Jorge L. Gonzalez-Velazquez

Numerical analysis of phase decomposition in A-B binary alloys using Cahn-Hilliard equations

The analysis of phase decomposition was carried out using the nonlinear and linear Cahn-Hilliard equations in a hypothetical A-B alloy system with a miscibility gap. These equations were solved by the explicit finite difference method assuming a regular solution model. The supersaturated solid solution and decomposed phases were considered to have an fcc structure. Different aging temperatures and thermodynamic interaction parameters ΩA-B were used to simulate different alloy systems. The numerical simulation results showed that the growth kinetics of phase decomposition in the alloy with 30at.% A was slower than that of 50 at.% A. Additionally, the start time and modulation wavelength of phase decomposition are strongly affected by the thermodynamic interaction parameter ΩA-B value. The numerical simulation results showed that the growth kinetics of phase decomposition with the linear equation is slower than that with the nonlinear one.

Phase transformations induced by mechanical alloying of Ag–28 at.-%Al alloy

Abstract A Ag–28 at.-%Al alloy was obtained by mechanical alloying in a SPEX 8000D mill from elemental Ag and Al powders. Two vials with different internal volume and two ball powder weight ratios were used to analyse the phase evolution. Microstructural characterisation was carried out by X-ray diffraction, SEM and TEM. The sequence of phase transformation during the milling process was as follows: Ag+Al (powders)→α→α+ζ→α+ζ+μ→μ+ζ→μ. The formation of the α and ζ phases is favoured by the energy of the impacts in the early stage of milling, while the formation kinetics rate of the μ phase was faster for the higher vial volume and ball/powder weight ratio. TEM images confirm the presence of the μ phase with a nanometric grain size.

Effect of phase transformations on hardness in Zn-Al-Cu Alloys

Zn-Al-Cu alloys were prepared by melting of pure elements. The as-cast alloys were homogenized at 350 °C for 180 h. Both the as-cast and homogenized alloys were analyzed with X-ray diffraction and scanning electron microscope with Energy Dispersive X-Ray analysis. The hardness of alloys was determined using Rockwell “B” hardness. The X-ray diffractograms and scanning electron micrographs indicated the presence of several phases in the as-cast alloys. Some of them do not correspond to those shown in the equilibrium Zn-Al-Cu phase diagram. On the other hand, the homogenized alloys showed most of the phases present in the equilibrium diagram. The hardness of alloys increases with the increase in Cu content because of the presence of Cu-containing phases such as, the θ and τ’ phases.

Environmentally-Assisted Fracture

This chapter describes the mechanisms and fractographic features of environmentally-assisted fractures. It begins with the general aspects of environmental fracture, such as definition, influencing factors and stages. To better understand the role of environment, the fundamentals of corrosion in metals are described. The environmentally-assisted fracture modes described are: Stress Corrosion Cracking, Creep Fracture and Hydrogen-Induced Cracking. In each of these modes of fracture, the mechanical aspects, the fractographic features and the basic mechanisms are described, with the aid of diagrams, graphs and photographs of fractures at both the macro- and microscopic levels.

The Fractographic Examination

This chapter describes the adequate methodology for the examination of fractured components, with emphasis being given to the specific features of fractured components that have to be recorded for further analysis. The correct procedures for the handling, cutting, cleaning and preservation of fractures are described in detail. Recommendations for taking good photographs of fractured surfaces are given, including a description of the fundamentals of digital photography. The chapter ends with a description of the elaboration of fracture replicas.

Failure Analysis of Fractured Components

This chapter describes in detail a proposed procedure for performing failure analysis of fractured components, taking as its foundation the knowledge of fractography presented in the previous chapters. It includes a critical analysis of the ASTM E2332 standard procedure, in order to give a broader description of its methodology, and is complemented with a discussion about the ethics in failure analysis. An entire section of this chapter presents the relations between fractography and the failure criteria of continuum mechanics and fracture mechanics, in order to provide a quantitative approach to failure analysis, which helps in estimating failure loads, critical flaw size and the fracture toughness of the material. In order to fully illustrate the usefulness of the proposed methodology, three examples of failure analysis of different fractured components are included. In these examples, the cause of failure, the calculation of the failure loads and the fracture sequence and mechanism are determined.

Elements of Fractography

This chapter begins with a description of the different classifications of fracture, according to mechanism and extent of plastic deformation. The mechanical aspects of fracture, from the continuum mechanics and fracture mechanics points of view, are briefly described. Based on the mechanical aspects of fracture, a General Fracture Model is introduced in order to facilitate the systematic study of fractures. The main features observed in the macroscopic examination of fractures are described, along with the formation mechanisms used to identify and analyze the fracture sequence, initiation sites and relations to mechanical properties. The chapter finishes with a proposed procedure for the examination of fractures at the microscopic level and a description of the main micromechanisms of fracture.

Brittle and Ductile Fractures

In this chapter, the main characteristics of brittle and ductile fractures, caused by single load application, are described. Each section begins with a description of the macroscopic features that allow for identification of brittle and ductile fractures, and continues with the description of the mechanisms of formation of the fractographic features of each type of fracture. Examples of each described feature are given, as well as the particular aspects of brittle and ductile fractures in non-metallic components.

Carbide Precipitation in a Low Alloy Ferritic Steel

the precipitation evolution was studied during aging of 2.25Cr-1Mo steel at 550 °C. The as-received steel was aged in two different ways: the first one was the a continuous isothermal treatment at 550 °C for times up to 1000 h and the second aging was performed using isothermal aging cycles which consisted of aging at 550 °C for 60 min and then water quenched at room temperature. M23C6 and M6C precipitation occurred intragranular and intergranular in both types of aging. The coarsening of the carbides was observed to occur with the increase in aging time. Nevertheless, the growth kinetics coarsening was faster in the case of isothermal aging cycles. The hardness decreased with aging time in both cases; however, it occurred in a shorter aging time for the cyclic aging. Nanoindentation tests indicated the increase of ductility as the aging time increased.

Effect of Precipitation on Cryogenic Toughness in Isothermally Aged Austenitic Stainless Steel

The effect of grain-boundary precipitates on cryogenic impact toughness of two corrosion steels (standard AISI 316 and a steel with a nitrogen additive) is studied. The steels are aged at 600 – 900°C with a hold of up to 1000 min. The KCV impact toughness at –196°C is determined. It is shown that the impact toughness of the nitrogen-containing steel decreases under cooling after the aging at 700 and 800°C more considerably than that of steel 316 after aging at 800 and 900°C. The causes of the embrittlement of the nitrogen-containing steel are determined.