Coarsening Of Oxides Research Articles

Effect of processing route on microstructure and tensile properties of hot isostatic pressed 30CrMnSiNi2A steel: Comparison of prealloy and master alloy routes

The prior particle boundaries (PPBs) are non-negligible powder metallurgy defect, which seriously weaken the mechanical properties of hot isostatic pressed (HIPed) steels. In present study, the master alloy route is proposed to improve the PPBs in HIPed 30CrMnSiNi2A steel, and the Fe-4.40Cr-4.80Mn-4.60Si-6.80Ni-1.00Mo-0.40V-1.12C master alloy system with low solidus temperature and wide solid-liquid temperature interval is designed. The microstructure, tensile properties and fracture behavior of HIPed compacts prepared by prealloy and master alloy routes are systematically compared. The results show that the structure in prealloyed sample is granular bainite (GB); and substantial Si oxides distributed along particle boundaries constitute serious PPBs. The yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE) of the sample are 723 ± 10 MPa, 1238 ± 16 MPa and 12.3 ± 0.4%, respectively. In master alloy route, the formation of local liquid disrupts the distribution of Si oxides along particle boundaries and promotes the coarsening of oxides, which results in dispersed distribution of oxides and significant reduction in oxide density. In addition, since the structural uniformity depends on element diffusion, a relatively uniform structure is obtained through the optimization of powder size. The structure of optimized sample mainly consists of GB and martensite; and good tensile properties with YS of 798 ± 7 MPa, UTS of 1310 ± 8 MPa and TE of 15.3 ± 0.6% are obtained. The higher tensile properties are mainly due to the improvement of PPBs, which avoids the initiation of microcracks along PPBs and further avoids premature failure of material.

Effect of Zr content on microstructure and hardness of ODS-FeCrAl alloys

Oxide dispersion strengthened (ODS) Fe-12Cr-5Al-2 W-xZr (x = 0, 0.3, 0.6, 1.0 wt%) alloys were produced by mechanically alloying and spark plasma sintering. The size and distribution of grain, number density, size distribution and types of nanoscale oxides as well as hardness of the ODS alloys were analyzed by EBSD, TEM, HRTEM and an indentation experiment, respectively. The results showed that the grain size is not sensitive to Zr content, but the increase of Zr content is helpful for the homogenization of grain size. With the increase of Zr content from 0 to 0.6 wt%, the number density of oxides is increased from 1.66 × 1021 m−3 to 9.94 × 1022 m−3, while the mean size of oxides is decreased from 21.2 nm to 11.1 nm. The coarsening of oxides occurs when Zr is up to 1 wt%. In Zr-free alloy, nano-oxides are YAlO3 with perovskite structure (YAP) and α-Al2O3. In Zr-containing ODS alloys, the trigonal δ-phase Y4Zr3O12, YAP and α-Al2O3 are observed. An appendage oxide, Y4Zr3O12/YAP/α-Al2O3, is found in the alloy with 1.0 wt% Zr and the possible formation mechanism is briefly analyzed. The hardness of 6.03 GPa is achieved in the 0.6 wt% Zr alloy.

On the thermal coarsening and transformation of nanoscale oxide inclusions in 316L stainless steel manufactured by laser powder bed fusion and its influence on impact toughness

The thermal evolution of nanoscale oxide inclusions in 316L stainless steel (SS) manufactured by laser powder bed fusion additive manufacturing (AM) was explored. The size, chemical composition, morphology, and distribution of the oxides were characterized as the function of heat treatment conditions. The study revealed the mechanistic driving force of the rapid oxide coarsening during recrystallization. Ostwald ripening governs oxide coarsening. The active grain boundary-oxide interaction at the early stage of recrystallization accelerated oxide coarsening via enhanced solute diffusion along grain boundaries. Pipe diffusion along dislocation cellular boundaries has a negligible contribution to oxide coarsening. At high temperatures (T > 1065 °C), although lattice diffusion primarily controlled the oxide growth, the contribution from the grain-boundary diffusion was necessary. The transformation from MnSiO3 to CrMn2O4 took place in the un-recrystallized grains but was not observed when recrystallization started. The interaction of grain boundary and oxides during recrystallization resulted in a high fraction of oxides accumulated at grain boundaries. While oxide coarsening does not significantly alter the toughness value, grain-boundary oxides promote microvoid formation and intergranular fracture under Charpy impact in the recrystallized AM 316L SS.

A new approach to synthesise high strength nano-oxide dispersion strengthened alloys

High oxygen content is usually harmful for metals and alloys. However, in this study, we propose a new approach, which involves direct additive manufacturing of a powder alloy that contains a high oxygen level, to synthesise nano-oxide dispersion strengthened (ODS) alloys. Thus, by selectively laser melting a FeNi alloy powder with an oxygen level of 3000 ppm, a new ODS alloy has been in-situ synthesised. The as-fabricated samples were also heat treated at different temperatures to understand their microstructural stability. The nano-sized oxides in the as-fabricated sample were identified to be face-centered cubic Fe3O4 and are entangled with dislocations. During deformation, dislocations loop around these particles suggesting that they have acted as effective dislocation barriers, which leads to enhanced tensile strengths. Heat treatment at or above 950 °C leads to depletion of oxides in grain interior and to segregation of oxides along grain boundaries (GBs). Transmission electron microscopy study reveals that at increased temperatures dislocations tend to act as bridges between oxides and GBs, promoting dissolution of oxides in grain interior and nucleation and coarsening of oxides at GBs. The results indicate that the proposed approach is an effective new way of synthesising high performance ODS alloys and also give a strong implication that the precipitate-dislocation interaction plays a significant role in microstructural evolution during annealing.

Oxide Morphology of a FeCrAl Alloy, Kanthal APMT, Following Extended Aging in Air at 300 °C to 600 °C

Iron-chromium-aluminum (FeCrAl) alloys are of interest to the nuclear materials community due to their resistance to high-temperature steam oxidation under accident conditions. The present work investigates oxide formation at temperatures relevant to light water reactor cladding operation following extended aging to assess growth kinetics, chemical composition, and microstructure of oxide formation on a commercial FeCrAl alloy, Fe-21 wt pct Cr-5 wt pct Al-3 wt pct Mo (Kanthal APMT). Aging treatments were performed for 100 to 1000 hours in stagnant air at 300 °C, 400 °C, 500 °C, and 600 °C, respectively. Oxide growth behavior under the investigated conditions follows a logarithmic time dependence. When the oxidization temperature is 400 °C or below, the oxide is amorphous. At 500 °C, isolated crystalline regions start to appear during short period aging time and expand with extended exposures. Crystalline α-Al2O3 oxide film develops at 600 °C and the correlated logarithmic rate constant decreases significantly, indicating enhanced oxidation resistance of the formed oxide film. In addition, Mo segregation at grain boundaries has been observed when the aging temperature exceeds 500 °C. The results of this study can be viewed as an upper bounding result for potential oxide coarsening during reactor operation.

Microstructural evolution and tensile properties of oxide dispersion strengthened Alloy 617 at elevated temperatures

This study investigated evolution of the microstructure of oxide dispersion strengthened Alloy 617 with annealing temperature. A mixture of prealloyed Alloy 617 and Y2O3 powders was mechanically alloyed and consolidated by hot-extrusion at 1100°C. Hot extrusion developed a submicron-sized grain structure with M23C6 carbides and finely dispersed Al2O3 and Y2Ti2O7 oxides. The fine-grained structure was stable during subsequent annealing at temperatures up to 1250°C. Further increase of annealing temperature to 1300°C resulted in a significantly coarsened grain structure, which was coincident with the abrupt coarsening of oxides. M23C6 carbides in the as-extruded conditions were transformed to M7C3 carbides with complex shapes when annealed at 1200°C, and their shapes changed to very coarse hexagonal prisms at 1250°C, which was followed by the formation of eutectic M2C carbides at grain boundaries at 1300°C. Tensile tests of the as-extruded ODS Alloy 617 showed that the yield strength decreased steeply at a transition temperature of around 600°C, which can be attributed to diffusional creep along the grain boundaries.

Influence of Oxygen Supply on Oxide Coarsening During the Internal Oxidation of Two-Phase Fe-Y Alloys

Fe-Y binary alloys underwent thermogravimetric analysis while exposed to low oxidant partial pressures using a mixture of 5 pct H2-95 pct Ar gas at two flow rates. The alloys experienced in situ internal oxidation of Y-rich intermetallic phases. Kinetics and microstructures were both affected by flowrate. Lower flow rates resulted in coarser oxides and decreased oxidation rates. Results show the possible impacts of rate-controlling processes in the gas phase on internal oxidation with dilute oxidants.

Temperature‐Sensitive Nanocapsules for Controlled Drug Release Caused by Magnetically Triggered Structural Disruption

AbstractSelf‐assembled nanocapsules containing a hydrophilic core and a crosslinked yet thermosensitive shell are successfully prepared using poly(ethylene‐oxide)‐poly(propylene‐oxide)‐poly(ethylene‐oxide) block copolymers, 4‐nitrophenyl chloroformate, gelatin, and 1‐ethyl‐3‐(3‐dimethylaminopropyl) carbodiimide. The core is further rendered magnetic by incorporating iron oxide nanoparticles via internal precipitation to enable externally controlled actuation under magnetic induction. The spherical nanocapsules exhibit a hydrophilic‐to‐hydrophobic transition at a characteristic but tunable temperature reaching 40 °C, triggering a size contraction and shrinkage of the core. The core content experiences very little leakage at 25 °C, has a half life about 5 h at 45 °C, but bursts out within a few minutes under magnetic heating due to iron oxide coarsening and core/shell disruption. Such burst‐like response may be utilized for controlled drug release as illustrated here using a model drug Vitamin B12.

Oxide coarsening and its influence on recrystallization in a mechanically alloyed Fe-base oxide-dispersion-strengthened alloy

Analytical transmission electron microscopy has been used to determine whether yttria particles readily coarsen in oxide-dispersion-strengthened ferritic stainless alloy (PM2000), in an attempt to contribute to a model that explains the directionality of the microstructure observed when this material undergoes recrystallization under isothermal conditions. Likewise, the role of Al2O3 particles in the recrystallization processes of alumina-enriched PM2000 alloy has been studied via the addition of 1 wt pct Al2O3 particles as dispersoids. A carbon extraction replica technique was used to show that no coarsening of the yttria dispersion occurred even under exaggerated heat treatment at 1380 °C for 1 hour. By contrast, a significant ripening of alumina particles occurred apparently without influence on the recrystallization processes.

Effect of Nano-Sized Oxides on Annealing Behaviour of Ultrafine Grained Steels

The annealing behaviour of ultrafine grained steels containing nano-scale dispersed oxides was studied in a temperature range of 600– 900� C by means of microstructural observations and hardness measurement. The starting materials with submicrocrystalline structures were developed by mechanical milling of Fe-Fe3O4 powders followed by consolidating bar rolling at 700 � C. Depending on the initial oxygen content and the mechanical milling time, the fraction of dispersed oxides varied from 0.3 to 3.0 vol%. During the heating up to 800� C( i.e. within the ferrite region), the initial ultrafine grained microstructures were essentially stable against any discontinuous grain growth. The grain coarsening and the softening can be roughly expressed by power-law functions of annealing time. The main mechanism of microstructure evolution that operated during annealing is considered as a normal grain growth accompanied by recovery. The grain coarsening is characterized by a rather high value of the grain-growth exponent of about 20. The grain growth kinetics correlates with the oxide coarsening. The effect of dispersed oxides on the annealing behaviour of submicrocrystalline oxide bearing steels is discussed in some detail.

Annealing behavior of submicrocrystalline oxide-bearing iron produced by mechanical alloying

The structural changes upon annealing of an ultrafine-grained iron containing dispersed oxides were studied. The starting material was subjected to mechanical milling followed by consolidating bar rolling. The fine-grained microstructures were essentially stable against discontinuous grain growth and/or primary recrystallization during annealing at temperatures up to 800 °C, where the specimens maintain a very fine ferrite grain size. The annealing behavior is mainly characterized by normal grain growth accompanied by recovery. The grain-growth kinetics correlates with the oxide coarsening. Both the grain growth and the oxide coarsening can be enhanced by an increase in the temperature. The static restoration/recrystallization processes operating in the ultrafine-grained matrices, as well as the effect of dispersed oxides particles, are discussed in some detail.

Oxide-Dispersed Iron Alloys

As part of a programme to develop novel iron-base superalloys, several oxide-dispersed pure iron alloys were prepared by the technique of mechanical alloying and hot extrusion of powders. The refractory oxides (AI 203, Ce02, Cr203, Si02, and Y203) were added at the 2 wt.-% level. Control alloys were also prepared without refractory oxide additions. The distribution of the oxide phase was observed metallographically in . as-extruded material and in material. exposed at 1250° C for 72 h. There was good correlation between .the observed microstructure and the roomtemperature mechanical properties. The dispersed oxides were extracted from the metal matrix' and identified by X -ray diffraction as mixed oxides with iron (spinels and perovskites) formed by reaction between the added refractory oxides and contaminant oxygen present during fabrication. The degree of oxide coarsening observed after heat-treatment varied with the oxide addition and was least for the chromiacontaining alloy. Attempts to correlate this effect with the parameters in the- Wagner equation, which is usually used: to describe such behaviour, proved unsuccessful. The coarsening could not be related to the known~thermodynamic stabilities of either the added. single oxides or the mixed oxides. However, a correlation with the melting temperatures of the mixed oxides was observed. Some theoretical implications, and practical consequences for the design of oxidedispersed-super alloys, are discussed.