Fine Subgrain Structure Research Articles

Evaluation of effects of niobium and manganese addition on nickel base weldments

The present work investigates the influence of different concentrations of Nb (from 0.1 to 3.35 wt-%) or Mn (from 0.78 to 3.32 wt-%) on the microstructure and mechanical properties of an Inconel 690 weldment. Welding electrodes are produced by coating Inconel filler metal 52 with a flux containing various percentages of Nb or Mn. Weldments with a bevel edge are butt welded via a manual shielded metal arc welding process, using identical parameters and procedures. The microstructure and mechanical properties of the resulting weldments are then analysed. The experimental results indicate that the subgrain structures of the welds are primarily dendritic. Under tensile testing, it is found that each specimen ruptures in the fusion zone and that the fracture surfaces exhibit entirely ductile features. It is noted that as the content of Nb increases, the welds tend to show a finer subgrain structure, i.e. having smaller dendritic spacing. Consequently, the tensile strength and microhardness of the fusion zone increase slightly and the tensile rupture mode changes from slant to flat fracture. It is determined that the interdendritic precipitates are mainly Nb rich eutectic type and Nb rich type constituents. The presence of these precipitates increases with higher concentrations of Nb in the flux and results in a significant decrease in the ductility of the weldment. Regarding the relative influence of the Mn additions, there appears to be no significant change in the subgrain structure as the percentage of Mn increases. However, the ductility tends to increase and it is also found that the tensile strength and microhardness of the fusion zone also increase slightly. Accordingly, the tensile rupture mode exhibits a slight tendency to change from flat to slant fracture. Although the interdendritic precipitates identified in the Mn series are similar to those in the Nb series, it should be noted that the precipitates appear in lower numbers and are smaller than their Nb counterparts.

Advances in Physical Metallurgy and Processing of Steels. Strengthening Mechanisms of Creep Resistant Tempered Martensitic Steel.

The creep deformation resistance and rupture life of high Cr ferritic steel with a tempered martensitic lath structure are critically reviewed on the basis of experimental data. Special attention is directed to the following three subjects: creep mechanism of the ferritic steel, its alloy design for further strengthening, and loss of its creep rupture strength after long-term use. The high Cr ferritic steel is characterized by its fine subgrain structure with a high density of free dislocations within the subgrains. The dislocation substructure is the most densely distributed obstacle to dislocation motion in the steel. Its recovery controls creep rate and rupture life at elevated temperatures. Improvement of creep strength of the steel requires a fine subgrain structure with a high density of free dislocations. A sufficient number of pinning particles (MX particles in subgrain interior and M 23 C 6 particles on sub-boundaries) are necessary to cancel a large driving force for recovery due to the high dislocation density. Coarsening and agglomeration of the pinning particles have to be delayed by an appropriate alloy design of the steel. Creep rupture strength of the high Cr ferritic steel decreases quickly after long-term use. A significant improvement of creep rupture strength can be achieved if we can prevent the loss of rupture strength. In the steel tempered at high temperature, enhanced recovery of the subgrain structure along grain boundaries is the cause of the premature failure and the consequent loss of rupture strength. However, the scenario is not always applicable. Further studies are needed to solve this important problem of high Cr ferritic steel. MX particles are necessary to retain a fine subgrain structure and to achieve the excellent creep strength of the high Cr ferritic steel. Strengthening mechanism of the MX particles is another important problem left unsolved.

On the extrusion microstructural evolution of Al–Al 3Ni in situ composite

A directionally solidified Al–Al 3Ni in situ composite with fine Al 3Ni eutectic fibers was extruded at 200 and 400°C. Evolution of the microstructures was then investigated by examining the partially and fully extruded portions of the billets. According to the results, extrusion at 200°C will generate numerous tangled dislocations initially. When fiber breakage occurs, subgrains which are formed preferentially near the cracks will grow to eliminate the tangled dislocations and separate the pieces of broken fibers further apart, resulting in a fine subgrain structure with rod-like Al 3Ni particles distributed intergranularly. On the other hand, the billet extruded at 400°C shows formation of large subgrains with many intragranular Al 3Ni fibers in the early stage. Upon further extrusion, the fibers are broken while the large subgrains are refined by forming dislocation bands and changing the bands to sub-boundaries. Consequently, the 400°C-extruded billet shows fine subgrains with intergranular and intragranular rod-like Al 3Ni particles.

Analysis of microstructure and mechanical properties in alloy 690 weldments using filler metals I-82 and I-52

This work investigated the weldability and mechanical properties of weldments made with Inconel filler metals I-52 and I-82 in the welding of Inconel alloy 690. Gas tungsten arc welding was used with different multipass sequences. The microstructures of the fusion and heat affected zones were examined and weldment properties were compared by tensile, hardness, and impact tests. Fracture surfaces were examined by scanning electron microscopy. Experimental results indicate that the subgrain structure near the fusion zone centreline was dominated by equiaxed dendrites in I-82 weldments but by columnar dendrites in I-52 weldments. In addition, the I-82 weldments had a finer subgrain structure near the fusion zone centreline and smaller cellular spacing near the fusion line than I-52 weldments. Mechanical test results demonstrate that the I-82 weldments had higher tensile strength (622–630 MPa) with rupture occurring in the base metal. In comparison, the I-52 weldments had lower tensile strength (568–578 MPa) and rupture occurred in the fusion zone. Filler metal I-52 showed better welding performance and greater impact toughness with only slight sensitivity to the number of passes. However, a gradual increase in the hardness of the weld was observed as the number of passes increased.

Comparative effects of the hydrogen and nitrogen gas treatment and re-calcination (GTR) routes on the composition, microstructure, and magnetic properties of conventionally synthesized Sr–hexaferrite

Optimized static hydrogen treated and recalcined (HTR) and static nitrogen treated and recalcined (NTR) Sr–hexaferrite powders synthesized conventionally in-house are compared with one another. The phase identification studies and lattice parameter measurements showed first that the Sr–hexaferrite decomposed, forming iron oxide (Fe2O3), which was then reduced during the static hydrogen or nitrogen treatment, and, second, that the hexaferrite phase was recovered albeit with a small change in the composition (as indicated by the lattice spacings) after the re-calcination treatment in static air. These effects were more pronounced in the hydrogen process than in the nitrogen process. The main effect of this gas-treatment and re-calcination (GTR) process on the microstructure of the Sr–hexaferrite was the transformation of the single-crystal particles into particles with a very fine sub-grain structure during the gas treatment, which resulted in the formation of polycrystalline hexaferrite particles with a much finer grain size during subsequent recalcination, compared to that of the initial hexaferrite powder. This finer structure was responsible for the higher coercivities observed after re-calcination. With regard to the hydrogen and nitrogen processes, the former resulted in a higher degree of oxide reduction and hence a higher coercivity on re-calcination. The coercivity of the initial Sr–hexaferrite increased from 310 kA/m (3.9 kOe) to ∼400 kA/m (5 kOe) after HTR and to 342 kA/m (4.3 kOe) after NTR. The initial magnetization behavior was also different for the HTR- and NTR-processed powders, with the former exhibiting behavior characteristic of single domains. This was consistent with the grain size being significantly less than the single-domain size (∼1 μ).

Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder synthesized conventionally in-house from strontium carbonate and hematite (Fe2O3) without using additives has been treated in a static nitrogen atmosphere and subsequently calcined in static air. The phase identification studies by means of X-ray diffraction (XRD) and thermal magnetic analysis (TMA) indicated the decomposition of the strontium hexaferrite and the reduction of the resultant iron oxide (Fe2O3) during the reaction with nitrogen. High-resolution scanning electron microscopy (HRSEM) studies show that the reduction occurring during nitrogenation results in the conversion of some of the large grains into much finer sub-grains. Strontium hexaferrite, Fe3O4, and Sr7Fe10O22 were the main phases obtained after reduction. However, weak traces of other phases, such as Fe2O3, were also detected. The hexaferrite phase re-formed on subsequent calcination. The magnetic measurements indicated a significant decrease in the intrinsic coercivity during nitrogenation due to the formation of Fe3O4. However, after a re-calcination process, the remanence and maximum magnetization (i.e., magnetization at 1100 kA/m) exhibited values close to the initial values before treatment, but the value of the intrinsic coercivity was higher than that prior to nitrogenation. Examination of the re-calcined microstructure showed that this could be attributed to the fine grains that originated from the fine sub-grain structures formed in the powder particles during nitrogenation.

Mechanical Behavior of Al-Al<SUB>2</SUB>O<SUB>3</SUB> MMC Manufactured by PM Techniques Part I—Scheme I Processing Parameters

Metal matrix composites (MMC) were manufactured using hot pressing followed by hot extrusion of aluminum (Al) powder reinforced by alumina (AI2O3) particles. Under tensile as well as compressive loads, a strength improvement of 64 to 100 % compared to the matrix material strength was obtained. The percent elongation to fracture ranged from 20 to 30%, which indicates good ductility as compared to the ductility of MMC manufactured by other techniques. Optical as well as scanning electron microscopy (SEM) examinations were used for characterization of the material microstructure and fracture behavior. Porosity retained in the microstructure was very limited in the case of pure aluminum billets. Microstructural examination revealed uniform distribution of Al2O3 particles in the Al-matrix. Under tensile loads, voids opened by decohesion between the matrix and reinforcement. Such behavior led to a decrease in strength properties of the MMC as a function of reinforcement volume fraction. The fracture surface is dominated by the ductile fracture features, that is, dimples. Voids were found to initiate at retained porosity sites at the AI/AI2O3 interface or in the matrix close to the interface due to stress concentration. The SEM revealed the formation of a complex fine subgrain structure. Such a polygonized structure is a major source of strengthening.

Fatigue properties of Al-Mg alloys with and without scandium

The fatigue properties, e.g. HCF strength, crack growth of small surface and long-through cracks, of two wrought Al-5Mg-0.3Mn-0.06Zr alloys without and with scandium (0.25%) were investigated in vacuum, air and salt water. The alloy with scandium showed a higher tensile yield stress, higher fatigue strength values and a higher resistance against microcrack growth, due to the presence of coherent Al 3 (Sc, Zr) precipitates and a very fine subgrain structure. The scandium free alloy exhibited a slightly higher resistance against macrocrack growth, due to the coarser grain structure.

Microstructural changes induced by dry sliding wear of a A357/SiC metal matrix composite

The sliding wear behaviour of a commercial (Duralcan) metal matrix composite containing 30 vol.-%SiC particles, and that of the unreinforced A357 matrix alloy were investigated in the load range 6-74 N and a sliding speed of 1 m s−1 against a grey castiron counterface. At the lowest load (6 N) the wear coefficient of the composite (3.1 × 10−6 mm3 N−1 m−1) was lower than that of the monolithic alloy (169.6 × 10−6 mm3 N−1 m−1 ). Between 18 and 40 N the wear performance of the two materials was verysimilar but at the highest load (74 N) the wear coefficient of the composite (176.8 × 10−6 mm3 N−1 m−1) was much higher thanthat of the monolithic alloy (89 × 10−6 mm3 N−1 m−1). The maximum depth offriction induced deformation wasfound to beabout three times higher for the composite than the monolithic alloy at a load of 74 N. Detailed TEM was performed on the surface of the composite. The SiC was found to be fragmented. Extensive formation of a alumina was observed, and the presenceof iron oxides indicated transfer from the counterface. Widespread cracking was found at the SiC/matrix and lI. alumina/matrixinterfaces. The matrix had developed a fine subgrain structure with little evidence of the Mg2Si which was present in the startingstructure.

Microstructural changes induced by dry sliding wear of a A357/SiC metal matrix composite

The sliding wear behaviour of a commercial (Duralcan) metal matrix composite containing 30 vol.-%SiC particles, and that of the unreinforced A357 matrix alloy were investigated in the load range 6-74 N and a sliding speed of 1 m s−1 against a grey castiron counterface. At the lowest load (6 N) the wear coefficient of the composite (3.1 × 10−6 mm3 N−1 m−1) was lower than that of the monolithic alloy (169.6 × 10−6 mm3 N−1 m−1 ). Between 18 and 40 N the wear performance of the two materials was verysimilar but at the highest load (74 N) the wear coefficient of the composite (176.8 × 10−6 mm3 N−1 m−1) was much higher thanthat of the monolithic alloy (89 × 10−6 mm3 N−1 m−1). The maximum depth offriction induced deformation wasfound to beabout three times higher for the composite than the monolithic alloy at a load of 74 N. Detailed TEM was performed on the surface of the composite. The SiC was found to be fragmented. Extensive formation of a alumina was observed, and the presenceof iron oxides indicated transfer from the counterface. Widespread cracking was found at the SiC/matrix and lI. alumina/matrixinterfaces. The matrix had developed a fine subgrain structure with little evidence of the Mg2Si which was present in the startingstructure.

Use of light and scanning electron microscope techniques to monitor microstructural changes in aluminium‐based metal matrix composites

SUMMARYMicrostructural developments in discontinuously reinforced aluminium composites have been monitored during a variety of thermo‐mechanical treatments. The material, made by powder blending or casting, followed by extrusion, contains particulate or short‐fibre alumina in a matrix of commercial purity aluminium. Attention is centred on the distribution of reinforcement, the fine alumina particles derived from the prior oxide layers on the aluminium powder, and the effects that these inclusions have on cavitation, recovery and recrystallization processes. Optical metallography, by viewing anodized sections in polarized light, is used to reveal grain and coarse subgrain structures. Scanning electron microscopy has also been employed to reveal fine oxide stringers, cavities and (in backscattered mode) fine subgrain structures. The emphasis in this paper is on the type of information that can be obtained for these materials from careful application of standard microscopy techniques.

Morphology of precursors and CuIn1−xGaxSe2 thin films prepared by a two-stage selenization process

The morphology of sequentially sputtered metallic precursors and CuIn1−xGaxSe2 thin films prepared using a novel two-stage selenization process was found to change from initially very smooth layers with a root-mean-square (rms) surface roughness of <10 Å to coalescing grains with a fine (∼200 Å) subgrain structure, three-dimensional ∼9000 Å size islands, and finally to compact, well-faceted, large ∼1 μm grain-size CuIn1−xGaxSe2 thin films with a rms roughness of 950–1500 Å. In situ homogenization of Cu-rich precursors prior to the first selenization and a maximum selenization temperature of 550–560 °C that provided beneficial fluxing action of copper selenide improved the morphology of completed CuIn1−xGaxSe2 thin films and the photovoltaic conversion efficiency of the solar cells.

The role of controlled recrystallization treatments on ductility of Fe 3Al alloys

Careful heat treatments after mechanical processing can lead to optimised mechanical properties of Fe 3Al-base intermetallics that is to high strength combined with maximum ductility. These heat treatments are shown to lead to recovery of the dislocation structure introduced by prior working such that a fine subgrain structure is established. The subgrain boundaries are constituted of networks of 〈100〉 dislocations that are hence essentially immobile at room temperature. These boundaries are effective obstacles to the 〈111〉 dislocations mobile during room temperature deformation and ensure that deformation is distributed uniformly throughout the grains. It is this homogeneous distribution of strain that is responsible for the good strength and ductility.

Determining fracture facet crystallography using electron backscatter patterns and quantitative tilt fractography

A methodology is presented to characterize the crystallography of individual fracture surface facets. Electron backscatter patterns (EBSP's) from a metallographic section through a facet identify grain orientation, and quantitative tilt fractography identifies facet orientation; these results are combined to establish fracture facet crystallography. For this technique, facet electropolishing is not required, the facet alignment procedure is accurate and quick, and the method can be generalized to different microstructures, test environments, or facet orientations. Method accuracy is illustrated for 25 to 50 μm fatigue crack facets in an unrecrystallized Al–Li–Cu alloy (AA2090) that has 5 μm thick subgrains in elongated grains that are 10 to 200 μm thick. The fine subgrain structure and tortuous fatigue crack profile precludes the use of other diffraction techniques for determining AA2090 facet crystallography. EBSP and tilt fractography results demonstrate that vacuum fatigue cracks in AA2090 are nearly parallel to local {111} planes.

Microstructural development in AlSiC composites made by resistance sintering of mechanically alloyed powders

Abstract SiC particles were found to be distributed homogeneously in the aluminum matrix by the mechanical alloying (MA) process. It was observed that the aluminum matrix of the MA powders consists mainly of fine subgrains with a wide range of misorientations. In addition, a band structure consisting of lamellae of elongated subgrains was observed. The substructure of the aluminum matrix of MA powders indicates that the average temperature in the MA process in quite low, i.e. near room temperature. It was found that the fine substructure of MA powders can be maintained after resistance sintering. The high strength of the resistance-sintered AlSiC composites may be attributed to the fine subgrain structure in the aluminum matrix and the high dislocation densities which exist owing to the presence of a worked structure arising from the MA process, and are generated by the thermal mismatch between aluminum and SiC particulates.

Mechanical properties of AlSiC composites made by resistance sintering of mechanically alloyed powders

This work has demonstrated the potential of making high strength AlSiC composites with reasonable ductility by resistance sintering of mechanically alloyed powders. The effects of SiC particle size and content on the mechanical properties of SiC-particulate-reinforced aluminium composites were systematically studied by the use of compressive tests. It was found that the compressive yield stress and ultimate strength increases with increasing SiC volume fraction and decreasing SiC particle size, while the compressive failure strain shows the reverse trend with the variation in SiC particle size and content. The high strength of the resistance-sintered AlSiC composites may be attributed to the fine subgrain structure in the aluminium matrix, the dislocations generated by differential contraction of aluminium and SiC particulates, and the previous worked structure in the aluminium matrix resulted from the mechanical alloying process. In addition, it was suggested that, besides the formation of liquid phase, the plastic deformation of AlSiC composite powders also makes a contribution to the densification process in resistance sintering.

Microstructural evolution during initial stage of high strain rate superplastic deformation in powder metallurgical 7475 Al–0·7Zr alloy

The 7475 Al–0·7Zr (wt-%) cold rolled alloy sheet fabricated by powder metallurgy exhibited remarkable superplasticity for strain rates between 2×10−3 and 3×10−1 s−1 at 793 K. During the initial deformation stage, the strain rate sensitivity m increased with increasing strain and attained a value greater than 0·5 after about 50% elongation. Thus, the microstructural evolution which occurred during this initial deformation stage was examined, especially at high strain rates. On heating to the tensile testing temperature of 793 K, the alloy developed a very fine subgrain structure, stabilised with finely dispersed Al3Zr precipitate particles. The average subgrain size was less than 1 μm, and the misorientation angle of most subboundaries was less than 5°. In the initial deformation stage showing an increase in m value, the subboundary misorientations and, thus, the contribution of boundary sliding tended to increase with strain. Even the subboundaries with misorientation of 5–15° could play an important role through boundary sliding in the limited strain rate range showing superplasticity. Subgrains rotated as a group owing to the different sliding rates at different boundary misorientations, including those of original grain boundaries. The accommodation dislocations for the boundary sliding were absorbed into the subboundaries. Both subgrain rotation and dislocation absorption were considered to contribute greatly to the increase in subboundary misorientation.MST/1367

急冷凝固Ａｌ‐Ｆｅ粉末合金の諸性質に及ぼすメカニカルアロイングの影響

Rapidly solidified flakes of Al-2 to 8wt%Fe alloys were produced by atomizing the alloy melt and subsequent splat quenching onto a water-cooled copper roll. Mechanical alloying treatment (MA) was applied to the rapidly solidified flakes by using a high energy ball mill with addition of stearic acid as a process control agent. Consolidation of flakes or MA powders was done by cold pressing, degassing and hot extrusion. A significant hardness increment was observed by MA treatment. After consolidation, P/M materials from MA powder showed very fine dispersoids and fine subgrain structures. Isochronal annealing experiments revealed microstructural stability was high at temperature up to 400°C. The MA treatment was significantly effective in improving the mechanical properties of rapidly solidified P/M Al-Fe alloys. Tensile strength of a P/M Al-8%Fe alloy increased to about 67kgf/mm2 at room temperature and to 27kgf/mm2 at 300°C. These improvements are attributed to the introduction of fine oxide/carbide dispersoids and fine subgrain structures formed during MA. However, decrease of ductility was accompanied in the mechanically alloyed P/M materials.

Microstructure Change During High Strain Rate Superplastic Deformation in a PM 7475 AL-0. 7ZR Alloy

ABSTRACTImprovements of superplasticity in the high strain rate range have been achieved in a 7475 Al alloy with 0. 7 wt% Zr produced from rapidly solidified powder (PM7475-0.7Zr alloy), and the superplastic properties and microstructural changes with strain have been investigated.The PM 7475-0.7 Zr alloy has a cold rolling structure, but on heating to the superplastic temperature of 520°C, the alloy developed a very fine subgrain structure of average subgrain size of less than 1 μm. The fine structure was stabilized with metastable cubic Al3Zr precipitate dispersions, and there exist a large number of less than 5° misorientation subboundaries together with a small number of highly misoriented ones.After 117% strain at the high strain rate of 2. 5×10−1 S−1 the subboundaries with the misorientation of 5° – 15° Were observed to increase. During the initial stage of the deformation, m value and the contribution of boundary sliding to the deformation also increased with strain, and thus the alloy behaved superplastically during the following deformation.The improvements of superplasticity with strain in the early stage of the high strain rate deformation of the alloy were interpreted to be closely related with the increase in misorientation of subboundaries stabilized with finely dispersed metastable Al3Zr precipitates.

Superplasticity in a thermomechanically processed High-Mg, Al-Mg alloy

Superplastic elongations in excess of 400 pct have been observed in tension testing at 573 K (300 °C) and strain rate έ= 2 × 10-3 s-1 for a thermomechanically processed Al-10.2 pct Mg-0.52 pct Mn alloy. The thermomechanical processing consists of solution treatment and hot working, followed by extensive warm rolling at 573 K (300 °C), a temperature below the solvus for Mg in the alloy. This processing results in a fine subgrain structure in conjunction with refined and homogeneously distributed β(Al8Mg5) and MnAl6 precipitates. This structure does not statically recrystallize when annealed at 573 K (300 °C) but appears to recrystallize continuously during deformation at such a temperature and the resulting fine grain structure deforms with minimal cavitation. At temperatures above the Mg-solvus,e.g., 673 K (400 °C), recrystallization and growth occur readily resulting in rela tively coarser structures which deform superplastically but with extensive grain boundary sliding and cavitation.