Increase In True Stress Research Articles

Effects of Heating Rates on Microstructural Evolution of Hot Extruded 7075 Aluminum Alloy in the Semi-Solid State and Thixotropic Deformation Behavior.

The non-dendritic microstructure plays a crucial role in determining the rheological properties of semi-solid alloys, which are of the utmost importance for the successful industrial application of the thixoforging process. To further understand the impact of the reheating process on the evolution of microstructure and thixotropic deformation behavior in the semi-solid state, a hot extruded and T6 treated 7075 aluminum alloy was reheated to the selected temperature ranges using varying heating rates. Subsequently, thixo-compression tests were performed. The study found that during reheating and isothermal holding, the elongated microstructure of the as-supplied alloy can transform into equiaxed or spherical grains. The presence of recrystallized grains was found to be closely linked to the penetration of the liquid phase into the recrystallized grain boundaries. Furthermore, it was observed that higher heating rates resulted in smaller grain sizes. The thixotropic flow behavior of the alloy with various microstructures was analyzed using the true stress-strain curves obtained by thixo-compression experiments, which exhibited three stages: a rapid increase in true stress to a peak value, followed by a decrease in true stress and a steady stress until the end of compression. The stress fluctuated with strain during the formation of the slurry at a strain rate of 10 s-1, indicating the significant role of strain rate in material flow during semisolid formation.

Improvement of tensile properties of low-carbon steels via short-time intercritical annealing

The effect of low volume fraction formation of martensite on the tensile properties of low carbon steel was evaluated. First, steel samples with ferrite-cementite microstructure were produced. The thermomechanical treatment used included austenitizing at 1000 °C and then quenching in ice brine solution, tempering the obtained martensitic structure for 1 h at 650 °C, 80% cold rolling, and re-tempering for 2 h at 650 °C. In order to form a low volume fraction of martensite, steel samples with ferrite-cementite microstructure were intercritically annealed for 30 seconds at 740 °C. As a result of intercritical annealing treatment, 6.2% martensite was formed. The results of tensile test showed that the formation of 6.2% martensite led to the elimination of yield point phenomenon and Lüders banding, decrease of yield stress and increase of true stress at maximum load, while true uniform strain did not change significantly. The work hardening rate also increased significantly. Based on the results of modeling of the flow behavior with the Holloman equation, the work hardening capability of the steel sample including ferrite-cementite decreased after a certain plastic strain, while the work hardening capacity remained constant with the formation of a low volume fraction of martensite in the microstructure.

Modified θ projection model-based constant-stress creep curve for alloy 690 steam generator tube material

Steam generator (SG) tubes in a nuclear power plant can undergo rapid changes in pressure and temperature during an accident; thus, an accurate model to predict short-term creep damage is essential. The theta (θ) projection method has been widely used for modeling creep-strain behavior under constant stress. However, many creep test data are obtained under constant load, so creep rupture behavior under a constant load cannot be accurately simulated due to the different stress conditions. This paper proposes a novel methodology to obtain the creep curve under constant stress using a modified θ projection method that considers the increase in true stress during creep deformation in a constant-load creep test. The methodology is validated using finite element analysis, and the limitations of the methodology are also discussed. The paper also proposes a creep-strain model for alloy 690 as an SG material and a novel creep hardening rule we call the damage-fraction hardening rule. The creep hardening rule is applied to evaluate the creep rupture behavior of SG tubes. The results of this study show its great potential to evaluate the rupture behavior of an SG tube governed by creep deformation.

A modified θ projection model for constant load creep curves-I. Introduction of the model

Estimating long-term creep deformation and life of materials is an effective way to ensure the service safety and to reduce the cost of long-term integrity evaluation of high temperature structural materials. Since the 1980s, the θ projection model has been widely used for predicting creep lives due to its ability to capture the characteristic transitions observed in creep curves obtained under constant true stress conditions. However, the creep rupture behavior under constant load or engineering stress conditions cannot be simulated accurately using this model because of the different stress states. In this paper, creep curves obtained under constant load conditions were analyzed using a modified θ projection model by considering the increase in true stress with creep deformation during the creep tests. This model is expressed as ε=θ11−e−θ2t+θ3eθ4eθ5εt−1, and was validated using the creep curves of K465 and DZ125 superalloys tested at a range of temperatures and engineering stresses. Moreover, it was shown that the predictive capability of the modified θ projection model was significantly improved over the original one, as it reduces the prediction uncertainty from a range of 10% to 20% to below 5%. Meanwhile, it was shown that the model can be reasonably used for predicting constant stress creep conditions, when appropriate parameters are used. The prediction performance of the modified model will be discussed in another paper. The results of this study show great potential for the evaluation and assessment of the service safety of structural materials used in applications where designs are limited by creep deformation.

Role of Hydrogen Bonding on Nonlinear Mechano-Optical Behavior of l-Phenylalanine-Based Poly(ester urea)s

The uniaxial mechano-optical behavior of a series of amorphous l-phenylalanine-based poly(ester urea) (PEU) films was studied in the rubbery state. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during deformation. When the materials were subjected to deformation at temperatures near the glass transition temperature (Tg), the photoelastic behavior was manifested by a small increase in birefringence with a significant increase in true stress. At temperatures above Tg, PEUs with a shorter diol chain length exhibited a liquid–liquid (Tll) transition (rubbery–viscous transition) at about 1.06Tg (K) under the tested strain rate of 0.017 s–1 (stretching speed of 20 mm/min), above which the material transforms from a heterogeneous “liquid of fixed structure” to a “true liquid” state. The initial photoelastic behavior disappears with increasing temperature, as the initial slope of the stress optical curves becomes temperature independent. Fourier transfo...

Compressive plastic metallic glasses with exceptional glass forming ability in the Ti–Zr–Cu–Fe–Be alloy system

We report seven TiZr-based bulk metallic glasses (BMGs) developed in the Ti–Zr–Cu–Fe–Be alloy system with critical sizes over 20mm for glass formation. Especially, the melts with compositions of Ti31.9Zr33.4Cu8.7Fe4Be22 (at.%, No. 6 alloy) and Ti32.8Zr30.2Cu9Fe5.3Be22.7 (No. 7 alloy) can directly solidify into 150g glassy ingots, and the No. 7 alloy could be cast into an amorphous rod with a diameter of 50mm and a height of 75mm by the simple copper mold casting method. Yield strength, maximum strength, the increase of true stress after yielding, and compressive plastic strain of No. 6 BMG are 1720MPa, 2010MPa, 165MPa and 8%, respectively. Other BMGs show similar mechanical properties with compressive plastic strains more than 5%, concomitant work hardening-like behavior. A parameter named as angles of spherical compositional vectors (ASCVs) was proposed to quantitatively evaluate the discrepancy between two multiple-component compositions. These plastic BMGs with exceptional GFA in the Ti–Zr–Cu–Fe–Be alloy system have enriched the family of BMGs and may have scientific significance on developing advanced BMGs for practical applications.

Analysis on flow stress of magnesium alloys during high temperature deformation

The flow stress of magnesium alloys during hot compression at different temperatures and strain rates was studied by experiments. Materials used were AZ91D alloys in as-cast, homogeneous treatment states, AZ31 and ZK60 alloys in as-cast state. The results show that the thermal simulation curves of different alloys differ from one another at the same deforming condition. The general curves of AZ31 and AZ91D alloys have the character of dynamic recrystallization. There are increase of true stress, drastic falling of true stress and increase of true stress in most curves of ZK60 alloy, while the other curves have the characteristics of dynamic recrystallization. From the analysis the reasonable deforming temperature should be selected from 523 to 673 K for AZ31 and the unhomogenized AZ91D alloy, from 473 to 673 K for the homogenized AZ91D alloy, and it was concluded to be 473 K or 673 K for ZK60 alloy.

Mechanical Behavior of Processed AZ91D by Equal Channel Angular Extrusion during Semi-Solid Isothermal Compression

Mechanical behavior of processed AZ91D magnesium alloy by equal channel angular extrusion during semi-solid isothermal compression was investigated using semi-solid isothermal compression test. The results show that there are four stages, drastic increase of true stress, drastic falling of true stress, steady stage and increase of true stress. With the increase of holding time or deformation temperature, required true stress for obtaining same true strain falls evidently and peak stress and steady stress also fall in true stress-strain curve. With increasing strain rate, steady stress also increase. Peak stress will increase or decrease with the sudden increase or fall of strain rate. During semi-solid isothermal compression, plastic deformation of semi-solid billet mainly depends on plastic deformation of solid phases and their rotation and sliding. Coarse grains occur in upper position adjacent to compression die’s surface and fine grains occur in the central and free surface positions of billets.

Al-4%Ge 合金の時効組織と疲労亀裂との関係

Fatigue crack initiation and propagation have been investigated in relation to the aged microstructure in an Al-4%Ge alloy by means of a fatigue test, microstructural characterization, and fractography. Specimens were solution-treated at 693 K, quenched in iced water, kept at 273 K for 60 s, aged at 473 K for times up to 72 ks and then fatigue-tested. It was confirmed that as the aging time increases, the tensile strength initially increases, reaches a maximum, and then decreases, while the fatigue strength monotonously decreases. Both in the interior and at the surface of the specimen, as the aging proceeded, the growth of precipitates inside the grain as well as on the grain boundary was observed while the width of precipitate free zones (PFZs) adjacent to the grain boundary was almost constant. Fractographic observations showed that intergranular fracture became predominant with increasing aging time and decreasing stress amplitude. From this result and close observations of the fatigue cracks initiation sites, the cracking was deduced to proceed in the following steps: 1) cracks initiate in the intergranular region of the specimen surface, 2) the cracks propagate along the intergranular region towards the interior of the specimen, 3) the crack path transits to the transgranular region because of the increase in true stress, 4) ductile fracture occurs with a marked reduction in area. The crack initiation was presumed to occur as a result of intrusion and extrusion formations along the soft PFZs that play a similar role to that of the persistent slip bands in most metallic materials.

Creep Lifetime Under Constant Load and Constant Stress: Theory and Experiment

Abstract Creep tests are usually performed under constant load. The remarkable reduction of time to rupture due to the increase of true stress with the elongation of the specimen and with necking is estimated on the basis of the theories by Hoff, Brooklyn, and Kachanov. The influence of necking in tests under constant load and constant stress is investigated using the finite element method (FEM) with specimens containing an initial geometric imperfection. Results of the two types of tests show specimens have different sensitivities to these imperfections. To verify the theoretical statements, two types of creep tests under constant stress were performed in which the behavior of the AISI 316 L(N) austenitic steel at 700°C was investigated. In the first type, tests under constant homogenized stress, necking of the specimen was ignored. In the second type, tests under constant true stress, the mean axial stress in the minimal cross-section was held constant. To perform this type of test, an optical measuring technique was employed to determine the minimal diameter of the specimen during the test. The observed data, in particular the time to rupture, are compared with those from creep tests performed under constant load. Remarkable differences are observed. Theory and experiments confirm that problems arise if creep tests under constant load on ductile materials yielding too conservative data are interpreted with lifetime rules or continuum damage models.

ミニチュア試験片によるクリープ破断特性の評価

Evaluation of service reliability of components, particularly the remaining service lifetime of materials, has become a very important matter for thermal power stations and other plants. The remaining lifetime is usually assessed with a fair precision from aggravation of mechanical properties, especially that of creep characteristics. One difficulty here is that, if the precision of prediction is to be improved over what is available with today's in-situ nondestructive methods, one needs to take samples directly from operating machines, whereby one has to be satisfied with a small quantity of samples. This calls for establishment of a trustworthy method for evaluating creep rupture characteristics with a small number of small-size specimens.The following observations and conclusions were made in the present study.(1) The miniature specimen creep tester, developed here with a testing chamber maintained airtight with a pair of O-rings, performed quite well, in which the combined effect of internal Ar pressure and O-rings could be safely ignored.(2) In Ar atmosphere to forestall oxidation, the rupture time obtained with the 2mm diameter miniature test specimens agreed quite well with that obtained with the 8mm diameter conventional test specimens.(3) The effect of oxidation to degrade the rupture time became manifest above 600°C.(4) The major cause of this degradation is an increase in true stress arising from the loss of material due to oxidation.

Change of molecular aggregation state in amorphous region with uniaxial extension of preoriented polypropylene

AbstractPreoriented isotactic polypropylene was uniaxially drawn at various testing directions and testing temperatures. Change of the molecular aggregation state in amorphous region with extension was elucidated by measurements of melting temperature, enthalpy of fusion, and birefringence at each stage of extension. Melting temperature depends on both crystallite thickness and orientation function of amorphous chains. It is assumed that the enthalpy change of amorphous region takes place when oriented amorphous chains are transformed into random state by heating. The ratio of the enthalpy change of amorphous region in the sample after extension to that in the sample before extension monotonously increased with increasing orientation function of amorphous chains, fa, independent of testing direction and testing temperature. Increase of true stress with drawing led to increase of fa. Increase of fa with extension depended on the testing angle θ between the testing direction and the direction of the crystal c‐axis of the preoriented sample, and fa most remarkably increased in extension at θ = 45°.