Microstructure and Mechanical Properties of Austenitic Stainless Steels after Dynamic and Post‐Dynamic Recrystallization Treatment

The effects of dynamic and post‐dynamic recrystallization (DRX and post‐DRX) on the microstructure and mechanical properties of austenitic stainless steels are critically reviewed. Particularly, the paper is focused on the grain refinement and strengthening by large strain deformation including severe plastic deformation conditions. The DRX and post‐DRX microstructures are considered with close relation to the operative recrystallization mechanisms. Specific emphasis is placed upon two recrystallization mechanisms, that is, discontinuous and continuous, and their dependence on the deformation/annealing conditions. The relationships between DRX microstructures and processing conditions are summarized and their effect on post‐DRX behavior is clarified. The structural strengthening mechanisms including the grain size and the dislocation density are elaborated.

     

Multi‐agent distributed intelligent system based on fuzzy decision making

Generally decision making for solving ill‐structured problems in DSS takes place in uncertain situations. The main drawbacks of existing traditional DSS are inefficiencies associated with dealing with complex models and large databases. Usually a fuzzy DSS has many input variables and, hence, its knowledge base, containing the totality of fuzzy rules, is very large. Large rule base leads to disadvantages in speed, reliability, and complexity of DSS. This paper introduces an alternative concept for designing fuzzy DSS based on multi‐agent distributed artificial intelligent technology and fuzzy decision making. The main idea of the proposed DSS is based on granulation of the overall system intelligence between cooperative autonomous intelligent agents capable of competing and cooperating with each other in order to propose a total solution to the problem and organization (combining individual solutions) of the proposed solution into the final solution. It is supposed that every agent in DSS is characterized by a set of fuzzy criteria of unequal importance and definition of a “winner” agent is based on multi‐criteria fuzzy decision making involving unequal objectives. © 2000 John Wiley & Sons, Inc.     

New grain formation during warm deformation of ferritic stainless steel

Microstructural evolution accompanied by localization of plastic flow was studied in compression of a ferritic stainless steel with high stacking fault energy (SFE) at 873 K (≈0.5 Tm). The structure evolution is characterized by the formation of dense dislocation walls at low strains and subsequently of microbands and their clusters at moderate strains, followed by the evolution of fragmented structure inside the clusters of microbands at high strains. The misorientations of the fragmented boundaries and the fraction of high-angle grain boundaries increase substantially with increasing strain. Finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The mechanisms operating during such structure changes are discussed in detail.     

Effect of Liquid Hot Isostatic Pressing on Structure and Mechanical Properties of a Sand-Cast A356.02 Alloy

The effect of liquid hot isostatic pressing (LHIP) on the structure and mechanical properties of an A356.02 alloy was examined. It was shown that LHIP provides a porosity decrease from ~0.9 to 0.2 pct due to the elimination of shrinkage voids. As a result, the yield stress (YS) increases from 200 to ~230 MPa, the ultimate tensile strength (UTS) increases from ~275 to 310 MPa, the total elongation increases from ~4 to ~7.5 pct, and the fatigue strength increases from ~88 to ~140 MPa. It was found that the sequence of LHIP and homogenization annealing highly affects the hardness and variability in fatigue strength. LHIP followed by homogenization annealing provides the lowest scattering of fatigue strength and, therefore, the fabrication of the most reliable casting components.     

Effect of Co on Creep Behavior of a P911?Steel

The microstructure and creep behavior of a 3 pct Co modified P911 steel and standard P911 steel were examined. It was shown that the nanoscale M₂₃C₆ carbides and MX carbonitrides in the 3 pct Co modified P911 steel are not susceptible to significant coarsening under creep conditions. Also, coarsening simulations of M₂₃C₆ particles were performed for both steels. The rates of lath and particle coarsening in the P911 + 3 pct Co steel are remarkably lower than those in the P911. Increased stability of a tempered martensite lath structure in the 3 pct Co modified P911 steel provides enhanced creep resistance at an exceptionally high temperature of 923 K (650 °C).     

Development of electroconductive polyacrylonitrile fibers through chemical metallization and galvanisation

The paper describes the modification of PAN fibres with metallic nickel in order to give the fibre electroconductive properties. This is achieved by two consecutively executed processes. The first process is chemical metallization, involving the immersion of the PAN fibres in a bath containing 0.5 mol l⁻¹ NiCl₂ and 1.5 mol l⁻¹ Rongalite at pH = 5.5 and T=288–293 K. Ni(II) is absorbed in the structure of the fibre and adsorbed at the surface through a complex reaction with the cyanide and carboxylic acid groups present in the PAN chemical structure and the Rongalite reduces the adsorbed Ni(II) to metallic Ni. The second process is galvanisation of the Ni-containing PAN surface (the Ni particles present at the surface form a “seed” layer) in an electrolyte bath using an electrolyte with 150 g l⁻¹ of NiSO₄ at pH 5–6 and room temperature. The resulting fibre showed a specific electrical resistance of the order of 10⁻⁶ Ω m, which is a promising result.