High creep exponents in a nearly-lamellar γ-based titanium aluminide intermetallic

Secondary creep data are reported for an extruded nearly-lamellar Ti-48Al-1.5Cr-alloy tested in a temperature range of 700 to 900°C. Within this temperature regime, this alloy exhibits a two-stage creep deformation behavior, with relatively high (approximately 8–12) creep exponents occurring in the high stress/high temperature regime. The high exponents in this regime are explained by dynamic recrystallization phenomena observed ₂ + in the nearly-lamellar microstructure.     

Effects of heat treatments on the creep properties of a hot pressed silicon nitride ceramic

Effects of heat treatment in an argon atmosphere at high temperatures for varying times on the creep properties of a Y₂O₃-Al₂O₃ (8-2 wt%) doped hot pressed silicon nitride (HPSN) ceramic were investigated. It was observed from the creep measurements that higher temperature, i.e. 1360C, and longer time, i.e. 8 h, heat treatment in an argon atmosphere improved the creep properties, (e.g. secondary creep rate) of this material. Heat treatment at a lower temperature of 1300C and for a shorter time of 4 h did not change the creep behaviour. Improvement of the creep properties was related to the crystallization of an amorphous grain boundary phase by heat treatment. Secondary creep rate parameters of the as-received material: stress exponent, n (2.95–3.08) and activation energy, Q (634–818 kJ mol^S–1), were in the range of values found by other investigators for various hot pressed silicon nitride ceramics.     

High temperature deformation behavior of permanent casting AZ91 alloy with and without Sb addition

The elevated temperature deformation behavior of permanent cast magnesium alloy AZ91 with and without Sb addition has been investigated using slow strain rate (5.0 × 10^–4s^–1) elevated temperature tensile and constant load creep testing at 150°C and 50 MPa. The alloy with 0.4 wt% Sb showed a higher elevated temperature tensile strength and creep resistance due to the formation of thermal stable Mg₃Sb₂ precipitates and a smaller microstructure as well as the suppressing of the discontinuous precipitation. Plastic deformation of AZ91 based alloys is determined by motion of dislocation in basal plane and non-basal slip systems. The dislocation motion in a slip system is influenced by temperature, precipitates and other lattice defects. Dislocations jog, grain boundaries and/or precipitates are considered as obstacles for moving dislocations. The deformation twinning were founded in the creep process by TEM. Cross slip of dislocations was taken into account as the main softening mechanism for permanent cast AZ91 alloy during elevated temperature deformation process.     

Effects of minor additions on microstructure and creep performance of RR2086 SX superalloys

A comparative investigation of creep performance in an experimental single crystal base alloy (designated RR2086) and modified (with additions of carbon, boron and hafnium) RR2086 superalloys has been undertaken. The alloys were creep tested over a temperature range of 850–1050°C. At 850°C/430 MPa, the creep behaviour of the modified alloy was slightly better than the base material, particularly in the tertiary creep stage. But at both 950°C/210 MPa and 1050°C/165 MPa, the creep strain of the modified alloy evolved more rapidly, resulting in a shorter creep life than the base RR2086. Microstructure investigation showed that the interdendritic regions of modified RR2086 contained relatively large, irregular particles which rafted to a limited level during creep, and that the amount of pores was significantly reduced in the modified alloy owing to MC carbide formation. Analysis of the correlation between microstructure and creep performance revealed that the reduction of pores in the modified alloy was beneficial to creep behaviour at the tertiary stage, and resulted in a longer creep life in the modified alloy than in the base RR2086 at 850°C/430 MPa. However, large, irregular and partially rafted particles and brittle MC phase in the interdendritic regions of the modified RR2086 caused a higher creep rate during the primary and secondary creep stages. The detrimental effect increased with creep temperature, counteracting the beneficial effect of porosity reduction. As a result the modified alloy had a shorter creep life than the base alloy at 950 and 1050°C.     

Creep behaviour of a 25wt % Cr-20wt % Ni austenitic stainless steel doped with antimony

The effect of antimony on the steady state creep rates, , of a 25 wt% Cr-20wt% Ni austenitic stainless steel with 0.005 wt % C is studied. The effect on vacancy viscous creep (Coble creep) is shown to be different to that on dislocation creep (power law creep). The effect on Coble creep is particularly striking. The threshold stress is significantly increased by antimony additions.     

Some Thermal Properties of a Copper–Tin Alloy

The thermal properties (heat capacity, thermal diffusivity, and electrical resistivity) of a Cu + 10 wt% Sn alloy in both solid and liquid phases have been reported. Using these values it was confirmed that the Lorenz relation is suitable for obtaining thermal conductivity from electrical resistivity in the liquid phase of this alloy. Also, the temperature differential (d/dT) obtained from such an approach was in excellent agreement with the thermal conductivity values calculated from thermal diffusivity.     

Study on tensile properties and microstructure of cast AZ91D/AlN nanocomposites

AZ91D is a widely used magnesium alloy, but its application is generally limited to below 150 °C because of its weak creep resistance and tensile properties at elevated temperatures. In this study, high temperature (200 °C) tensile properties including yield strength and tensile strength of AZ91D are much improved by adding only about 1.0 wt% AlN nanoparticles in the AZ91D matrix through an innovative ultrasonic cavitation based dispersion of nanoparticles. The good ductility of AZ91D is also retained in AZ91D/1%AlN nanocomposites. It is found that ultrasonic cavitation based solidification processing is very effective to disperse AlN nanoparticles in AZ91D melts, which is difficult to obtain by traditional mechanical stirring methods. With a good combination of high temperature yield strength, tensile strength and ductility, AZ91D/1%AlN nanocomposite is promising as a new class of structural materials to be used at temperatures up to 200 °C or higher.     

Steady-state creep of an alloy based on the intermetallic compound Ni3Al(γ′)

An investigation of the steady-state creep of a Ni₃Al.10 at% Fe alloy () has shown that two creep mechanisms were operative over the temperature range 530 to 930° C. The experimental data at low temperatures (below 680° C) were not consistent with any of the established creep theories. However, the experimental data were in good agreement with a proposed model for cross-slip from octahedral {111} planes on to cube {100} planes in Li₂ crystals. Above 680° C, the rate-controlling mechanism, which had an activation energy of 3.27eV atom^–1, is considered to be the removal/production of APBs during climb.     

Primary and Anelastic Creep of a Near α-Ti Alloy and Their Dependencies on Stress and Temperature

The primary creep behaviour of a high temperature near -Ti alloy Ti6242Si has been investigated in the temperature range from 500 to 625°C, and the stress range from 80 to 450 MPa. The results are analysed in terms of the dependencies of stress on strain (strain hardening) and on strain rate (strain rate sensitivity). Furthermore, full unloading experiments were conducted in order to gain additional information as to the nature of primary creep. It is shown that primary creep can be described by an athermal component, strain hardening, with a mean strain hardening coefficient of 0.37, and a thermally activated component, strain rate sensitivity, with a strain rate sensitivity coefficient suggesting a mechanism based on climb controlled recovery. This is confirmed by the activation energy of 259 kJ/mol determined at different stresses, which is similar to the activation energy of Ti self diffusion in -Ti. The anelastic strain obtained on full unloading was analysed in its fast stage in a similar way. The kinetics of anelastic creep and its activation energy are in many aspects very similar to those of primary creep. It is thought that, in the stress and temperature range investigated, primary creep is to a relatively high extent anelastic in nature, and is controlled by the climb controlled bow out of pinned dislocation segments, particularly dislocations pinned at lath boundaries.     

Strengthening in a DS casting Ni3Al base alloy IC6

A unidirectionally solidified casting base alloy IC6, with the chemical composition (at.%) 16.3–17.6% Al, 8.0–8.3% Mo, 0.16–0.78% B, Ni balance has been developed for advanced gas turbine blades and vanes. The experimental results show that this alloy has high yield strengths from room temperature to 1100°C, excellent creep resistance at temperatures up to 1100°C. The microstructural observations and analysis indicate that the superior mechanical properties of this alloy may be attributed to solid solution hardening by the large molybdenum addition, second phase strengthening by phase and other minor phases that precipitate in various temperature ranges, the formation of a raft structure during creep, and to the existence of high density misfit dislocation networks at / interface areas due to a high value of / misfit.Abbreviations BSE back scattered electron - DS unidirectionally solidified - EDS energy dispersive spectroscopy     

Influence of rhenium on the microstructures and mechanical properties of a mechanically alloyed oxide dispersion-strengthened nickel-base superalloy

The influence of a 3 wt% Re addition on the creep strength and microstructure of a mechanically alloyed and oxide dispersion-strengthened nickel-base superalloy was investigated. Two alloys, Ni–8Cr–6.5Al–6W–3Ta–1.5Mo–6Co–1Ti–3Re–0.15Zr–0.05C–0.01B–0.9Y₂O₃ (3Re alloy) and a non-rhenium containing (0Re) alloy were prepared for this study.The 3Re alloy showed two-fold improvement in creep life compared with that of 0Re alloy, presumably due to a change in the mode of the precipitate-dislocation interaction. For the 3Re alloy, finer, more cuboidal and aligned precipitates are formed, which force the mobile dislocations at the – interfaces to cut precipitates in order to proceed. Shearing of precipitates is evinced by the existence of stacking faults and results in an increase of creep strength. In constrast, lower creep strength was observed for 0Re alloy because a dislocation looping mode is dominant with coarser and more irregularly shaped precipitates present in this alloy. Another possible explanation for an improved creep strength of 3Re alloy is related to the tangled dislocation structure formed by the interaction between glide dislocation and interfacial dislocation, which also acts as an effective barrier for further glide dislocation motion. A 3 wt% Re addition significantly retards coarsening kinetics. Rhenium acts as a rate-controlling species upon the volume diffusion-controlled coarsening process because it is a heavy elemenet and also it almost solely partitions to the matrix. X-ray diffraction experiments showed that the magnitude of the lattice mismatch between and increased with the 3 wt% Re addition from 0% to –0.26% at room temperature. Increased lattice mismatch for 3Re alloy causes the formation of more aligned and cuboidal precipitates rather than random and odd-shaped precipitates for 0Re alloy, and it also accelerates the coalescence between cuboidal precipitates.     

Comparison about effects of Ce,Sn and Gd additions on as-cast microstructure and mechanical properties of Mg–3.8Zn–2.2Ca (wt%) magnesium alloy

In this paper, the effects of Ce, Sn and Gd additions on the as-cast microstructure and mechanical properties of Mg–3.8Zn–2.2Ca (wt%) magnesium alloy are investigated and compared. The results indicate that adding 1.0 wt% Ce, 1.0 wt% Sn or 1.0 wt% Gd can effectively refine the grains of the Mg–3.8Zn–2.2Ca alloy, and the refinement efficiency of Ce addition is relatively high, followed by the additions of Sn and Gd, respectively. Accordingly, the tensile properties of the as-cast Mg–3.8Zn–2.2Ca alloy are improved by the additions of Ce, Sn or Gd, with the improvement resulting from the Ce addition being best and followed by the additions of Sn and Gd, respectively. In addition, adding 1.0 wt% Ce, 1.0 wt% Sn or 1.0 wt% Gd to the Mg–3.8Zn–2.2Ca alloy can also improve the creep properties of the as-cast alloy. Among the Ce-, Sn- and Gd-containing alloys, the creep properties of the Sn- and Gd-containing alloys are similar but lower than that of the Ce-containing alloy.     

Soft Magnetic Properties of Fe-5wt%Al Alloy

In this investigation, a new soft magnetic material (iron with 5 wt% aluminum) has been developed using powder metallurgy processing. The microstructure and the magnetic properties of this new P/M alloy have been characterized at both room and elevated temperatures (up to 500°C). The influence of post-sintering (after initial processing) on the porosity and magnetic properties of this material has also been examined.

Test results show that the room temperature soft magnetic properties of this alloy are comparable to other commercially available soft magnetic materials such as P/M pure Fe, Fe-Si, Fe-P, etc. Post-sintering at 1316°C resulted in significant grain growth and lower porosity with more rounded pore morphology and improved the magnetic properties. While the magnetic induction of the alloy was essentially constant from room temperature to 500°C, the coercivity of the material decreased significantly at elevated temperature. This new P/M alloy may be a suitable soft magnetic material for high temperature (up to 500°C) applications.     

Creep of reaction-bonded silicon nitride

The high temperature mechanical properties, mainly the creep behaviour of reaction-bonded silicon nitride (RBSN), a new engineering ceramic for the gas turbine, have been a point of considerable interest. During the recent development a remarkable increase of the creep resistance of RBSN has been reached and the latest data show creep rates of below 10^–6 h^–1 at 1300° C and 70 to 100 MM m^–2. Activation energies between 540 and 700 kJ mol^–1 and stress exponents of 1in vacuo. Methods to determine the amount of internal oxidation,namely X-ray diffraction analysis, electron microprobe analysis and Rutherford backscattering of -particles were used. The deleterious effects of the internal oxidation are explained in terms of the microstructure, mainly porosity and pore size distribution, and ways to avoid this effect are discussed.     

Elevated-temperature deformation properties of a HfC modified Ti-48Al-2Mn-2Nb matrix particulate composite

Rapid solidification techniques in combination with HIPing have been used to produce Ti-48Al-2Mn-2Nb and a Ti-48Al-2Mn-2Nb+15 wt% HfC composite. While the composite does contain several second phases within the + ₂ matrix, none was identified to be HfC. The elevated-temperature properties were determined by constant velocity compression and constant load tensile testing in air between 1000 and 1173 K. Such testing indicated that the elevated temperature strengths of the HfC-modified aluminide was superior to those of the unreinforced matrix with the best 1100 K temperature slow strain rate properties for both materials being achieved after high-temperature annealing prior to testing. Examination of the microstructures after deformation in combination with the measured stress exponents and activation energies suggest that creep resistance of the HfC-modified form is due to solid-solution strengthening from carbon and hafnium rather than the presence of second phases.     

Incipient melting of Al5Mg8Si6Cu2 and Al2Cu intermetallics in unmodified and strontium-modified Al–Si–Cu–Mg (319) alloys during solution heat treatment

The present work was performed on seven alloys containing in common Al–6.5 wt%Si–3.5 wt%Cu, with magnesium in the range 0.04–0.45 wt%, and strontium in the range 0–300 p.p.m. The alloys were cast in the form of tensile test bars, solution heat treated in the temperature range 480–540°C for times up to 24 h. Two types of solution heat treatment were applied: (i) single-stage, where the test bars were solution treated at a certain temperature for 12 h prior to quenching in hot water (60°C); (ii) two-stage, where the test bars were solution treated for 12 h/510°C+12 h/T°C (T=510, 520, 530, 540°C), followed by quenching in hot water. In the low-magnesium alloys (i.e. with Mg0.04 wt%), melting of the Al2Cu phase commenced at 540°C. Increasing the magnesium content to 0.5 wt% reduced the incipient melting temperature of the Al5Mg8Si6Cu2 phase to 505°C. The mechanism of incipient melting and its effect on the tensile properties have been discussed in detail. © 1998 Chapman & Hall     

The high temperature creep deformation of Si3N4-6Y2O3-2Al2O3

The creep properties of silicon nitride containing 6 wt % yttria and 2 wt% alumina have been determined in the temperature range 1573 to 1673 K. The stress exponent, n, in the equation ⁿ was determined to be 2.00±0.15 and the true activation energy was found to be 692±25 kJ mol^–1. Transmission electron microscopy studies showed that deformation occurred in the grain boundary glassy phase accompanied by microcrack formation and cavitation. The steady state creep results are consistent with a diffusion controlled creep mechanism involving nitrogen diffusion through the grain boundary glassy phase.     

Effect of multiple strain-anneal cycles on the 1000° C creep behaviour of γ/γ′-α

An investigation of the influence of multiple strain-anneal cycles on the 1000° C creep behaviour of the directionally solidified eutectic alloy /- has been undertaken. Cycles consisted of swageing at room temperature or 900° C by about 5 to 10% per pass followed by annealing at 900° C, and were repeated to total strains of approximately 10, 30 and 50%. Transmission electron microscopy (TEM) of strain-annealed materials revealed that three-dimensional dislocation networks were introduced into the matrix, but very little work remained in the fibres. Constant-velocity creep testing indicated that all thermomechanical processing schedules improved the creep strength for strain rates 2 x 10^–6sec^–1; however only strain-annealing with a total of 13% work at room temperature (RT13) improved the behaviour at strain rates 2 x 10^–7sec^–1. The advantage of RT13 processing over as-grown materials at lower strain rates was confirmed by constant-load creep testing. It was also shown that 900° C annealing slightly improves the 1000° C creep properties in comparison to as-grown alloys. TEM of crept materials indicated that the active creep mechanism had been changed from dislocation pile-ups at fibres in as-grown alloys to dislocations being stopped by sub-boundaries in the matrix for RT13.     

Rapidly solidified titanium alloys for high-temperature applications

The application of rapid solidification technology (RST) to titanium alloy systems is relatively new and became the subject of active research since it was demonstrated that novel titanium alloys of higher temperature capability can be synthesized through new alloy design based on rapid solidification processing. The effects of rapid solidification on the occurrence of metastable phases, microstructures and mechanical properties in binary and ternary titanium alloys are reviewed. In particular, earlier results from RS-Ti alloy research have shown that many different novel dispersoids, some of which are coarsening-resistant at elevated temperatures (600 to 800° C), can be created in the matrix through RST. The alloys containing novel dispersoids also exhibit good creep resistance at elevated temperatures. Further studies on/- and-Ti alloys through RST, in conjunction with the development of various processing technologies for bulk alloy manufacturing, are clearly desirable.     

Fluidity of mica particle dispersed aluminium alloy

Cast aluminium alloy-mica particle composites were made by dispersing mica particles in a vortex produced by stirring the liquid Al-4 wt% Cu-1.5 wt% Mg alloy and then casting the melt containing the suspended particles into permanent moulds. Spiral fluidity and casting fluidity of the alloy containing mica particles in suspension were determined. Both the spiral fluidity and the casting fluidity of the base alloy were found to decrease with an increase in volume or weight percent of mica particles (of a given size), and with a decrease in particle size (for a given amount of particles). The fluidities of Al-4 wt% Cu-1.5 wt% Mg alloys containing suspended mica particles were found to correlate very well with the surface area of suspended mica particles. The regression equation for spiral fluidity Y (cm) as a function of surface area of mica particles per gram of spiral X (cm² g^–1) at 700° C was found to be Y=42.62–0.42X with a correlation coefficient of 0.9634. The regression equations for casting fluidity Y (cm) as a functiono of surface area of mica particles per gram of fluidity test piece X (cm² g^–1) at 710 and 670° C were found to be Y=19.71–0.17X and Y=13.52–0.105X with correlation coefficients of 0.9194 and 0.9612 respectively. The percentage decrease in casting fluidity of composite melts containing up to 2.5 wt% mica with a drop in temperature is quite similar to the corresponding decrease in the casting fluidity of base alloy melts (without mica). The change in fluidity due to mica dispersions has been discussed in terms of changes in viscosity of the composite melts. However, the fluidities of these composite alloys containing up to 2.5 wt% mica are adequate for making a variety of simple castings including bearings for which these alloys have been developed.