Showing papers in "Materials Science Forum in 1998"

Ferrous and Nonferrous Bulk Amorphous Alloys

Abstract: Bulk amorphous alloys with thicknesses up to 75 mm and a wide supercooled liquid region reaching 127 K before crystallization have been found to be fabricated in a number of multicomponent systems which satisfy the three empirical rules for the achievement of large glass-forming ability, i.e., (1) multicomponent alloy systems consisting of more than three constituent elements, (2) significantly different atomic size ratios above 12 % among the main constituent elements, and (3) negative heats of mixing among their elements. The scientific significance of the three empirical rules has been proved based on a number of experimental data as well as on the kinetic theories of the nucleation and growth of a crystalline phase. By choosing appropriate compositions which satisfy the three empirical rules, bulk amorphous alloys in Mg-, lanthanide metal-, Zr-, Pd-, Fe- and Co-based systems were produced in cylindrical and sheet forms by various solidification processes. The bulk amorphous alloys exhibit high tensile strength, good ductility, high elastic energy, high impact fracture energy and high corrosion resistance for Zr-based system and good soft magnetic properties for Fe-based system. Furthermore, their bulk amorphous alloys heated in the supercooled liquid region can be deformed into various shapes by viscous flow. The ideal Newtonian flow has been achieved in the supercooled liquid. The utilization of the ideal superplasticity enabled the achievement of an extremely large elongation exceeding 15000 %. These excellent data allow us to expect that the bulk amorphous alloys with a wide thickness range up to 75 mm develop as a new type of engineering material.

Bioactive Glasses and Glass-Ceramics

Abstract: Volume I: Bioactive Glasses and Glass-Ceramics General Science and Development Composities and Coatings Tissue and Organ Culture Studies Preclinical and Clinical Results Index

Microscopic Model for Mechanical Alloying

Abstract: We present a microscopic model for the mechanical alloying (MA) of a mixture of two or more elemental ductile powders. The MA produces fresh metal/metal surfaces and a high density of dislocations. The model is based on dislocation kinetics and the diffusion of solutes along the dislocation cores. For short time intervals T 1 , on the order of microseconds, powder particles are subjected to high stresses which exceed their mechanical strength. However, for times T 2 on the order of seconds, the powder particles are unaffected by the milling process. During T 2 , solutes diffuse along the cores of these dislocations. During successive T 1 periods, the dislocations are mechanically forced to glide, leaving behind strings of solutes in states of supersaturation. The model is used to explain the formation of extended solid solutions and amorphous alloys, as well as many other unusual effects observed during MA.

The production and mechanical properties of ultrafine ferrite

Abstract: A new thermomechanical process has been developed to produce ultrafine (1 μm) equiaxed ferrite grains in hot rolled steel strip. This process is remarkably simple and is applicable to a wide range of steel chemistries, including low and high carbon and microalloyed steels. Strips are reheated to produce a coarse austenite grain size, then rolled in a single pass at or just above the austenite to ferrite transformation temperature. It is suggested that the observed refinement is due to strain induced transformation from austenite to ferrite. The requirements for this appear to be high strain induced by shear in the strip surface layers, and thermal gradients created by heavy quenching of the strip surface by the work rolls. The yield strength was markedly higher than conventionally processed strip, although there was little work hardening even though total elongation of over 20% was achieved.

Strain Broadening Caused by Dislocations

Abstract: Numerous experiments have shown that strain broadening caused by dislocations can be well described by a special logarithmic series expansion of the Fourier coefficients of Bragg reflection peak profiles. In the present work it will be shown that this formalism can be incorporated into the classical methods of Williamson-Hall and Warren-Averbach. The new procedures are suggested to be called modified Williamson-Hall and modified WarrenAverbach methods, respectively. Based on the examples of a submicron grain size copper specimen and a ball-milled iron powder sample it is going to be shown that the modzjied methods can yield physically well justified data for particle size, dislocation densities and twinning and faulting.

Combustion phenomena induced by ball milling

Abstract: Ball milling induces self-sustaining reactions in sufficiently exothermic powder mixtures. Combustion occurs after some activation time, when the powder reaches a well defined critical state. Investigating the nature of this state and the processes leading to ignition are used as a vehicle to learn about the mechanism of mechanochemical reactions in general. A possible framework is proposed to describe the process. The effects of a single collision play the central role. The more fundamental atomic scale events and the global kinetics of the milling process are also considered. Experimental investigations of the ignition of combustion are reviewed with emphasis on systematic studies relating the reaction parameters and the ignition time. Unusual combustion phenomena such as the interrupted combustion effect and the mutual suppression of combustion are discussed.

Nanocrystallization of Al-Ni-Y and Al-Ni-Nd Metallic Glasses

Abstract: Crystallization behavior and microstructural development during annealing of Al 90 Ni 6 Nd 4 and Al 85 Ni 5 Y 10 glasses were investigated in detail by quantitative TEM. Isothermal annealing for various periods at temperatures between 140°C and 250°C lead to partially nanocrystalline microstructures consisting of a fine dispersion of α-Al nanocrystals embedded in an amorphous matrix. The Al 85 Ni 5 Y 10 glass exhibits a higher thermal stability than Al 90 Ni 6 Nd 4 glass. Nucleation was studied by crystallization statistics of partially crystallized glasses by fitting measured crystal size distributions. Crystallization statistics revealed that the α-Al phase forms by transient heterogeneous nucleation with nucleation rates in the order of 10 20 - 10 21 m -3 s -1 . The nanocrystal growth proceeds rapidly at the beginning, but was found to slow down probably due to the formation of a RE-rich diffusion barrier between the α-Al crystals and the matrix; Nd seems to be even more effective in slowing down the growth than Y.

Microalloyed Forging Steels

Abstract: Microalloyed forging steels have been developed to improve the competitiveness of wrought steel components, especially in the automotive sector, by achieving the desired properties in the as-forged condition, thus eliminating the need to subsequently heat treat, straighten and stress relieve the previously specified low alloy steels. Significant cost reductions are realised by adopting microalloyed steels. This paper reviews the metallurgical principles on which microalloyed forging steels are based, including the relationships between steel composition, thermomechanical processing, microstructure and the resulting properties, highlighting the various strengthening mechanisms that are invoked. The properties, characteristics and applications of the initial development grade, 49MnVS3, are described. Research and development then focussed on increasing the strength and/or the toughness of this steel to improve its appeal to the market, especially for safety critical applications. the various metallurgical options are described and discussed. Attention has also been placed on maximising the machinability of these steels by controlled additions of sulphur, the adoption of inclusion modification techniques and other free machining additives. The fatigue properties and toughness of microalloyed steel forgings have been demonstrated to be fit for purpose, but compared with heat treated low alloy steels their fracture toughness is lower, albeit still significantly superior to castings. A wide range of forged automotive applications had been successfully converted to air cooled microalloyed steels over the past 25 years, with a large proportion of crankshaft and connecting rods now being made by this route. Future challenges have been identified to further extend the attainable properties and to improve the combination of strength and toughness, to broaden the market applications and the product range to include bar and rod. The use of warm near net shape forming processes for microalloyed steel is also anticipated. Greater exploitation of computer aided modelling and design techniques is encouraged to facilitate rapid prototyping, in order to improve further the competitiveness of forged engineering steels.

Experimental and Theoretical Analysis of the High Temperature Thermal Conductivity of Monocrystalline SiC

Abstract: The thermal properties of monocrystalline 6H-SiC have been studied within the temperature range 300-2300 K. The thermal diffusivity a was measured with the laserflash method up to 2300 K. The specific heat capacity c{sub p} was determined from room temperature up to 1650 K. From thermal diffusivity data, the c{sub p} data fitted by the Debye-model and the density {rho} taken partly from measurements and partly from literature the thermal conductivity {kappa} was evaluated from {kappa} = a(T) x {rho}(T) x c{sub p}(T) to give {kappa}/[Wm{sup -1}K{sup -1}]{approx_equal}451700 x (T/[K]){sup -1.29}. The theoretical analysis of the given high temperature thermal conductivity data and of low temperature data taken from literature shows, that the phonon contribution is dominant within the whole temperature range (8-2300 K). Within a Callaway analysis a Grueneisen-parameter {gamma} = 1.5 was found. Unipolar and bipolar electronic contributions to the thermal conductivity seem to be negligible. At temperatures {>=}1500 K additionally heat transport by radiation becomes relevant. (orig.) 15 refs.

Stability of retained austenite in a Nb microalloyed Mn-Si TRIP steel

Abstract: The effect of thermomechanical processing on the volume fraction and stability of retained austenite in a Nb microalloyed Mn-Si TRIP steel was investigated. In one set of tests, specimens were isothermally held at 400 °C for a series of different times, then air cooled to room temperature. In another set of tests, specimens were continuously cooled to room temperature at a cooling rate of 2 °C/s or 10 °C/s. The former generated a mixed microstructure of ferrite + bainite + retained austenite, while the latter produced a microstructure of bainite + martensite + retained austenite. It was found that the stability of retained austenite against a strain induced transformation to martensite increases with increasing carbon concentration, decreasing size and hard surrounding phase.

Junction Barrier Schottky Diodes in 4H-SiC and 6H-SiC

Abstract: The Junction Barrier Schottky (JBS) diode in silicon carbide is a promising candidate for a low-leakage power rectifier for high switching frequencies and elevated temperature operation It has the advantage of a low forward voltage drop while keeping a low leakage current at high blocking voltage JBS devices have been fabricated in 4H SiC and 6H SiC and then electrically characterised in comparison with pn and Schottky diodes on the same wafer The JBS devices reached blocking voltages up to 10 kV at a leakage current density of 13 IŒA/cm2 and the forward conduction was limited by an on-resistance close to the theoretical value

Tie Lines of the Grain Boundary Wetting Phase Transition in the Zn-Rich Part of the Zn-Sn Phase Diagram

Abstract: The temperature dependence of the contact angle θ at the intersection of Zn grain boundaries with Zn/liquid Zn-Sn boundaries have been studied. For this purpose, two Zn bicrystals were grown with {1000} tilt boundaries having misorientation angles of o=16° (small angle grain boundary) and o=60°. These boundaries possess different energies. The temperatures T w of the grain boundary wetting phase transition for these two boundaries were determined and the corresponding tie lines in the two-phase (Sn)+L field of the Zn-Sn phase diagram were constructed. Above T w the contact angle θ is 0° and a layer of liquid phase substitutes for the grain boundary. The temperature of the wetting transition (T w1 = 382±1°C) for the boundary with the high energy (o=60°) is lower than that (T w2 = 386.5±1°C) for the boundary with the low energy (o=16°). Above the temperature interval where all the grain boundaries become wetted, the solid phase may exist only as isolated single crystalline islands in the sea of melted phase.

SiC Merged p-n/Schottky Rectifiers for High Voltage Applications

Abstract: A method of reducing reverse currents and increasing breakdown voltages without inducing negative effects on switching behavior in silicon carbide Schottky diodes is proved successfully. Implantation of p-regions in the surface of the n-drift region below the Schottky metal form face to face p-n junctions which screen the Schottky contact from high electrical fields. This results in a reduction of the reverse current and an increase of the breakdown voltage to the limit of a `pure` SiC p-n diode. It is shown, that in contrast to silicon based devices, SiC merged p-n/Schottky (MPS) rectifier preserve their excellent unipolar switching behavior. (orig.) 5 refs.

The Role of Microstructure in Toughness Behaviour of Microalloyed Steels

Abstract: The micromechanisms involved in cleavage propagation in the brittle and ductile-brittle behaviour of microalloyed steels are analysed. The cleavage process is divided into three different steps which must take place dynamically in order to succeed: nucleation of a microcrack in a microstructural feature, propagation of the microcrack across the particle-matrix boundary and finally, propagation across the matrix surmounting high angle boundaries. The microstructural parameters involved in the three steps are considered, together with their effective surface energy values. Depending on testing temperature, the microstructural feature controlling cleavage fracture changes. In relation to the ductile-brittle transition, the main parameter is the volume fraction of coarse microstructural units surrounded by high angle boundaries.

Study of the Dielectric Characteristics of Cement Paste

Abstract: The characterization of the microstructure of cementitious materials by means of Impedance Spectroscopy (IS) is attracting an increasing interest. This is due not only to the importance of the cement cover on steel corrosion, but because of the possibility of characterise aspects like hydration degree of cement or water content. The present study analyses the impedance of cement paste specimens previously oven-dried or conditioned at 50% and 100% RH, in order to calculate their dielectric constant, e. The results show that when oven-dried the cement paste has an almost perfect capacitive behaviour with reasonably low e values. However, when the cement paste contains a certain amount of moisture, the shape of the IS diagram changes significantly and, according to the literature, abnormally high e values are obtained. These changes are considered to be due mainly to some bulk and interfacial phenomenon still not well identified. Appropriate electrical insulation of the electrode/paste interface by means of mylar sheets allows to obtain reasonable e values for moisten specimens and to advance in the identification of the phenomenon defining the shape of the obtained IS diagrams.

Raman Microprobe Measurement of Under-Damped LO-Phonon-Plasmon Coupled Mode in n-Type GaN

Abstract: Raman scattering spectra from LO-phonon-plasmon coupled modes in n-type hexagonal GaN epitaxial layers have been measured in detail for different carrier densities in n=10{sup 17}-10{sup 18} cm{sup -3}. Both the upper and lower branches of the coupled modes were clearly observed, showing striking changes in lineshape and peak frequency with the carrier density. A spectral lineshape analysis shows that n-type GaN belongs to a system of under-damped plasmon satisfying {omega}{sub P}{tau}>or{approx}1, where {omega}{sub P} and {tau} are the plasmon frequency and scattering time, respectively. This is in contrast with other wide band-gap semiconductors such as SiC, which shows clearly the character of over-damped plasmon, {omega}{sub P}{tau}<1. A measurement of spatial distribution of carrier density in epitaxial layers is also presented for an application of the coupled-mode analysis. (orig.) 9 refs.