The effects of austenite phase deformation on microstructure and magnetic properties in Fe–13.4%Mn–5.2%Mo alloy

The effects of austenite phase deformation on martensitic transformations and magnetic properties in Fe–13.4%Mn–5.2%Mo have been investigated by scanning electron microscopy, transmission electron microscopy, and M?ssbauer Spectroscopy. The increase of plastic deformation rates on austenite phase created considerable changes in amounts of ε (h.c.p.) and α′(b.c.c.) martensite, and austenite grains size decreased. Analysis of microstructure and M?ssbauer spectra show that the amount of ε martensite increased at low deformation rates whereas it decreased at high deformation rate. Besides, M?ssbauer spectra of the alloy reveal a ferromagnetic character with a broad sextet for α′ martensite phase and a paramagnetic character with a singlet for the γ (f.c.c.) austenite and ε martensite phases. In the other hand, the magnetic character of the alloy exhibits a different magnetic order depending on strain rates.     

Effects of high magnetic fields on solidification microstructure of Al–Si alloys

The effects of high magnetic fields on the solidification microstructure of Al–Si alloys were investigated. Al–7.2 wt%Si and Al–11.8 wt%Si alloys were solidified in various high magnetic fields at different cooling rates. The secondary dendrite arm spacing (SDAS) of the primary Al dendrites and the lamellar spacing (LS) of the eutectics were measured. It was found that the application of a high magnetic field could decrease the SDAS of the primary Al dendrites in Al–7.2 wt%Si alloys and the LS of the eutectics in Al–11.8 wt%Si alloys. The effects of the high magnetic field on the SDAS decreased with increasing cooling rate. The decrease in the SDAS and LS can be attributed to the decrease of the solute diffusivity in the liquid ahead of the solid/liquid interface during the growth of the dendrite and eutectic. This decrease is caused by the high magnetic field which can damp the convection and avoid its contributions to the diffusion.     

Effect of high temperature exposure on the microstructure of Udimet-500 super alloy

Udimet-500 alloy is a high-strength nickel-bases super alloy used for turbine blades applications. It is primarily strengthened by very fine precipitates called gamma prime (γ′) which coarsened with time when exposed to high temperature. In this study, Udimet-500 alloy in the standard aged condition was exposed at 850–1100 °C for 25–100 h in air. The change in the size of γ′ with respect to the high temperature exposure was characterized using scanning electron microscope. A systematic coarsening of the γ′ was observed with the change in the exposure temperature/time. The γ′ size in the virgin sample was 0.1 μm while after high temperature exposures it coarsened to 1 μm. The Larson-Miller parameter (LMP) of the high temperature exposure was also measured. It was found that the LMP had power relation with the γ′ size. In addition, degradation of the primary as well as secondary carbides was also observed which can be used as add-on microstructural information in high temperature exposure. Elevated temperature exposure (≥1000 °C) also lead to (a) near surface de-alloying, (b) precipitation of needle-like Ti-rich phase and (c) depletion of γ′. It may contribute in the accelerated corrosion and loss of strength of alloy.     

Microstructure and stress rupture properties of single crystal superalloy CMSX-2 under high thermal gradient directional solidification

The interface morphologies of single crystal superalloy CMSX-2 were studied over a range of cooling rate with large variations in withdrawal speeds in directional solidification. A superfine cellular structure was obtained under both high thermal gradient up to 1000 K/cm and fast withdrawal rate up to 1 mm/s. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of γ′ phase, reduced microsegregation of alloying elements and smaller size of γ-γ′ eutectics. The rupture life and plasticity of fine structure samples produced in high thermal gradient directional solidification increase significantly than that in conventional directional solidification process at 1323 K.     

Formation mechanism and coarsening behavior of fan-type structures in a new Ni–Cr–Co-based powder metallurgy superalloy

The formation mechanism and coarsening behavior of fan-type structures in a new Ni–Cr–Co-based powder metallurgy superalloy were investigated by means of field scanning electron microscope, transmission electron microscope, electron backscattered diffraction, and differential scanning calorimetry. The results show that the fan-type structures consist of finger-shaped γ′ dendrites and γ matrix between them. They nucleate in the chemical segregation regions on grain boundaries and grow by diffusion. There are three types of solute atoms flow: (a) rapid diffusion along grain boundary; (b) the diffusion from supersaturated γ matrix to fan-type γ′ phases; and (c) short-distance diffusion from the previous formed γ′ phases at high temperature to γ′ phases formed at low temperature within the branches of fan-type structures. These γ′ dendrites are perpendicular to grain boundaries and grow asymmetrically, resulting in grain boundary serration. In addition, the fan-type structures coarsen within the γ′ depletion zone after the standard aging treatment.     

Thermal shock behaviour of a kaolinte refractory prepared using a natural organic binder

The thermal shock resistance of a kaolinite-based refractory prepared with a binder derived from a plant (corchorus olitorius) was investigated. Thermal shock tests employing water-quenching technique were performed at quench temperature differences of between 80 and 580 °C and then the damage parameter, and thermal shock parameters (R, R′, and R′′′) evaluated using measured strength and modulus values. The results showed that R and R′ parameters increased with increasing binder concentrations, an indication that the damage due to crack initiation was progressively impaired with increased binder concentrations. The R′′′ parameter on the other hand decreased with increasing binder concentrations indicating poor resistance of the material to crack propagation. These observations were also matched with thermal shock results which showed that samples plasticized with higher binder concentration had relatively very high strengths before thermal shock, compared to those plasticized with plain water, but they experienced rather large strength losses (over 60% of their initial values) at quench temperature difference (ΔT) exceeding 325 °C. The critical temperature difference (ΔT _C) for the samples tested was in the range 250–325 °C. Scanning electron microscope (SEM) micrographs of samples quenched at temperature difference of 580 °C showed that samples plasticized with high binder concentration (0.68) experienced severe cracking of the matrix compared to their counterparts plasticized with plain water, whose microstructures showed an inhibited crack propagation.     

Effect of very high temperature short exposures on the dissolution of the γ′ phase in single crystal MC2 superalloy

Time-temperature dependence of the γ′ phase volume fraction was investigated for a second generation single crystal nickel-based superalloy exposed to very short high temperature regimes (1,100–1,200 °C). In this temperature range, the dissolution of the strengthening γ′ phase occurs. Evolution of the γ′ volume fraction in transient regimes has been established for each temperature and activation energy of the dissolution phenomenon were determined. They directly attest from the activity of the diffusing species involved during this phenomenon. From these analyses, the volume fraction at equilibrium was established for the entire temperature range where dissolution occurs. A model, based on a time/temperature equivalence, is proposed to quantify the γ′ volume fraction dissolved during short high temperature exposure.     

Unidirectional solidification of Ni-Mo-Al eutectic alloys

The composition triangle of the Ni-Mo-Al ternary system contains a monovariant trough between theγ′ (Ni₃Al)-α(Mo) pseudobinary eutectic and a guessedγ(Ni)-γ′ (Ni₃Al)-α(Mo) ternary eutectic. Alloys with compositions on this trough were directionally solidified at various growth rates. The microstructure of the alloys consists of fine Mo fibres of rectangular cross-section in an Ni₃Al/Ni matrix. The determined crystallographic relationship does not correspond to an interface of low lattice mismatch. The eutectic trough strongly extends towards increasing Mo contents so that the volume fraction of the Mo fibres varies between 18 and 25% dependent on composition. It was not possible to determine the position of a ternary eutecticγ-γ′-α because the liquidus temperatures along the trough are almost equal and the distribution of the three phases is strongly dependent on the growth rate.     

Effect of Solidification Parameters on the Microstructure and Creep Property of a Single Crystal Ni-base Superalloy

A single crystal Ni-base superalloy was processed with withdrawal rates between 2 and 7 mm/min.The ascast microstructures,heat treatment response and creep property have been characterized as a function of the withdrawal rate.As expected,the primary and secondary dendrite arm spacing decreased with increasing withdrawal rate;microsegregation degree and porosity distribution were also varied with different withdrawal rates.The withdrawal rate of 2 mm/min resulted in a noticeable residual microsegregation eve...     

Effect of magnesium content on thermal and structural parameters of Al–Mg alloys directionally solidified

In this study, the influence of magnesium content on thermal and structural parameters during the unsteady-state unidirectional solidification of Al–Mg alloys is analyzed. Using a special device, Al–Mg alloys containing 5, 10, and 15 wt% Mg were submitted to unidirectional solidification. Using a data acquisition system, the temperature variations along the casting during solidification were measured. From these results, the variations of solidification parameters as growth rate of dendrite tips, thermal gradient, cooling rate, and local solidification time were determined. The variation of global heat transfer coefficient at metal/mould interface was estimated through the adjustment of experimental temperature variation close to the interface and numerical predictions. Primary and secondary dendrite arms spacing variations during solidification were measured by optical microscopy. From these results, comparative analysis were developed to determine the influence of magnesium content.     

Effect of Heat Treatment on the Specific Heat Capacity of Nickel-Based Alloys

Alloy-718 and Udimet alloy 720 are gamma prime strengthened superalloys with excellent mechanical and thermal properties at elevated temperatures, as well as at cryogenic temperatures. The nickel-based alloys were improved to be resistant to creep and become stronger by changing the heat-treatment conditions. The measurement of the specific heat capacity of a nickel-based alloy is a very useful tool to investigate the effect of heat treatment. The specific heat capacity of nickel-based alloys Alloy-718 and Udimet alloy 720 were measured using a differential scanning calorimeter in the temperature range of 100 – 1000 K. The specific heat capacity of the nickel-based alloys increases monotonically with temperature; however, above 800 K, it is strongly dependent on the heat treatment conditions and it is thought to be influenced by the precipitation phase (γ′, γ′′). Optical and scanning electron microscopies are used to investigate the microstructure of the phases. The microstructures of the precipitates are examined.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Gamma prime (γ′) precipitating and ageing behaviours in two newly developed nickel-base superalloys

Two cast nickel-base superalloys with superior creep rupture lives at high temperatures and high stresses were developed. In the present study, the microstructural characteristics of γ′-precipitates and the hardening behaviour were studied by electron microscopy and micro-hardness testing. The alloys were solution-treated at 1553 K for 2 h and air-cooled followed by step ageing heat treatments. Ageing up to 300 h at 1143, 1223 and 1293 K was carried out to examine the effects of ageing temperature and ageing time on the growth of γ′-precipitates. The growth kinetics of γ′-precipitates was analysed. The experimental evidence reveals that the morphology of γ′ does not change in shape during heat treatments. The γ′ is usually cubic in both Alloy B and Alloy C. Growth of γ′ precipitates proceeds by Ostwald ripening controlled by volume diffusion of solute atoms despite the high volume fraction of γ′ and complex compositions in these two alloys. The activation energies for the growth are evaluated as 272 and 277 kJ mol^-1 for Alloy B and Alloy C, respectively, which correlate well to coarsening of γ′ in other commercial and developed superalloys. The hardness increases to a peak value at about 20 to 60 h for Alloy B and at about 10 to 20 h for Alloy C during ageing. The hardness of the alloys hardly decreases and still maintains a high value at high temperatures after passing the hardness peak. This revised version was published online in November 2006 with corrections to the Cover Date.     

Electron microscopy study of martensite in Cu–11.2 wt% Al–3 wt% Ni

The structures of martensites in a Cu–11.2 wt% Al–3 wt% Ni specimen, which was quenched from 1173 K, have been studied by high resolution electron microscopy (HREM) and diffraction techniques. Two kinds of martensites, i.e. β′1 and γ′1, coexisting adjacent to each other in the specimen were observed. The γ′1 matensite consists of microtwins with monoclinic structure. Three variants of the twin structure, i.e. {1 2 1}, {2 1 0} and {1 0 1} twins, are arranged within a single plate. The β′1 martensite possesses basically ordered N9R structure, but mixed with thin 2H domains. Some diffraction spots of this martensite shift along the [0 0 1] direction. In addition, extra weak reflections appear in the diffraction pattern due to heterogeneous atomic displacements. The microstructural features of the martensites are examined and discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Gamma-ray irradiation induced structural and optical constants changes of thermally evaporated neutral red thin films

Neutral red (NR) is polycrystalline in powder form, it transforms to nanocrystallite phase upon thermal deposition. Gamma-ray irradiation with doses 1.25–6 KGy induced partial transformation of nanocrystallite phase to amorphous structure. The changes of optical constants with γ-ray doses were calculated using spectrophotometer measurements of transmittance and reflectance at normal incidence of light over spectral range 200–2500 nm. The complex refractive index of NR film is highly influenced by exposure to γ-ray irradiation, the onset and optical energy gaps decrease with increasing γ-ray doses, and Urbach tail increases linearly with increasing irradiation dose. The type of electronic transition, oscillator, and electric dipole strengths and dispersion parameters were determined before and after irradiation. The spectral behavior of dielectric constant with γ-ray doses was also estimated.     

Investigation of microhardness and thermo-electrical properties in the Sn–Cu hypereutectic alloy

Sn–3 wt% Cu hypereutectic alloy was directionally solidified upward with different growth rates (2.24–133.33 μm/s) at a constant temperature gradient (4.24 K/mm) and with different temperature gradients (4.24–8.09 K/mm) at a constant growth rate (7.64 μm/s) in the Bridgman-type growth apparatus. The measurements of microhardness of directionally solidified samples were obtained by using a microhardness test device. The dependence of microhardness HV on the growth rate (V) and temperature gradient (G) were analyzed. According to these results, it has been found that with the increasing the values of V and G the value of HV increases. Variations of electrical resistivity (ρ) and electrical conductivity (σ) for casting samples with the temperature in the range of 300–500 K were also measured by using a standard dc four-point probe technique. The variation of Lorenz coefficient with the temperature for Sn–3 wt% Cu hypereutectic alloy was determined by using the measured values of electrical and thermal conductivities. The enthalpy of fusion for same alloy was determined by means of differential scanning calorimeter from heating trace during the transformation from eutectic liquid to eutectic solid.     

Pearlite in hypoeutectoid iron–nitrogen binary alloys

In this study, hypoeutectoid Fe–N binary specimens have been prepared by gas nitriding pure iron in austenite domain at 840 °C. The slow cooling of these specimens led to the α-ferrite + γ′-Fe₄N pearlitic microstructure which is similar to the pearlite in Fe–C binary system. This pearlitic microstructure has been characterized by electron microscopy. The crystal structure of the γ′-Fe₄N nitride has been identified by electron microdiffraction and the Nishiyama–Wassermann (N–W) and near Kurdjumov–Sachs (K–S) orientation relationships have been found between the α-ferrite and the γ′-Fe₄N.     

A simple application of the Bailey-Orowan creep model to Fe-39.8 at % AI and γ/γ′-α

A method which allows calculation of recovery rates and work hardening coefficients for creep from simple constant velocity compression experiments is described. Stressing rates and straining rates are computed from the measured load-time curves, and these quantities are then fitted to the universal form of the Bailey-Orowan equation for creep. The techniques were applied to B2 crystal structure Fe-39.8 at % Al intermetallics and the directionally solidified eutectic alloyγ/γ′-α compression tested between 1200 and 1400 K. The recovery rates were found to be functions of nominal strain rate, stress, and temperature; the hardening coefficients were dependent only on temperature. Hardening coefficients for the aluminides are small (<0.002 of the elastic modulus) and are believed to be due to the lack of dislocation barriers. A more typical value for the hardening coefficient (∼ 0.05 of the elastic modulus) was observed inγ/γ′-α. Recovery rates and hardening coefficients can be described in terms of stress and temperature, and the equations used to estimate creep rates.     

Effects of heat treatments on the serrated flow in a Ni–Co–Cr-base superalloy

Serrated flow in a Ni–Co–Cr-base superalloy was studied in three microstructural conditions (SUB, SUBA, and SUPER) from 25 to 750 °C by tensile test at initial strain rates ranging from 8 × 10⁻⁵ to 3 × 10⁻³ s⁻¹. The results showed that the SUB and SUBA samples had fine grain size of about 9 μm, whereas the SUPER samples had coarse grain size of about 600 μm. The tertiary γ′ fraction was about 0 in the SUB, 5% in the SUBA, and 15% in the SUPER samples, respectively. The types and temperature ranges of serration were different in the alloy with SUB, SUBA, and SUPER microstructures. It is proposed that the tertiary γ′ fraction and size had great effects on the serrated flow of the alloy with different microstructures.     

Creep behavior of INCOLOY alloy 617

The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700 and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)₂₃C₆ carbides and the δ-Ni₃Mo phase. M₆C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni₃(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)₂₃C₆ carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good creep response observed at 700 °C, compared with that found at 800 °C.     

Dislocation configurations and internal stresses in the creep of Nimonic 91

The internal stress, σ_i, developed during the creep of Nimonic 91 was determined as a function of applied stress, σ_a, using the strain transient dip technique. Transmission electron microscope observations of thin films of crept specimens showed Orowan dislocation loops to exist aroundγ′ phase particles at low stresses with partial dislocation loops around faultedγ′ particles at high stresses. The numbers of loops per specimen volume were counted and the resulting internal stress calculated. The results indicate that a significant part of the mechanically measured internal stress can be attributed to Orowan loops aroundγ′ particles which are stabilized against climb by the superlattice fault resulting from partial penetration of theγ′ particle by the dislocation. The variation of internal stress with applied stress can be accounted for qualitatively by the variation of loop density with stress at low stresses and the initiation of relaxation processes involving partial or complete shearing ofγ′ particles by loops at high stresses. It is suggested that creep in Nimonic 91 is dependent on the magnitude of the effective (σ_a-σ_i) and that the internal stress is determined largely by the density of dislocation loops aroundγ′ particles.