Influence Of Temperature Cycles Research Articles

Experimental Investigation of Allotropic Transformation of Cobalt: Influence of Temperature Cycle, Mechanical Loading and Starting Microstructure

The allotropic phase transformation in polycrystalline high-purity cobalt is incompletely reversible and exhibits a temperature hysteresis. This leads to the presence of a FCC metastable phase at room temperature, which alters the mechanical properties. Moreover, this phase transformation seems to be induced by the plastic deformation. The influence of thermal cycling and initial microstructure on the phase transformation has been analyzed with different experimental approaches, namely in situ x-ray diffraction, differential scanning calorimetry and high-temperature digital image correlation analysis. A multiscale analysis, under an in situ tensile test, has been adopted to follow the phase transformation induced by the plastic deformation. The main result shows that the transformation is initiated by basal slip mechanisms, in competition with twinning mechanisms during the second work-hardening stage.

Experimental study on the influence of temperature cycle on low-rank coal permeability

The formation temperature is a key factor affecting coalbed methane (CBM) migration in reservoirs. Both the prediction of CBM production and prevention of mine gas disasters require the understanding on the controlling mechanism of temperature on coal permeability. We experimentally examined the evolution of permeability for natural low-rank coal samples under various stresses and cyclic temperature conditions. Apparent permeability and intrinsic permeability decrease significantly when the temperature increases and they only partially recover after the temperature returns. The permeability loss decreases greatly with the increasing number of temperature cycles. The permeability loss due to the rising temperature and the irreversible permeability loss for a whole temperature cycle decrease prominently with increasing confining stress. The impacts of swelling/shrinking of coal matrix, roughness of surface, pore compressibility and weakly bound water in coal on temperature sensitivity of coal permeability are investigated.

Influence of temperature cycles on strength and microstructure of spray-deposited Si–Al CE9F alloy

This paper investigates the effects of temperature cycles between −55 °C and 125 °C (the temperature range for intended space application) on the strength and microstructure of spray deposited Si–Al CE9F alloy. After 500 cycles, the strength is significantly improved of 9% (i.e., 26 MPa). Moreover the Weibull modulus also undergoes a significant increase. Microstructure characterizations by XRD, SEM, EBSD and DSC evidence recovery and recrystallization processes in the Al phase, and subgrain development at grain boundaries in the Si phase, leading to lower crystallite size. A Hall–Petch dependence of mean strength to average crystallite size in Si is proposed as strengthening mechanism.

Influence of temperature cycling and pore fluid on tensile strength of chalk

Calcite has a highly anisotropic thermal expansion coefficient, and repeated heating and cooling cycles can potentially destabilize chalks by breaking cement bonds between neighboring particles. Based on tensile strength measurements, we investigated how temperature cycles induce weakening of chalk. Tensile strength tests were performed on chalk specimens sampled from Kansas (USA) and Mons (Belgium), each with differing amounts of contact cement. Samples of the two chalk types were tested in dry and water-saturated states, and then exposed to 0, 15, and 30 temperature cycles in order to find out under what circumstances thermally induced tensile strength reduction occurs. The testing results show that the dry samples were not influenced by temperature cycling in either of the chalk types. However, in the water-saturated state, tensile strength is increasingly reduced with progressive numbers of temperature cycles for both chalk samples, especially for the more cemented Kansas chalk. The Kansas chalk demonstrated higher initial tensile strength compared to the less cemented Mons chalk, but the strength of both chalks was reduced by the same relative proportion when undergoing thermal cycles in the water-saturated state.

Influence of Temperature Cycles on Bond between Glass Fiber-Reinforced Polymer and Concrete

Reinforced concrete (RC) beams externally strengthened with glass fiber-reinforced polymer (GFRP) strips bonded to the soffit may see their load-carrying capacity reduced due to environmental conditions-especially due to the deterioration of bond between the adhesively bonded laminates and concrete, causing premature failure. More research has been published on the detachment of the laminate progressing from the anchorage zone than on failure induced by the formation of flexural or shear-flexural cracks in the midspan followed by fiber-reinforced polymer (FRP) separation and failure designated as intermediate crack (IC) debonding. An experimental program to study degradation of the GFRP laminate beam specimens after accelerated temperature cycles, namely: 1) freezing-and-thawing type; and 2) cycles of the same amplitude (40°C [104°F]) and an upper limit approximately 70% of the glass vitreous transition temperature of the resin, Tg, is described. Effects on the bond stress and ultimate capacity are reported. Substantial differences between shear and bending-induced failure and a decrease of bond stresses and engagement of the laminates on the structural response are analyzed.

Mechanical behavior and clinical application of nickel-titanium closed-coil springs under different stress levels and mechanical loading cycles

The main advantage of superelastic nickel-titanium (NiTi) products is their unique characteristic of force plateaus, which allow for clinically precise control of the force. The aims of this study were to define the mechanical characteristics of several currently available closed-coil retraction springs and to compare these products. A universal test frame was used to acquire force-deflection diagrams of 24 NiTi closed-coil springs at body temperature. Data analysis was performed with the superelastic algorithm. Also, the influence of temperature cycles and mechanical microcycles simulating ingestion of different foods and mastication, respectively, were considered. Mechanical testing showed significant differences between the various spring types (ANOVA, < or =0.05), but constant intrabatch behavior (t test). Four groups were formed according to the mechanical properties of the springs: strong superelasticity without bias stress, weak superelasticity without bias stress, strong superelasticity with bias stress, and weak superelasticity with bias stress. In sliding mechanics, the strongly superelastic closed-coil springs with preactivation are recommended. In addition, we found that the oral environment seems to have only a minor influence on their mechanical properties.

Influence of temperature cycling near the curie point on the effects of thermomagnetic treatment in soft magnetic materials

It has been found that thermocycling near the Curie temperature improves magnetic properties of magnetically soft alloys and, in particular, decreases the coercive force of the Fe-70 wt % Ni by 15% in comparison with that observed after conventional thermomagnetic treatment.

Effects of temperature cycling and nitrogen on the stability of microstructures in austenitic stainless steels

The influence of temperature cycling from room temperature to 77 K on nitrogen-alloyed austenitic stainless steels, Fe24Mn13Cr0.4N, Fe24Mn18Cr3Ni0.6N, Fe1Mn19Cr8Ni0.2N and Fe17Mn14Cr1Ni0.4N, has been studied by optical microscopy and scanning electronic microscopy (SEM). Results show that the phase transformation from austenite to ε martensite takes place due to the temperature cycling and stress concentration, whereas nitrogen can stabilize the austenitic microstructures greatly. Finally, the mechanism of the phase transformation and the structure of ε martensite are discussed in detail.

Influence of temperature cycling on the kinetics of new phase growth in two-phase solid 3He– 4He mixtures

Precise pressure measurements at constant volume were carried out to study the influence of thermal cycling on the kinetics of new phase growth and dissolution in 3He– 4He solid mixtures in the phase separation region. It is found that the equilibrium pressure in the sample reduces after cycling. This points to improvement of the crystal quality, which is supported by the acceleration of the growth and dissolution processes. These data indicate that cooling–warming cycles in the two-phase region are more effective in eliminating defects than the traditional annealing near the melting temperature.

A novel high performance adhesion enhancing Zn-Cr leadframe coating for popcorn prevention

A novel adhesion enhancing Zn-Cr (A2) leadframe coating has been reported to be a highly effective solution for popcorn prevention in plastic surface mount packages. In this paper, we report our recent understanding of the adhesion and degradation mechanisms of such a coating system. TEM and TOF-SIMS were employed to study the structure of A2-coating and the molding compound/leadframe (MC/LF) interface. The A2-coating was found to be a partially thin, crystalline layer comprised of a continuous interfacial layer and a network of whiskers. Zn silicate compound, ZnO and Cr oxide(s) were identified as possible phases present in the A2-coating. The adhesion enhancing and degradation mechanisms of the A2-coating were investigated by lead pull test to study the influence of temperature cycling, pressure cooker testing, moisture preconditioning, and leadframe oxidation. The adhesion data showed that the A2-enhanced MC/Cu LF interface is not susceptible to thermomechanical stress and moisture degradation. The wetting of the molding compound coupled with mechanical interlocking mechanism offered by the whiskers are believed to be key contributing factors of adhesion enhancement. Degradation of the A2-enhanced MC/Cu LF adhesion was observed after 100 min exposure at 300/spl deg/C by Cu oxide formation from Cu outward diffusion through the A2-layer and was validated by AES analysis operated in the depth profiling mode. Lead pull test specimens were cleaved to have access to the MC/Cu LF interface for RBS analysis to identify the locus of failure.

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.

Influence of temperature cycling on the production of aflatoxins B1 and G1 by Aspergillus parasiticus.

The effect of temperature cycling on the relative productions of aflatoxins B1 and G1 by Aspergillus parasiticus NRRL 2999 was studied. The cycling of temperature between 33 and 15 degrees C favored aflatoxin B1 accumulation, whereas cycling between 35 and 15 degrees C favored aflatoxin G1 production. Cultures subjected to temperature cycling between 33 and 25 degrees C at various time intervals changed the relative productions of aflatoxins B1 and G1 drastically. Results obtained with temperature cycling and yeast extract-sucrose medium with ethoxyquin to decrease aflatoxin G1 production suggest that the enzyme system responsible for the conversion of aflatoxin B1 to G1 might be more efficient at 25 degrees C than at 33 degrees C. The possible explanation of the effect of both constant and cycling temperatures on the relative accumulations of aflatoxins B1 and G2 might be through the control of the above enzyme system. The study also showed that greater than 57% of aflatoxin B1, greater than 47% of aflatoxin G1, and greater than 50% of total aflatoxins (B1 plus G1) were in the mycelium by day 10 under both constant and cyclic temperature conditions.

Accelerated Stage Creep and Creep Rupture under Temperature Cycling

In the present paper, the influence of temperature cycling on the accelerated stage creep and the rupture life is discussed by employing AISI 318 type stainless steel. In this discussion, the strength of material under temperature cycling was compared with that under steady temperature by introducing the idea of the equivalent steady temperature for the rupture life of material under temperature cycling. Embrittlement of a creeping material subjected to temperature cycling is significantly larger than in the case of steady temperature. It would probably be because of a difference in the metallographic structure change occuring during the tertiary stage of creep, for both series of tests.