High temperature creep of lithium zinc silicate glass-ceramics

The creep and recovery behaviour in compression of two lithium zinc silicate glassceramics is established over the temperature range 590 to 750° C at stresses up to 91.4 MN m^–2. It is shown that the transient creep obtained is linearly viscoelastic and obeys the Boltzmann superposition principle. The activation energy of the rate-controlling process is the same as that found for secondary creep and is due to viscous flow of the residual glass phase. A simple method of analysis of the strain-time curves is presented, which can be modified to apply to stress relaxation tests.     

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al₂O₃, Al₂O₃-ZrO₂) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10^–8–7.70 × 10^–6s^–1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol^–1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.     

Mechanical properties of precipitation-strengthened Ni-Al-Cr alloy based on an NiAl intermetallic compound

Mechanical properties of a ternary alloy Ni-30.3 at. % Al-6.6 at. % Cr have been studied in the temperature range 25 to 1100° C. The material was heat-treated to produce a stable dispersion of incoherent rod-shaped Ni₃Al precipitates, 1m in diameter and 20m long. The tensile properties were found to be temperature dependent. Below 750° C the material had high strength, low ductility and low strain-rate sensitivity, whilst above 750° C the strength fell, ductility increased and the material became strain-rate sensitive. The room temperature fracture toughness of the single-phase material was 6 MN m^–3/2 and increased to 50 MN m^–3/2 in the two-phase material. This can be attributed to the effect of Ni₃Al on crack nucleation and propagation.     

Plastic bending of CdxHg1−xTe

Preliminary results are reported of three-point plastic bending tests on Cd _x Hg_1–x Te single crystal samples, for an x value of about 0.2, conducted in air at strain rates of the order of 10^–5 sec^–1, and at temperatures in the range 303 K (30° C) to 363 K (90° C) (in the region of 0.35T _m ⁰ , where T _m ⁰ is the absolute melting point). Single crystal samples were cut from polycrystalline ingots, and the orientation, although measured in each case, was not consistent from sample to sample, being determined by the available grain shape. The stress-strain curves resemble those found for Group IV and III–V semiconductors. They display a yield drop, followed by a region of zero work hardening. All tests were stopped in this region, and in no case did the overall glide strain exceed 3%. The upper and lower yield stresses (outer fibre glide stress values) varied from 16 MN m^–2 and 10 MN m^–2, respectively, at 363 K (90° C) to 24 MN m^–2 and 17 MN m^–2, respectively, at 303 K (30° C).     

Creep behaviour of Cu-30% Zn at intermediate temaperatures

The creep properties of single-phase Cu-30% Zn alpha brass were investigated in the intermediate temperature range 573–823 K (0.48–0.70T _m, whereT _m is the absolute melting point). Inverse, linear, and sigmoidal primary creep transients were usually observed above 573 K under stresses resulting in minimum creep rates between 10^–7 and 2×10^–4s^–1, while normal primary creep occurred under all other conditions. The creep stress exponent decreased from about 5.4 at 573 K to about 4.1 above 623 K, and the activation energy for creep varied between 170 and 180 kJ mol^–1. A detailed analysis of the data, as well as a review of the literature, suggests that no clearly defined class M to class A to class M transition exists in this alloy, although the characteristics of both class A and class M behaviour are observed under nominally similar stresses and temperatures. It is concluded that Cu-30% Zn does not conform to the normally accepted characterization of class A or class M solid solution alloys.     

Fabrication of cellular structure composite material from recycled soda-lime glass and phlogopite mica powders

A homogeneous composite material with different physical structures has been fabricated from recycled colourless soda-lime glass powders and phlogopite-type mica powders by mixing the two powder components and sintering the mixture at a temperature above 850° C for a period of 30 min or longer. The physical structure of the composite material can be fabricated into either a cellular structure consisting of both closed and open cells or a highly densified ceramic body. The cellular structure composite material is found to have a compressive strength of about 0.877 MN m^–2 and thermal conductivity values in the range of 0.290 to 0.306 W m^–1 °C^–1 when measured at temperatures in the range of 25 to 100° C. The highly densified composite material, on the other hand, is found to have a compressive strength of about 53.0 MN m^–2 and thermal conductivity values in the range of 0.198 to 0.250 W m^–1 °C^–1. The composite material, when compared with other common building materials, is found to be potential material for construction applications because of its superior mechanical and thermal properties.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Creep of Pb–2·5Sb–0·2Sn alloy at low stresses

Abstract

The creep of a Pb–2·5Sb–0·2Sn alloy has been studied at stresses up to 6·5 MN m^?2 in the temperature range 318–348 K (0·53–0·58T_m) using helical specimens. At 333 K, a transition in the stress exponent from ~1 to 3 occurred at ~3 MN m^?2. The observed good agreements below the transition stress, both for experimental dE/do and predictions for Coble diffusional creep of lead, and for measured activation energy for creep and the activation energy for grain boundary self-diffusion in lead, suggest that grain boundary diffusional creep is the dominant mechanism. at low stresses. The presence of antimony does not seem to affect the magnitude of dE/do appreciably, and the results suggest that the grain boundary self-diffusivity of lead is not influenced by the presence of segregated antimony on the grain boundaries. The diffusional creep occurred above a threshold stress of magnitude ~0·5 MN m^?2, and its temperature dependence was characterised by an activation energy of ~20 kJ mol^?1, similar to the value of 23 ± 7 kJ mol^?1 typical of pure metals in the temperature range investigated. The stress exponent of ~3 observed for the power law regime suggests control by viscous glide of dislocations constrained by dragging of solute atmospheres. Preliminary tests on sagging beam specimens of as-worked material at an applied stress of 2·5 MN m^?2 and a test temperature of 333 K has provided the first direct evidence that anisotropic grain shape affects Coble creep. The specimen with the longest grain dimension along the stress axis underwent slower creep than the specimen with the longest grain dimension perpendicular to the stress axis. This observation is in qualitative agreement with theoretical predictions.

MST/1139     

Microplasticity in Elgiloy

The room temperature microstrain characteristics of Elgiloy (composition in wt%, 40 Co, 20 Cr, 15 Ni, 7 Mo, 2 Mn, 0.1 C, balance Fe) have been determined for a range of microstructures corresponding to various alloy treatments reported previously. The friction stress was found to be constant ( _F=12±3 MN m^–2) for the solution treated (after prestrain), cold-worked and cold-worked plus aged at 500° C conditions, but decreased to _f=5±3 MN m^–2 after ageing at 800° C. The microscopic yield stress (MYS) increased with cold-working and subsequent ageing at 500° C to a maximum value of 210 MN m^–2 reflecting the increase in the long range internal stress field. An increase in the MYS of the solution treated strip was noted with ageing at 800° C, whereas the MYS of the cold-worked samples was decreased by this treatment. The solution treated and aged at 800° C samples exhibited a two-stage stress-plastic microstrain curve, whereas the cold-worked and cold-worked plus aged at 500° C conditions showed a three-stage stress-plastic microstrain behaviour.     

The effect of water on the critical stress intensity factor of unsaturated polyester resins

The effect of water on the critical stress intensity factor,K _IC, of a highly unsaturated isophthalic polyester resin has been measured for periods of immersion of up to 2200 h in water at temperatures of 35, 70 and 80° C.K _IC decreased from 0.84±0.1 MN m^–3/2 to 0.24±0.03 MN m^–3/2 after immersion for 2200 h in water at 80° C. The observed changes in the critical stress intensity factor indicate that a constant value is eventually reached. This is consistent with the decline being attributable to the leaching of extractable matter from the polyester resin.     

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.     

Creep behaviour of cast TiAl based intermetallics

Abstract

Constant load tensile creep tests were carried out on the cast TiAl based intermetallics Ti–47 Al–2Mn, Ti–47 Al–2Zr, and Ti–48Al (at.-%), prepared by plasma arc melting. Two microstructural conditions dependent on heat treatment were evaluated as follows: a fully lamellar (FL) scheme consisting of a fully transformed coarse lamellar structure with α₂ laths plus γ laths within the grain interiors; and a duplex scheme consisting of fine equiaxed grains of γ with α₂/γ lamellae. The steady state creep behaviour of both microstructural conditions, for each composition, was studied under stresses of 70–300 MN m^?2 in the temperature range 700–900°C. The microstructure was found to have a pronounced influence on the creep resistance. The FL microstructure exhibited superior creep resistance to the duplex microstructure. At temperatures and stress levels at which direct comparisons can be made, the steady state creep rates of the FL structures are an order of magnitude lower than those of the duplex structure. The apparent creep activation energies and stress exponents were measured for both microstructural conditions for each composition. The temperature and stress dependence of the steady state creep rate of both microstructures can be described by the power law creep equation, suggesting dislocation motion as the operative deformation mechanism.

MST/1962     

Fracture mirrors in Columbia Resin CR-39

The relationship of fracture stress to mirror size for Columbia Resin, CR-39, is investigated. The fracture mirror constant is influenced by strain rate and test temperature, and gives values of 3.28 and 2.87 MN m^–3/2 for static and impact tests at room temperature, and 1.36 MN m^–3/2 for static tests at –150° C. The fracture surface energy for the initiation of unstable fracture decreases at high loading rate and at low temperature. Fractographic observations made on the fracture surface show some characteristic features reflecting the microstructure of this thermosetting plastic. The changes in the mirror constant and the fracture surface energy are discussed from the viewpoint of fracture mechanics.     

Strength of a weld-seam metal and glass-enamel coating composite under conditions of cyclical creep

In the temperature interval 293–573 K we have experimentally established the relationship of the failure stress of a coating (UES 300 with a base sublayer of brand 3132) in a weld-seam metal (Sv08GA steel) composite with a glass-enamel coating under conditions of cyclical creep at levels of up to t_test=6.10⁵ sec. It was noted that below 448 K the failure stress of the coating over the metal of the weld seam is lower by 50–60 MN/m² than in a composite with a base layer of a hot-rolled thick-sheeted 08GT-brand steel exhibiting greater ductility in tests of short duration, but under identical temperature conditions. The derived results are explained by the appearance within the weld-seam metal in contact with the coating of additional local stresses generated by volumetric changes within the process of residual austenite decay.Translated from Problemy Prochnosti, No. 8, pp. 58–61, August, 1991.     

Fatigue endurance of aged glass fibre reinforced cement

The resistance to flexural fatigue of glass fibre reinforced cement (GRC) stored in water for six years, has been studied. Peak stresses of between 6.0 and 18.2 MN m^–2 were used. At stresses of 10.0 and 18.2 MN m^–2 the median times before failure were 1.95×10⁵ and 2.0×10³ cycles, respectively. At a stress of 8.1 MN m^–2, six out of sixteen samples tested survived 4.65×10⁶ cycles. At a stress of 6 MN m^–2, all of the samples survived 1.75×10⁶ cycles. An unreinforced mortar specimen was also studied and its fatigue endurance showed greater scatter than the GRC samples.     

The solid state bonding of nickel,chromium and nichrome sheets to α-Al2O3

This paper describes experiments to measure the interfacial shear strengths of bonds formed between nickel, chromium and nichrome sheets hot pressed on to -Al₂O₃ single crystal plaques. Nickel developed bonds of 8×10³ psi (56 MN. m^–2) in shear when pressing was carried out under non-reducing conditions at 1100° C, 1 tsi (15 MN. m^–2) for 2 h: in the case of chromium, the effect of pressing temperature (1000 to 1300° C) pressing pressure (1/2 to 3 tsi [7.7 to 45 MN. m^–2]) and pressing time (1/2 to 7 h) on the bond was investigated. Nickel-chrome alloys produced from alternate nickel and chromium strips showed bond strengths up to 20×10³ psi (140 MN. m^–2) whilst commercial nickel-chrome (containing a silicon impurity) was not as effective in bonding to alumina. In material prepared from alternate strips of nickel and chromium, the degree of alloy homogenisation was investigated using microprobe analysis and suggestions made as to the mechanism of the interfacial reactions with alumina.     

Fracture of a plasticized epoxide under superposed hydrostatic pressure

The deformation and fracture behaviour of rubber-coated and uncoated epoxy specimens has been studied under superposed hydrostatic pressures extending to 300 MN m^–2. Maximum shear stress at yield for this epoxy were about 25 MN m^–2 at atmospheric pressure and rose to about 48 MN m^–2 at 300 MN m^–2 superposed pressure. Yielding and failure of all specimens tested beyond pressures of 75 MN m^–2 took place when all the (macroscopic) principal stresses, though unequal, were compressive. Fractographic examination revealed three distinct zones of the failure surfaces at atmospheric pressure. The behaviour of all uncoated specimens and those coated and tested below 100 MN m^–2 was similar. A fracture-mechanics interpretation of failure could be applied to these tests assuming the deformation-produced first zone was the fracture initiating site. Coated samples tested beyond 100 MN m^–2 superposed pressure failed with no evidence of Zones II or III of failure; Zone I appeared to spread over the entire failure surface. An interpretation involving fluid penetration of Zone I failure nuclei, along the lines suggested by Duckett, can account for the failure stresses of the uncoated specimens but is not tenable for the coated samples. It appears that crack nucleation and (slow) growth, as opposed, perhaps, to (catastrophic) crack propagation, can take place in this polymer when all the principal stresses are compressive.     

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m^–2 for unreinforced Si₃N₄, to 4770 J m^–2; its fracture toughness,K _lc , increased from 3.7 MN m^–3/2 for unreinforced material, to 15.6 MN m^–3/2. The strength remains about the same as unreinforced Si₃N₄ and the thermal expansion coefficient is only 2.51×10^–6 ° C^–1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.     

Order–Disorder Transformations in Ln2Ti2O7(Ln = Lu, Yb, Tm, Gd)

The ordering processes in the rare-earth titanates Ln₂Ti₂O₇ with Ln = Lu, Yb, Tm, and Gd are studied by x-ray diffraction, thermal analysis, IR spectroscopy, electron microscopy, and electrical conductivity measurements. The compounds are prepared via hydroxide coprecipitation, followed by freeze-drying and heat treatment in the temperature range 350–1700°C. The compounds Ln₂Ti₂O₇ with Ln = Lu, Yb, and Tm are found to have the fluorite structure between 600 and 800°C. Above 800°C, they undergo a fluorite-to-pyrochlore transformation, with cation disordering and the formation of Ln_Ti + Ti_Ln antistructure pairs. Gd₂Ti₂O₇ has the pyrochlore structure over the entire temperature range studied and contains no antistructure defects. In contrast to Gd₂Ti₂O₇, the compounds Ln₂Ti₂O₇ with Ln = Lu, Yb, and Tm undergo a high-temperature pyrochlore-to-fluorite phase transition around 1700°C. The 750°C conductivity of Ln₂Ti₂O₇ (Ln = Lu, Yb, Tm) samples sintered at 1700°C is 5 × 10^–3 to 10^–2 S/cm, which is two orders of magnitude higher than that of ceramics of the same composition prepared at lower temperatures (950 or 1400°C). The conductivity of the Gd₂Ti₂O₇ ceramic prepared at 1500°C is two orders of magnitude lower than that of Ln₂Ti₂O₇ with Ln = Lu, Yb, and Tm.     

Evaluation of hot workability of particle reinforced aluminum matrix composites by using deformation efficiency

The high temperature deformation behavior of Al 6061 composites reinforced with SiC and Al₂O₃ particles has been studied in the temperature range of 300–550°C and the strain rate range of 0.1–3.0/sec by hot torsion test. The deformation efficiency , given by (2m/m + 1), where m is the strain rate sensitivity, is calculated as a function of temperature and strain rate to obtain iso-efficiency contour map. The composite reinforced with SiC particle exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of 40% at the temperature range of 450–500°C and strain rate range of 0.2–0.5/sec. On the other hand, the composite reinforced with Al₂O₃ particle showed the DRX domain at the temperature range of 450–480°C and strain rate range of 0.1–0.2/sec. The characteristics of these domain have been investigated with the help of microstructural observation and hot ductility measurements.