Effect of the precipitation state on high temperature tensile and creep behaviors of Mg-15Gd alloy

Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will transform to other phases, resulting in severe performance degradation. In this study, we investigated the effect of precipitation state achieved by different heat treatments on high temperature tensile and creep behaviors of the Mg-15Gd alloy by comparing the properties of the as-cast, solid-solutioned(T4) and peak-aged(T6) alloys. The results showed that the tensile mechanical properties of the T6 alloy were always highest from room temperature to 300 ℃, in spite of an abnormal strength increase with temperature existed in the T4 alloy. For tensile creep properties, the T6 alloy exhibited the lowest steady creep rate below 235 ℃ while the T4 alloy possessed the best properties above 260 ℃. Microstructure characterization revealed that the transition was caused by the stress-promoted precipitation of β phase in the T4 alloy and rapid phase transformation in the T6 alloy at high temperatures. At 260 ℃, the calculated stress exponent n was 3.1 and 2.8 for the T4 and T6 alloys, respectively, suggesting the creep deformation mechanism was dislocation slip, which was further confirmed by the microstructure after creeping. Our findings can provide new insights into the heat treatment process of Mg-Gd alloys served at high temperatures.

An Analytical Method for Predicting Direct Tensile Creep Fracture in Brittle Solids Containing Initial Microcracks

The tensile creep fracture behaviors in brittle solids are of great significance for the safety evaluation of brittle solid engineering.However,micromechanics-based tensile creep fracture behavior is rarely studied.In this study,a micromechanics-based method for predicting direct tensile creep fractures is presented.This method is established by combining the suggested expression of the mode-I stress intensity factor,the subcritical crack growth law,and the relationship between wing crack length and axial strain.This suggested mode-I stress intensity factor is formulated by the use of the basic theory of fracture mechanics under different loading modes.The rationality of the proposed tensile creep fracture model is verified by comparing with the experimental results.The correspondences of time-dependent axial strain,strain rate,wing crack length,and crack velocity are plotted under constant stress and stepping stress during tensile creep fracture.The effects of the initial crack size,inclination angle and density on the crack initiation stress,tensile strength,tensile creep fracture time,steady-state strain rate,initial strain,crack coalescence strain,and failure strain are discussed.

COMPARISON OF FATIGUE AND CREEP BEHAVIOR BETWEEN 2D AND 3D-C/SiC COMPOSITES

The differences of tension-tension fatigue and tensile creep characters of 2D-C/SiC and 3D-C/SiC composites have been scrutinized to meet the engineering needs. Experiments of tension-tension fatigue and tensile creep are carried out under vacuum high temperature condition. All of the high temperature fatigue curves are flat; the fatigue curves of the 2D-C/SiC are flatter and even parallel to the horizontal axis. While the tension-tension fatigue limit of the 3D-C/SiC is higher than that of the 2D-C/SiC, the fiber pullout length of the fatigue fracture surface of the 3D-C/SiC is longer than that of the 2D-C/SiC, and fracture morphology of the 3D-C/SiC is rougher, and pullout length of the fiber tows is longer. At the same time the 3D-C/SiC has higher tensile creep resistance. The tensile curve and the tensile creep curve of both materials consist of a series of flat step. These phenomena can be explained by the non-continuity of the damage.

A Research on the Air Permeability of High Polymer Materials Used to Produce Sports Clothing Fabrics

Composite fabrics based on Polytetrafluoroethylene(PTFE)polymer displays several notable properties.They are waterproof,windproof,permeable to moisture and thermally insulating at the same time.In the present study,PTFE fibers are used as raw material to make fiber membranes.The film is formed by crisscrossing interconnected fiber filaments and the related air permeability:tensile creep characteristics and other properties are tested.The results show that the pore size,thickness,and porosity of the film itself can affect the moisture permeability of the film.The water pressure resistance of the selected fabric is 8.5 kPa,and the moisture permeability is 7038 g/(m^(2)·24 h).

Time-dependent Early-age Behaviors of Concrete under Restrained Condition

To investigate the early-age behaviors of concrete under a restrained condition, a set of apparatus was developed. In this way, the tensile creep and other early-age properties can be investigated in depth. By measuring the modulus of elasticity of concrete, synchronous shrinkage of concrete and steel rings and free shrinkage of concrete, the deformations of concrete ring can be quantified respectively. The experimental results show the tensile stress in concrete is time-dependent, and the stress at cracking is much lower than the tensile strength at that age; the tensile creep plays an important role in relaxing the tensile stress and postponing the cracking of concrete.

Cracking Tendency of Restrained Concrete at Early Ages

A modified testing system characterized by full automation, steady operation and high accuracy of strain and stress measurements was developed to determine the cracking tendency of high strength concrete （HSC） in restrained condition at early ages. The shrinkage stress and the tensile creep behavior of HSC at early ages were investigated. The influence of W/C ratio and curing conditions on the early-age shrinkage stress and tensile creep was evaluated. It was found that the lower W/C ratio and drying curing condition resulted in higher shrinkage stress, stress induced tensile creep and greater cracking tendency.

Mechanical Analysis of Concrete Specimen under Restrained Condition

In order to quantify the development of the tensile stresses and obtain a reliable estimation of the cracking risk, the concrete was subjected to restrained conditions. The fully restrained condition was achieved by keeping the length constant of a concrete specimen. Comparing the free shrinkage with the restrained shrinkage, tensile creep could be discriminated from shrinkage. The testing method was introduced in details, and the mechanical behoviors under tensile lond were analyzed. Results show that conerete exhibits a pronounced viscoelasticity. Under restroined condition, the self-induced tensile stress increases with time. The lower the water to cement ratio, the larger the tensile stress at the same age. The tensile creep of hardening concrete is much larger than that of hardened concrete. The relationships among autogenous shrinkage unter free condition, elastic strain and creep under restrained condition are described, and the mathematical model for the calculation of elastic strain and creep is propased.

Superb Strengthening Effect of Net-Like Distributed Amorphous Al_(2)O_(3)on Creep Resistance of(B_(4)C+Al_(2)O_(3))/Al Neutron-Absorbing Materials

B4C reinforced Al composites are widely used as neutron absorbing materials(NAMs)due to excellent neutron absorbing efficiency,however,such NAMs exhibit poor high-temperature properties.To meet the requirement for structure-function integration,NAMs with enhanced high-temperature mechanical properties are desired.In this work,a novel(B4 C+Al_(2)O_(3))/Al NAM with netlike distribution of Al_(2)O_(3)was fabricated by powder metallurgy method and subjected to high-temperature tensile creep test.It was shown that the creep resistance was enhanced by several orders of magnitude via the addition of only2.1 vol.%netlike-distributed Al_(2)O_(3).(B_(4)C+Al_(2)O_(3))/Al exhibited high apparent stress exponents ranging from 16 to 25 and high apparent activation energy of 364 kJ/mol.The creep behaviour could be rationalized using the substructure-invariant model and its rupture behaviour could be described by the Dobes-Milicka equation.

Microstructure and mechanical properties of as-cast Mg-5Sn-2Ce-xZn alloys

The microstructure, mechanical properties and compression creep resistance of as-cast Mg-5Sn-2Ce-xZn（x=0, 2 wt.%, 4 wt.%） alloys were investigated in this paper. The results showed that the Ce-containing compounds were MgS nC e phase and an unknown phase, and they were the dominant intermetallics in as-cast Mg-5Sn-2Ce-xZn alloys. At x=0 wt.%, the Ce-containing compounds formed in interdendritic regions with the shape of rod. As the Zn content was increased to 2 wt.%, the Ce-containing compounds partly changed from rod-like to sheet-like. However, there were few further changes in the Ce-containing compounds with further increase in Zn content to 4 wt.%. Compared with Mg-5Sn-2Ce alloy, the tensile strength and elongation of the alloy containing 4 wt.% Zn increased by 39% and 112%, respectively, and the secondary creep rate（175 °C/55 MPa） decreased at least by an order of magnitude. These improvement could be attributed to the combined effects of the high content of Zn dissolving in a-Mg matrix and the modification of Ce-containing compounds by Zn addition.