Effect of dynamic strain ageing on the tensile properties of a modified 9Cr-1Mo steel
TL;DR: In this article, a modified 9Cr-1Mo steel in two microstructural conditions (710T and 550T) was tested in the temperature range 25-450°C, under strain rates of 2.3×10-5-2.
Abstract: Tensile testing of a modified 9Cr–1Mo steel in two microstructural conditions (710T– normalized at 1100°C, tempered at 710°C, and 550T – normalized at 1100°C tempered at 550°C) in the temperature range 25–450°C, under strain rates of 2.3×10-5–2.3×10-3s-1, exhibited serrated flow curves, with serrations appearing almost at the onset of deformation and disappearing before ultimate strengths were attained. The serrated flow curves (characteristics of dynamic strain ageing) were accompanied by increased ultimate strengths, loss of ductility and negative strain-rate sensitivity, relative to the ambient temperature properties. However, the increase in ultimate strength and the reduction in ductility were much larger for 710T specimens, as compared to 550T ones. In the dynamic strain ageing regime, the work-hardenability of 710T specimens increased rapidly while that of 550T specimens remained practically unaffected. Based on the microstructural consideration and the observed activation energy of 45 kJ mol-1, it is proposed that serrations are initiated by a nitrogen atmosphere formation on the waiting dislocations by a pipe diffusion mechanism, and they disappear by diffusion to the precipitate sinks during deformation. Because the fine alloy carbide precipitates in 550T specimens are more effective sinks than those of 710T ones, they can cause much faster depletion of the atmosphere, resulting in a much smaller effect of dynamic strain ageing on the tensile properties of 550T specimens.
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15 Feb 2008-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: The tensile data involving austenitic Alloy C-276 suggest that this alloy is capable of maintaining appreciable structural strength at temperatures relevant to the sulfuric acid decomposition process related to the nuclear hydrogen initiative.
Abstract: The tensile data involving austenitic Alloy C-276 suggest that this alloy is capable of maintaining appreciable structural strength at temperatures relevant to the sulfuric acid decomposition process related to the nuclear hydrogen initiative. Reduced failure strain and formation of serrations, characteristics of dynamic strain ageing (DSA), were noted within susceptible temperature regimes. An average activation energy of 55 kJ/mol, and work hardening index ranging from 0.68 to 0.75 were determined as functions of different testing temperature and strain rates. The occurrence of DSA was also associated with enhanced dislocation density. Depending on the testing temperature, a combination of ductile and intergranular brittle failures was observed with the tested specimens.
56 citations
TL;DR: The authors suppress DSA via a unique microstructure obtained using additive manufacturing and propose a new dislocation-arrest model in nickel superalloys to elucidate the criterion for DSA to occur or to be absent as a competition between dislocation pipe diffusion and carbide–carbon reactions.
Abstract: Dynamic strain aging (DSA), observed macroscopically as serrated plastic flow, has long been seen in nickel-base superalloys when plastically deformed at elevated temperatures. Here we report the absence of DSA in Inconel 625 made by additive manufacturing (AM) at temperatures and strain rates where DSA is present in its conventionally processed counterpart. This absence is attributed to the unique AM microstructure of finely dispersed secondary phases (carbides, N-rich phases, and Laves phase) and textured grains. Based on experimental observations, we propose a dislocation-arrest model to elucidate the criterion for DSA to occur or to be absent as a competition between dislocation pipe diffusion and carbide–carbon reactions. With in situ neutron diffraction studies of lattice strain evolution, our findings provide a new perspective for mesoscale understanding of dislocation–solute interactions and their impact on work-hardening behaviors in high-temperature alloys, and have important implications for tailoring thermomechanical properties by microstructure control via AM. Detrimental serrated plastic flow via dynamic strain aging (DSA) in conventionally processed nickel superalloys usually occurs during high temperature deformation. Here, the authors suppress DSA via a unique microstructure obtained using additive manufacturing and propose a new dislocation-arrest model in nickel superalloys.
55 citations
01 Mar 2013-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the authors examined serrated flow behavior in 9Cr-1Mo ferritic steel with different types of serrations, namely type A, B and C serrations at intermediate temperatures, and found that serrations were strongly dependent on temperature and applied strain rate.
Abstract: Tensile tests were performed on specimens in normalised and tempered condition at temperatures ranging from 300 to 873 K and at four strain rates in the range 6.33×10 −5 to 6.33×10 −3 s −1 to examine serrated flow behaviour in 9Cr–1Mo ferritic steel. At all strain rates, the steel exhibited different types of serrations namely type A, B and C serrations at intermediate temperatures, and the nature and type of serrations were strongly dependent on temperature and applied strain rate. Serrations were observed only after a specimen was deformed beyond a critical plastic strain. Critical strain for type A and A+B serrations decreases with increase in temperature and decrease in strain rate. Inverse temperature dependence of critical strain for type C serrations was observed. The activation energy of 86 kJ mol −1 obtained for serrated flow suggested that diffusion of an interstitial solute such as carbon is responsible for dynamic strain ageing in 9Cr–1Mo steel.
54 citations
05 Jan 2015-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
Abstract: Tensile behavior of modified 9Cr–1Mo steel was studied over the temperature range from room temperature (RT) to 600 °C at three strain rates 10 −5 , 10 −4 and 10 −3 s −1 Serrated plastic flow was observed from 250 to 350 °C at strain rate of 10 −4 s −1 , signifying occurrence of dynamic strain ageing (DSA) Characteristic features of DSA such as plateau/peak in yield and tensile strength, minima in ductility, negative strain rate sensitivity, and peak in internal friction were also observed Activation energy for DSA was found equivalent to that for diffusion of nitrogen, hence nitrogen was considered to cause DSA Dislocation substructure in the region of DSA revealed dislocation debris, kinks, bowing of dislocations and high density of dislocations Irrespective of temperature from 200 to 450 °C there was formation of dislocation cell structure; however, cell size in the region of DSA was smallest Fractographic analysis showed rosette type fracture at RT resulting from longitudinal splitting It was associated with decohesion of interface of carbide particles and prior austenite grain boundaries Non uniform and shallow dimples were observed in fibrous zones in the region of DSA Contours of dual shear lip zone were observed at elevated temperatures from 200 to 450 °C
53 citations
08 Sep 2014-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the low cycle fatigue (LCF) behavior of China Low Activation Martensitic (CLAM) steel is studied under fully reversed tension-compression loading at 823 K in air.
Abstract: China Low Activation Martensitic (CLAM) steel is considered to be the main candidate material for the first wall components of future fusion reactors in China. In this paper, the low cycle fatigue (LCF) behavior of CLAM steel is studied under fully reversed tension–compression loading at 823 K in air. Total strain amplitude was controlled from 0.14% to 1.8% with a constant strain rate of 2.4×10 −3 s −1 . The corresponding plastic strain amplitude ranged from 0.023% to 1.613%. The CLAM steel displayed continuous softening to failure at 823 K. The relationship between strain, stress and fatigue life was obtained using the parameters obtained from fatigue tests. The LCF properties of CLAM steel at 823 K followed Coffin–Manson relationship. Furthermore, irregular serration was observed on the stress–strain hysteresis loops of CLAM steel tested with the total strain amplitude of 0.45–1.8%, which was attributed to the dynamic strain aging (DSA) effect. During continuous cyclic deformation, the microstructure and precipitate distribution of CLAM steel changed gradually. Many tempered martensitic laths were decomposed into subgrains, and the size and number of M 23 C 6 carbide and MX carbonitride precipitates decreased with the increase of total strain amplitude. The response cyclic stress promoted the recovery of martensitic lath, while the thermal activation mainly played an important role on the growth of precipitates in CLAM steel at 823 K. In order to have a better understanding of high-temperature LCF behavior, the potential mechanisms controlling stress–strain response, DSA phenomenon and microstructure changes have also been evaluated.
27 citations
References
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TL;DR: In this article, the rate sensitivity of the flow stress was investigated as a function of pre-strain for a commercial nickel alloy (INCONEL 600) and for polycrystalline Al-1% Mg. over a wide temperature range encompassing the regime of jerky flow.
517 citations
Book•
01 Sep 1991
TL;DR: The Atlas of Time Temperature Diagrams for Nonferrous Alloys as mentioned in this paper is the most comprehensive collection of time-temperature diagrams for irons and steels ever collected, containing both commonly used curves and out-of-print and difficult-to-find data.
Abstract: The most comprehensive collection of time-temperature diagrams for irons and steels ever collected. Between this volume and its companion, Atlas of Time Temperature Diagrams for Nonferrous Alloys, you'll find the most comprehensive collection of time-temperature diagrams ever collected. Containing both commonly used curves and out-of-print and difficult-to-find data, these Atlases represent an outstanding worldwide effort, with contributions from experts in 14 countries. Time-temperature diagrams show how metals respond to heating and cooling, allowing you to predict the behavior and know beforehand the sequence of heating and cooling steps to develop the desired properties. These collections are a valuable resource for any materials engineer Both Collections Include: Easy-to-Read Diagrams Isothermal transformation Continuous cooling transformation Time-temperature precipitation Time-temperature embrittlement Time-temperature ordering Materials Included in the Irons and Steels Volume: Low-carbon High Strength Low Alloy Stainless (Maraging, austenitic, ferritic, duplex) Chromium, molybdenum, vanadium, silicon Structural Quenched and tempered Spring and Rail High-temperature creep-resistant Tool and die Eutectoid, hypereutectoid carbon Deep hardening Titanium bearing Irons: Gray cast, malleable, white, white cast, ductile.
167 citations
TL;DR: The activation energy for strain aging was found to be 0.57 to 0.68 eV which is consistent with a carbon atmosphere aging mechanism as mentioned in this paper, where stress decrement ( σ D ) was used as a criterion for determining the activation energy.
116 citations
TL;DR: In this article, the authors investigated the dynamic strain-ageing behavior of a nuclear structural steel, designated ASTM A203 grade D, in tempered martensitic and ferritic-pearlitic microstructural conditions.
35 citations