Sub-bandgap photocurrent response and carrier transport properties of undoped semi-insulating LEC GaAs as a composite

Undoped (ND) semi-insulating (SI) liquid encapsulated Czochralski (LEC) GaAs crystals were investigated by photocurrent and temperature-dependent Hall measurements. It is indicated that strong nonuniformities in the distributions of impurities and defects can occur for the NDSILEC GaAs crystal grown under a condition with strong constitutional supercooling. In such case, the deep level that dominates Fermi level is spacial location dependent, and the GaAs crystal becomes a composite consisting of a large number of elementary domains with different conductivities. The sub-bandgap photocurrent response and the carrier transport properties for this kind of composite are quite different from those for homogeneous NDSILEC GaAs.     

Sub-bandgap photocurrent response and carrier transport properties of undoped semi-insulating LEG GaAs as a composite

Undoped (ND) semi-insulating (SI) liquid encapsulated Czochralski (LEG) GaAs crystals were investigated by photocurrent and temperature-dependent Hall measurements. It is indicated that strong nonuniformities in the distributions of impurities and defects can occur for the NDSILEC GaAs crystal grown under a condition with strong constitutional supercooling. In such case, the deep level that dominates Fermi level is spacial location dependent, and the GaAs crystal becomes a composite consisting of a large number of elementary domains with different conductivities. The sub-bandgap photocurrent response and the carrier transport properties for this kind of composite are quite different from those for homogeneous NDSILEC GaAs.     

Process modeling in machining. Part I: determination of flow stress data

In the present study, two-dimensional orthogonal slot milling experiments in conjunction with an analytical-based computer code are used to determine flow stress data as a function of the high strains, strain rates and temperatures encountered in metal cutting. The workpiece materials selected for the present study are AISI P20 mold steel (DIN 1.2330, 35CrMo4) hardened to 30 HRC, AISI H13 tool steel (DIN 1.2344, X40CrMoV51) hardened to 46 HRC and Aluminum EN AW 2007 (DIN 1725 T1: AlCuMgPb, 3.1645) cold hardened to 100 HB. The methodology of flow stress determination for metal cutting, suggested in the present study, has advantages when compared with methods such as the Hopkinson's bar technique. This paper summarizes the first part of the study, conducted to estimate process variables in machining operations. The second part of the study, summarized in a different paper, addresses the application of flow stress data for predicting forces, stresses and temperatures in machining.     

Estimation of mechanical properties of ferritic steel welds. Part 1: Yield and tensile strength

Abstract

The yield strength and ultimate tensile strength of ferritic steel weld metal have been expressed as functions of chemical composition, the heat input during welding, and the heat treatment given after welding is completed. The method involved a neural network analysis of a vast and fairly general database assembled from publications on weld metal properties. The outputs of the model have been assessed in a variety of ways, including specific studies of model predictions for the so called C–Mn and 2.25Cr–1Mo systems. Where possible, comparisons have also been made with corresponding methods which use simple physical metallurgical principles. The models created are believed to have been trained on the largest weld metal database to date, and are shown to capture vital metallurgical trends. The computer programs associated with the work have been made freely available on the World Wide Web.     

The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence

In this investigation, tensile test specimens were fabricated with increasing levels of compressive and tensile residual stress on the surface and through the thickness of the specimen. These residual stresses were then measured by neutron diffraction at multiple points along the length and through the depth of the specimens. The specimens were then exposed to a neutral pH aqueous soil environment in combination with an applied cyclic stress for various lengths of time in order to initiate and propagate stress corrosion cracking (SCC). The formation of micro-pitting was found to occur preferentially in areas where the tensile residual stresses were the highest (approximately 300 MPa), while SCC initiation occurred with a 71% normalized frequency in areas where the surface residual stress was in the range 150–200 MPa. The difference between residual stress levels occurring at SCC locations versus pitting locations resulted from both the change of residual stress during cyclic stress application during SCC testing and the residual stress gradient in the depth direction.     

High precision linear slide. Part II: control and measurements

Design and construction of a linear slide have been discussed in Mekid [1]. Very high precision in nanometric scale depends on mechanical design and servo control with a very precise and adequate metrology. However, servo technology is employed as a method for going beyond mechanical accuracy limits. For that purpose the linear slide is controlled using two methods: Proportional–Integral–Derivative (PID) controller and Internal Model Controller (IMC) that would compensate automatically for unmodeled mechanical behaviors such as prerolling phenomena.The paper focuses on the design of a numerical controller able to handle imprecision in the model of behavior of mechanical and electromechanical components of the bench. The metrology frame including laser interferometer and optic linear encoder was used for the measurements.     

Bandsawing. Part III: stress analysis of saw tooth microgeometry

This article presents a stress analysis of a saw tooth. The analysis aims to determine the stress distribution for various egde load and varying microgeometry. The analysis is performed on a three-dimensional finite element model representing the tip of a saw tooth. The 3D model makes it possible to model the load distribution and study stress distributions across the width of the teeth with a sufficient accuracy. The FEM calculations are based on experimental measurements of the individual cutting forces during bandsawing. The previous studies of nonuniform contact relationships on the rake face is used when modelling the load distribution.     

RuAl and its alloys. Part I. Structure,physical properties,microstructure and processing

The structure, physical properties, processing and the related microstructure of RuAl and its alloys are presented in Part I of this overview, with a particular emphasis on the recent studies. The structure and physical properties reviewed include electronic structure, bonding, crystal structure, phase stability, as well as thermodynamic, elastic, electrical and thermal properties. Crystal defects are also covered and special attention is given to constitutional point defects, dislocations and planar faults. Ingot and powder metallurgy, mechanical alloying and thin film deposition are considered as processing techniques for the production of single or multiphase RuAl alloys. The typical characteristics of the microstructure related to different processing methods are presented. The entire overview is completed in Part II where mechanical properties, deformation and fracture behavior, environmental resistance and applications are discussed.     

On the determination of residual stress and strain fields by sharp indentation testing.: Part I: theoretical and numerical analysis

Sharp indentation tests, presently represented by cone and Vickers indenters, are analysed theoretically and numerically in order to explore how equi-biaxial residual stress and strain fields can be determined from the global properties, i.e. the size of the contact area between indenter and material and the hardness, given by such tests. It is shown that the residual strain fields can be accurately correlated with the hardness value while residual stresses are related to the size of the contact area. The latter feature is explained by the fact that the size of the contact area is sensitive to elastic effects. The results are summarized in simple closed form relations, well-suited to be used in an experimental situation, and the range of validity for the resulting formulae is discussed. The predictions are compared with corresponding results taken from the literature and good agreement is found. An experimental scheme for determination of residual fields by indentation is also suggested.     

Machining of free-form surfaces. Part I: Analytical chip load

In the machining simulations of 3D free-form surfaces with ball-end mill, determinations of instantaneously changing tool-workpiece engagement regions and chip load play very critical roles in force and surface quality predictions. Cutting force models for ball-end milling are based on the undeformed chip load. Undeformed chip load can be constructed from the boundaries of instantaneous engagement region between the ball-end mill and workpiece. In order to predict the cutting forces accurately, precise determination of the varying engagement regions is important. In the literature, there are two main engagement region constructing methods; one is the Z-mapping and the other one is using solid modeler based on Boolean operation method. Both methods construct the engagement region within accuracy limits, on the other hand the computational time for these methods are long such that it is not possible to calculate the forces at the same time of CL-point construction. This paper presents development of a new analytical method for fast and accurate determination of the instantaneous engagement regions in the 3D machining of monotonic free-form surfaces. The analytical tool is grid size independent, thus it is much faster than discretization and Boolean methods. In addition to that, the analytical tool gives the most precise and exact engagement regions.     

Mechanical and fatigue properties of stress relieved type 302 stainless steel wire

Type 302 stainless steel wire is manufactured using a cold extrusion process. The cold working increases the ultimate strength of the wire from approximately 690 MPa to 2070 MPa. However, the cold working process creates residual stresses and surface microcracks in the wire. The residual stresses and microcracks reduce the fatigue life of the wire considerably. Elimination or minimization of residual stresses and microcracks is necessary if longer fatigue life is required. Residual stresses or microcracks in a wire can be minimized by a heat treatment process, which could improve both its mechanical properties and fatigue properties. Experimental evidence shows that the stress relieving process yields maximum mechanical properties between 316 and 482 ° (600 and 900 °F). The fatigue properties of the wire are optimum at a stress relieving temperature of 649 ° (1200 °F). However, the mechanical properties such as yield strength, modulus of resilience, modulus of toughness, and ultimate strength, etc., are reduced by as much as 30% if compared to the similar properties of 316 ° (600 °F) stress relieved wire.     

ECAP with back pressure for optimum strength and ductility in aluminium alloy 6016. Part 2: Mechanical properties and texture

The simultaneous increase in strength and ductility of aluminium alloy 6016 processed by equal channel angular pressing (ECAP) was investigated. A complete study of microstructure, texture and mechanical properties after ECAP processing with and without back pressure was carried out for the O temper. The simultaneous increase in strength and ductility of AA6016-O with number of ECAP passes was explained by the use of back pressure during ECAP. A maximum ductility of ～100% was obtained at the temperature of 200 °C and strain rate of 10^?4 s^?1, which is a significant improvement on the ductility exhibited by AA6016 (～89%) after a conventional thermomechanical treatment at a much higher temperature of 500 °C. The mechanical behaviour was interpreted in the context of the textures developed in the material. A significant amount of texture rotation due to applied back pressure was found.     

High precision linear slide. Part I: design and construction

The purpose of the linear slide project was to gain knowledge and experience in high precision machine design dedicated to short stroke single and multiple axes. The work presented is part of a research program developed by the Université de Technologie de Compiègne to design a high precision linear slide.This paper discusses the design and construction of a linear stage with 16 nm positioning precision. A steel slide with a mass of 100 kg is fully floated by three hydrostatic bearings. The working motion length of the system is 220 mm. Very high axis stiffnesses are achieved.The linear axis is driven with a maximal translational speed of 10 mm/s using a friction drive actuator. A closed loop servo position control i.e. Internal Model Controller (IMC) is used to compensate automatically for unmodeled mechanical behaviors such as prerolling phenomena. The metrology frame including laser interferometer and optic linear encoder was used for the measurements.     

Modeling micro-end-milling operations. Part I: analytical cutting force model

A new analytical cutting force model is proposed for micro-end-milling operations. The model calculates the chip thickness by considering the trajectory of the tool tip while the tool rotates and moves ahead continuously. The proposed approach allows the calculation of the cutting forces to be done accurately in typical micro-end-milling operations with very aggressively selected feed per tooth to tool radius (f_t/r) ratio. The difference of the simulated cutting forces between the proposed and conventional models can be experienced when f_t/r is larger than 0.1. The estimated cutting force profile of the proposed model had good agreement with the experimental data.