Showing papers in "Materials Transactions Jim in 2003"

Effect of microstructural factors on tensile properties of an ECAE-processed AZ31 magnesium alloy : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: Mg-3%Al-1%Zn (AZ31) alloy was subjected to ECAE (Equal Channel Angular Extrusion) processing under various processing conditions. Then tensile tests were carried out at room temperature to investigate the relationship between tensile properties and microstructural parameters that include grain size and the texture generated by ECAE processing. In 4-pass ECAE specimens processed at 523 K, tensile ductility is improved as a result of easy basal slip during tensile test along the extrusion direction, because such specimens have textures in which the basal plane is inclined at 45° to the extrusion direction. On the other hand, in the specimens processed at 573 K, 0.2% proof stess is higher than those of specimens processed at lower temperatures, but elongation is smaller. This is because of difficult basal slip caused by the textures in which the basal plane is oriented parallel to the extrusion direction. However, 8-pass specimens processed at 473 K and subsequently annealed, which have similar textures but different grain sizes (d), exhibit clear grain size dependencies of 0.2% proof stress (σ 0.2 ) according to Hall-Petch relationship; σ 0.2 = 30 + 0.17d -1/2 . Therefore, crystallographic orientation has a profound effect on the tensile properties of AZ31 alloy, and grain size has a little effect.

Anisotropy and non-uniformity in plastic behavior of AZ31 magnesium alloy plates : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: On the AZ31-O magnesium alloy plates of 20 mm in thickness, basal plane texture was studied for the samples taken from different layers. Compression tests were carried out in the rolling, width and thickness directions at room temperature. Tensile tests were conducted for the specimens taken from the different layers of the plates in different planar directions. The formability in deep drawing and stretch forming was evaluated for the thin sheet specimens taken from different layers of the plates and the results were discussed in relation to the texture and mechanical properties. The severity of the basal plane texture is higher at the surface layer than the inner layers. In tensile tests at room temperature, proof stress is higher for the surface layer than the inner layers, whereas elongation is lower and r-value is higher at the surface layer. In compression tests at room temperature, yield stress in the rolling and width directions is appreciably lower than in the thickness direction. At 573 K, anisotropic and non-uniform deformation behavior disappeared. Thin sheet specimens taken from inner layers of the plates showed higher formability than those from the surface layer in deep drawing and stretch forming. It is concluded that the formability of magnesium alloy sheets can be improved by decreasing the severity of the basal plane texture.

Development of nanostructures in metallic materials with low stacking fault energies during surface mechanical attrition treatment (SMAT) : Nano-Hetero Structures in Advanced Metallic Materials

Abstract: Surface mechanical attrition treatment (SMAT) technique was developed to synthesize a nanostructured surface layer on metallic materials for upgrading their overall properties and performance. In this paper, the grain refinement process during SMAT was investigated in materials with low stacking fault energies (SFE, Inconel 600 alloy and AISI 304 stainless steel) by means of transmission electron microscopy and high-resolution electron microscopy, respectively. Grain subdivision was performed by the interaction of mechanical microtwins with dislocations in Inconel 600. For AISI 304 stainless steel with a lower SEE, twin-twin intersections subdivide initial grains into refined blocks with sizes ranging from nanometers to submicrometers. Such grain subdivision processes of the interaction of microtwins with dislocations or microtwins obviously differ from those observed in the high SFE materials in which dislocation interactions predominate the grain refinement.

Dynamic processes for nanostructure development in Cu after severe cryogenic rolling deformation : Nano-Hetero Structures in Advanced Metallic Materials

Abstract: The dynamic grain refinement behavior upon severe plastic deformation has been systematically studied in commercial purity copper that was heavily cold rolled to large deformations at cryogenic temperatures. The low-temperature rolling allows the accumulation of extraordinarily high densities of dislocations in Cu, enabling an investigation of the various dynamic recrystallization phenomena upon further deformation. The eventual steady-state grain sizes achieved, as well as the dynamic recrystallization mechanisms, are studied using controlled deformation tests combined with transmission electron microscopy. The dominant mechanisms observed to contribute to grain refinement in Cu include the classical migration dynamic recrystallization process, the deformation twinning, as well as the continuous dynamic recrystallization via progressive lattice rotation upon deformation to extremely large strains. The strain rate and deformation temperature have strong effects on the operative mechanisms and the grain sizes achieved in the ultrafine and nanocrystalline regimes.

Influence of Sr and Mn additions on intermetallic compound morphologies in Al-Si-Cu-Fe cast alloys : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: The influence of Sr or Sr and Mn combined additions on the Fe-containing intermetallic compounds in Al-Si-Cu-Fe cast alloys has been investigated using Al-6.5%Si-3.5%Cu-1.0%Fe and Al-6.5%Si-3.5%Cu-1.0%Fe-0.3%Mn alloys (in mass%) with a similar composition to the 319 aluminum alloy. The results show that Sr successfully modifies the large, highly branched β-needle-like phase (β: Al 5 FeSi) into the individual, less-branched and finer one. The combined addition of Mn and Sr results in modification of the needle-like β-phase as well as promotion of Chinese script and sludge morphology formation. The mechanism of the above morphological changes has been discussed in accordance with the mechanism of nucleation and growth of the β-needle-like phase during solidification.

Experimental consideration of multistage martensitic transformation and precipitation behavior in aged ni-rich Ti-Ni shape memory alloys : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: It has been demonstrated with systematic experiments that the appearance and disappearance of multistage martensitic transformation in aged Ni-rich Ti-Ni alloys depend on the heat treatment atmosphere. No multistage transformation occurs when the evaporation of Ti and Ni and/or the preferential oxidation of Ti in the specimen are prevented and the purification of heat treatment atmosphere in an evacuated quartz tube is achieved. The heterogeneity in precipitation morphology of Ti 3 Ni 4 phase which is responsible for the multistage transformation can be suppressed with the regulation of heat treatment atmosphere as mentioned above. We have concluded that the multistage martensitic transformation in aged Ni-rich Ti-Ni alloys is an extrinsic nature, i.e., a kind of artifact during the heat treatment.

Influence of Fe and Co on phase transitions in Ni-Mn-Ga alloys : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: Differential scanning calorimetry (DSC) and magnetic measurements were performed to study the influence of ferromagnetic 3-d transition elements Fe and Co on structural and magnetic properties of ferromagnetic shape memory alloys Ni 2 MnGa. Addition of Fe or Co on the Ni sites decreases the temperature of martensitic phase transition T m , whereas addition of Co on the Mn sites results in a considerable increase of T m . Magnetic measurement revealed that Curie temperature T C increases upon substitution of Fe or Co for Ni. This observation is of importance for design of high temperature ferromagnetic shape memory alloys.

Quenched and annealed microstructures of hot worked magnesium AZ31 : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: The microstructures of magnesium AZ31 are examined following hot compression testing and annealing. The grain size, fraction dynamically recrystallized and, in a couple of cases, the crystallographic texture are reported. The progress of dynamic recrystallization and the recrystallized grain size were sensitive to processing conditions, as expected. This effect was more marked in the former than in the latter, compared to other metals. It was also found that, for structures containing between 80 and 95% dynamic recrystallization, abnormal grain growth occurred during annealing. Irrespective of the whether or not abnormal grain growth occurred, the annealing step weakened the crystallographic texture.

Random anisotropy model for nanocrystalline soft magnetic alloys with grain-size distribution : Nano-Hetero Structures in Advanced Metallic Materials

Abstract: A simple model considering grain-size distribution is proposed based on the random anisotropy model. When the maximum grain size (D m ) is less than the exchange correlation length and induced anisotropies are sufficiently small, the effective magnetic anisotropy constant ({K 1 }) is given by using a distribution function ( f(D)) for the grain size (D) as (K 1 ) K 4 1{∫ D m 0 D 3 f(D)dD} 2 /(φ 6 A 3 c), where K 1 is the magnetocrystalline anisotropy constant, φ is a parameter which reflects both the symmetry of (K 1 ) and the total spin rotation angle over the exchange-correlated coupling chain and A c is the exchange stiffness constant. The log-normal distribution function reproduces well the observed grain-size distribution and yields (K 1 ) K 4 1(D) 6 exp(6σ 2 D)/(φ 6 A 3 c), where (D) is the mean grain size and σ D is the geometric standard deviation for the distribution. This result satisfies the well-known (D) 6 law. However, (K 1 ) increases with increasing σ D even if (D) is constant. Our model has been extended by taking into account the effect of the coherent induced anisotropies on the exchange correlation length. The coercivity (H c cc (K 1 )/J s , where J s is the saturation magnetization) of the nanocrystalline Fe-Nb-B(-P-Cu) alloys with different grain-size distribution have been calculated. Our model explains well the dependence of H c on the grain-size distribution. These results suggest that one should pay attention on not only the mean grain size but also on the grain-size distribution since the inhomogeneity of the grain size increases H c .

Analysis of yield rate in single crystal casting process using an engineering simulation model : Special issue on solidification science and processing for advanced materials

Abstract: A 2-D engineering model for grain selection has been developed taking the columnar dendrite growth theory into consideration. After evaluating this model via a unidirectional solidification experiment, the single-crystal casting process was simulated. Since the time required for calculation is rather short, a statistical analysis has been performed for the first time. The yield rate of well-oriented single crystal is increased by increasing the initial number of grains on the chill plate. However, the yield rate does not exceed approximately 90%. A detailed investigation of the formation mechanism of misorientation has revealed two possible processes (Type A and Type B) that may occur during single crystal casting process.

Microstructures and tensile properties of ECAE-processed and forged AZ31 magnesium alloy : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: In order to achieve same level of high strength and high ductility as 6061 aluminum forging alloy that is currently used for automobile applications, AZ31 magnesium alloy rod with a large diameter of 40 mm was subjected to ECAE-processing, and the microstructures and mechanical properties of the ECAE-processed specimens were investigated. Furthermore, automobile knuckle arm was produced by forging using the ECAE-processed material, and the mechanical properties of the forged product and their strain rate dependencies were investigated under impact tensile load conditions. 4pass-ECAE-processed specimen has fine and uniform microstructure and a texture whose basal planes are mainly parallel to the extrusion direction with some inclined at angles up to 45° to the extrusion direction. Therefore, they show high ductility even if the tensile direction is parallel to the extrusion direction. The knuckle arm forged using the ECAE-processed material exhibits high elongation even in the high strain rate region. Furthermore, the tensile strength, fracture elongation and absorption energy of the forged product increase with increasing strain rate and their values are higher than those of T6-treated 6061 aluminum forging alloy specified by JIS.

Grain refinement in a Mg AZ91 alloy via large strain hot rolling : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: Significant grain refinement has been achieved in a cast Mg-based AZ91 alloy via large strain hot rolling. This is a simple processing method consisting of one rolling pass at intermediate temperatures with a large thickness reduction. The as-cast material was first homogeneized in order to obtain a homogeneous and equiaxed grain structure with a random texture. During processing, a double-peak basal texture develops, typical of rolled Mg alloys, that results from the operation of basal, prismatic and pyramidal (c + a) slip. The stabilization of this deformation texture during rolling suggests that the mechanism for grain refinement is continuous dynamic recrystallization.

Microstructure and voltage-current characteristics of anodic films formed on magnesium in electrolytes containing fluoride : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: Formation behavior of anodic oxide films on magnesium in various electrolytes including fluoride was investigated with attention to the effects of anodizing voltage, pH and aluminum content. In the range of formation voltage between 2V and 100V, porous film was formed in alkaline fluoride solution associated with high current density at around 5 V and at breakdown voltage. The critical voltage of breakdown to allow maximum current flow was approximately 60V and relatively independent on substrate purity. Barrier type films or semi-barrier type films, which were composed of hydrated outer layer and inner layer, were formed at the other voltages. A peculiar phenomenon of high current density at around 5 V, which may be caused by trans-passive state, was not observed for anodizing in acidic fluoride solutions such as Dow 17 and ammonium fluoride. In the case of AZ91D, the critical voltage increased to 70 V and peculiar phenomenon at 5 V was not observed, so that only barrier films were formed at less than the critical breakdown voltage. When AlO - 2 ion was added in the electrolytes, the critical voltage remarkably increased and current density effectively decreased with increasing AlO - 2 content. The passivation effect of aluminum addition in the electrolytes is more remarkable than the addition in magnesium substrates. The depth profiles of constituent elements showed that aluminum migrated into oxide film to reach near oxide/substrate interface. Atomic ratio of aluminum to magnesium increased with increasing voltage to attain 0.42 at 80 V and crystalline MgAl 2 O 4 and MgO were found in the film.

Tensile properties of directionally solidified AZ91 Mg alloy : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: AZ91 Mg alloy consisting of elongated grains was processed by a directional solidification method and mechanical properties of the directionally solidified Mg alloy were compared with those of the non-directionally solidified Mg alloy by tensile tests at room temperature and at 473 K. The directionally solidified Mg alloy exhibited higher strength at 473 K than the non-directionally solidified Mg alloy because grain boundary sliding was suppressed. Also, the directionally solidified alloy exhibited higher strength and larger elongation at room temperature than the non-directionally solidified alloy. Suppression of intergranular fracture was responsible for high ductility for the directionally solidified alloy.

Advanced plasma nitriding for aluminum and aluminum alloys : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: Low hardness and wear resistance of aluminum alloys limit their use in practical application to automotive parts. Formation of hard aluminum nitride (AlN) layer on the surface can prolong the life time of aluminum automotive parts. Plasma nitriding was selected in the present study to form AIN layer on aluminum alloys. This processing is an environmental friendly method because of its low gas and energy consumption. Normal plasma nitriding requires long processing time to successfully form AIN layer. For advancing this surface treatment, refining microstructure and micro-alloying processes are proposed to activate the formation of AIN by plasma nitriding. Bulk Mechanical Alloying is used not only to make grain-size refinement but also to carry out micro alloying with addition of 1 mass%Ti. The formation rate of AlN layer is improved from 4.2 x 10 -5 μm/s to 20.8 x 10 -5 μm/s by microstructure refining. In particular, since the co-formed TiN with AIN works as a template in the initial state nitriding, the micro-alloyed aluminum can be nitrided even without pre-sputtering.

Effect of Co and Ni on martensitic transformation and magnetic properties in Fe-Pd ferromagnetic shape memory alloys : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: Effects of Co and Ni addition on fcc-fct transformation temperature and magnetic properties have been investigated for Fe-Pd ferromagnetic shape memory alloys. The addition of Co shifts the fcc-fct transformation temperature slightly higher while the Ni addition shifts it lower. The results suggest that the relative stability between fcc and fct phases is significantly affected by the electronic structure. Saturation magnetization values for Fe-Pd-Co and Fe-Pd-Ni were found to be slightly higher than those for binary Fe-Pd.

Modeling of microstructure changes in FePt nano-granular thin films using the phase-field method : L10-type ultra-high density magnetic recording materials

Abstract: Since the factors that influence microstructure formation are extensive (e.g., alloy composition, heat treatment condition, etc.), quite a lot of experimental trial-and-error is often necessary when searching for the best combination of desired microstructure and material properties, even when the basic mechanism of microstructure formation is understood. During the last decade, the phase-field method has emerged across many fields in materials science as a powerful tool to simulate and predict complex microstructure evolution. Since phase-field methodology can model complex microstructure changes quantitatively, it is possible to search for the most desirable microstructure by using this method as a design simulation, i.e., through computer trial-and-error testing. In order to establish this methodology, first of all, quantitative modeling of complex microstructure changes using the phase-field method is required. The objective of this study is to model the FePt nano-granular structure formation and the order-disorder phase transition of FePt nano-particles as a typical example of phase-field modeling. We show that it is possible to model FePt nano-granular structure formation quantitatively using the phase-field method. This modeling method may also be applicable to various types of granular structure formation. The simulation result also suggests that there is a size dependence on the ordering of FePt nano-particles. Using the phase-field method to model the microstructure evolutions proved to be a very effective strategy in predicting and analyzing the complex microstructure formation.

High-purity magnesium coating on magnesium alloys by vapor deposition technique for improving corrosion resistance : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: Microstructures in coated magnesium alloy with high purity magnesium fabricated by applying a vacuum deposition technique were investigated. Moreover, relationships between microstructures in coated and un-coated magnesium alloys and corrosion behaviors were interpreted by in-situ laser microscopic observations during salt immersion tests. Magnesium with 3N-grade and AZ31 magnesium alloy were used for an evaporation source and a substrate for deposition. Temperature of the substrates was changed resulting in change in temperature profile in a furnace in order to optimize deposition coating conditions for obtaining homogeneous microstructures and thickness in deposited layer. The coated specimen revealed superior corrosion resistance to those on 3N-Mg, AZ31 and AZ91E alloys, and comparable to that on 6N-Mg in salt immersion tests using 3% NaCI solution at 300 K for 587 ks. In-situ observations showed that inhomogeneity in microstructures, such as second phases and grain boundary segregations, deteriorate corrosion resistance in magnesium alloys. Therefore, pure magnesium coated layer without inhomogeneity in metallographic and electrochemical meanings can improve the corrosion resistance on magnesium alloys.

Combination of triboelectrostatic separation and air tabling for sorting plastics from a multi-component plastic mixture : New systems and processes in recycling and high performance waste treatments

Abstract: This paper examines separation of waste plastics obtained from automotive shredder operations. Thus, a mixture of three kinds of plastic components (i.e. acrylonitrile-butadiene-styrene (ABS), polypropylene (PP), and polyvinyl chloride (PVC)) is sorted by using dry techniques i.e. without using water and hazardous chemicals. The separation process is characterized by the utilization of properties of triboelectric charge and density. Considering the relative position in triboelectric series (TES) and density of the each plastic component of the ABS, PP and PVC mixture, it is concluded that a satisfactory separation could not be obtained by either triboelectrostatic separation or air tabling alone. Thus, a system utilizing a combination of the two processes is developed. The triboelectrostatic separator is initially used to collect the ABS flakes as positively charged fraction and to produce a negatively charged fraction of PP and PVC flakes for further treatment on air table. Hence, at the end of the process, products of ABS, PP and PVC with a grade of 94.3%, 92.5% and 93.7% respectively are collected with a percentage mass distribution higher than 76.7% for all of them.

High strength Ni-Fe-W and Ni-Fe-W-P alloys produced by electrodeposition : Nano-Hetero Structures in Advanced Metallic Materials

Abstract: Ni-Fe-W alloys were produced by electrodeposition from an ammoniacal citrate bath having nickel sulphate, ferric sulphate and sodium tungstate as sources of nickel, iron and tungsten, respectively. The alloys prepared at low current densities have nanocrystalline structure, while those prepared at high current densities are amorphous. X-ray diffraction results show that the structure changes gradually from nanocrystalline to amorphous phase with an increase of current density. As the current density increases, tungsten content increases while iron content decreases. The hardness of the alloys increases with increasing tungsten content. The best mechanical properties among all the alloys are obtained for 51Ni-29Fe-20W alloy prepared at 600A/m 2 . Effect of sodium hypophosphite on the composition of the alloys produced at 2000 A/m 2 was also studied. Addition of hypophosphite causes a decrease in tungsten content of the alloys. Further increase in the hypophosphite content causes a decrease of both iron and tungsten contents and an increase of phosphorus and nickel contents. For the alloys deposited from solutions containing sodium hypophosphite more than 0.03 mol/L, the total molar content of tungsten and phosphorus remains constant at 20 at%.

Novel recycling system of aluminum and iron wastes-in-situ Al-Al3Fe functionally graded material manufactured by a centrifugal method : New systems and processes in recycling and high performance waste treatments

Abstract: In this study, the concept of novel recycling system using waste Al and Fe is described. Taking advantage of the fact that due to its cyclic usage, aluminum scrap unavoidably contains iron and steel wastes, an in-situ Al-Al 3 Fe functionally graded material (FGM) is planned to be fabricated. A centrifugal method is applied to a model master alloy, Al-10mass% Fe, obtained from virgin materials, the content of which is decided from the liquidus temperature. The resulting product is a thick-walled tube having a graded distribution of second phase particles in the Al matrix. It has been established that the shape of the particles varies depending on their position along the radial direction. The second phase is confirmed to be a stable Al 3 Fe intermetallic compound. Thin plates of Al-Al 3 Fe having homogeneously distributed Al 3 Fe particles, considering both the composition gradient and the particle morphology, were machined from the thick-walled Al-Al 3 Fe FGM tube and their mechanical properties measured. Based on the experimental observations, the potential and the advantages of the Al-Al 3 Fe alloy as a recyclable eco-FGM are discussed.

Processing of refractory organic waste water using ozone and novel agitation method : New systems and processes in recycling and high performance waste treatments

Abstract: A novel agitation method using ozone was applied to removing BOD 5 , COD Mn , color, and NH 4 -N in organic waste water filled in a cylindrical vessel. The organic waste water was agitated by injecting the same organic waste water into the bath through a centered bottom nozzle. Its flow rate was adjusted to form a jet above the nozzle. A pump was used for draining the organic waster water through four nozzles settled on the bottom of the vessel and circulating it until the aforementioned four quantities were sufficiently decreased. The swirl motion of the jet appeared under certain injection conditions. The bath was strongly agitated in the presence of the swirl motion. An ozone and air mixture therefore was supplied into the nozzle and then introduced into the bath with the jet. The rate constants of BOD 5 , COD Mn , color, and NH 4 -N were highly enhanced by this method compared to the conventional method using aeration of an ozone-air mixture supplied through a perforated plate.

Volume and grain boundary diffusion of chromium in Ni-base Ni-Cr-Fe alloys : Diffusion in materials and its application: Recent development

Abstract: Volume and grain boundary diffusion of chromium in Ni-16mass%Cr-7mass%Fe alloys containing 0.004, 0.015 and 0.07 mass% of carbon have been measured using radioactive tracer 51 Cr over the temperature range of 858-1424K by serial radio-frequency sputter-microsectioning technique. While the volume diffusion coefficients are largely unaffected by the presence of carbon, the increase in carbon content markedly reduces the mobility of chromium atoms along the grain boundaries. As a result, the difference between the activation energy for lattice and grain boundary diffusion decreases with increase in carbon content. In fact, for 0.07 at% carbon, the activation energies for volume and the grain boundary diffusion are nearly same. Among all three alloys, the difference between the volume diffusion coefficient, D v , as well as δD gb (δ = grain boundary width; D gb = grain boundary diffusion coefficient) decreases gradually in a regular manner with the rise of temperature. Grain bounadry energy is reduced by addition of carbon and enhanced by temperature. At higher temperatures, there is no difference between the grain boundary energies of the three alloys.

Tribological properties of magnesium composite alloy with in-situ synthesized Mg2Si dispersoids : Special issue on platform science and technology for advanced magnesium alloys, II

Abstract: The tribological properties of the magnesium composite alloys reinforced with solid-state synthesized Mg 2 Si or Mg 2 Si/MgO dispersoids are evaluated under wet conditions (in engine oil lubricants) by using pin on disc type wear test equipment. Every composite shows a dependence of the friction coefficient on the applied load, which corresponds to Sribeck diagram based on elasto-hydrodynamic lubrication (EHL). The increase of Mg 2 Si content causes the increase of both the friction coefficient and the total wear by plowing, because Mg 2 Si dispersoids protruded from the surface are much harder than S35C mild steel counter material. When including MgO dispersoids formed in deoxidizing SiO 2 particles by magnesium, the friction coefficient reduces to 0.01-0.02 in oil lubricant. This is due to the mild offensive effect by MgO dispersoids which ease the attacking or plowing phenomenon on the counter material, because they are not so hard.

Fabrication of lotus-type porous brass by zinc diffusion into porous copper : Diffusion in materials and its application: Recent development

Abstract: Lotus-type porous brass has been fabricated by utilizing solid-state diffusion of zinc into porous copper which has been made by unidirectional solifdification in pressurized hydrogen gas. The Zn diffusion has been carried out by thermal annealing of the porous copper. Zn was deposited by electroplating or vapor deposition method. It has been shown that the diffusion method is a very efficient way to synthesize the lotus-type porous alloys with a suitable composition.

Alignment of BiMn crystal orientation in Bi-20at%Mn alloys by laser melting under a magnetic field : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: This paper presents results of preliminary work done on developing a new technique to produce magnetically aligned structure in selected regions of rapidly solidified alloys. Rapid solidification of the Bi-20at%Mn alloy resulted in dispersion of the fine BiMn grains in the supersaturated Bi-rich matrix. Low magnetization could be achieved in the rapidly solidified specimen, as compared to the equilibrium condition. The rapidly solidified specimen when further subjected to the semi-solid processing under a magnetic filed of 4 T achieved the alignment of the BiMn grains due to the magnetic anisotropy and consequently exhibited a hysterisis loop in the magnetization curve. The semi-solid casting by laser melting of the rapidly solidified structure under a magnetic field of 10 T achieved the BiMn alignment in a small selected region (10μm in diameter). The experimental results pointed out the potential of the semi-solid casting by the laser melting under a magnetic filed to fabricate the aligned structure in the desired region.

Shape memory behavior of Ni-Mn-Ga sputtered films under a magnetic field : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: The shape memory behavior under the magnetic field up to 5 T was observed in Ni-Mn-Ga sputtered films. The sputtered films were heat-treated at 1073 K for 36 ks for homogenization and aged at 673 K for 14.4 and 56.7 ks in a constraint condition. The martensitic transformation temperatures and Curie temperature of the obtained films were higher than room temperature. The martensitic transformation temperatures increased under a magnetic field. Furthermore, a shape memory effect was observed by increasing and decreasing of the magnetic field.

Effect of carbon on nano-crystallization in steel during mechanical milling treatment : Nano-Hetero Structures in Advanced Metallic Materials

Abstract: Mechanical milling (MM) treatment of metallic powder is suitable for fabricating nano-crystallized materials, because milling action by steel balls enables to give a huge amount of strain with multi-directional plastic deformation to powder particles. In this study, effect of carbon on the grain refining behavior during MM treatment was investigated in high purity iron material and Fe-C materials. The powder used in this study is electrolytic pure (9 ppmC) iron and cementite (6.2 mass%C) powder particles. The powders are mixed to set the chemical composition to be Fe-(0-2)mass%C. The mixed powder is subjected to MM treatment for various times (3.6-360ks) and then microstructure was examined by means of X-ray diffractometry, TEM observation. With MM treatment, cementite decomposes into ferrite matrix and ferritic single structure is obtained after 360ks MM treatment. On the other hand, microstructure of ferrite develops from dislocation cells structure to fine-grained structure through dynamic continuous recrystallization (DCR). The grain size is reduced gradually with MM treatment. However the grain size after reaching steady state is different between high purity iron and Fe-C materials. The grain size after 360 ks MM treatment decreases with increasing carbon content, and nano-crystallized structure with about 15 nm grain size was obtained in the Fe-0.8 mass%C. This indicates that carbon addition enhances grain refining and is necessary for nano-crystallization by severe plastic deformation. Considering the interaction between carbon atoms and dislocations, carbon addition would assist the increment of stored dislocations which contributes to DCR. This results in the effectiveness for the formation of nano-crystallized structure in carbon added iron.

Formation of crystallographically aligned grains during coarsening in a magnetic field : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: A model that considers combined effects of the interfacial energy and the magnetic anisotropy energy is proposed to examine development of the crystallographically aligned microstructure during annealing under a magnetic field. For ferromagnetic materials with rather large magnetic anisotropy energies, the model indicates the coarsening can take place in accordance with the magnetic anisotropy. The grain with favorite orientation survive during annealing, resulting in the alignment. The narrow distribution in the grain diameters significantly improves the alignment by coarsening. For Paramagnetic and diamagnetic materials, the coarsening is also effective to develop the aligned microstructures when low degree of the alignment can be achieved by using a magnetic field in the initial microstructure prior to the coarsening.

High temperature in-situ observations of magnetic domains in Fe-Co alloys : Structural and functional control of materials through solid-solid phase transformations in high magnetic field

Abstract: In-situ observations of magnetic domain structure in Fe-Co alloys with Co concentrations of 4.6 ∼ 50 at% were made by the optical Kerr microscopy in the temperature range from R.T. to the Curie temperature (1103 K ∼ 1253 K). The magnetic domain structures could be successively observed up to the Curie temperature (1103 K ∼ 1253 K) and the average domain width was found to increase with increasing temperature. The contrast of magnetic domains in an Fe-4.6 at%Co alloy gradually decreased with increasing temperature and disappeared near the Curie temperature (1103 K). On the other hand, the magnetic domain structure in an Fe-50 at%Co alloy could be clearly observed at immediately below the α/γ phase transformation temperature (1253K). Furthermore, it was found that the magnetic domain walls tended to orient perpendicularly to the a/y interphase boundaries in an Fe-50 at%Co alloy at the a/y phase transformation temperature.