa and plastic strain was 2%. Microstructure studies showed low porosity of composites of less than 1%, uniform distribution of the silver phase and a transition zone between both components, about 150 nm thick, where diffusion of nickel, niobium and zirconium into silver was observed. High‐resolution TEM allowed identifying the structure of nanocrystalline inclusions in the amorphous matrix after hot pressing as either Ni3Zr or Ni17Nb3. The identification was performed basing on measurements of angles and interatomic distances using inverse Fourier transformed images with enhanced contrast using Digital Micrograph computer program.

Identification of Mg 2 Cu particles in Cu-alloyed austempered ductile iron

In the present work, the Mg2Cu precipitates in copper-alloyed austempered ductile iron (ADI) were identified by analyzing techniques such as TEM and SEM with EDS. It was revealed that, in castings made of ADI-containing copper, highly dispersed particles of Mg2Cu are formed, whose size does not exceed <1 μm. The research work was carried out on ductile iron that was austenitized at 900 °C, followed by austempering at 380 °C. The microstructure was investigated using various techniques, including optical microscopy, XRD, SEM, and TEM. In addition to this, the exhibited impact properties of castings with Cu, Ni, and Cu+Ni were also determined. This study casts a new light on the formation of the structure of Cu-alloyed ADI. The highly-dispersive and brittle Mg2Cu particles that are located in the vicinity of the graphite nodules have a negative effect on the impact properties of ADI. It has also been shown that impact strength decreases from levels of 160-180 J (for copper-free ADI) to 90-120 J (for copper-and copper-nickel-alloyed ADI).