Nanostructured Oxide Dispersed Strengthened Steels: Preparation and Investigation (original) (raw)
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Metallurgical and Materials Transactions A, 2013
Oxide-dispersion strengthened (ODS) ferritic steels were produced by mechanical alloying and subsequent spark plasma sintering. Very fast heating rates were used to minimize porosity when controlling grain size and precipitation of dispersoids within a compacted material. Sintering cycles performed at 1373 K (1100°C) induced heterogeneous, but fine grain size distribution and high density of nano-oxides. Yield strengths at room temperature and at 923 K (650°C) are 975 MPa and 298 MPa, respectively. Furthermore, high-temperature ductility is much increased: total strain of 28 pct at 923 K (650°C).
Powder Technology, 2014
Ferritic steel powder was mechanically milled in a dual drive planetary ball mill, under different milling conditions to optimize the milling parameters. The resulted powder was characterized, using particle size analyzer, X-rays and electron microscope. X-ray peak broadenings were investigated to estimate crystallite size, lattice strain and deformation stress. Better Pearson's coefficient was observed for uniform stress deformation model (USDM) (0.988) in comparison to uniform deformation model (UDM) (0.64) which shows better estimation of lattice parameter. An increase in fineness was observed with an increase in ball to powder ratio as well as for an increase in rotational speed. At the optimized condition, ferritic steel powder, together with Y 2 O 3 , was milled in the dual drive mill to produce oxide dispersion strengthened ferritic steel powder, suitable to be used in nuclear applications. Convoluted morphology, desired for better alloying, was confirmed using an electron microscope. A significant increase in per unit surface area was noticed due to milling using BET surface area analysis. Negligible contamination was observed due to milling atmosphere and mill container. The steel powder produced, was sintered using spark plasma sintering and its density and hardness were measured. High hardness and lower crystallite size were recorded using spark plasma sintering. Addition of Y 2 O 3 shows decreases in the thermal expansion coefficient. Effect of added titanium was studied and an adverse effect on oxide dispersion strengthened steel was noticed.
Metallurgical and Materials Transactions A, 2014
Oxide-dispersion strengthened ferritic steel was produced by high-energy attrition, leading to a complex nanostructure with deformed ferritic grains. After mechanical alloying, the powder was then consolidated by spark plasma sintering (SPS) using various thermo-mechanical treatments. Hot isostatic pressing (HIP) was also performed on the same powder for comparison. Above 1123 K (850°C), SPS consolidation-induced heterogeneous microstructure composed of ultrafine-grained regions surrounded by larger grains. Spatial distribution of the stored energy was measured in the bimodal microstructure using the Kernel average misorientation. In contrary to large recrystallized grains, ultra-fine grains are still substructured with low-angle grain boundaries. The precipitation kinetics of the nano-oxides during consolidation was determined by small-angle neutron scattering. Precipitation mainly occurred during the heating stage, leading to a high density of nanoclusters that are of prime importance for the mechanical properties. Other coarser titanium-enriched oxides were also detected. The multiscale characterization allowed us to understand and model the evolution of the complex microstructure. An analytical evaluation of the contributing mechanisms explains the appearance of the complex grain structure and its thermal stability during further heat treatments. Inhomogeneous distribution of plastic deformation in the powder is the major cause of heterogeneous recrystallization and further grain growth during hot consolidation. Then, the thermal stability of coherent nano-oxides is responsible for effective grain boundary pinning in recovered regions where the driving pressure for recrystallization is lowered. This scenario is confirmed in both SPSed and HIPed materials.
Lanthana-bearing nanostructured ferritic steels via spark plasma sintering
Journal of Nuclear Materials, 2016
A lanthana-containing nanostructured ferritic steel (NFS) was processed via mechanical alloying (MA) of Fe-14Cr-1Ti-0.3Mo-0.5La2O3 (wt.%) and consolidated via spark plasma sintering (SPS). In order to study the consolidation behavior via SPS, sintering temperature and dwell time were correlated with microstructure, density, microhardness and shear yield strength of the sintered specimens. A bimodal grain size distribution including both micron-sized and nano-sized grains was observed in the microstructure of specimens sintered at 850, 950 and 1050 o C for 45 min. Significant densification occurred at temperatures greater than 950 o C with a relative density higher than 98%. A variety of nanoparticles, some enriched in Fe and Cr oxides and copious nanoparticles smaller than 10 nm with faceted morphology and enriched in La and Ti oxides were observed. After SPS at 950 o C, the number density of Cr-Ti-La-O enriched nanoclusters with an average radius of 1.5 nm was estimated to be 1.2 ×10 24 m 3. The La + Ti : O ratio was close to 1 after SPS at 950 and 1050 o C; however, the number density of nanoclusters decreased at 1050 o C. With SPS above 950 o C, the density improved but the microhardness and shear yield strength decreased due to partial coarsening of the grains and nanoparticles.
2012
Ferritic steel with compositions 83.0Fe-13.5Cr-2.0Al-0.5Ti (alloy A), 79.0Fe-17.5Cr-2.0Al-0.5Ti (alloy B), 75.0Fe-21.5Cr-2.0Al-0.5Ti (alloy C) and 71.0Fe-25.5Cr-2.0Al-0.5Ti (alloy D) (all in wt%) each with a 1.0wt% nano-Y2O3 dispersion were synthesized by mechanical alloying and consolidated by pulse plasma sintering at 600, 800 and 1000 degrees C using a 75-MPa uniaxial pressure applied for 5 min and a 70-kA pulse current at 3Hz pulse frequency. X-ray diffraction, scanning and transmission electron microscopy and energy disperse spectroscopy techniques have been used to characterize the microstructural and phase evolution of all the alloys at different stages of mechano-chemical synthesis and consolidation. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus were determined using a micro/nano-indenter and universal testing machine. All ferritic alloys recorded very high levels of compressive strength (850-2850 MPa), yield strengt...
Advanced Engineering Materials, 2015
Elemental powder mixture of Fe-14Cr-1Ti-0.3Mo-0.5La 2 O 3 (wt%) composition is mechanically alloyed for different milling durations (5, 10 and 20 h) and subsequently consolidated via spark plasma sintering under vacuum at 950 C for 7 min. The effects of milling time on the densification behavior and density/microhardness are studied. The sintering activation energy is found to be close to that of grain boundary diffusion. The bimodal grain structure created in the milled and sintered material is found to be a result of milling and not of sintering alone. The oxide particle diameter varies between 2 and 70 nm. Faceted precipitates smaller than 10 nm in diameter are found to be mostly La-Ti-Crenriched complex oxides that restrict further recrystallization and related phenomena.
Philosophical Magazine, 2012
Ferritic steel with composition of 83.0Fe-13.5Cr-2.0Al-0.5Ti (alloy A), 79.0Fe-17.5Cr-2.0Al-0.5Ti (alloy B), 75.0Fe-21.5Cr-2.0Al-0.5Ti (alloy C) and 71.0Fe-25.5Cr-2.0Al-0.5Ti (alloy D) (all in wt %) each with 1.0 wt% nano-Y 2 O 3 dispersion were synthesized by mechanical alloying and consolidated by pulse plasma sintering at 600, 800 and 1000 °C using 75 MPa uniaxial pressure applied for 5 min and 70 kA pulse current at 3 Hz pulse frequency. X-ray diffraction, scanning and transmission electron microscopy and energy disperse spectroscopy techniques have been extensively used to characterize the microstructural and phase evolution of all the alloys at different stages of mechanochemical synthesis and consolidation. Mechanical properties in terms of hardness, compressive strength, yield strength and Young's modulus were determined using micro/nano-indentater and universal testing machine. The present ferritic alloys record very high levels of compressive strength (850-2850 MPa), yield strength (500-1556 MPa), Young's modulus (175-250 GPa) and nanoindentation hardness (9.5-15.5 GPa) and measure up to 1-1.5 times greater strength than other oxide dispersion strengthened ferritic steel (< 1200 MPa). These extraordinary levels of mechanical properties can be attributed to the typical microstructure comprising uniform dispersion of 10-20 nm Y 2 Ti 2 O 7 or Y 2 O 3 particles in high-alloy ferritic matrix.
Materials & Design, 2016
A new class of ultrafine-grained high-Mn steels containing nanoscale oxides has been developed by sparkplasma sintering of ball-milled powders. During spark-plasma sintering, nanoscale manganese oxides were generated in Fe-15Mn steel, while nanoscale aluminum oxides were produced in Fe-15Mn-3Al-3Si steel because of the high affinity of aluminum for oxygen. Ultrafine-grained high-Mn steels that contain nanoscale oxides exhibited superior strength without significant loss of toughness, owing to the combined effects of grain refinement, twinning-induced plasticity (TWIP), and dispersion of nanoscale oxides. These new materials have potential for application in powder metallurgy components used in the automotive industry, such as gear sets, connecting rods, and bearing caps, which require high surface hardness as well as good core toughness.
Ferritic oxide dispersion strengthened alloys by spark plasma sintering
Journal of Nuclear Materials, 2013
Spark plasma sintering (SPS) was used to consolidate a Fe-16Cr-3Al (wt.%) powder that was mechanically alloyed with Y 2 O 3 and Ti powders to produce 0.5 Y 2 O 3 and 0.5 Y 2 O 3-1Ti powders. The effects of mechanical alloying and sintering conditions on the microstructure, relative density and hardness of the sintered oxide dispersion strengthened (ODS) alloys are presented. Scanning electron microscopy indicated a mixed fine-grain and coarse-grain microstructure that was attributed to recrystallization and grain growth during sintering. Analysis of the transmission electron microscopy (TEM) and atom probe tomography (APT) data identified Y-O and Y-O-Ti nanoclusters. Elemental ratios of these nanoclusters were consistent with that observed in hot-extruded ODS alloys. The influence of Ti was to refine the grains as well as the nanoclusters with there being greater number density and smaller sizes of the Y-O-Ti nanoclusters as compared to the Y-O nanoclusters. This resulted in the Ti-containing samples being harder than the Ti-free alloys. The hardness of the alloys with the Y-O-Ti nanoclusters was insensitive to sintering time while smaller hardness values were associated with longer sintering times for the alloys with the Y-O nanoclusters. Pressures greater than 80 MPa are recommended for improved densification as higher sintering temperatures and longer sintering times at 80 MPa did not improve the relative density beyond 97.5%.
Materials Science and Technology, 2014
Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe-14Cr-5Al-3W) and (Fe-20Cr-5Alz3W) with additions of Y 2 O 3 and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.