Fine Dispersoids Research Articles

Superplasticity of Al-Mg-Zr Alloy

The microstructure and superplasticity of the thermomechanically proceeded sheets of Al - 3wt.%Mg - 0.25wt.%Zr were investigated. High density of L12-structured fine dispersoids of Al3Zr metastable phase was observed by TEM analysis. Alloy demonstrated high recrystallization resistance at elevated temperature due to Al3Zr dispersoids. The tensile tests were carried out in a temperature range of 440-500 °C and a strain rate of 1×10-3to 1×10-2s-1. The maximum elongation to failure of 370% was observed at 480 °C at the constant strain rate values of 2×10-3and 5×10-3s-1.

Correlating oriented grain number density of recrystallisation in particle-containing aluminium alloys

Abstract The recrystallized grain structure of Al–(Mn)–Fe–Si alloys after isothermal annealing was studied by electron backscatter diffraction (EBSD) technique. Statistical correlation suggests that the frequency of forming P-oriented ({011}«566») grains at a particle larger than the critical diameter (about 1.1 μm) is about 2% when the effect of fine dispersoids and concurrent precipitation is negligible. The overall grain number density is correlated linearly with the number densities of P and Cube ({001}«100») grains. The grain number densities of typical orientations (P, ND-rotated cube {001}«310» and Cube) and the overall recrystallized grains increase as rolling strain increases, following a similar exponential law.

Constrained groove pressing, cold-rolling, and post-deformation isothermal annealing: Consequences of their synergy on material behavior

Stress-relieved Al-Mn-Si specimens were constrained groove pressed (CGPed) and in the following, cold-rolled under different strains of 0.47, 0.8 and 1.27. Dual strained sheets were isothermally heat treated at 150, 250, and 350 °C. Microstructure survey revealed that generated shear-bands by CGP acted as talent sites for further strain-induced grain boundary migration (SIGBM) during annealing. SEM micrographs pointed out that coarse particles (1 μm <) had not preferential positions within the aluminum matrix and often comminuted into fine dispersoids (0.5 μm >) under heavy strains. Assessment of the softening fraction (Rrec) depicted that greater accumulated strains along with higher post-annealing temperatures accelerated the thermal recovery and hence, reduced the Rrec. Tensile testing was performed repeatedly while fluctuation range for obtained stress-strain curves for each specimen as a criterion of structural uniformity was considered. It was found that post-deformation treatment of severely deformed sheets at 250 °C supplied maximum structural uniformity due to perfect restoration and evolution of inhomogeneity agents such as shear-bands, and precipitates within the matrix. In this regard, processing route contained cold-rolling by 0.8 strain of two-pass CGPed specimen and subsequent annealing at 350 °C ascertained to be an adequate combination of deformation and heat treatment with respect to achievement of the maximum uniform elongation (UE) of 22.96% and contribution of uniform deformation energy (UD) in total mechanically absorbed energy equal to 84.18% that provided the optimal formability for the applied alloy. Fracture analysis was performed by fracture angle measurement and SEM fractography. Thermal annealing of CGPed sheet at 150 and 250 °C declined its fracture angle from 55° to 50° and 46.5°, respectively. However, annealing at 350 °C changed the dominant fracture mode from shear to necking by incidence of fibrous morphology and enhanced the angle to 61°. Likewise, it was revealed that in spite of fracture angle increment by rolling reduction, post-rolling prevented the necking fracture in CGPed sheet treated at 350 °C and this might be shifted to higher temperatures.

Surface modification and deuterium retention in reduced-activation steels under low-energy deuterium plasma exposure. Part I: undamaged steels

In this paper, reduced-activation ferritic/martensitic (RAFM) steels including Eurofer (9Cr) and oxide dispersion strengthening (ODS) steels by the addition of Y2O3 particles with different amounts of Cr, namely, (9-16)Cr were exposed to low energy deuterium (D) plasma (~20–200 eV per D) up to a fluence of 2.9 × 1025 D m−2 in the temperature range from 290 K to 700 K. The depth profile of D in steels was measured up to 8 µm depth by nuclear reaction analysis (NRA) and the total retained amount of D in those materials was determined by thermal desorption spectroscopy (TDS). It was found that the D retention in ODS steels is higher compared to Eurofer due to the much higher density of fine dispersoids and finer grain size. This work shows that in addition to the sintering temperature and time, the type, size and concentration of the doping particles have an enormous effect on the increase in the D retention. The D retention in undamaged ODS steels strongly depends on the Cr content: ODS with 12Cr has a minimum and the D retention in the case of ODS with (14-16)Cr is higher compared to (9-12)Cr. The replacing of Ti by Al in ODS-14Cr steels reduces the D retention. The formation of nano-structure surface roughness enriched in W or Ta due to combination of preferential sputtering of light elements and radiation-induced segregation was observed at incident D ion energy of 200 eV for both Eurofer and ODS steels. Both the surface roughness and the eroded layer enhance with increasing the temperature. The surface modifications result in a reduction of the D retention near the surface due to increasing the desorption flux and can reduce the overall D retention.

The Effects of Ni Content and Intermediate Annealing Condition on Recrystallization of Al-Fe-Ni-Si Alloy Fin Stocks

The effects of Ni content and intermediate annealing conditions on recrystallization behavior during brazing heat treatment of Al–Fe–Ni–Si alloys fin stocks of automotive heat exchangers were studied by means of microstructural analysis. Following brazing heat treatment, coarsening of recrystallized grain structure was observed in both 0.5% Ni and 1.0% Ni fin stocks, which were intermediately annealed in a salt bath. This coarsening was particularly marked in 1.0% Ni fin stocks. The results of the microstructural observations indicated that fine dispersoids were heterogeneously dispersed in 1.0% Ni fin stocks that were annealed at 550°C for 10 s in a salt bath. It was proposed that the preferential recovery and recrystallization during the brazing heat treatment in the low density area of the fine dispersoids contributed to the larger recrystallized gain structure. The coarse recrystallized grains obtained by controlling the salt bath annealing conditions are expected to contribute in improving the properties of Al–Fe–Ni–Si alloy fin stocks of automotive heat exchangers.

High resolution microstructural studies of the evolution of nano-scale, yttrium-rich oxides in ODS steels subjected to ball milling, selective laser melting or friction stir welding

Coarse-grained ferritic Oxide Dispersion Strengthened (ODS) FeCrAlY alloys were initially developed for use in high temperature environments due to their excellent creep strength and oxidation resistance. More recently, ODS materials capable of forming protective α-alumina scales have attracted increasing interest for nuclear applications, due to their potential corrosion resistance in super-critical water and liquid metal environments. However, there is still controversy about how and when the fine dispersoids form; and whether they are transformed during fabrication processes.In this paper high spatial resolution electron microscopy is used to monitor the evolution of the dispersoids under different conditions including: heat treatment of as-milled powder, selective laser melting and friction stir welding.

Two-stage annealing of a cold-rolled Al–Mn–Fe–Si alloy with different microchemistry states

A detailed investigation of an Al–Mn–Fe–Si model alloy during two-stage annealing after cold rolling has been carried out. Two homogenized variants, one with a high supersaturation of Mn and fine dispersoids and another with a much coarser dispersoid structure together with less Mn content in solid solution, were cold rolled to different strains before subsequent annealing. The effect of pre-existing dispersoids, introduced prior to cold rolling through homogenization, and concurrent precipitation during annealing on the softening behaviour are both discussed in terms of grain structure and texture evolution. The results show that more and finer dispersoids together with strong concurrent precipitation lead to a coarser and pan-cake shaped grain structure with strong P {011}〈566〉 and medium strength ND-rotated cube {001}〈310〉 texture components regardless of the target temperature. On the other hand, a refined grain structure and medium strength P and Cube {001}〈100〉 were obtained when the pre-existing dispersoids are coarser and fewer, and concurrent precipitation is limited. However, even though significant differences were observed during the two-stage annealing in terms of recrystallization microstructure and texture brought about by the different microchemistries, the stress–strain behaviour of the respective as-recrystallized states exhibited negligible differences.

Dispersoid strengthening of a high temperature Al–Si–Cu–Mg alloy via Mo addition

Abstract The influence of Mo addition on the microstructure and mechanical properties of an Al–7Si–0.5Cu–0.3Mg alloy (wt%) was investigated. The Mo-containing alloy exhibited significant improvement in creep resistance over the base alloy. At 300 °C and 30 MPa, the minimum creep rate decreased by 95% while creep time-to-fracture was increased by 2 orders of magnitude, from 50 min to 1500 min. The tensile yield strength at 300 °C was also increased by 25%. These effects were attributed to the formation of novel Al–(Fe,Mo)–Si dispersoids during solution treatment in the grain interiors (intradendritic regions). Unlike the age-hardening precipitates, which coarsened resulting in loss of strength, these dispersoids were thermally stable and retained their strengthening effect at 300 °C. TEM investigations showed that the dislocation motions were effectively hindered by these fine dispersoids, leading to the reduction in the minimum creep rate. The onset of the tertiary creep stage was delayed by postponing the dislocation pile-up at the interdendritic Si particles. It was found that Mo addition suppressed the formation of the brittle plate-like β-Al5FeSi intermetallics and formed a blocky phase in the cast microstructure, which resulted in 34% increase in elongation at 300 °C.

Influence of dispersoids on grain subdivision and texture evolution in aluminium alloys during cold rolling

Al-Mn alloys containing similar amounts of solutes but various dispersoid densities were cold rolled. The grain subdivision and micro-texture were examined by electron backscatter diffraction and orientation imaging microscopy. Macro-texture was measured by X-ray diffraction. It is found that a high density of fine dispersoids enhances the development of the copper and S textures at large strains (∼3), and also induces a higher fraction of high-angle grain boundaries. At smaller strains, the texture and high-angle grain boundaries are not evidently influenced by the density of dispersoids. It is suggested that the texture evolution, which is enhanced by dispersoid pinning effect, contributes to the grain subdivision and the formation of high-angle grain boundaries.

Recrystallization Behavior of Twin-Roll Cast 3xxx Series Aluminum Alloy

Twin roll casting (TRC) is an efficient process used to produce thin plates directly from molten metal. However, TRC has a much higher cooling rate during solidification compared with conventional direct chill casting. Accordingly, the constituent phases are refined and the added elements are supersaturated by this casting method. Supersaturation of the added elements hinders both recovery and recrystallization because fine dispersoids precipitate from the supersaturated solid solution during annealing [ and coarsen the grain size of the final plate, which leads to degradation of formability. Therefore, it is important to understand the influence of the solute element and dispersoids on recrystallization behavior to obtain an appropriate grain structure for forming. In this study, the effects of the homogenization conditions and cold rolling reduction on the recrystallization behavior of twin-roll cast 3XXX series aluminum alloy were investigated.

Microstructure and properties in friction stir weld of 12Cr steel

Friction stir welding was employed for joining of 12Cr heat resistant ferritic steel. Microstructure and mechanical properties in the obtained welds were examined and their relationship was established. The stir zone microstructure was found to consist of quenched martensite with numerous fine dispersoids uniformly distributed in grain interiors. The formation of this microstructure remarkably strengthened the stir zone. Tempering for 3·6 ks at 923 K reduced the as welded hardness of friction stir weld to nearly base material level.

Hall&amp;ndash;Petch Breakdown at Elevated Temperatures

The Hall­Petch effect responsible for the strength of fine-grained and ultrafine-grained (UFG) metals is almost exclusively measured at room temperature. One reason for this is that at elevated temperatures grains tend to coarsen, and this negates the strengthening. The grains may, however, be stabilized by small volume fractions of fine dispersoids. These dispersoids cause direct Orowan strengthening and, by stabilizing the so-called Zener grain size, indirect strengthening due to Hall­Petch. We show that for most metals the critical Zener grain size above which Hall­Petch strengthening is more important than Orowan strengthening is lower than, and sometimes even considerably lower than 1μm, i.e., in the range of UFG metals. Breakdown of the Hall­Petch relationship, which occurs at elevated temperatures once mechanisms weaker than Hall­ Petch start to control the strength, is best studied for grain sizes well above this critical grain size. The Hall­Petch breakdown due to either Coble creep or grain size-dependent dislocation creep is modeled. We present model calculations for copper and verify our approach by comparing with experimental results for ferritic steels containing nanoscale dispersions. [doi:10.2320/matertrans.MA201309]

Effects of Microstructure and Rare-Earth Constituent on the Oxidation Behavior of Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd Titanium Alloy

The isothermal oxidation behavior in air of high-temperature titanium alloy Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd with bimodal and lamellar microstructures was investigated at 600–800 °C. The results revealed that the alloy with lamellar microstructure has better oxidation resistance than that with bimodal microstructure. The porous oxide scales that form mainly contain TiO2. A noticeable observation concerns the preferential attack around rare-earth particles, associated rapid oxygen diffusion along the incoherent rare-earth precipitate/matrix interface and cracks formed during oxidation. The resulting internal attack caused fragmentation of rare-earth particles and further oxidation of substrate to form TiO2 scale with some fine dispersoids of Al2O3. Tensile tests showed that the ultimate strength and ductility of the specimens with removed surface were higher than that with a surface scale.

Characterization the Softening Behavior of Cold Rolled AlMnFeSi-Alloys during Conditions of Concurrent Precipitation

In the present work an Al-Mn-(Fe-Si) model alloy has been subjected to different homogenization treatments, to achieve materials with different microchemistry states in terms of constituents, levels of Mn in solid solution (potential for concurrent precipitation) and dispersoid densities, followed by cold rolling and back-annealing. Characterization of the microchemistry state after homogenization and the evolution in dispersoid precipitation and its effects on the softening behavior after deformation has been performed. It is demonstrated that variations in microchemistry may have dramatic effects on the softening kinetics and the final grain structures, where both pre-existing fine and dense dispersoids before back annealing as well as precipitation concurrent with recovery and recrystallization strongly retard kinetics and generally lead to a coarse grain structure, while conditions with no or limited concurrent precipitation softens much faster and generally results in an even, fine and equi-axed grain structure. The different softening behaviors have been discussed in terms of Zener drag effects derived from the dispersoid evolutions.

Influence of dispersoids on microstructure evolution and work hardening of aluminium alloys during tension and cold rolling

The influence of dispersoids on work hardening of aluminium during tension and cold rolling has been studied by comparing Al–Mn alloys containing similar amounts of solutes but various dispersoid densities. The microstructure evolution with deformation strain was examined in transmission and scanning electron microscopy. It is found that a high density of fine dispersoids strengthens the materials significantly, but their strengthening effect diminishes as the strain increases. From a series of Bauschinger tests, it is found that the internal stress, due to particles, increases rapidly at the initial stage of deformation, but saturates at strains larger than 5%. It is concluded that the internal stress makes a small contribution to the work hardening and contributes to less than 10% of the total flow stress during monotonic loading at strains larger than 5%. The work-hardening behaviour has been correlated to the corresponding microstructure, and the strengthening mechanisms are discussed.

Interaction Between Eutectic Intermetallic Particles and Dispersoids in the 3003 Aluminum Alloy During Homogenization Treatments

The transformation of primary eutectic Al6(Mn,Fe) intermetallics into α-Al(Mn,Fe)Si and the precipitation of dispersoids were studied in the commercial in the form of 3003 series cast aluminum alloys, mainly under isothermal conditions between 673 K and 873 K (400 °C and 600 °C). After solidification, both the solid solution and the primary eutectic intermetallics were far from equilibrium. During further heat treatment, the precipitation of fine dispersoids and eutectoid transformation of the primary eutectic particles occurred simultaneously. Having characterized these evolutions under industrial homogenization conditions, the evolution of the microstructure (in terms of its nature, and the quantity, size, and chemical composition of the phases) was characterized during isothermal heat treatment, using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, quantitative image analysis, and transmission electron microscopy–energy-dispersive spectroscopy (TEM-EDS). The experimental results are analyzed, and changes in chemical composition are discussed and compared with the calculated equilibrium compositions. It is shown that (1) the chemical composition of eutectic intermetallics evolves and tends toward an equilibrium composition; (2) during precipitation, the chemical composition of dispersoids is constant, and close to the expected equilibrium composition when the initial mean composition of the solidification cell is taken into account; (3) after the formation of dispersoids, the quantity of α-Al(Mn,Fe)Si formed from the initial eutectic intermetallics increased, with the kinetics being controlled by long-range manganese diffusion; and (4) the latter evolution is associated with the dissolution of dispersoids located close to eutectic intermetallics and contributes to the formation of a dispersoid-free zone (DFZ).

Work hardening and mechanical properties of severely deformed AA3003 by constrained groove pressing

A severe plastic straining method, called constrained groove pressing, is used to improve the microstructure and mechanical properties of non-hardenable 3003 Al–Mn alloy. Williamson–Hall method is utilized to measure the grain size from X-ray diffraction patterns of deformed samples. The results show that ultra-fine grained 3003 alloy having grain size of 580nm can be successfully produced by constrained groove pressing at room temperature. Mechanical properties of sheets, examined by tensile test, are approximately exhibited 170% improvement in comparison with those of annealed sample. Also, comparing the achieved results with those of aluminum 1100 alloy confirms the rapid rate of grain refinement and mechanical properties improvement in 3003 alloy. This is attributed to the presence of fine dispersoids of Al6Mn in microstructure of the alloy. Tensile tests demonstrate that the work hardening rate of constrained groove pressed sheet is increased in comparison with that of annealed samples. Investigating of the work hardening by a model reveals why improvement stops after third pass and also work hardening rate diagrams show that only one stage of work hardening is detectable for the sheet after three passes of constrained groove pressing, while in lower pass numbers, more work hardening stages can be observed.

Elevated Temperature Mechanical Properties of Al-8.5Fe-1.3V-1.7Si/SiCP Composite Prepared by Multi-Layer Spray Deposition

Al-8.5Fe-1.3V-1.7Si/SiCP composite prepared by multi-layer spray deposition in different states were investigated. Ultimate tensile strengths of the composite sheets as-rolled tested at 315°C and 400°C are up to 300MPa and 186MPa respectively. The fine dispersoids and fine grains, and homogeneous SiC particles contribute to the excellent mechanical properties of the composite at elevated temperature. Specially, partial dissolution of SiC particles contributes to the stability of Al12(Fe,V)3Si phase.

Microstructure and texture evolution of particle-containing AA3104 alloy cold rolled to large strains

The evolution of the microstructure in a particle-containing AA3104 aluminium alloy cold rolled to large strains has been investigated using the electron channelling contrast imaging and electron backscattered diffraction techniques. It is found that the microstructure evolves from cell-block structure at low strains into a lamellar structure composed of lamellar boundaries (LBs) and interconnecting cell boundaries at high strains. The orientation dependence of deformation microstructure is also observed in this particle-containing alloy even at high strains. Different particles have different effects on the microstructure evolution. It is found that there is no clear effect of the fine dispersoids on the evolution of lamellar structure even at large strains, while significant distortion of LBs structure occurs around the coarse constituent particles. For both types of boundaries, the average boundary spacing decreases and the average misorientation angle increases with increasing strain. No saturation of these structure parameters is reached. The texture evolves in a typical manner with finely distributed deformation texture components composed of small volume elements of the ideal texture components (brass, S and copper) separated by LBs. It is found that the particles have no strong effect on the texture evolution.

Microstructural studies on nanocrystalline oxide dispersion strengthened austenitic (Fe–18Cr–8Ni–2W–0.25Y2O3) alloy synthesized by high energy ball milling and vacuum hot pressing

In the present work, nanostructured (Fe–18Cr–8Ni–2W) austenitic base and oxide dispersion strengthened (ODS) alloy powders were produced through mechanical alloying and these nano powders were consolidated by vacuum hot pressing. The results showed that initially bcc solid solution formed in both the alloys and this transformed to fcc with continued milling. The bcc solid solution formation and the subsequent transformation to fcc were significantly faster in the ODS alloys when compared to the base alloy. In the ODS alloy, a grain size of ~25 nm is achieved within 5 h of milling. Study of variation of microhardness of mechanically alloyed powder particles with grain size showed linear Hall–Petch kind of behavior. Following vacuum hot pressing of mechanically alloyed powders, nearly fully dense (>99% of theoretical density) compacts were obtained with a grain size of ~80 nm. The bulk hardness of base and ODS alloys are ~530 and ~900 HV, respectively. These are significantly higher than the values reported in the literature so far. The enhanced strength the ODS alloy is due to increased dislocation density and presence of fine dispersoids along with the nanocrystalline grains.