A study of grinding damage in magnesium oxide single crystals

The sub-surface and surface damage produced by alumina and diamond grinding wheels has been studied as a function of material removal rate. Sub-surface damage is characterised in all cases by a discrete, dense layer of dislocations adjacent to the machined surface. The depth of the layer increases with material removal rate.The nature of the surface damage depends on the mechanism of material removal. As long as the cutting points are unloaded and the residue can escape the material is removed in a brittle manner. This is always the case for the diamond wheel and occurs for low rates of material removal with the alumina wheel. When the cutting surface is loaded with residue, the tool-workpiece interface temperature becomes so high that the material is removed by plastic flow. The finished surface is burnished. Unfortunately as the burnished surface cools down thermal quenching produces a network of surface cracks.     

Variation of cutting forces in machining of f.c.c. single crystals

In this study, micro-machining of f.c.c. single-crystal materials was investigated based on a hybrid modelling approach combining smoothed particle hydrodynamics and continuum finite element analysis. The numerical modelling was implemented in the commercial software ABAQUS/Explicit by employing a user-defined subroutine VUMAT for a crystal plasticity formulation to gain insight into the underlying mechanisms that drive a plastic response of materials in high deformation processes. The numerical studies demonstrate that cutting force variations in different cutting directions are similar for different f.c.c. crystals even though the magnitudes of the cutting forces are different.     

Temperature dependence of yielding and work-hardening rates in magnesium oxide single crystals

The mechanical properties of MgO single crystals have been analysed for their yielding and work-hardening behaviour as a function of purity and deformation temperature. The Fleischer theory accounts for the observed temperature dependence of the critical resolved shear stress well. Depending on temperature and impurity content, the MgO single crystals are shown to exhibit two-stage work-hardening behaviour, with the hardening shown to be athermal in nature. At and below room temperature, a temperature-dependent contribution results from dislocation-point defect interactions, while the primary athermal contribution arises out of dislocation-dislocation dipole interactions. The CRSS and work-hardening behaviour of MgO single crystals indicates a close relationship to the deformation characteristics of NaCl-structure alkali halides.     

Grinding induced machining damage in beryllium

An investigation has been made of the machining damage induced in beryllium by surface grinding. A series of damage free specimens was prepared from a bar extruded from a block of hot-pressed beryllium powder. Each specimen was then ground on one large surface using a different set of grinding conditions, as dictated by the requirements of a quarter-replicate factorial experiment which involved eight grinding parameters each taken at two levels. The damage induced by the surface grinding was evaluated by testing each specimen in a four point bending mode. The tests were conducted so that the maximum tensile strain due to bending occurred at the experimentally ground surfaces. A statistical analysis of the results enabled the grinding conditions giving rise to the least and the greatest amount of machining damage to be predicted. In a second series of experiments, these predictions were found to correlate with the metallographically observable damage and the mechanical test data obtained from a further series of bend test specimens.     

Electrical transport parameters of heavily-doped zinc oxide and zinc magnesium oxide single and multilayer films heteroepitaxially grown on oxide single crystals

Heavily doped epitaxial ZnO:Al and Zn_1−xMg_xO:Al films were grown by radio frequency magnetron sputtering onto single crystalline substrates (sapphire, MgO, silicon) and characterized by structural and electrical measurements. It is the aim of this investigation to better understand the carrier transport and the doping mechanisms in heavily doped transparent conducting oxide (TCO) films. It was found that the crystallographic film quality determines only partly the mobilities and the carrier concentrations: ZnO:Al films on a-plane (110) sapphire and on MgO (100) exhibit the highest mobilities. The oxygen partial pressure during the deposition from ceramic targets is more important influencing especially the carrier concentration N of the films. Though the films grew epitaxially grain boundaries are still existent, which reduce the mobility due to electrical grain boundary barriers for N < 3 · 10²⁰ cm^− 3. From annealing experiments the role of point defects and dislocations for the carrier transport could be estimated. For carrier concentrations above 3 · 10²⁰ cm^− 3 ionized impurity scattering limits the mobility, which is in agreement with our earlier review [K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097].     

Dislocations produced in magnesium oxide crystals due to contact pressures developed by softer cones

An experimental investigation shows that dislocations, encompassing a predictable dislocated volume, are produced in MgO single crystals subjected to circular contact pressures due to cones of other solids which may be an order of magnitude softer. The mean contact pressure necessary to produce localized plastic flow, without fracture, is shown to be directly related to the critical resolved shear stress of the MgO. Dislocation etching is used to investigate the influence of cone hardness and normal loading on the nature of the deformed zone in the harder crystal and the results are discussed in terms of the deformation characteristics of the softer cones and the crystal.     

Tool life and cutting forces when machining XC 45 steel

Machinability equations have been developed for prediction of tool life when machining XC 45 steel with P 10 and P 30 carbide inserts. A marked difference in cutability was observed between separate batches of P 10 inserts. A new criterion of tool life is suggested.

Equations are developed relating cutting forces to cutting conditions and to tool wear measurements. It is suggested that the percentage increase of the feed force may be suitable as a monitor of tool performance. The effect of cratering upon feed force is discussed and it is concluded that cratering should not necessarily affect feed force.

A slight positive correlation between cutting force and cutting speed has been observed for the radial and feed forces but not for the vertical cutting force.     

Synthesis of ultrafine magnesium oxide and titanium oxide powders

The formation of magnesium and titanium hydroxides (differing in the rate of primary particle formation) and conversion of the hydroxides to ultrafine oxide powders of controlled size have been studied using scanning electron microscopy, X-ray diffraction, differential thermal analysis, and analysis of the particle size distributions of solid materials from scattered laser light intensity. The particle size of the synthesis products has been shown to be influenced by the initial concentration and nature of precursors and precipitants, ultrasonic processing, additions of low-molecular-weight organic compounds, and dehydration temperature and time.     

Agglomeration of magnesium oxide particles formed by the decomposition of magnesium hydroxide

The agglomeration process of MgO powder derived from Mg (OH)₂ was investigated at fixed temperatures of 600, 800, 900 and 1200° C; these temperatures were chosen as representative of four regions, i.e. below 600° C, 650 to 850° C, 850 to 1050°C and 1050 to 1200° C previously reported. At 600° C, coherent crystallites coalesced within the heating time of 60 min; on further heating till 300 min, the primary particles which consisted of crystallites grew rapidly. The original Mg (OH)₂ framework or pseudomorphs, composed of minute crystallites and primary particles, still remained in the powder. At 800° C, the pseudomorphs had disintegrated into fragments. The crystallite growth and primary particle growth were accelerated with increasing the heating times beyond 60 min. At 900° C, a further fragmentation of agglomerates occurred with increasing the heating times; the crystallite and primary particle growth in fragments brought about the pore coalescence. At 1200° C, the crystallite and primary particle growth proceeded with the coarsening of pores; on heating beyond 240 min, the crystallites and primary particles grew rapidly due to the entrapment of pores within them.     

Agglomeration of magnesium oxide particles formed by the decomposition of magnesium hydroxide

Agglomeration of magnesium oxide (MgO) particles was studied by decomposing magnesium hydroxide (Mg(OH)₂). The properties of agglomerates varied according to the decomposition temperature region: (i) below 650° C, (ii) 650° C to 850° C, (iii) 850° C to 1050° C, and (iv) 1050° C to 1200°C. In region (i), the original Mg(OH)₂ frameworks or pseudomorphs remained in the powder and showed agglomeration. The strength of agglomerates containing the pesudomorphs was about 50 MPa; the primary particles in pseudomorphs are bonded chemically by the interaction of MgO and residual water. In region (ii) the pseudomorphs began to show some fragmentation: the bonding strength of these pseudomorphs reduced rapidly. In region (iii), both crystallite and primary particles were grown by the sintering; this growth may be due to an increase in contact area based on the collapse of pseudomorphs. The primary particles whose necks were grown by the sintering could be easily pulled apart by grinding. In region (iv) pore growth due to the rearrangement of primary particles caused the suppression of both densification rate and crystal growth of MgO.     

Segregant-enhanced fracture in magnesium oxide

The fracture toughness of MgO has been studied as a function of the concentration of an intergranular LiF phase, added to enhance densification. The fracture toughness increases as lithium and fluorine are volatilized from the MgO while the fracture morphology changes from intergranular to transgranular. The data are interpreted with respect to competing embrittlement and crack-deflection toughening mechanisms, and a scheme is proposed for determining conditions for embrittlement and toughening.     

Simultaneous measurement of forces and machine tool position for diagnostic of machining tests

A diagnostic system of the milling process through the cutting forces and cutting tool position (coordinates X, Y, and Z) simultaneously collected is detailed in this paper. After the milling test, the cutting forces generated during the machining and the machine position where they occurred can be correlated. After collecting and recording of both the force and position signals, a graphic representation of force components can be applied. In this way, the machine user is able to check those events that happened during the milling; thus, a more useful diagnosis of machining problems is possible. The collecting system is composed by a dynamometric plate for the measurement of the three components of the cutting force, as well as an acquisition device connected to the analog output of the position control loops. After machining, the file containing position and force data is post-processed and chromatic and vector maps are generated. The analysis tool shows its capabilities being applied to three examples, referring to three research projects.     

Mechanism of thermoluminescence in magnesium oxide

Thermoluminescence (TL) induced by γ-rays in nominally pure polycrystalline MgO powders has been investigated. In addition to a high temperature peak around 650 K, which is also present in the unirradiated specimens, TL glow peaks are observed around 365, 435, 510 and 545 K. The role of transition metal impurities on TL behaviour has been studied by the doping technique. It is observed that Cr³⁺, Mn²⁺ and Ni²⁺ act as emission centres while Fe³⁺, Co²⁺ and Cu²⁺ act as quenchers. For a particular sample all γ-induced glow peaks give a similar emission spectrum which is characteristic of the activators present. Optical bleaching, isothermal decay and ESR studies have been used to study the trapping centres. It is shown that the charge carriers involved are holes from V-type defects. The dominant 365 K peak has been assigned to V_M centres. A hole-release mechanism involving recombinations with electrons trapped at activators is proposed to explain the TL behaviour. Based on the thesis submitted by A. Sathyamoorthy to the University of Bombay (1976).     

Numerical formability assessment in single crystals of magnesium

In this paper a rate-sensitive elastic–viscoplastic crystal plasticity constitutive model (CPCM) together with the Marciniak–Kuczynski (M–K) approach have been used to assess the formability of a magnesium single crystal sheet by simulating the forming limit diagrams (FLDs). Sheet necking is initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field is assumed inside and outside the band, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs have been simulated under two conditions: (a) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, and (b) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, as well as extension and contraction twinning systems. The FLDs have been generated for two grain orientations. In the first orientation pyramidal 〈c + a〉 and extension twinning systems, and in the second orientation basal and pyramidal 〈c + a〉 slip systems, as well as contraction twinning systems have favourable orientation for activation. The effects of shear strains outside the necking band, strain rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations have been individually explored.     

Etching studies of beryllium oxide crystals

Chemical etching behaviour of BeO crystals was investigated for selected etchants ranging from basic to acidic in nature. Surface-emergent defects (dislocations, enclosed platinum crystals, point-defect concentrations, surface mechanical damage, twin boundaries) give rise to recognisable etch figures that are characteristic of etchant, defect type, and orientation of emergence face. Gross etching rates (in the absence of surface-emergent defects) are strongly anisotropic for both basic and acidic etchants, reflecting the crystallographic polarity of the BeO crystal structure; intermediate etchants give nearly isotropic etching. The details of gross and figure etching are described.     

Cavity growth in neutron-irradiated magnesium oxide

The growth of cuboidal cavities in neutron-irradiated magnesium oxide after annealing in the temperature range 1475 to 1775° C has been followed using the techniques of transmission electron microscopy and electron spin resonance. Microscopic examination has shown that, while cavities are nucleated on annealing at about 1500° C, most of their growth does not occur until a temperature approaching 1625° C is exceeded. Electron spin resonance spectra from the same samples annealed in the temperature range 1475 to 1575° C indicate that some of the vacancies, which are released to the lattice when cavities are nucleated, are used in the formation, from iron present at impurity level in the crystals, of Fe³⁺ in octahedral symmetry at magnesium sites. This corresponds to the region of negligible cavity growth. For annealing treatments at and above 1625° C however, the fractional volume of crystal occupied by cavities increases by a factor of ten and simultaneously the octahedral symmetry Fe³⁺ transitions disappear. The role of iron in controlling vacancy movement and cavity growth is discussed.