Creep behavior of Mg–2 wt.%Nd binary alloy

A binary magnesium alloy, Mg–2 wt.%Nd, has been prepared. Under the condition of temperature between 150 and 250 °C and applied stress between 30 and 110 MPa, the alloy exhibits good creep resistance due to both solution-hardening and especially precipitation-hardening. Tiny precipitates forming dynamically during creep have been observed, which play an important role in restricting dislocation movements. When the creep tests are carried out at the temperature range between 150 and 250 °C, the stress exponents lie in the range of 4.5–7.1 at low stresses, which is consistent with the “five-power-law”. The values of stress exponent increase up to 9.8–29.5 at high stresses indicate power-law breakdown. When the creep tests are carried out under the applied stress between 30 and 90 MPa, the apparent activation energy values vary from 70.0 to 96.0 kJ/mol at low temperatures, but increase to 199.9–246.1 kJ/mol at high temperature range. Dislocations in basal plane are activated in the primary creep stage, but as creep goes on, they are observed in non-basal plane. The creep is mainly controlled by both dislocation-climb and cross-slip.     

Effect of electric current on the creep deformation of lead

Using the impression test, the effect of electric current on the creep deformation of Pb was studied in the temperature range of 343-403 K and under the punching stress of 11.1-30.5 MPa. During the impression test of constant load, a direct electric current in a range of 0-5 A passed through the punch into the Pb-sample. A steady state creep with sudden jumps in the impression curves was observed. A power-law relation was used to describe the dependence of the steady state impression velocity on the punching stress. The average stress exponent increased from 3.2 to 4.0 with increasing electric current and the apparent activation energy was 36.0 kJ/mol for small punching stresses. For large punching stresses, the average stress exponent decreased with increasing electric current and the apparent activation energy decreased from 63.5 kJ/mol to 49.7 kJ/mol with increasing electric current from 0 to 5 A.     

Ultrahigh-temperature deformation of high-purity HIPed Si3N4

High-temperature tensile deformation behavior of high-purity HIPed silicon nitride material was investigated in the temperature range between 1600°C and 1750°C. Recoverable anelastic and non-recoverable deformation was observed in high-purity HIPed silicon nitride. A power-law deformation model analogous to rheological models was used to distinguish the different deformation components. A stress exponent n = 1.64 and an activation energy Q ₁ = 708 kJ/mol was determined for the non-recoverable deformation. For the anelastic deformation a stress exponent p = 4 and an activation energy Q ₃ = 619 kJ/mol was observed. Diffusional creep and grain boundary sliding with the accomodation process responsible for the anelastic component are discussed as deformation mechanisms.     

Creep behavior in SiC-whisker reinforced silicon nitride composite

Tensile and flexural creep tests of 20 vol % SiC whiskers reinforced Si₃N₄ composite processed by gas pressure sintering have been carried out in air in the temperature range of 1000–1300°C. The stress exponent for flexural creep is 16 at 1000°C. However, at 1200 and 1250°C the stress exponents for both tensile and flexural creep vary with increasing stress. In the low stress region, the activation energy for creep is 1000 kJ/mol. In the high stress region, it is 680 kJ/mol. The different creep mechanisms dominate in the low and high stress regions, respectively. This revised version was published online in September 2006 with corrections to the Cover Date.     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

Tensile Creep Behaviour of Silicon Carbide Particulate Reinforced Aluminium Composite

Tensile creep tests were carried out on 15SiC (vol.pct) particulate reinforced commercial pure aluminum (15%SiCp/Al) composite at 573 and 623 K. The steady state creep stage exists at the applied stresses under the condition of tension. The 15%SiCp/Al composite exhibits an apparent stress exponent of about 13 and an apparent activation energy of 253 kJ/mol. The creep data were normalized using a substructure invariant model with a stress exponent of 8 together with a threshold stress.     

High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Impression creep behavior of Al–1.9%Ni–1.6%Mn–1%Mg alloy

In the present study, microstructure and creep behavior of an Al–1.9%Ni–1.6%Mn–1%Mg alloy were studied at temperature ranging from 493 to 513 K and under stresses between 420 and 530 MPa. The creep test was carried out by impression creep technique in which a flat ended cylindrical indenter was impressed on the specimens. The results showed that microstructure of the alloy is composed of primary α(Al) phase covered by a mantle of α(Al)+Ni₃Al intermetallic compound. Mn segregated into Al_xMn_yNi_z or Al₆Mn phases distributed inside the matrix phase. It was found that the stress exponent, n, decreases from 5.2 to 3.6 with increasing temperature. Creep activation energies between 115 kJ/mol and 151 kJ/mol were estimated for the alloy and it decreases with rising stress. According to the stress exponent and creep activation energies, the lattice and pipe diffusion- climb controlled dislocation creep were the dominant creep mechanism.     

Creep of chemically vapour deposited SiC fibres

The creep, thermal expansion, and elastic modulus properties for chemically vapour deposited SiC fibres were measured between 1000 and 1500°C. Creep strain was observed to increase logarithmically with time, monotonically with temperature, and linearly with tensile stress up to 800 MPa. The controlling activation energy was 480 ± 20 kJ mol^–1. Thermal pretreatments near 1200 and 145O° C were found to significantly reduce fibre creep. These results coupled with creep recovery observations indicate that below 1400°C fibre creep is anelastic with negligible plastic component. This allowed a simple predictive method to be developed for describing fibre total deformation as a function of time, temperature, and stress. Mechanistic analysis of the property data suggests that fibre creep is the result of -SiC grain boundary sliding, controlled by a small percentage of free silicon in the grain boundaries.     

Creep Behavior of As-Cast Ti-48Al-2Cr Intermetallic Alloy for Aerospace and Automotive Applications

The creep properties of as-cast Ti-48Al-2Cr (at%) alloy which had been strengthen with addition of 2 at% Cr were investigated. Tensile creep experiments were performed in air at temperatures from 600-800°C and initial stresses ranging from 150 to 180 MPa. Stress exponent and activation energy were both measured. Data indicates that the alloy exhibits steady state creep behavior and the steady state creep rate is found to depend on the applied load and temperature. The measured power law stress exponent for steady state creep rate is found to be close to 3 and the apparent activation energy for creep is calculated to be 15.7 kJ/mol. The creep resistance of the present alloy was also compared with binary Ti-48Al (at%) to evaluate the effect of Cr addition on creep resistance of TiAl. It is concluded that addition of 2 at% of Cr does not have significant effect on the creep resistance of TiAl.     

The influence of temperature cycling on the creep properties of Nickel 201 and Inconel 600 in combustion gas

The effect of temperature cycling on the creep behaviour of Nickel 201 and Inconel 600 in combustion gas has been studied. Specimens were tested both at constant temperature, 900° C, and at 900° C interrupted by temperarature drops down to 510° C. The creep straining has been analysed with respect to a weighted time parameter which includes the creep contribution during the lower temperatures of each cycle. With respect to this compensated time parameter, the temperature variations were generally observed to result in a strong acceleration in creep. The effect seemed to increase with increasing frequency of temperature drops, increasing grain size and decreasing stress. Thus, at low stress levels, large-grained specimens of both alloys experienced an acceleration even inabsolute creep rate upon cycling. The grain size dependency indicates that the destructive effect of the cycles is caused by crack formation. Surface cracking associated with grain boundary oxidation seemed to be the dominant cracking mode. It is suggested that, during creep in oxidizing environments, repeated periods of cooling might strongly accelerate the growth of surface creep cracks due to the difference in thermal expansion between metals and oxides. This difference causes high tensile stresses to arise in the metal in front of the grain boundary oxides, and the stresses are assumed to be high enough to nucleate microcracks along the boundary.     

Investigation of the compressive creep behavior of AZ91D magnesium alloy

The creep resistance of AZ91D alloy has been studied in uniaxial compression tests at temperature ranges from 275 °C to 325 °C. The initial microstructure of the alloy consists of α phase and β phase precipitated in the grain boundary. The minimum creep rate dependence on applied stress and the temperature is also analyzed in detail. We find that the stress exponent n is close to the theoretical values (3 or 5) and the activation energy Q for creep varies from 121 kJ/mol to 171 kJ/mol. Creep could be controlled by high-temperature climb and cross-slip of dislocation at different temperatures.     

Temperature effect on synthesis of different carbon nanostructures by sulfur-assisted chemical vapor deposition

The temperature effect on synthesizing different carbon nanostructures in the range of 820−1020 °C by sulfur-assisted chemical vapor deposition is investigated. When the growth temperature is no more than 900 °C (e.g. 820, 860, and 900 °C), carbon onions can be obtained, accompanying with some fishbone-like carbon nanofibers (CNFs), graphite sheet and carbon nanotubes (CNTs). When the growth temperature is increased to 940 °C or above (e.g. 980 and 1020 °C), the products are mainly CNTs. Furthermore, by comparing the nitrogen adsorption-desorption results of samples obtained with and without sulfur addition at each temperature, it is found that the specific surface area (SSA) of products can be remarkably enlarged after introducing small amount of sulfur during growth. This is favorable to their applications in areas like electrodes of supercapacitors, adsorbents, catalyst supports, and so on.     

Creep and microstructure in powder metallurgy 15 vol.% SiC<Subscript>p</Subscript>–2009 Al composite

The creep behavior and microstructure of powder metallurgy (PM) 15 vol.% silicon particulate-reinforced 2009 aluminum alloy (SiCp–2009 Al composite) and its matrix PM 2009 Al were investigated over six orders of magnitude of strain rate and at temperatures in the range 618–678 K. The results show that the creep behavior of PM 15% SiC_p–2009 Al composite resembles that of PM 2009 Al with regard to (a) the variations in both the apparent stress exponent and the apparent activation energy for creep due to applied stress, (b) the value of the true stress exponent, (c) the value of the true activation energy for creep, (d) the interpretation of creep in terms of a threshold stress, and (e) the temperature dependence of threshold stress. This resemblance implies that deformation in the matrix governs deformation in the composite. Analysis of the creep data in terms of creep rate against an effective stress shows that the creep behaviors of the composite and unreinforced alloy are consistent with the operation of viscous glide creep at low stresses. A comparison between the creep data of the composite and those of the unreinforced matrix revealed that the composite exhibited more creep-resistant characteristics than its matrix over the entire range of applied stresses.     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

Creep behaviour of Si3N4/Y2O3/Al2O3/AIN alloys

The compression creep behaviour of pressureless sintered Y₂O₃/Al₂O₃/AIN-doped Si₃N₄ was studied between 1473 and 1673 K, under stresses ranging from 100–300 MPa. Strain rate versus stress and temperature analysis give a stress exponent n1 and an activation energy Q=860 kJ mol^–1. Microstructural change was investigated by transmission electron microscopy. The observed strain whorls, the stress exponent and the activation energy are indicative of a solution-diffusion-precipitation accommodated grain-boundary sliding where the diffusion through the glass is rate controlling.     

Dimensional changes and creep of silica core ceramics used in investment casting of superalloys

Dimensional changes and creep deformation of a silica/zircon (74%/24%, respectively) and a high silica (93% silica and 3% zircon) ceramic were characterized and compared. All specimens were tested with a thermal profile that consisted of a 300°C/h heating rate to 1475 or 1525°C, followed by a one-hour isothermal hold (where each specimen was compressively crept under a static stress of 2.07, 4.14, or 6.21 MPa). The specimens were cooled at a rate of 900°C/h under stress. Dimensional changes were interpreted from apparent thermal expansion behavior during heating as well as before-and-after dimensional measurements. The silica/zircon ceramic generally exhibited less total contraction than the high silica ceramic for a specific test condition even though it crept faster at all stresses and temperatures during the one-hour isothermal/isostress segment. This indicates that the total contraction for both was dominated by reinitiated sintering and subsequent cristobalite formation that occurred during the heating segment. Minimum creep rate during the one-hour isothermal/isostress segment was examined as a function of stress and temperature for both ceramics using a power-law creep model. Creep-rate stress exponents (n) and activation energies (Q) were equivalent (within 95% confidence) for both ceramics showing that their different contents of zircon (3 vs. 24%) did not affect them. Lastly, n 1.3–1.4 and Q 170 kJ/mol indicate that diffusion-assisted crystallization of cristobalite, combined with power-law sintering owing to the high concentration of porosity (28–30%) was likely the rate-limiting mechanism in the creep deformation for both ceramics.     

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti₃Al and Ti₃Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m^–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m^–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m^–2, a transition regime with a lower stress-exponent value was obtained.     

Superplastic deformation and postformed mechanical properties of high temperature titanium alloy IMI 834

Abstract

Superplastic forming is particularly attractive for high temperature Ti alloys because of the much lower forming stresses compared with those encountered during forging. The superplastic deformation parameters of IMI 834 sheet were obtained at 900, 940, and 990°C. At 990°C, IMI 834 shows low flow stresses, high values of strain rate sensitivity, and minimum strain anisotropy, however, 300% superplastic elongation was readily obtained at the lower forming temperature of 940°C but with a higher flow stress. A reduction in the room temperature and 600°C tensile properties with superplastic strain resulted from strain enhanced grain growth during superplastic deformation; this effect was greatest at 990°C. Aging of post 990°C superplastically formed material was studied. The creep performance of IMI 834 was found to be slightly reduced by superplastic forming. These properties and the changes in the microstructure and texture are compared with other Ti alloys under superplastic conditions.

MST/1822     

Mechanical properties of vanadium beryllide,VBe12

The mechanical properties of VBe₁₂, both at room and elevated temperatures (up to 1200°C), have been measured. Room-temperature properties, including Young's modulus, flexural strength, and fracture toughness are reported. The material behaved elastically at room temperature but became plastic at temperatures above 1000°C. Creep properties of VBe₁₂ were also studied in temperature ranges from 1000–1200°C and applied stress ranges from 33–58 MPa. At low strain rates (approximately < 10^–5s^–1), the stress exponent was about 4, suggesting deformation was controlled by dislocation climb. Microstructural examination indicated that fracture was initiated from grain boundaries subjected to tensile stresses. The creep behaviour of VBe₁₂ is briefly compared with that of other intermetallics.