Lanthana-bearing nanostructured ferritic steels via spark plasma sintering (original) (raw)
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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.
Mechanical alloying of lanthana-bearing nanostructured ferritic steels
Acta Materialia, 2013
A novel nanostructured ferritic steel powder with the nominal composition Fe-14Cr-1Ti-0.3Mo-0.5La 2 O 3 (wt.%) was developed via high energy ball milling. La 2 O 3 was added to this alloy instead of the traditionally used Y 2 O 3. The effects of varying the ball milling parameters, such as milling time, steel ball size and ball to powder ratio, on the mechanical properties and microstructural characteristics of the as-milled powder were investigated. Nanocrystallites of a body-centered cubic ferritic solid solution matrix with a mean size of approximately 20 nm were observed by transmission electron microscopy. Nanoscale characterization of the as-milled powder by local electrode atom probe tomography revealed the formation of Cr-Ti-La-O-enriched nanoclusters during mechanical alloying. The Cr:Ti:La:O ratio is considered "non-stoichiometric". The average size (radius) of the nanoclusters was about 1 nm, with number density of 3.7 Â 10 24 m À3. The mechanism for formation of nanoclusters in the as-milled powder is discussed. La 2 O 3 appears to be a promising alternative rare earth oxide for future nanostructured ferritic steels.
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.
Journal of Nuclear Materials, 2017
A nanostructured ferritic steel with nominal composition of Fe-14Cr-1Ti-0.3Mo-0.5La 2 O 3 (wt.%) was irradiated with Fe þ2 ions at 475 C for 100, 200, 300 and 400 dpa. Grain coarsening was observed for the samples irradiated for 200e400 dpa resulting in an increase of the average grain size from 152 nm to 620 nm. Growth of submicron grains at higher radiation doses is due to decreased pinning effect imparted by Cr-O rich nanoparticles (NPs) that underwent coarsening via Ostwald ripening. Dislocation density consistently increased with increasing irradiation dose at 300 and 400 dpa. The mean radius of lanthanum-containing nanoclusters (NCs) decreased and their number density increased above 200 dpa, which is likely due to solutes ejection caused by ballistic dissolution and irradiation-enhanced diffusion. Chromium, titanium, oxygen and lanthanum content of nanoclusters irradiated at 200 dpa and higher got reduced by almost half the initial value. The reduction in size of the nanoclusters accompanied with their higher number density and higher dislocation density led to significant radiation hardening with increasing irradiation dose.
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, 2016
This research optimized spark plasma sintering (SPS) process parameters in terms of sintering temperature, holding time and heating rate for the development of a nano-structured duplex stainless steel (SAF 2205 grade) reinforced with titanium nitride (TiN). The mixed powders were sintered using an automated spark plasma sintering machine (model HHPD-25, FCT GmbH, Germany). Characterization was performed using X-ray diffraction and scanning electron microscopy. Density and hardness of the composites were investigated. The XRD result showed the formation of FeN 0.068. SEM/EDS revealed the presence of nano ranged particles of TiN segregated at the grain boundaries of the duplex matrix. A decrease in hardness and densification was observed when sintering temperature and heating rate were 1200 • C and 150 • C/min respectively. The optimum properties were obtained in composites sintered at 1150 • C for 15 min and 100 • C/min. The composite grades irrespective of the process parameters exhibited similar shrinkage behavior, which is characterized by three distinctive peaks, which is an indication of good densification phenomena.
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...
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.
The International Journal of Advanced Manufacturing Technology, 2019
Duplex stainless steel (SAF 2205) reinforced with various weight percent of titanium nitride (TiN) nanoparticles is fabricated in vacuum via spark plasma sintering (SPS) using optimized SPS process parameter of 1150°C for 10 min and 100°C/min. The influence of TiN addition on the densification mechanism, microstructure, hardness, and fracture surface of the fabricated duplex stainless steel composite fabricated is evaluated. The results indicate even dispersion of the TiN nanoparticles in the steel matrix during turbular mixing. The displacement and shrinkage rates show three densification stages relating to micro-nanoparticle rearrangement, plastic deformation of the particles, and rapid densification of the composite. The microstructure revealed ferrite, austenite, and TiN phase at grain boundaries. There was phase transformation of ferrite to austenite with the addition of TiN nanoparticles due to diffusion of nitrogen as austenite stabilizer. The evolution of Cr 2 N nitride precipitates along grain boundary, and a dendrite-like austenite structure was evident during sintering. The hardness of the composite was enhanced while the density decreased with TiN content. The fracture surface analysis showed a transition from ductile to brittle fracture with increase in TiN addition.
Journal of Nuclear Materials, 2013
In this work the spark plasma sintering (SPS) technique has been explored as an alternative consolidation route for producing ultra-fine grained Fe-14Cr model alloys containing a dispersion of oxide nanoparti-cles. Elemental powders of Fe and Cr, and nanosized Y 2 O 3 powder have been mechanically alloyed in a planetary ball mill and rapidly sintered in a spark plasma furnace. Two alloys, with nominal compositions Fe-14%Cr and Fe-14% Cr-0.3%Y 2 O 3 (wt.%), have been fabricated and their microstructure and mechanical properties investigated. The results have been compared with those obtained for other powder metallurgy processed alloys of the same composition but consolidated by hot isostatic pressing. The SPS technique under the present conditions has produced Fe-14Cr materials that apparently exhibit different microstructures yielding inferior mechanical properties than the counterpart material consolidated by hot isostatic pressing. Although the presence of a dispersion of Y-rich particles is evident, the oxide dispersion strengthened (ODS) Fe-14Cr alloy consolidated by SPS exhibits poor tensile properties. The extensive decoration of the powder particle surfaces with Cr-rich precipitates and the residual porosity appear to be responsible for the impaired properties of this ODS alloy consolidated by SPS.