Fine Subgrains Research Articles

Creep Behavior of Double Tempered 8%Cr-2%WVTa Martensitic Steel

Creep testing was carried out at around 650°C for a martensitic 8Cr-2WVTa steel (F82H), which is a candidate alloy for the first wall of the fusion reactors of the Tokamak type. Rupture strength of the double tempered steel (F82HD) is lightly higher than that of simply tempered steel (F82HS). On the other hand, creep rate of F82HD is obviously smaller than that of F82HS in acceleration creep, though creep strain of F82HD in transition creep, where creep rate decreases with increasing strain, is larger than that of F82HS. Hardness of the crept F82HD decreases with increasing creep strain, which corresponded with the transmission electron microscopy (TEM) observation. On the contrary, X-ray diffraction and electron back-scattered diffraction pattern measurements show that fine sub-grains are created during transition creep. The creep curves were analyzed using an exponential type creep equation and the apparent activation energy, the activation volume and the pre-exponential factor were calculated as a function of creep strain. Then, these parameters were converted into two parameters, i.e. equivalent obstacle spacing (EOS) and mobile dislocation density parameter (MDDP). While EOS decreases with increasing creep strain, MDDP increases with increasing strain during transition creep. The decrease in EOS and the increase in either EOS or MDDP are rate-controlling factors in transition and acceleration creep, respectively. On the other hand, in case of F82HS, EOS increases and MDDP decreases during transition creep. In this case, the decrease in MDDP controls the creep rate during transition creep of F82HS. It is concluded that both EOS and MDDP are representative parameters of the change in substructure during creep.

Effect of Annealing Temperature on Microstructure and Shape Memory Characteristics of Ti–22Nb–6Zr(at%) Biomedical Alloy

Effect of annealing temperature on microstructure and shape memory characteristics of Ti–22Nb–6Zr(at%) biomedical alloys wasinvestigatedbyusingtensiletests,XRDmeasurement,scanningelectronmicroscopy(SEM)andtransmissionelectronmicroscopy(TEM).Afterseverecold-rolling,theplatewasannealedattemperaturesbetween773and1173K.The= transustemperatureinthisalloywasdeterminedtobe between 823 and 873K. The specimen annealed at 823K for 3.6ks exhibited a fine subgrain structure. A fully recrystallized structure wasobserved in the specimens annealed above 873K. The annealing temperature less affected the transformation temperature and recovery strain.However, the critical stress for slip decreased considerably with increasing annealing temperature, because the grain size increased. Allspecimens annealed above 823K exhibited stable superelastic behavior at room temperature.(Received November 4, 2005; Accepted December 21, 2005; Published March 15, 2006)Keywords: shape memory effect, superelasticity, biomaterials, titanium based alloy, titanium–niobium–zirconium, Hall–Petch relationship

A Study on Fatigue Properties of Sc Added Al 2519 Alloy

The high cycle fatigue properties of two kinds of wrought Al 2519 alloys without and with scandium of 0.10% were investigated. The fatigue strength was determined at R = 0.1 under constant amplitude loading conditions in air. The alloy with scandium of 0.10% showed a little lower tensile yield strength and higher fatigue strength values. The fine grained Al-0.10Sc alloy exhibited a higher resistance against fatigue crack nucleation despite the lower yield stress in comparison to the coarse grained Al 2519 alloy. The results can be explained mainly with the microstructural differences between both alloys. This results are due to the presence of coherent Al3 (Sc, Zr) precipitates and a very fine subgrain structure.

Microstructure and creep strength of welds in advanced ferritic power plant steels

The microstructure and creep strength of simulated heat affected zone (HAZ) specimens and welded joints have been investigated for advanced 9-12%Cr steels in order to understand the mechanisms responsible for Type IV cracks and to improve the creep strength of welded joints at high temperature. The creep and creep rupture tests were carried out at 650° C (923 K) for up to about 104 h. The creep crack growth tests were also carried out for welded joints, base metal and simulated HAZ specimens using the CT specimens. The creep rupture time of simulated HAZ specimens has its minimum after heating to AC3 temperature, which produces fine-grained martensitic microstructure. Decreasing the width of HAZ by means of electron beam (EB) welding is effective for the extension of creep life but the brittle Type IV fracture appears even in the EB welded joints at low stress and long time conditions. Most of the welded joint specimens were fractured in fine-grained HAZ and resulted in shorter creep life than those of base metals as a result of the formation of creep voids and cracks. It should also be noted that in the fine-grained zone, the recovery of martensitic microstructure during creep is inhomogeneous as shown by the formation of coarse subgrains in the region of fine subgrains. Using a specially designed FEM code for Type IV crack growth behaviour, the vacancy diffusion under multi-axial stress conditions of welded joints in HAZ is analysed. The effect of creep ductility and void formation ahead of the crack tip on creep crack growth rate is successfully simulated.

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.

The effect of plastic deformation on structure and properties of chosen 6000 series aluminium alloys

In order to determine the effect of increased copper addition on the strengthening behaviour during deformation and subsequent ageing, two types of Al alloys were continuously cast and extruded: 6013 containing 1.15% Mg, 1.08% Si, 0.7% Mn, 0.3% Fe and 1.1% Cu, and 6XXX alloy containing 1.09% Mg, 0.9% Si, 0.1% Fe and 1.6%Cu (in wt.%). The 6XXX alloy aged at 165°C showed a hardness maximum of 150 HV, while alloy 6013 revealed lower precipitation hardening at higher strain hardening. A similar maximum hardness of about 170 HV was observed for both alloys after ageing in a deformed state. Transmission electron microscopy performed after deformation of 60% and 90% by cold rolling showed the presence of large, elongated subgrains. Narrow deformation bands consisting of fine subgrains of large misorientation were observed in the alloys deformed 60% by rolling, and their density increased in the alloys deformed up to 90%. Fine precipitates were observed after an ageing time corresponding to the maximum of hardness for alloy 6XXX deformed in the as quenched state. They formed predominantly on dislocations as confirmed by dark field imaging. In the alloys deformed by 60% the precipitates were identified as belonging to the metastable B2 phase. In the alloy deformed by 90%, larger needle-like precipitates were observed. They seemed to possess a quaternary Q-precipitate structure. A frequent rotational moire pattern observed in this state allowed a misorientation as 2–3° to be determined. The presence of fine subgrains formed after ageing indicated the activation of recovery process at the ageing temperature.

Fine scale structures from deformation of aluminium containing small alumina particles

Microstructures and mechanical properties have been studied in aluminium containing a fine dispersion of alumina particles, deformed by cold-rolling to strains between 1.4 and 3.5. The microstructure was characterised by TEM. The deformation structures evolved very rapidly, forming a nanostructured material, with fine subgrains about 0.2 μm in diameter and a fraction of high-angle boundaries which was already high at a strain of 1.4, but continued to increase with rolling strain. The yield stress and ductility of the rolled materials were measured in tension, and properties were similar for all materials. Yield stress measurements were correlated with estimates made using microstructural models. The role of small particles in forming and stabilising the deformation structure is discussed. This nanostructured cold-deformed alloy has mechanical properties which are usefully enhanced at comparatively low cost. This gives it, and similar particle-strengthened alloys, good potential for commercial exploitation.

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.

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.

Hot Deformation Microstructures of Al-1%Mn Single Crystals

To determine the basic mechanisms of substructure development within the grains of hot rolled Al alloys, the influence of processing conditions on the hot channel-die compression structures of Al-1%Mn single crystals has been systematically characterised by automatic EBSD orientation maps. Analysis of the microstructures gives the subgrain sizes and misorientation angle distributions, which can be described by physically based mathematical functions. The average subgrain size decreases, as expected, with increasing Zener-Hollomon parameter Z (or increasing flow stress). Most crystals have similar subgrain sizes and distributions. In contrast, the Cube oriented crystals are composed of bands of fine and coarse subgrains. The average adjacent disorientation are also quite similar and are shown to increase with flow stress (or Z). However, the spatially heterogeneous structures of the cube grains are associated with significantly higher average disorientations.

Effect of strain rate on deformation behaviour of a Mg–8.5Li–1Zn alloy sheet at room temperature

Abstract The formability of a two-phase Mg–8.5Li–1Zn alloy thin sheet at room temperature is investigated in this study. Uniaxial tension tests are carried out for various strain rates between 1.4×10−5 and 8.3×10−2 s−1, and the microstructural and textural changes during the tests are observed. The sheet has high formability at comparatively low strain rates. Maximum elongation amounts to 100%. However, ductility decreases with the increase in strain rate. Even at room temperature, the stress is also sensitive to the strain rate. It is clarified that the grains are divided into fine sub-grains without any obvious textural change at high strain rates, and that the grains are elongated with textural change mainly in the β phase at low strain rates.

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.

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.

Structural observations of the interface of explosion-bonded Mo/Cu system

The structure of the interface of explosion-bonded Mo/Cu system has been examined by optical and electron (TEM, AEM) microscopy. Molybdenum and copper can be directly bonded along the flat interface without a diffusion zone, but only on the copper side of the interface is a thin (about 10 μm thick) “bond layer,” which consists of very fine subgrains with an ill-defined boundary. The microstructure in the bond layer is generated by severe dynamic deformation due to jetting and subsequent recovery with frictional heating. On the molybdenum side, originally-existing and elongated subgrains are observed just adjacent to the interface, even after bonding. These results indicate that jetting can occur only on the copper side, with a strength much lower than molybdenum because bonding must be carried out at an impact pressure as low as possible to avoid cracking of molybdenum as well as melting of copper during welding.

Ａｌ‐３％Ｆｅ‐３％Ｎｉ‐３％Ｃｒ‐０．７％Ｚｒ Ｐ／Ｍ合金のメカニカルグラインデング処理による高強度化

Mechanical grinding (MG) of air atomized Al–3Fe–3Ni–3Cr–0.7Zr (in mass%) alloy powder was performed with a vibrational ball mill for various times up to 154.8 ks in an argon atmosphere. Subsequently, MG powders as well as atomized powder were hot extruded to rectangular bar shapes (20 mm in width and 3.5 mm in thickness) at 673 K. The effect of MG time on the mechanical properties of extrudates up to 673 K and on the microstructures of MG powders and extrudates was investigated. Vickers hardness of MG powders increased to about 300 with increasing of MG time up to about 30 ks. The results of DSC, XRD and TEM revealed that the primary intermetallic compounds in atomized powder were finely ground and dispersed by MG. The results also suggest that elements consisting of the compounds were dissolved in the matrix beyond their equilibrium solubility by prolongation of MG time. MG and subsequent hot extrusion processes at 673 K resulted in giving extremely fine sub-grains. On boundaries or interior of the sub-grains, the transition elements dissolved in the matrix by MA were precipitated finely as Al3Fe, Al3Ni and Al13Cr2 phase particles. The tensile strength at ambient temperature and at 573 K were about 431.9 MPa and 208.7 MPa for extrusion specimen from atomized powder, and 675.7 MPa and 281 MPa for extrusion specimen from 154.8 ks-MG powder, respectively. However, the ductility was poor in MG materials. Improvements in the mechanical strength are mainly attributed to the presence of extremely fine sub-structure and finely dispersed particles which were introduced during MG and extrusion process.

Microstructures of surface white layer and internal white adiabatic shear band

After being impacted in an impact wear tester simulating the conditions in a mill, the surface white layer and internal white adiabatic shear band were observed in a low alloy steel (0.40% C0.85% Cr1.55% Ni0.20% Mo). It was found that cracks can easily nucleate and propagate in the surface white layer and internally formed adiabatic shear band during impact wear processes. Surface white layers with different hardness values possess very fine subgrains of various sizes, as do the internal white adiabatic shear bands. It was supposed that dynamic recovery may occur in the surface white layer and internal white adiabatic shear band in impact wear.

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.

Formation of a submicrocrystal FCC structure under irradiation with high-energy particles

The influence of a high-dose (up to 200 dpa) irradiation with 1.5 MeV Kr + ions at 500–700 °C on the formation of a submicrocrystal structure of Cr-Ni stainless steels was analyzed. In Cr16Ni15Mo3 and Cr16Ni8Mo3 unaging steels dislocations rearrange themselves to subgrains subject to the irradiation dose of 10 dpa at 600 °C, thus favoring formation of fine (30–50 nm) subgrains misoriented up to 1–3°. An increase in the irradiation dose up to 160 dpa leads to recrystallization and appearance of fine misoriented crystals 50–100 nm in size. If dislocations are pinned with dispersed Ni 3Ti particles (Cr16Ni15Mo3Ti1 aging steels), the subgrains are formed only after the irradiation dose of 160–200 dpa. Therefore the radiation method of producing nanocrystals is comparable as to its efficiency with a strong deformation.

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.