Materials Science Forum Vol. 710

Abstract: In developing compositions for new ultra-high strength steels it is important to understand the effects of alloying additions on the microstructure, strength and fracture resistance. Cobalt has been widely used in the development of high toughness ultra-high strength steels and the effects of cobalt on the strength of steels has been studied extensively but there is remarkably little known about how cobalt influences toughness. In this article are reviewed the effects of cobalt on the toughness of steel. The literature suggests that cobalt in solid solution will act to raise the ductile-to-brittle transition temperature but that it can act to increase the upper-shelf toughness of steel.

Abstract: Grain boundary embrittlement and de-embrittlement observed in age hardening iron alloys were reviewed. Fe-Mn-Ni and Fe-Ni-Ti alloys show excellent hardening response during aging treatment. However these alloys all suffer grain boundary embrittlemnt and show no tensile ductility even after very short aging treatment. Precipitation of intermetallic phases, θ-MnNi in Fe-Mn-Ni alloys and η-Ni3Ti in Fe-Ni-Ti alloys, at grain or lath boundaries was suggested as the reason for the weakening of grain boundary strength. Grain boundary strength recovered when these precipitates transform to austenite after extended aging. Dislocation glide or dislocation climb did critical role in conversion of these grain boundary precipitates to austenite.

Abstract: While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. Ultrafine-grained steels with different carbon contents from ultralow carbon to high carbon were produced through warm caliber rolling. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensite steels for all materials. Formability of ultrafine-grained steel is examined by applying to form a M 1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews.

Abstract: The purpose of this investigation is to develop the nondestructive tools for microstructure assessments in carbon alloyed steels. The role of carbon in steel and its effects on electromagnetic property and also the free electron model have been reviewed. The fundamental electromagnetic principle behind low frequency impedance measurements has been included. The systematic analysis of phonon vibration and ultrasonic resonance spectroscopy for elastic wave perturbation in T22 Cr-Mo steel has been presented. A brief examination of nondestructive microstructure evaluation techniques has been described. The induced microstructure variations in Grade T22 Cr-Mo steel, including the correlations of changes in physical (microstructure) and mechanical (hardness data) properties during annealing have been measured. The explanations of aged carbide precipitates, martensitic and pearlitic nucleation and growth have been illustrated. The possibility of simultaneous use of two nondestructive wave techniques is discussed. The electron model and electron interactions are associated and are shown to support the results of low frequency impedance measurements enhanced elastic wave perturbations.

Abstract: This paper is concerned with the tensile behavior of various Al alloys during solidification obtained by using an initially solid specimen heated locally until it becomes fully liquid and then partially solidified at a controlled cooling rate. It is shown that for Al-Cu as well as for Al-Si-Mg alloys, a similar behavior is observed with a sharp transition on the stress-solid fraction curve when the coalescence solid fraction of the dendrites is reached. Below the transition fracture occurs along liquid films for very low stresses whereas beyond this transition, ductile fracture is observed leading to higher stresses.

Abstract: Research activities on aluminium production technology focus on the successive steps in the production chain of aluminium wrought products. Direct-chill casting of aluminium alloys is a well-developed technology with a long history. But only in the last 20 years, the development of computer modelling offered a means of better understanding of the physical phenomena involved in solidification. The main scientific challenge is to obtain a fundamental insight into the processing of aluminium alloys and to establish quantitative relationships between materials, processes, and performance. A systems approach is employed, covering theoretical and experimental studies on processing into semi-finished products. Special emphasis is placed on experimental verification and industrial applicability with the availability of pilot scale experimental facilities at M2i-TUD. This facilitates the design of process conditions as desired for experimental validation studies. In this paper we will consider main mechanisms of structure and defect formation during solidification of DC cast billets, mostly based in the results obtained in cooperation between Delft University of Technology and Materials Innovation Institute (M2i).

Abstract: This article discusses the fundamental principles associated with the selection of aluminium alloys for specific purposes, alloy design & heat treatment procedures and development of key microstructures responsible for obtaining desired properties in selected wrought Al alloys for aerospace and defence applications. Influence of micro/trace additions of suitable alloying elements on the microstructure and properties of high strength 7xxx series Al alloys is further highlighted.

Abstract: We review the effect of processing on structure and texture in titanium alloys, focusing on the understanding of this relationship that has evolved over the last decade. Thermomechanical processing cycles for these alloys involve deformation and heat treatment in single phase β and two phase, α+β, phase fields, and involves a complex interplay between deformation and recrystallization textures of the individual phases, textures arising from the crystallographic relationship between the two phases, and the scale of microstructure evolution. We explore these interactions and trace the strong dependence of thermomechanical pathways on the final structure and texture.

Abstract: Titanium (Ti) alloys are used in critical, fatigue limited applications in aircraft and aircraft engines. Current design practices are, of necessity, conservative in order to minimize risk of unexpected failures. Among the sources of this conservatism are the inherent variations in the number of loading cycles the materials can withstand prior to fatigue crack initiation, the uncertainty in crack propagation lifetime prediction and the need to set safe minimum component life values. While the stochastic nature of fatigue is well-known, improved characterization methods have begun to provide a better understanding of the crack initiation process. This paper describes recent work designed to provide an improved understanding of the relationship between thermomechanical processing history, microstructure, texture and the fatigue behavior of α+β Ti alloys. Due to length limitations, the paper focuses on two important aspects of fatigue life variation: the effects of microstructural discontinuities on fatigue and the role of facet formation during crack initiation and the early stages of fatigue crack growth.