The role of dislocations in the precipitation of M23C6 in Co-Cr-C alloys

An investigation is reported of the mechanisms of M₂₃C₆ precipitation from supersaturated fcc solid solution in two Co-Cr-C alloys of compositions (wt %): Co-25.3 Cr-0.26 C and Co-32.9 Cr-0.18 C. Electron microscope investigations were made with particular reference to the role of dislocations in the nucleation and growth of carbide particles formed on ageing in the range 650 to 800° C. Carbide precipitation occurred initially as a fine homogeneously distributed dispersion of matrix-nucleated particles. Extrinsic stacking faults were nucleated at some of the matrix particles. Stacking faults grew and repeated carbide precipitation occurred on the bounding Frank partials. The fault density increased with ageing time, and various stacking fault interactions were observed. Carbide precipitation occurred also on undissociated dislocations and on Shockley partials.Formerly in the Department of Metallurgy and Materials Science, Imperial College, London, UK.     

Precipitation of vanadium carbides in 0.8% C-13% Mn-1% V austenitic steel

The precipitation mechanism of vanadium carbides in 0.8% C-13% Mn-1% V austenitic steel was investigated by transmission electron microscopy and energy dispersive X-ray spectroscopy techniques. The precipitate size and nucleation sites were observed to differ with different heat-treatment cycles applied. Carbide particles were mainly seen to precipitate on grain boundaries, in the matrix and in association with stacking faults, depending on the ageing conditions. Characteristic stacking fault contrast was observed after ageing at low temperatures and stacking fault precipitation occurred in the samples that were water quenched and aged at high temperatures; whereas samples which were not water quenched showed only local matrix precipitation. Energy dispersive X-ray spectroscopy showed that the carbide particles are rich in vanadium. The observations suggest that the nucleation of these particles on stacking faults depends on the point defect concentration and dislocation density in the matrix, prior to ageing.     

Stress rupture properties and deformation mechanisms of K4750 alloy at the range of 650 °C to 800 °C

The stress rupture properties and deformation mechanisms of K4750 alloy at 650 °C, 700 °C, 750 °C and 800 °C were investigated. As the decrease of temperature and stress, the stress rupture life gradually increased. A Larson-Miller Parameter (LMP) method was used for analyzing the stress rupture life under different conditions. The linear fitting formula between stress (σ) and LMP was derived as σ = 3166.455 − 119.969 × LMP and the fitting coefficient was 0.98. After testing, the dislocation configurations of all stress rupture samples were investigated by transmission electron microscopy (TEM). The temperature and stress had a significant impact on the deformation mechanism, thereby affected the stress rupture life of K4750 alloy. As the increasing stress at a given temperature, the deformation mechanism gradually transformed from Orowan looping to stacking fault shearing. Based on experimental results, the threshold stress at 650 °C, 700 °C, 750 °C and 800 °C for the transition of deformation mechanism was estimated to be about 650 MPa, 530 MPa, 430 MPa and 350 MPa, respectively. Below the threshold stress, γ' phase effectively hindered dislocation motion by Orowan looping mechanism, K4750 alloy had a long stress rupture life. Slightly above the threshold stress, Orowan looping combining stacking fault shearing was the dominant mechanism, the stress rupture life decreased. As the further increase of stress, stacking fault shearing acted as the dominant deformation mechanism, the resistance to dislocation motion decreased rapidly, so the stress rupture life reduced significantly.     

Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Carbide precipitation in low carbon niobium steel containing manganese and chromium

Abstract

Precipitation of NbC inferrite has been examined in a low C (0·03 wt-%) steel in which the austenite-ferrite transformation kinetics has been slowed down substantially by the combined addition of 2%Mn and 4%Cr. This has enabled the carbide precipitation sequence at 700°C to be defined more precisely, starting with precipitation on dislocations, followed by interphase precipitation and, finally, carbide free ferrite. At 650°C the formation at the γ/α interface of V-shaped carbides and also the growth of carbide fibres have been related to the morphology of the interfaces. The retardation of the γ–α transformation by alloying additions encouraged the growth of these alternative carbide morphologies.

MST/1158     

The important role of titanium microalloying in refining austenite grain of heavy-duty H-beam steel during rough rolling produced by a new technology

A new technology of austenite grain refinement, fine austenite enhanced ferrite transformation, is proposed for heavy-duty hot-rolled H-beam steels in this work. Titanium microalloying is very important and necessary for the new technology. The effect of titanium on the prior austenite grain size of steels during simulated rough rolling was investigated. The results show that the prior austenite grain sizes of specimens with titanium and niobium elements are much finer than those of specimens with niobium but without titanium deformed at the same parameters. For the alloying composition of studied steels, titanium nitride particle maybe precipitated in specimens with titanium at above 1,200 °C, however, niobium carbide particles can't form in specimens without titanium at above 1,150 °C. The thermodynamically stable titanium nitride particles can impede the grain growth at high temperature for example furnace heating before rough rolling and bring the epitaxial growth of niobium carbide on pre-existing themselves which induces a large number of titanium nitride-niobium carbide composite precipitates. These fine precipitates can pin austenite grain boundaries effectively and ensure austenite grain refinement.     

Phase composition of 1.5Cr—1.0Mo—0.3V rotor steel as a factor of cooling rate from austenitic region

Carbide reactions in 1.5Cr—1.0Mo—0.3V steel are investigated at various thermal treatments at 550–650°C. The cooling rate control was carried out by cooling the environment. Metal tempering was conducted in the range 650–720°C for up to 100 h. It is shown that the rate of preliminary cooling from the austenitic region has the principal effect on the carbide reaction nature. At rapid cooling (martensite structure) the major carbide reaction during tempering and ageing is M₃C → M₇C₃ whereas for cooling rate decreasing the reaction M₃C → M₂₃C₆ prevails. Tempering and ageing change the quantitative characteristics of the carbide state without having an effect on the major mechanism of transformation. The carbide reaction character is compared with metal properties under the same time—temperature conditions and the correlation between the structural state and metal service properties is examined.     

Stress and dislocations in diamond–SiC composites sintered at high pressure,high temperature conditions

Diamond–silicon carbide composites were sintered at 10 GPa and three different temperatures: 1600, 1800, and 2000 °C. Distributions of residual surface stresses in diamond crystals were obtained by the analysis of Raman band shifts and splitting. It was noted that stresses concentrate around points of contacts between diamond crystals. Average stress increase with increasing sintering temperature. Complementary information on average sizes of crystallites, concentration of stacking faults, and population of dislocations in both diamond and SiC were obtained from X-ray diffraction profile analysis. It was observed that for both diamond and silicon carbide phases the average crystallite sizes decrease. The population of dislocations in the diamond phase increases with increasing sintering temperature and the population fluctuates in the SiC phase. Concentration of stacking faults was significant only in SiC.     

Dynamic recrystallisation during hot working of Zr–2·5Nb: characterisation using processing maps

Abstract

The characteristics of the hot deformation of Zr–2·5Nb (wt-%) in the temperature range 650–950°C and in the strain rate range 0·001–100 s^?1 have been studied using hot compression testing. Two different preform microstructures: equiaxed (α+β) and β transformed, have been investigated. For this study, the approach of processing maps has been adopted and their interpretation carried out using the dynamic materials model. The efficiency of power dissipation given by [2m/(m+1)], where m is the strain rate sensitivity, is plotted as a function of temperature and strain rate to obtain a processing map. A domain of dynamic recrystallisation has been identified in the maps of equiaxed (α+β) and β transformed preforms. In the case of equiaxed (α+β), the stress–strain curves are steady state and the dynamic recrystallisation domain in the map occurs with a peak efficiency of 45% at 850°C and 0·001 s^?1. On the other hand, the β transformed preform exhibits stress–strain curves with continuous flow softening. The corresponding processing map shows a domain of dynamic recrystallisation occurring by the shearing of α platelets followed by globularisation with a peak efficiency of 54% at 750°C and 0·001 s^?1. The characteristics of dynamic recrystallisation are analysed on the basis of a simple model which considers the rates of nucleation and growth of recrystallised grains. Calculations show that these two rates are nearly equal and that the nucleation of dynamic recrystallisation is essentially controlled by mechanical recovery involving the cross-slip of screw dislocations. Analysis of flow instabilities using a continuum criterion revealed that Zr–2·5Nb exhibits flow localisation at temperatures lower than 700°C and strain rates higher than 1 s^?1.

MST/3103     

Coarsening behavior for M23C6 carbide in 12 %Cr-reduced activation ferrite/martensite steel: experimental study combined with DICTRA simulation

Based on the multi-component aspects of thermodynamics and diffusion, coarsening behavior of M₂₃C₆ (M = Cr, Fe, W) carbide at 650 °C in 12 %Cr-reduced activation ferrite/martensite steel has been investigated experimentally using scanning transmission electron microscopy, combined with DICTRA simulation. Both the experimental measurements as well as the simulations indicate that the interfacial energy of M₂₃C₆ carbide in this steel at 650 °C is probably 0.5 J m^?2, and the coarsening rate of M₂₃C₆ carbide is very low. The influence of a change in Mn, V, and Ta content and temperature on the coarsening rate of M₂₃C₆ carbide is also investigated. The results show that the coarsening rate is increased by adding Mn and reduced by V and Ta addition, respectively, while an increase in the coarsening rate by an order of magnitude with increasing temperature per 50 °C between 600 and 750 °C. Precipitation of Laves (Fe₂W) phase during aging has a negligible effect on the coarsening of M₂₃C₆.     

Microstructural evolution of ASTM grade 91 after long-term creep testing

Modified 9Cr–1Mo steel (ASTM grade P/T91) is conventionally applied in the construction of high efficiency supercritical and ultra-supercritical boilers, reaching metal temperatures up to 600–610 °C. As a part of the characterisation process of commercial pipes, Tenaris has performed over the years several long-term creep tests, with duration up to over 100,000 h, for temperatures between 550 and 650 °C. Microstructural characterisation has been performed on broken specimens, including analysis of the state of precipitation after TEM imaging of extraction replicas. The evolution of M₂₃C₆ coarse carbides, and MX carbo-nitrides, as well as the nucleation of new phases, such as Laves and Z are described in this paper. Observations reveal Laves phase nucleation and growth peak at 600 °C, whereas almost no particles were observed after ageing at higher temperature; moreover, transformation of MX carbo-nitrides into Z-phase is also faster at 600 °C, and slows at higher temperature.     

Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

Abstract

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s^-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ～790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s^-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ～700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ～225 kJ mol^-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ～0.1 s^-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s^-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s^-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s^-1 and temperatures above 700°C, the material exhibits flow localisation.     

A Study on the Effect of Thermal Ageing on the Specific-Heat Characteristics of 9Cr–1Mo–0.1C (mass%) Steel

The effect of thermal ageing on the heat-capacity behavior of 9Cr–1Mo–0.1C (mass%) ferritic/martensitic steel has been studied using differential scanning calorimetry (DSC) in the temperature range from 473 K to 1,273 K. The DSC results in the case of slow cooled, normalized and tempered, and subsequently thermally aged samples (500 h to 5,000 h at 823 K (550 °C) and 923 K (650 °C), clearly marked the presence of both magnetic and α-ferrite + carbide → γ-austenite phase transformations that take place successively upon heating. Furthermore, for the case of fully martensitic microstructure, an additional exothermic transformation at about 920 K(647 °C), arising from carbide precipitation is noticed. This event is characterized by a sharp drop in C _P . It is found that the α-ferrite + carbide → γ-austenite phase transformation temperature is only mildly sensitive to microstructural details, but the enthalpy change associated with this phase transformation, and especially the change in specific heat around the transformation regime, are found to be dependent on the starting microstructure generated by thermal ageing treatment. Prolonged ageing for about 500 h to 5,000 h in the temperature range from 823 K to 923 K (550 °C to 650 °C) contributed to a decrease in heat capacity, as compared to the normalized and tempered sample. This is due to the increase in carbide volume fraction. The martensitic microstructure is found to possess the lowest room-temperature C _P among different microstructures.     

Effect of rolling conditions on the structure and shape memory properties of Fe–Mn–Si alloys

Lattice defects play an important role in controlling the γ → ε martensitic transformation in shape memory ferrous alloys. This work focuses on the relation between various rolling and annealing processes, the microstructure resulting from the processes, and strain recovery of two Fe–Mn–Si alloys with different stacking fault energies (SFEs). Rolling experiments, conducted over a temperature range from 20 °C to 1000 °C, produce quite different microstructures, which vary from a high dislocation density to a structure containing only few isolated dislocations. In addition, annealing temperature has a very important influence not only on the dislocation arrays but also on the stacking faults remaining in the austenite, whose density depends on the SFE value for the alloy. Within the framework of the processing parameters selected for this work, i.e. roll speed, rolling reductions, processing temperatures and schedules, rolling at intermediate temperatures and annealing at a temperature of 650 °C seem to be the most appropriate methods to obtain a microstructure favorable for a nearly full degree of shape recovery.     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Small-angle X-ray scattering study of precipitates in AlZnMgCu-1.0 wt.% Li alloy

The precipitate size, distribution and volume fraction in an AlZnMgCu-1.0 wt.% Li alloys during various aging conditions were investigated by small-angle X-ray scattering and transmission electron microscopy. According to the selected area diffraction patterns, the prime precipitates in this alloy are η′ phases, not δ′ phases. During the temperature ranging from 120 to 160 °C, the growing of precipitates in the 7055-1.0 wt.% alloys is not apparent. From 120 to 150 °C, the varieties of precipitate volume fraction depend on the competition between dissolution rate of GP zones and nucleation rate of η′. At 160 °C, the growing of η′ precipitates is undergoing a Lifshitz–Slyozov–Wagner coarsening process, and the prime thermodynamic reaction is the nucleation growth of η′ precipitates.     

Partially pyrolyzed composites with basalt fibres – Mechanical properties at laboratory and elevated temperatures

Mechanical properties of partially pyrolyzed at 650 °C or 750 °C unidirectional basalt fibre composites with polysiloxane matrix were studied at laboratory and elevated temperatures. Ten pyrolysis processes differing mutually in heating courses and ultimate temperatures were compared. The material treated at 650 °C revealed at laboratory temperature flexural strength around 850 MPa. Fracture toughness of this material exceeded that of the cured only (at 250 °C) and treated at 750 °C composites. However, the composite pyrolyzed at 750 °C is more suitable for applications at elevated temperatures because of its slower degradation in hot air.     

Effects of cooling process on microstructure and hardness for 1·0C–1·5Cr bearing steel

Effects of cooling rate (V_cr) and final cooling temperature (T_ft), after hot deformation, on microstructure and hardness for 1·0C–1·5Cr bearing steel were investigated. The results show that if V_cr increases from 2 to 25°C s^?1 and T_ft remains at 650°C, pearlite colony size and grain size both decrease, hardness increases. When V_cr exceeds 8°C s^?1, carbide network can be restrained effectively. TEM micrographs indicate that there exist branches in the local region of lamellar cementite and ferrite, and a ferrite thin film is also found around the proeutectoid carbide. Under the cooling rate of 10°C s^?1, with the increase in T_ft, the microstructure changes from martensite into pearlite, carbide network becomes more serious and hardness decreases.     

The effects of ruthenium additions on tensile deformation mechanisms of single crystal superalloys at different temperatures

The tensile behavior of two experimental nickel-base single crystal superalloys has been studied from room temperature to 1100 °C. Emphasis is placed on elucidating the effects of ruthenium (Ru) additions on the deformation mechanisms using transmission electron microscopy (TEM). Furthermore, the partitioning behavior of the alloy elements between the γ and γ′ phases for both experimental alloys has been studied using three-dimensional atom probe (3DAP). Detailed analysis demonstrates that at low and medium temperature ranges, the stacking faults present in the γ matrix of the 3Ru alloy but no trace of stacking fault in the γ matrix of the 0Ru alloy have been observed; during high temperature range, as a result of Ru additions, the γ/γ′ interfacial dislocation space of the 3Ru alloy is smaller than that of the 0Ru alloy due to further decreasing the lattice misfit. Apart from that, Ru additions result in more Re partitioning to the γ′ phase, and thus the solution strengthening for the γ phase is decreasing. Thus, during tests below and at the temperature corresponding to the peak strength, the yield strength of the 3Ru alloy is lower than that of the 0Ru alloy. At last, in the light of the TEM observations, the changing trends of the stacking fault energy in the γ matrix and the transformation points (the temperature related to the stacking faults formation) for the two experimental alloys have been drawn. The temperature range of the stacking faults formation in the γ matrix is expanded after Ru additions. The energy conditions of the stacking faults formation of the 0Ru and 3Ru alloys have been analyzed in detail. The changing of lattice misfit with temperature can be considered as one of the principal reasons for the stacking faults formation.     

Precipitation hardening in Inconel* 625

Abstract

The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760°C for different intervals of time ranging from 1 to 335 h, has been measured. Peak hardening is found to occur much earlier at 760°C than at 700°C. Also the peak hardness is higher at 700°C than at 760°C. The results have been discussed in terms of precipitation. Scanning electron microscopy revealed the presence of precipitates in specimens aged at 760°C for a longer time. Electron probe microanalysis results show these precipitates to be rich in Ni, Nb, and Mo indicating that these are γ″ precipitates of Ni₃ (Nb, Mo) type. Transmission electron microscopy confirmed that these are γ″ precipitates. It also suggests that nucleation takes place heterogeneously on dislocations and stacking faults. Longer aging causes somewhat uniform nucleation but still preferentially on the secondary defects. At 700°C γ′ precipitates have been observed in addition to γ″ precipitates. The orientation relationship between the precipitates and the matrix has also been determined.