Showing papers in "Materials Transactions Jim in 1995"

High strength bulk amorphous alloys with low critical cooling rates (overview)

Abstract: This paper aims to review our recent research results on new amorphous alloys. The main topics consist of the following five parts ; (1) the finding of new amorphous alloys with extremely large glass-forming ability in a number of alloy systems, (2) the mechanism for the achievement of the large glass-forming ability, (3) the clarification of fundamental properties of the new amorphous alloys, (4) the successful examples of producing bulk amorphous alloys by four different techniques of water quenching, metallic mold casting, arc melting and unidirectional zone melting, and (5) the high tensile strength of the bulk amorphous alloys. These new results enable the elimination of the limitation of sample shape which has prevented the development of amorphous alloys as engineering materials and are expected to give rise to the revisit age to amorphous alloys.

Thermal and Magnetic Properties of Bulk Fe-Based Glassy Alloys Prepared by Copper Mold Casting

Abstract: Bulk glassy Fe 73 Al 5 Ga 2 P 11 C 5 B 4 alloys in cylindrical form with diameters of 0.5 and 1.0 mm were found to form by a copper mold casting method. The further increase in diameter causes the formation of coexistent glassy, Fe 3 (B, C), Fe 2 B and Fe 3 P phases for the 1.5 mm ? sample and coexistent Fe 3 (B, C), Fe 2 B and Fe 3 P phases for the 2.0 mm ? sample. It is to be noticed that the maximum thickness for glass formation is about 10 times larger than the largest thickness for Fe-based glassy alloys reported up to date. The glass transition temperature (T g ), crystallization temperature (T x ) and heat of crystallization of the 1.0 mm ? glassy alloy are 732 K, 785 K and 3.76 kJ/mol, respectively. No appreciable difference in the thermal stability and magnetic properties is seen between the bulk glassy alloys and the melt-spun ribbon. The 1.0 mm ? glassy alloy has ferromagnetism with a Curie temperature of 606 K and exhibits 1.26 T for saturation magnetization (B s ), 82 A/m for coercivity (H c ) and 0.38 for the ratio of residual magnetization to B s at room temperature. The large ΔT x ( = T x - T g ) and large glass-forming ability can be obtained for the Fe-based alloy containing simultaneously the five solute elements. The effectiveness of the multiplication is presumably due to the combination of the following three effects ; (1) the suppression of crystalline nuclei due to the increase in dense random packing density for the glassy structure containing P, C and B with significantly different atomic sizes, (2) the difficulty of atomic rearrangements for the precipitation of the Fe-metalloid compounds caused by the generation of Al-metalloid pairs with strongly attractive bonding nature, and (3) the decrease in the preferential precipitation tendency of Fe-B and Fe-C compounds by the dissolution of Ga which is immiscible to B and C and soluble to Fe.

Fe-Based Ferromagnetic Glassy Alloys with Wide Supercooled Liquid Region

Abstract: A multicomponent Fe 72 Al 5 Ga 2 P 11 C 6 B 4 alloy was found to form a glassy phase with a wide supercooled liquid region before crystallization and of ferromagnetism at room temperature. The glass transition temperature (T g ), crystallization temperature (T x ) and temperature intervals of supercooled liquid, ΔT x (=T x -T g ) for the glassy alloy are 732, 793 and 61 K, respectively. It is to be noticed that the supercooled liquid region exceeds 60 K which is about 3 times larger than the largest ΔT x value for the Fe-based glassy alloys reported up to date. The amorphous alloy also exhibits good bending ductility and rather good soft ferromagnetic properties at room temperature. The soft magnetic properties are improved by annealing at temperatures above the Curie temperature (T c ). The magnetic properties at room temperature for the glassy alloy annealed for 600 s at 723 K are 605 K for T c , 1.07 T for saturation magnetization, 5.1 A/m for coercivity, 9000 for permeability at 1 kHz and 2.0 x 10 -6 for magnetostriction. The finding of the ductile Fe-based glassy alloy exhibiting simultaneously the wide supercooled liquid region and good soft ferromagnetic properties is expected to create a new ferromagnetic bulk material with good deformability.

Nanocrystalline Soft Magnetic Fe–M–B (M=Zr, Hf, Nb) Alloys Produced by Crystallization of Amorphous Phase (Overview)

Abstract: This paper reviews our results on the development of a new type of soft magnetic material with high saturation magnetic flux density (B s ) combined with excellent soft magnetic properties. A mostly single bcc structure composed of bcc grains with about 10-20 nm in size surrounded by a small amount of intergranular amorphous layers was obtained by crystallization of amorphous phases prepared by melt-spinning and sputtering technique in Fe-rich regions of Fe-M-B (M=Zr, Nb, Hf) ternary systems. The typical nanocrystalline bcc Fe 90 Zr 7 B 3 , Fe 89 Hf 7 B 4 and Fe 84 Nb 7 B 9 alloys subjected to the optimum annealing exhibit high B s above 1.5 T as well as high effective permeability (μ e ) at 1 kHz above 20000. The high B s for the Fe-M-B alloys is resulting from the high Fe concentrations owing to high glass-forming ability of M(Zr, Hf, Nb) and B. The origin of the good soft magnetic properties for the alloys are listed as follows. (1) The apparent anisotropy is decreased by the combined effects of the formation of the nanoscale bcc structure and the achievement of rather strong magnetic coupling between the bcc grains through the intergranular ferromagnetic amorphous phase. (2) The small saturation magnetostriction (λ s ) results from the nonequilibrium bcc phase. The solute-rich inter-glanular amorphous phase with high Curie temperature (T c ) and high thermal stability has an important role in the achievement of the good soft magnetic properties through the formation of the nanoscale bcc structure and the attainment of the rather strong magnetic coupling between the bcc grains. The soft magnetic properties of the nanocrystalline Fe-M-B alloys were improved through the decrease in the bcc grain size and the increase in T c of the intergranular amorphous phase by optimizing heating rate in the crystallization process and adding small amounts of elements. For example, the improved Fe 84 Zr 3.5 Nb 3.5 N 8 Cu 1 alloy shows the high μ e of 100000 combined with the high B s of 1.53 T. This excellent μ e is comparable to those of nanocrystalline Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 and the zero-magnetostrictive Co based amorphous alloys, and the high B s is comparable to those of the Fe based amorphous alloys with good soft magnetic properties. The Fe-M-B based alloys also have very low core losses, the sufficient thermal stability and the low stress-sensibility of the soft magnetic properties. Therefore, the nanocrystalline Fe-M-B alloys are expected as practical magnetic materials for magnetic transformers, inductors, and other devices and parts.

Preparation of Bulky Amorphous Zr–Al–Co–Ni–Cu Alloys by Copper Mold Casting and Their Thermal and Mechanical Properties

Abstract: Bulky amorphous alloys were found to form in Zr-Al-M (M=Co, Ni, Cu) systems by arc melting on a copper hearth. The largest thickness for glass formation is 6. 1 mm for Zr 60 Al 10 Co 3 Ni 9 Cu 18 , 6.8 mm for Zr 60 Al 15 Co 5 Ni 15 Cu 5 and 6.2 mm for Zr 55 Al 20 Co 17.5 Ni 2.5 Cu 5 . The optimum composition for glass-forming ability shifts from the Cu-rich side to the Co-rich side through the Ni-rich side with increasing Al content from 10 to 20%. The use of a metallic mold casting process enabled the formation of amorphous cylinders with the largest diameter of 7 mm for the three alloys. The compositional effect for the large glass-forming ability has also been discussed by taking the present data into consideration. The cast amorphous Zr 60 Al 10 Co 3 Ni 9 Cu 18 alloy subjected to tensile testing exhibits distinct serrated flow before final fracture. The generation of the serrated flow is noticed because the alloy has a ductile nature which enables the momentary stop of the shear sliding. The Young's modulus, tensile fracture strength and fracture elongation are 97 GPa, 1510 MPa and 2.0%, respectively. The fracture occurs along the maximum shear plane and the fracture surface consists of a well-developed vein pattern. The size of their veins is about 10 times as large as that for the melt-spun ribbon and hence the shear deformation region occurs in a much wider region for the cast alloy, indicating the necessity of a larger amount of energy up to final fracture. The finding of the amorphous alloys with the large glass-forming ability and the extremely ductile nature is important for the subsequent development of metallic glassy materials.

Effect of additional elements on glass transition behavior and glass formation tendency of Zr-Al-Cu-Ni alloys

Abstract: The glass transition temperature (T g ) and crystallization temperature (T x ) of the Zr 65 Al 10 Cu 15 Ni 10 base glassy alloys containing additional M (M=Ti, Hf, V, Nb, Cr, Mo, Fe, Co, Pd or Ag) elements were examined as a function of M elements, with the aim of finding an effective element for the increase in ΔT x ( = T x - T g ) and of confirming the appropriateness of the previous empirical rules for the appearance of large ΔT x . As the additional amount of the M elements except Hf increases, T g increases gradually, whereas T x decreases significantly and leads to the decrease in ΔT x . No effective M element leading to the increase in ΔT x is found. The ineffectiveness is attributed to the partial generation of repulsive bonding nature of Cu-M (M=V, Nb, Cr, Mo, Fe, Co, Pd or Ag) and Zr-M (M=Ti of Hf) pairs which does not satisfy the empirical rules. The area ratio of the glassy region in the arc-melted ingots with a maximum thickness of about 8 mm was found to increase from 67% for the Zr-Al-Cu-Ni alloy to about 90% for the Zr-based alloys containing 5%Ti, 2.5%Nb or 5%Pd, though ΔT x decreases significantly by the addition of these elements. The great effectiveness of the three elements on the glass-formation tendency in the arc-melted ingots is interpreted to originate from the suppression of the growth reaction of crystalline nulcei which pre-exist in the arc-melted alloy. Furthermore, the disagreement between the glass-formation tendency evaluated by ΔT x and the area ratio of the glassy region is thought to result from the difference in the dominant factors which are the crystalline nucleation and growth reactions for the ΔT x of the melt-spun glassy alloys and the growth reaction for the area ratio of the glassy region in the arc-melted ingot. The finding of the additional elements leading to the increase in the glass-formation tendency of the arc-melted alloys, regardless of the magnitude of ΔT x , seems to be a very encouraging event for future development of bulk glassy alloys.

New Recycling Process by Extrusion for Machined Chips of AZ91 Magnesium and Mechanical Properties of Extruded Bars

Abstract: Hot extrusion has been carried out at 573, 673 and 753 K on machined chips of an AZ91 magnesium alloy and mechanical properties of the extrusions have been investigated. The extrusions processed from machined chips showed a good combination of high ultimate tensile strengths of 320-410 MPa, high 0.2% proof stresses of 220-300 MPa and elongations to failure of 5 ∼ 12% at room temperature. Grain refinement and dispersion of oxide layers by hot extrusion were responsible for the good mechanical properties at room temperature. Also the extrusions processed from machined chips at 573 and 673 K showed superplastic behavior at 573 K, though the elongations of the extrusions processed from machined chips were lower than those of the extrusion processed from the cast ingot.

Fabrication of Bulky Zr-Based Glassy Alloys by Suction Casting into Copper Mold

Abstract: Bulk glassy Zr 55 Al 10 Ni 5 Cu 10 alloys in a cylindrical form with a diameter up to 16 mm and a length of 70 mm were produced by sucking the molten alloy into a copper mold. The sucking force was generated from the rapid movement (5.0 m/s) of piston with a diameter of 16 mm which was set at the center of the copper mold. The sucking velocity was evaluated to be as high as 6.3 kg/s. Neither cavity nor hole is seen on the outer surface of the bulk glassy alloy and the glass transition temperature (T g ), crystallization temperature (T x ) and supercooled liquid region defined by ΔT x (=T x -T g ) are the same as those for the melt-spun glassy ribbon with a thickness of about 30 μm. Furthermore, there is no appreciable difference in T g , T x and ΔT x over the whole cast sample. The tensile fracture strength is 1620 MPa which is comparable to the compressive strength. The fracture takes place along the maximum shear plane and the size of the veins is about ten times as large as that for the melt-spun glassy ribbon. The significant difference indicates that the deformation for the cast bulk alloy takes place over the thicker and wider region. Thus, the direct production of the bulk glassy alloy with good deformability by the suction casting method is important for the future development of bulk glassy alloys.

Energy Transfer in Mechanical Alloying ( Overview )

Abstract: It is essential to understand in which way and how much energy is transferred from the milling tools to the powder in the milling process. We have attempted to quantify the energy transferred per impact and per unit of mass in a planetary mill assuming that collision is the dominant energy transfer event. Further, attrition mechanism (rolling/sliding) has been considered by evaluating the effect of filling the milling vessel. The energy transfers evaluated by the above modelization have been proved to be able to correlate the input energy and the different solid state reaction undergone by the Pd-Si system submitted to the milling action in both planetary and shaker mills. Measurements of power consumption during the milling process have been also performed and compared with those based on theoretical modelization. The fairly good agreement obtained gives a strong support to the validity of the theoretical background.

Formation of Nanostructural Materials Induced by Mechanical Processings ( Overview )

Abstract: Mechanical alloying (MA) was firstly developed to synthesize metallic matrix composite by mechanically incorporating preformed oxide and or carbide particles into a metallic matrix. A consecutive compaction process is applied to obtain bulk materials. During MA, powders are repeatedly welded, fractured and rewelded in a high energy mill leading to an intimate mixing on a nano/micro-scale with the possible formation of far from equilibrium phases. The versatility of MA is well known ; high volume, low energy mills can be used to commercially produced dispersion strengthened Al, Ni and other transition metal alloys. Various intermetallics and inorganic compounds (amorphous and/or nanocrystalline) have been synthesized by using higher energy mills which have been specially developed in some cases. Mechanical alloying, it appears, as suggested by T. H. Courtney et al., is the Alladin's lamp of powder processing. All the published works have shown that the reaction and end products of the MA process strongly depend on the milling conditions. As a consequence, it is obvious that an improved understanding of the dynamics of MA process is required to gain a full appreciation of the industrial potential of the technique for synthesizing materials. Recently, M. Abdellaoui and E. Gaffet have shown that the crystal to amorphous phase transition (at least in the case of the model Ni 10 Zr 7 ) only depends on the injected mechanically power, allowing a direct comparison among experiments performed using distinct type of milling apparatus (planetary milling machine, horizontal apparatus). An alternative method has been recently proposed by N. Malhouroux-Gaffet and E. Gaffet, for the solid state synthesis of disilicide powders exhibiting a wide contamination during the direct MA preparation : the mechanically activated annealing process (M2AP). Such a M2AP method has been applied to the synthesis of FeSi 2 , MoSi 2 , WSi 2 compounds. Such a method appears as being a well suitable one for the low temperature synthesis of refractory nanomaterials. Recent applications have been successfully performed to mechanically activated sintering (MAS).

Application of Mechanical Alloying to Chemical Refining (Overview)

Abstract: Since its discovery the process of mechanical alloying has demonstrated significant potential for the synthesis of materials with novel structures and properties. In this paper recent studies of the application of mechanical alloying to solid state refining processes are reviewed. These studies show that mechanical alloying enables displacement reactions, which conventionally require high temperatures to be thermally activated, to be mechanically activated during milling in a ball mill at ambient temperatures. It is shown that mechanical alloying can provide the basis of a single stage process which combines the separate processes of refining, alloying and powder manufacture. Of particular interest is the application of the process to the synthesis of rare earth permanent magnet materials.

Thermodynamic Properties of Amorphous Solids —Glass Formation and Glass Transition— (Overview)

Abstract: Glasses are generally produced from the highly undercooled liquid state by rapid quenching methods or quasi-statically at slow cooling by the effective control of potent heterogeneous nucleation sites. For metallic systems the latter method recently has led to the development of bulk metallic glass with a complex multicomponent chemistry and advanced engineering properties. Besides the formation of deep eutectics due to the strongly varying atomic size of the constituents an enhanced oxygen solubility is necessary in order to control heterogeneous nucleation and produce a bulk metallic glass. As long as crystallization can be avoided the relevant thermodynamic properties of the metastable glassy and undercooled liquid phases can be measured below and above the glass transition temperature, respectively. The obtained data give new insight into the nature of the glass transition suggesting that it is not a phase transition in the classical sense but kinetic freezing triggered by an underlying entropic instability. However, different types of glasses distinguished as fragile and strong exhibit different densities of configurational states. Therefore, the thermodynamic and transport properties become dependent on the time scales of their exploration. Furthermore, glass formation can be achieved by solid-state-processing without passing through the liquid state. This crystal-to-glass transition is observed under a number of different experimental conditions when a sufficiently high energy level is reached and kinetic conditions prevent the establishment of equilibrium. In some instances it can be shown that basically the same glassy state can be reached approaching it from the liquid or the solid state. In both cases the stability of the undercooled liquid and the non-equilibrium solid against glass formation is limited by an isentropic condition. Conceptually, the formation of the glassy state from the liquid and the solid can then be understood within a thermodynamic framework under appropriate kinetic constraints resulting in a universal phase diagram with a pseudocritical point.

Atom Probe Studies of Nanocrystalline Microstructural Evolution in Some Amorphous Alloys

Abstract: This paper reviews our recent atom probe studies of nanocrystalline alloys fabricated by primary crystallization of amorphous alloys. Partitioning and segregation of alloying elements in the course of primary crystallization in Fe-Si-B-Nb-Cu, Fe-Ta-C, Fe-Zr-B and Al-Ni-Ce(-Cu) alloys were examined by atom probe field ion microscopy. Concentration fluctuations in alloying elements are commonly observed in the early stage of annealing, which is believed to induce a large number density of nucleation sites. During the growth stage of the crystalline phases, one or two following mechanisms work to control the crystal grain growth : (1) stabilization of the remaining amorphous phase by partitioning of amorphous forming elements, (2) segregation of slow diffusing solute element at the amorphous/crystal interface, (3) precipitation of thermally stable compound such as TaC.

Effect of Heat Treatment on Shape Memory Behavior of Ti-rich Ti–Ni Thin Films

Abstract: Thin films of Ti-rich Ti-Ni were prepared by sputtering. The sputter-deposited films were annealed at three different temperatures of 773, 873 and 973 K for three different times of 1, 10 and 100 h in order to crystallize them. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The shape memory behavior was found to be very sensitive to the annealing conditions, being different from that in bulk specimens. The martensitic and reverse martensitic transformation temperatures increased with increasing annealing temperature and time. The film annealed at 973 K for 100 h showed a single-stage shape change associated with the martensitic transformation, while the other annealed films showed a two-stage shape change associated with both the martensitic and R-phase transformations. It was found from transmission electron microscopy that the difference in the shape memory behavior between thin films and bulk specimens comes from a difference in the distribution of Ti 2 Ni particles. That is, fine Ti 2 Ni particles distribute inside the TiNi grain in the annealed films except the one annealed at 973 K for 100 h. On the other hand, the structure of the film annealed at 973 K for 100 his similar to that normally observed in bulk specimens, where Ti 2 Ni particles distribute along the grain boundaries.

Process modeling of mechanical alloying: overview

Abstract: Recent efforts at modeling the mechanics and dynamics of mechanical alloying are summarized. The modeling is described from two perspectives. One considers the deformation response and the fracture and welding tendencies of powder particles entrapped between colliding grinding media in an individual collision. We call this local modeling. Fundamental and phenomenological approaches to local modeling have been undertaken, and these are described here as are numerical applications of the approaches. We have termed the second type of modeling global modeling. Global modeling concerns itself with device specific characteristics, and has potential for improving MA devices. Global modeling also serves to provide parameters that are necessary to best utilize local modeling.

Multicomponent Fe-Based Glassy Alloys with Wide Supercooled Liquid Region before Crystallization

Abstract: The glassy alloy containing 2 at% Ge in the Fe 74-x Al 5 P 11 C 6 B 4 Ge x system was found to cause the extension of the supercooled liquid region, ΔT x (= T x -T g ) defined by the difference between crystallization temperature (T x ) and glass transition temperature (T g ) to 60 K, though the ΔT value of the Ge-free Fe 74 Al 5 P 11 C 6 B 4 glassy alloy is 45 K. The remarkable increase in ΔT x is attributed to the increase in T x exceeding the degree of the increase in T g . The Fe 72 Al 5 P 11 C 6 B 4 Ge 2 glassy alloy has good bending ductility and ferromagnetism with Curie temperature at 590 K, in addition to the appearance of the wide supercooled liquid region. The crystallization of the glassy alloy containing 2%Ge takes place through a single stage and the crystallized structure consists of α-Fe, Fe 3 P, Fe 3 C, Fe 3 B and Fe 2 B phases. There are no appreciable changes with Ge content in the crystallized structure and the single-stage crystallization process leading to the simultaneous precipitation of the five crystalline phases. The effectiveness of 2%Ge on the increase in ΔT x is presumably due to the retardation of crystallization caused by the decrease in the formation tendency of Fe 3 C, Fe 3 B and Fe 2 B phases resulting from the existence of Ge which is soluble to Fe and insoluble to B and C.

Thermodynamics and kinetics of nanostructure formation in soft-magnetic nanocrystalline alloys (overview)

Abstract: Thermodynamic and kinetic conditions for nanocrystallization of Fe-based amorphous precursors into a duplex structure of α-Fe nanocrystals within a residual amorphous matrix are considered in view of recent literature. The effect of copper addition on nanograin size is explained in terms of the CuFe metastable phase diagram and the nature of the various heteroatomic interactions. Addition of larger atoms such as Nb and Zr that are rejected together with boron atoms at the α-Fe nanocrystallization interface is found to generate diffusion double-layers with sharp concentration gradients. The diffusion double-layers explain measured differences in Curie temperatures T c of the remaining grain boundary amorphous phase and bulk glasses of the same composition. They also successfully predict the occurence of a broad maximum in T c of the remaining amorphous phase during nanocrystallization annealing. The concentration gradients have an additional thermodynamic stabilization effect on the amorphous interlayers by reducing the driving force for the formation of intermetallics.

Raman Spectra of Graphite and Diamond Mechanically Milled with Agate or Stainless Steel Ball-Mill

Abstract: The reduction of crystalline size and amorphization of graphite and diamond during agate and stainless ball-milling are investigated by Raman spectroscopy. The ultimate crystalline size of graphite, estimated by the Raman intensity ratio, of 2.5 nm for the agate ball-mill is smaller than that of 3.5 nm for the stainless ball-mill, while the milling time to reach the ultimate size for the former is about 10 times larger than for the latter, indicating more stability of the nanocrystalline graphite. After reaching the ultimate crystalline size, a significant broadening of the Raman spectra, which indicates the completion of amorphization, is detected only for the stainless ball-milled graphite at ∼ 500 h of milling. Also the increase rate of the Raman peakwidth for the stainless ball-milled graphite before amorphization is higher than that for the agate ball-milled graphite, indicating a larger introduction of disorder from the start of milling. Amorphization of diamond is also observed for the stainless ball-mill. The difference in the results between the agate and the stainless ball-mill is discussed in terms of the effect of impurity mixed from the milling apparatus on the stability of nanocrystalline carbon materials.

Microstructural Changes during Annealing of Work-Hardened Mechanically Milled Metallic Powders (Overview)

Abstract: The behavior of work-hardening which occurs during mechanical milling (MM) treatment in metallic powders, and the process of recovery and recrystallization which occurs during annealing in the MM powders were over-viewed showing the results obtained by the authors using an industrial pure iron powder. Through the MM treatment, metallic powders stores extremely large strain energy, and this results in the marked work-hardening and the formation of a fine structure with nanocrystalline grains. In the case of iron, the hardness of powder can be increased to DPH1024 in practice, and the crystalline grain size is to be reduced to the limiting value of 3.4 nm in principle. The polycrystallization of dislocation cells and subgrains also proceeds on the grain refining process. When the MM powders are annealed, the powders undergo different microstructural changes depending on the degree of work-hardening subjected by the prior MM treatment. In the case that powders are not work-hardened so much, usual recovery and recrystallization occur with raising annealing temperature. However, when powders are extremely work-hardened to the level where crystalline grain size nearly reaches the limiting value, only the process of normal grain growth occurs during annealing and this results in the softening of MM powders. Under the usual milling conditions, the degree of work-hardening of powders is in the middle stage, so that both of the above two processes are possible with overlapping each other. It was confirmed in both of MM iron powders and annealed iron powders that the relation between hardness and polycrystalline grain size gives a good fit to the Hall-Petch relationship in a wide grain size range up to 6 nm. In addition, some examples are introduced at the end, in order to excellent properties of materials produced from MM powders.

Effects of Cold Work on Precipitation Hardening of Cu-4.5 mass%Ti Alloy

Abstract: The structure and properties of a solution treated, cold worked and aged Cu-4.5%Ti alloy were studied by means of optical, scanning and transmission electron microscopy as well as hardness, tensile tests and electrical conductivity measurements. Prior cold work (rolling) increases the peak hardness from 340 to 425 VHN, the tensile strength from 890 to 1380 MPa and the conductivity from 10 to 25% IACS on aging the alloy in the temperature range from 400 to 450°C. Maximum strengthening of the alloy is attributed to the precipitation of a coherent, metastable β' phase (Cu 4 Ti). The alloy is deformed through profuse twinning. The ductile mode of fracture of the aged alloy is not affected by the prior cold work.

Observation of Trapping Sites of Hydrogen and Deuterium in High-Strength Steels by Using Secondary Ion Mass Spectrometry

Abstract: Hydrogen and deuterium trapping sites in high-strength steels have been observed by secondary ion mass spectrometry (SIMS). High-strength steels with 1400 MPa tensile strength are stressed and dipped in D 2 O and 20% NH 4 SCN solution at 323 K to occlude hydrogen and deuterium. Depth profiles by SIMS show the presence of deuterium, which indicates hydrogen trapping sites occluded during the delayed fracture test. Secondary ion image analysis by SIMS has made it possible to observe hydrogen and deuterium trapping sites in high-strength steels. Hydrogen tends to accumulate at grain boundaries, in segregation bands, and on inclusions. Line scans by SIMS have shown that the accumulation ratios of hydrogen for the grain boundaries, segregation bands of P and inclusions are 7.8, 5.0 and 11.0 times higher than that in the matrix, respectively. Hydrogen trapping sites at the grain boundaries can be observed by measuring within 24 h after the delayed fracture test.

Preparation of Thermoelectric β-FeSi2 Doped with Al and Mn by Mechanical Alloying (Overview)

Abstract: MA has been applied to the Fe-Si system near the composition FeSi 2 starting from elemental powders using a horizontal ball mill. Powders were examined by means of XRD and DSC. After substantially long time milling, the premixed powder of nominal composition Fe 3o Si 70 was found to transform to a homogeneous β-FeSi 2 phase in the as milled condition. The appropriate milling time of a powder for sintering was considered to be 720 ks. At this stage the powder consists of very fine Fe and Si crystals in the as milled condition. This transforms into a homogeneous β phase with an exothermic reaction at around 720 K. The transformation rate from (α + e) to β in the MA powder was found to be faster than that in the specimen prepared from an eutectic alloy ingot. Specimens prepared by hot pressing from MA powder showed smaller grain size and lower thermal conductivity than those prepared by ingot metallurgy. The p-type thermoelectric properties were measured for β-FeSi 5 hot-press sintered specimens doped with both Al and Mn. Due to a small grain size, the specimen prepared from the MA powder showed a higher figure of merit and hence is, higher in conversion efficiency than that prepared by ingot metallurgy. The sequence of phase formation by mechanical alloying and post-milling annealing is considered. The effects of grain size on the thermal conductivity and Seebeck potential are discussed.

Structure of Surface-Modified Layers of Calcium-Ion-Implanted Ti–6Al–4V and Ti–56Ni

Abstract: The structures of surface-modified layers of calcium-ion (Ca 2+ )-implanted Ti-6 mass%Al-4 mass%V and Ti-56 mass%Ni were characterized using Auger electron spectroscopy with argon-ion-sputtering and X-ray photoelectron spectroscopy. Calcium ions were implanted in Ti-6Al-4V and Ti-56Ni in the amount of 10 22 ions/m 2 with an acceleration energy of 2.88 fJ (18 keV). The surface oxide layers of these alloys grew to approximately 12 nm as a result of the Ca 2+ -implantation. The oxide layers contained hydroxide. The outermost layer of the surface oxide was 3-nm-thick calcium oxy-hydroxide. A large part of the surface-modified layer, existing inside the calcium oxide, consisted of a mixture of titanium oxide, aluminum oxide and calcium oxide in Ca 2+ -implanted Ti-6Al-4V, and a mixture of titanium oxide, nickel oxide and calcium oxide in Ca 2+ -implanted Ti-56Ni. In Ca 2+ -implanted Ti-6Al-4V, Ca 2+ -free oxide layers existed in the deepest region and this layer consisted of a mixture of titanium oxide with aluminum oxide, whereas this layer was absent in Ca 2+ -implanted Ti-56Ni.

Abrasive Wear Characteristics of Zn–37.2Al–2.5Cu–0.2Mg Alloy Dispersed with Silicon Carbide Particles

Abstract: Observations made during two-body (high stress) abrasion tests conducted on a zinc-aluminium alloy reinforced with SiC particles have been reported in this investigation. The matrix alloy, processed under identical conditions, was similarly studied in order to understand the response of the reinforced second phase particles (SPPs), i.e. SiC, on the behaviour of the former. The influence of load and abrasive (SiC) size on the abrasion rate of the specimens was also studied. The study indicates that the presence of the SPPs (in the matrix) reduced the abrasion rate of the matrix when abraded against fine abrasive particles by providing protection to the matrix. However, the trend reversed when coarser abrasive was used. This was also evident from practically no damage to the SPPs (on the abraded surface) by the fine abrasive particles and substantial microcracking and fracturing of the SiC (abrasive as well as the SPPs) when coarser abrasive was used. Material removal mechanisms in the composite were observed to comprise the simultaneous exposure of the matrix and the reinforced SPPs followed by microcracking, fracturing and removal of the latter along with the matrix. The abrasive particles also were damaged during the process whose extent depended on the abrasive size and load. The observations have been explained with the help of the analyses of the wear surfaces, subsurfaces, debris and the abrasive particles.

Preparation of Amorphous Fe–Si–B and Co–Si–B Alloy Wires by a Melt Extraction Method and Their Mechanical and Magnetic Properties

Abstract: Amorphous (Fe, Co)-Si-B ternary and quaternary wires were found to be produced over a wire diameter range of 20 to 80 μm by a melt extraction method using a copper wheel with an edge angle of 60 degrees. The wires without any concaves in the transverse cross section were prepared in the extraction condition where the circumferential velocity of the wheel and the rising velocity of the molten alloy are in the range of 20 to 40 m/s and 0.1 to 4.0 mm/s, respectively. The resulting amorphous wires have good bending ductility in the wire diameter range below 80 μm for the Fe 75 Si 10 B 15 alloy and below 60 μm for the Co 72.5 Si 12.5 B 15 alloy. The thermal stability and mechanical properties except tensile fracture strength (σ f ) for the amorphous wires are nearly the same as those for the melt-spun amorphous ribbons. The σ f is 3800 MPa for the Fe 75 Si 10 B 15 wire and 3600 MPa for the Co 72.5 Si 12.5 B 15 wire, being about 10% higher than those for the corresponding ribbon samples because of the low degree of stress concentration resulting from the good smoothness on the outer surface. The magnetic properties of saturation magnetization, coercivity and effective permeability at 1 kHz for the wires are nearly the same as those for the ribbons. The squareness of the hysteresis B-H loop for the Fe-Si-B amorphous wire is better than that for the same Fe-based ribbon and the sharp output voltage due to the reversion of magnetic flux is also observed for the present Fe 78 Si 10 B 12 amorphous wire. The combined properties of good mechanical properties and unique magnetic properties leading to the generation of the sharp output voltage seem to be promising for the subsequent development of the present fine amorphous wires.

Research on Metastable Structures Using High Energy Ball Milling at North Carolina State University ( Overview )

Abstract: This paper reviews the research of the author's group at North Carolina State University over the last ten years in the field of metastable materials prepared by mechanical attrition. A brief historical perspective of the author on the beginnings of this sub-field is presented. Much of the research discussed is devoted to the crystalline-to-amorphous phase transformation induced by mechanical attrition. This includes mechanical alloying (MA) where dissimilar powders react and material transfer occurs. Equilibrium structures prepared by MA are also discussed. Amorphization by mechanical milling (MM), where a single composition (e.g. intermetallic compound) is milled requires the accumulation of defects to induce the transformation. It is concluded that anti-site chemical disorder and nanocrystalline grain boundaries are the important defects in this regard. A brief description of studies on MM of normally immiscible systems is presented. Non-equilibrium solid solubilities can be achieved by MM of systems such as Ge-Sn and Si-Sn. The paper concludes with a summary of recent work on nanocrystalline materials prepared by ball milling. The questions of 1) the mechanism for synthesis of nanocrystalline microstructures, and 2) the thermal stability of such microstructures are discussed.

Solidification analyses of bulky Zr60Al10Ni10Cu15Pd5 glass produced by casting into wedge-shape copper mold

Abstract: The liquid-crystalline transformation behavior during continuous cooling and the transformation-induced structure were examined for a Zr 60 Al 10 Ni 10 Cu 15 Pd 5 molten alloy which was ejected into a wedge-shape cavity in a copper mold. The wedge-shape cavity has a constant depth of 50 mm and different vertical angles (θ) ranging from 5 to 15 degrees. The ejection temperature of the molten alloy was also changed in the range of 1273 to 1573 K. The cast structure consists only of a glassy phase in the θ range smaller than 10 degrees and changes to a mixed structure consisting of glassy and nonequilibrium crystalline Zr 2 Ni and Zr 2 Cu phases in the higher θ range. The glass transition temperature and crystallization temperature of the cast metal glass are 683 and 778 K, respectively, which agree with those for the melt-spun glassy ribbon. The start (Cs) and termination (Ct) points for the transformation from the supercooled liquid to crystalline phases during continuous cooling were determined from the thermal analytical data obtained at different sites in the wedge-shape cavity and the continuous-cooling-transformation (C.C.T.) curves were constructed. The nose temperature (T n ) and the time (t n ) up to the nose point in the C.C.T. curves were 1018 K and 0.93 s respectively. The critical cooling rate for glass formation defined by (T m - T n )/t n is evaluated to be 110 K/s. Further, the time interval between Cs and Ct is as short as 0.2 s and the fast growth reaction is attributed to the easy formation of the nonequilibrium crystalline phases and the increase in temperature caused by the precipitation-induced recalescence.

Microscopic Studies on Stress-induced Martensite Transformation and Its Reversion in an Fe–Mn–Si–Cr–Ni Shape Memory Alloy

Abstract: Martensitic transformation induced by extension at room temperature and its reverse transformation on heating in an Fe-14Mn-6Si-9Cr-5Ni (mass%) shape memory alloy were studied in detail by optical microscopy, electron microscopy and electron diffraction. The stress-induced transformation is initiated by random formation of extremely thin martensite plates with 1-2 nm width and then these plates are clustered and some of them coalesce to form thicker martensite plates with increasing deformation. The clustered regions are 400-600 nm wide and considered to correspond to the thinnest martensite plates observable with optical microscope. It is presumed that these clustered regions may also correspond to the lamella structures of a mixture of f.c.c. and h.c.p. phases reported in a previous work. In optical microscopic scale, the thin martensite plates with the smallest width are formed rather uniformly in an austenite grain, and with further increasing deformation, they are clustered and coalesce into thicker plates with 3-8 μm width. On heating, the reverse transformation is accomplished exactly by the reverse process of the martensite formation mentioned above. Stacking disorders in martensite revealed by electron diffraction analysis are such that the stacking sequence at the fault represented by parameter α is equal to that of the parent phase, in other words, extremely thin layers of f.c.c. phase (parent phase) with the minimum width of 3 layers are contained in the martensite with a high density.

Computer Simulation of Milling Ball Motion in Mechanical Alloying (Overview)

Abstract: A model simulation of milling-ball motion during mechanical alloying done by the author's group was reviewed. The Kelvin's dashpot-spring model and the discrete element method were employed in the simulation. One, two and three dimensional milling-ball motions for three types of ball milling ; vibratory, tumbling and planetary ball milling, were studied. The simulation formalism was first given, which takes into account the translational and rotational motion and three types of forces ; visco-elastic, frictional and gravitational force. The whole motion of the milling balls was traced contact by contact. The simulation provided for each of the milling types the ball motion trajectories, impact velocities and frequencies of the balls, as well as the energy consumption during mechanical alloying. The results of the simulation were supported by the experimental observation.

Hard Magnetic Properties of Nanocrystalline Fe-Rich Fe–Nd–B Alloys Prepared by Partial Crystallization of Amorphous Phase

Abstract: When Fe-rich Fe-Nd-B amorphous alloys containing 88 to 90 at% Fe are annealed for 60-300 s at 923-1023 K, the annealed alloys have the nanostructure consisting of bcc-Fe, Fe 14 Nd 2 B and remaining amorphous phases and exhibit rather good hard magnetic properties. The best hard magnetic properties of remanence (B r ), coercive field ( i H c ) and maximum energy product ((BH) max ) are 1.14 T, 260 kA/m and 117 kJ/m 3 , respectively, for Fe 90 Nd 7 B 3 1.28 T, 252 kA/m and 146 kJ/m 3 , respectively, for Fe 89 Nd 7 B 4 and 1.22 T, 240 kA/m and 130 kJ/m 3 for Fe 88 Nd 8 B 4 . The mean particle sizes of these crystallites in the optimum annealing treatment are 20 to 40 nm and the thickness of the intergranular amorphous layer is 10 to 30 nm. The amorphous layer contains Nd concentrations much higher than the nominal concentration and the enrichment seems to be the reason for the residual existence of the amorphous phase at the high temperatures. The nanoscale Fe 14 Nd 2 B particles are surrounded by the bcc-Fe and amorphous phases. The three constituent phases have ferromagnetism and their Curie temperatures for the Fe 89 Nd 7 B 4 alloy annealed for 300 s at 923 K are about 1040 K for bcc-Fe and 630 K for Fe 14 Nd 2 B. The further increase in annealing temperature and time causes the decrease in hard magnetic properties presumably because of the grain growth of bcc-Fe and Fe 14 Nd 2 B phases resulting from the disappearance of the residual amorphous phase. The coexistence of bcc-Fe, Fe 14 Nd 2 B and amorphous phases on a subnanoscale is important for the achievement of the rather good hard magnetic properties and hence the bcc-Fe and amorphous phases seem to act as an effective magnetic exchange-coupled medium. The simultaneous achievement of the high B r and (BH) max values in the residual existence of the amorphous phase for the Fe-rich alloys containing about 90 at% Fe is believed to be the first evidence and has significant engineering importance because of the expectations of high deformability and high cost performance.