Evolution Of Tensile Properties Research Articles

Effect of W and Mo contents on the microstructural evolution and properties in the 9% Cr martensitic rotor steel during long-term aging at 630 °C

The evolution of tensile properties and microstructure in three 9% Cr martensitic rotor steel with varying contents of W and Mo elements is investigated after aging at 630 °C for different durations, with a primary focus on the characteristics of martensite laths and blocks, grain boundaries, dislocations, and precipitates. The results indicate that the presence of W and Mo elements not only influences the precipitation and coarsening behavior of Laves phases but also that of M23C6 carbides, which differs from conventional beliefs. The elevated content of Mo and reduced content of W in the steel leads to a reduction in the equilibrium Cr concentration, promoting enhanced precipitation of Cr atoms and increases nucleation sites of M23C6 carbides. During the aging treatment, the M23C6 carbides in the 2.0W0.5Mo steel exhibits the highest content and the lowest coarsening rate. The strength of 9% Cr martensitic rotor steel is primarily enhanced through dislocation strengthening and grain boundary strengthening. The presence of fine M23C6 carbides effectively immobilizes dislocations and interfaces, resulting in the exceptional stability of the microstructure in the 2.0W0.5Mo steel and a minimal reduction in yield strength.

Effect of Aging State on the Microstructure and Tensile Properties of Al-7.0Zn-2.5Mg-2.0Cu-0.1Zr-0.2Sc Alloy

Tensile experiments were conducted for Al-7.0Zn-2.5Mg-2.0Cu-0.1Zr-0.2Sc alloy in different aging states (18 h, 24 h, 36 h) with temperature environments including room temperature, −10 °C and −30 °C. Comparative studies were made on the evolution of the precipitate phase in alloys at three kinds of aging times and the evolution of tensile properties in alloys under different ambient temperatures. The findings showed that the precipitates in Al-7.0Zn-2.5Mg-2.0Cu-0.1Zr-0.2Sc alloy were mainly in the GP zone after the solution + aging treatment η’ phase, the secondary Al3 (Sc, Zr) phase and the θ’ (Al2Cu) phase. As the aging time was prolonged, the η’ phase gradually grew and the PFZ gradually widened. At the three test temperatures, the tensile strength (TS) and yield strength (YS) of the alloys both showed a trend of first increasing before decreasing with the extension of aging time, while the elongation (A) and section shrinkage (Z) showed a decreasing trend. As the test temperature decreased, the TS and YS of the alloys increased and the A and Z of the alloys decreased. At room temperature, alloys showed a ductile fracture mode, which changed to mixed ductile and brittle fracture with decreasing test temperature.

Assessment of Polyethylene Geomembrane Properties after Accelerated Thermal Ageing

Application temperatures and exposure environments are among the causes of premature antioxidant loss in polymer geomembranes. Therefore, over the long term, polymer degradation takes place which is reflected in geomembrane properties. In this work, accelerated thermal ageing of polyethylene geomembrane was carried out in a climatic chamber at 70℃ with 21% oxygen. The evolution of tensile properties, puncture resistance, and shore D hardness with exposure time was assessed. Changes in chemical and crystalline structure and melt flow index were made. According to the findings, geomembrane surface oxidation takes place in the polyethylene matrix after 9 months of exposure. This fact is confirmed by evaluating carbonyl, vinyl, and hydroxyl indexes. Uniaxial tensile properties, puncture resistance, and shore D hardness indicated slight changes after one year of exposure. While the crystallinity index, determined by XRD measurement, has shown an increase with exposure. Further, melt flow index measurements confirmed the absence of chain scission during the early exposure periods. However, for longer periods, chain scission reactions occurred and MI values increased moderately after five months of accelerated aging. All results indicated no significant bulk degradation while maintaining a certain mechanical resistance with applications under a temperature of 70℃. This temperature value is relevant in a waste storage center, but the studied geomembrane is not recommended for this case.

Evolution of Tensile Properties in AA2219 Plates Due to Varying Modes and Percentages of Cold Work before Artificial Aging

Evolution of tensile properties in 8-mm-thick AA2219-T8X plates with the pre-artificial aging (here onward termed pre-age) cold work applied independently by cold rolling and stretching was monitored by tensile tests. It is shown that the desired minimum 0.2% proof stress (0.2% PS) of 352 MPa, ultimate tensile strength of 441 MPa and percentage elongation of 7 in T87 temper could be achieved in the longitudinal direction of the alloy following 6% of pre-age cold work by stretching. However, these minimum tensile properties could be achieved in both longitudinal and long transverse directions following 7% of pre-age cold work independent of the methods by which the cold work is employed. The tensile strength properties subsequently decreased when the percentage of pre-age cold work equaled or exceeded 8%. The coarsening of θ′ (metastable Al2Cu-θ) precipitates caused the decrease in strength. It is further observed that suitable percentages of pre-age cold work involving both cold rolling and stretching not only provide the minimum desired T87 tensile properties but considerably reduce the differences in 0.2% PS values between longitudinal and long transverse directions of the plates. This is discussed in terms of the tendency of the pre-age cold work by stretching to introduce directional properties in the plates.

Evolution of tensile properties with transformation temperature in medium-carbon carbide-free bainitic steel

Different morphologies of carbide-free bainite were obtained through a series of isothermal heat treatments of a new medium-carbon bainitic steel, and the evolution of microstructures during tensile deformation was then observed. The results showed that the strength-ductility balance could reach its highest value near 350 °C. The retained austenite sustaining martensitic transformation, long bainite ferrite sheaf, and phase transformation dynamics were the main factors that caused high plasticity of the steel at 350 °C isothermal transformation. It is noteworthy that 350 °C is also a phase change sensitive point for most bainitic steels. Maintaining high work hardening rate at high strain is beneficial to increase elongation, which is attributed to the continuous martensitic transformation of the retained austenite with high volume fraction. The (200) austenite peak was separated using the Gaussian multi-peaks fitting method. It was found that the (200) austenite peak moves to the left with the increase of strain. The proportion of low angle peaks also increased with strain. This indicates that the transformation of the retained austenite always occurs in the low carbon region.

Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization (Tnr) Behavior of a High Niobium Microalloyed Steel

The non-recrystallization temperature (Tnr) of high niobium microalloyed steel was determined from both multihit and double-hit compression tests obtained under plane strain condition. The Tnr was determined to be in the range of 985 °C to 1010 °C for multihit conditions. The double-hit tests carried out at an interpass time of 5 seconds gave very low Tnr in the range of 860 °C to 900 °C. In order to understand this, double-hit experiments were carried out for different interpass times (2, 15, 100 seconds) at three different temperatures (950 °C, 1050 °C, and 1150 °C). The negative softening behavior was observed at 950 °C for higher interpass times of 15 and 100 seconds. This implies that the Tnr of this steel is well above 950 °C. This was due to the high amount of strain-induced precipitation of niobium carbonitrides at higher interpass times as revealed from TEM investigation. The precipitate size evolution considering the Dutta and Sellars nucleation condition in TC-PRISMA agrees well with the experimentally observed precipitate sizes. However, the evolution of Zener pinning forces considering TC-PRISMA nucleation condition is similar to both reported and experimentally determined values. The evolution of tensile properties also correlates well with the observed austenite recrystallization softening behavior. Therefore, a minimum interpass time of 15 seconds is required during double-hit compression tests to effectively precipitate Nb(C, N) and delay the static recrystallization softening behavior of this steel. This lead to the determination of comparable Tnr values between double-hit and multihit methods.

Effect of ratcheting fatigue damage on the evolution of tensile properties of Al–Mg alloy

This work aims to investigate the effect of damage accumulation on the evolution of tensile properties due to prior ratcheting fatigue in as-received and artificially aged conditions of Al–Mg alloy (AA 5754). Uniaxial asymmetric stress controlled tests for various combinations of mean stress and stress amplitude are performed at room temperature. Progressive degradation in the mechanical properties of structures and machines subjected to cyclic loading during service period can affect the structural integrity. In view of this, ratcheting fatigue tests are interrupted at various life fractions for particular mean stress and stress amplitude followed by tensile tests to understand the damage evolution during ratcheting for both conditions of alloy. Also, the effect of mean stress and stress amplitude on the post ratcheting tensile behaviour is analysed by conducting the ratcheting tests up to 150 cycles. Cyclic hardening is exhibited in the initial cycles followed by saturation for both as-received and artificially aged alloy. Ratcheting strain increases with the increase in both mean stress and stress amplitude. Drastic changes in tensile properties are noticed with ratcheting fatigue damage evolution for both conditions of alloy. The evolution of tensile properties due to prior loading history such as ratcheting fatigue can provide fundamental understanding in the design of structural components.

Effects of Melt Superheating on the Microstructure and Tensile Properties of a Ternary Al-15 Wt Pct Si-1.5 Wt Pct Mg Alloy

Unsteady solidified microstructures of the Al-15 wt pct Si-1.5 wt pct Mg alloy under two degrees of melt superheating are examined: 4 and 21 pct above the alloy liquidus temperature. The dendritic array and the eutectic mixture have been investigated, which are affected not only by the solidification kinetics but also by the melt superheating. The directional solidification experiments permitted that aligned Al-rich dendrites could be formed for both high- and low-melt superheatings. These dendrites appear as ‘islands’ isolated from each other and dispersed within the α-Al+Si+Mg2Si eutectic matrix. Such configuration arises in samples under the entire range of examined cooling rates, i.e., from 0.5 to 50 K/s. Directionally solidified samples having different α-Al dendrite interphase spacings have been characterized and subjected to tensile tests. Such microstructural spacings translate the effectiveness in blocking the dislocations motion during loading, which is promoted mainly by the dendritic/eutectic boundaries. This mechanism is especially operative since the applied tensile load is perpendicular to the dendritic growth path. As such, the evolution of tensile properties as a function of these spacings was assessed. Both elongation (δ) and the ultimate strength (σu) are enhanced nearly by 40 pct due to the reduction in the dendrite interphase spacing. The highest properties (σu: ~ 310 MPa and δ: ~ 7 pct) are associated with a microstructure formed by aligned dendrites with 70 μm in spacing embedded in a eutectic mixture. Three factors appear to contribute to the higher strength values observed for the alloy processed under lower-melt superheating, i.e., higher fraction of dendrites, solid solution strengthening of Mg in α-Al, and finer eutectic Si spacing.

On the use of low-density polyethylene as disposable drip tape for field irrigation

ABSTRACTIn the present study, micro-level analysis of seven months of field irrigation-induced evolution of tensile properties of a disposable drip tape made from a virgin polyethylene mixture and 2 wt% carbon black was conducted. After field irrigation, the molecular weight of the soluble small molecules in the drip tape was reduced, while their content was increased. Meanwhile, the crystallinity of the PE constituents in the drip tape increased, whereas the crystallinity of the non-PE constituents decreased. After the field irrigation, the melting point and the corresponding value caloric of the drip tape increased. During heating, the as-received drip tape showed no weight changes, whereas the drip tape after irrigation showed a continuous small weight loss. The difference in the intrinsic properties of low-density polyethylene, linear LDPE, and high-density polyethylene was the reason for the field irrigation-induced evolution of the mechanical properties of the disposable drip tape.

Effects of carbon and nitrogen on precipitation and tensile behavior in 15Cr-15Mn-4Ni austenitic stainless steels

Precipitation behavior and tensile properties have been investigated for 15Cr-15Mn-4Ni austenitic steels containing 0.2wt% carbon and/or 0.2wt% nitrogen. During aging at 600–1000°C, precipitates such as σ, M23C6, and Cr2N were formed and time-temperature-precipitation diagrams were constructed. The type, size, and density of precipitates depended on the carbon and nitrogen contents. The nitrogen addition suppressed carbide formation while the effect of carbon addition on nitride precipitation was comparatively small. The evolution of tensile properties after aging is explained by the changes in solid solution strengthening and deformation modes caused by the precipitation. The precipitation of M23C6 slightly decreased yield strength and elongation while it increased ultimate tensile strength. On the other hand, the precipitation of Cr2N hardly affected tensile properties due to its low volume fraction.

Relationship between spacing of eutectic colonies and tensile properties of transient directionally solidified Al-Ni eutectic alloy

Eutectic Al-Ni alloys are widely faced as materials to be considered for advanced structural components. Nevertheless, still there is a lack of research on microstructural aspects of the eutectic Al-6.3 wt%Ni alloy and important points need to be addressed, such as, the morphology of the unsteady solidified eutectic, size and distribution of phases under different solidification cooling rates and the effects of the resulting microstructural features on mechanical properties. As such, in the present study, an Al-6.3 wt%Ni alloy is directionally solidified under a wide range of cooling rates and the resulting microstructure is shown to be formed by eutectic colonies. Three general microstructural features characterize the colony: a fine central zone composed of fibrous α-Al + Al3Ni eutectic, a boundary coarse zone formed by lamellar eutectic, and an Al-rich zone delimiting the colony. A quantitative analysis relating solidification thermal parameters to Al3Ni and colony spacings is outlined. Furthermore, the evolution of tensile properties as a function of these spacings was examined, and the highest strength and elongation of 160 MPa and 15%, respectively, are associated with an ultrafine bimodal structure formed by eutectic colonies with 55 μm in spacing containing very fine fibers (300 nm in spacing) and lamellae with 750 nm in spacing.

Correlation between microstructure, tensile properties and fatigue life of AA1050 aluminum alloy processed by pure shear extrusion

This research is focused on the influence of multiple passes of pure shear extrusion (PSE) on grain refinement and evolution of tensile properties in AA1050 alloy. It is observed that the average cell size reduces and the misorientation angle increases with increasing number of PSE passes. High fractions of elongated grains are observed in the microstructures of the samples deformed by 1, 2 and 3 PSE passes. However, the fraction of elongated grains reduces by increasing number of PSE passes and diminishes at the 4-pass of PSE. In fact, by increasing the number of PSE passes, more homogeneous grain refinement occurs which is evidenced by formation of equiaxed cell structures with remarkable percentage of high angle grain boundaries. The most significant increase in yield and tensile strength occurs at the first pass of PSE. Strengthening continues with further PSE processing while elongation gradually improves which is attributed to reducing cell size and increasing the fraction of high angle grain boundaries. Despite of increasing elongation and strength, the fatigue life of the samples continuously reduces with further PSE processing. This is attributed to occurrence of cyclic softening in deformed samples with ultra-high strength.

A Comparative Evaluation of the Effect of Low Cycle Fatigue and Creep–Fatigue Interaction on Surface Morphology and Tensile Properties of 316L(N) Stainless Steel

In the present work, the deformation and damage evolution in 316L(N) stainless steel during low cycle fatigue (LCF) and creep–fatigue interaction (CFI) loadings have been compared by evaluating the residual tensile properties. Towards this, LCF and CFI experiments were carried out at constant strain amplitude of ±0.6 pct, strain rate of 3 × 10−3 s−1 and temperature of 873 K (600 °C). During CFI tests, 30 minutes hold period was introduced at peak tensile strain. Experiments were interrupted up to various levels of fatigue life viz. 5, 10, 30, 50, and 60 pct of the total fatigue life (N f) under both LCF and CFI conditions. The specimens subjected to interrupted fatigue loadings were subsequently monotonically strained at the same strain rate and temperature up to fracture. Optical and scanning electron microscopy and profilometry were conducted on the untested and tested samples to elucidate the damage evolution during the fatigue cycling under both LCF and CFI conditions. The yield strength (YS) increased sharply with the progress of fatigue damage and attained saturation within 10 pct of N f under LCF condition. On the contrary, under CFI loading condition, the YS continuously increased up to 50 pct of N f, with a sharp increase of YS up to 5 pct of N f followed by a more gradual increase up to 50 pct of N f. The difference in the evolution of remnant tensile properties was correlated with the synergistic effects of the underlying deformation and damage processes such as cyclic hardening/softening, oxidation, and creep. The evolution of tensile properties with prior fatigue damage has been correlated with the change in surface roughness and other surface features estimated by surface replica technique and fractography.

Effect of preform heat treatment on the flow formability and mechanical properties of AISI4340 steel

The present study reports the effects of variation in preform heat treatment on the flow formability and mechanical properties of flow formed tubes of AISI4340 steel. Preforms are subjected to three different types of heat treatments namely, annealing (ANN), spheroidizing (SPH) and hardening and tempering (HT). Sound quality seamless tubes have been produced by a two-pass backward flow forming method. Tensile properties in terms of yield strength, ultimate tensile strength (UTS), and percentage of elongation have been evaluated for both the preforms and subsequently flow formed tubes. The present study demonstrates that a combination of very high strength (UTS=1320MPa) with reasonable ductility could be achieved for flow formed HT tubes, whereas a combination of moderate strength (UTS>1050MPa) with good ductility has been obtained for flow formed ANN and SPH tubes in the as-flow formed condition. Evolution of tensile properties and flow formability of the differently heat-treated preforms are correlated to the microstructural features. Effects of microstructural features and mechanical properties of the preforms on the flow formability have further been evaluated by monitoring spindle load, spring-back, and ovality of the formed tube after each pass. It has been shown that microstructural features such as size and shape of secondary phases and strain hardening characteristics of the preform play a critical role in obtaining good flow formability in the present material.

Mechanical properties of 9Cr martensitic steels and ODS-FeCr alloys after neutron irradiation at 325 °C up to 42 dpa

Reduced activation ferritic/martensitic steels (F82H, JLF-1, 9Cr2WTaV and EUROFER-9Cr1WTaV), advanced ODS Fe–14%Cr alloys, standard and modified 9Cr1Mo conventional martensitic steels were irradiated as specimens for mechanical tests in different neutron irradiation experiments performed in the range 300–325 °C. The objective of this paper is to present the tensile and impact properties as well as the irradiation creep of these materials after irradiation to high doses (32–42 dpa) in BOR60. The evolution of tensile properties is discussed as a function of the dose up to 42 dpa including results from irradiations carried out in SM2 and OSIRIS reactors. In general, RAFM steels presented a lower level of hardening and embrittlement compared to 9Cr1Mo conventional steels. ODS-Fe–14%Cr exhibited the lowest hardening and a relatively high residual ductility at 42 dpa.