Initial Hardening Stage Research Articles

Unveiling deformation microstructure during cyclic straining of 316 L stainless steel at 873 K

Cyclic plastic deformation behaviour of 316L steel with an average grain size ∼ 30 μm was investigated at high temperature (873K) under completely reversed straining at ±0.5% strain amplitude with a constant total strain rate of 3 × 10−3S−1. Maximum true tensile stress initially increased due to cyclic hardening until 100 cycles, then stress saturation occurred and finally failed at 1250 cycles. Deformation microstructure varied significantly with imposed fatigue cycles. Occurrence of planar glide and isolated dislocation walls were observed in the initial hardening stage, while dislocation walls accompanied by connected equiaxed dislocation cells were predominant in the saturation stage. The deformation microstructure at failure stage mainly composed of elongated dislocation cells and diffusive dislocation walls. Dislocation densities estimated by X-ray diffraction were ∼ 1013m-2.

Phase transformation and deformation behavior under isothermal compression in β-quenched metastable Ti-10V-2Fe-3Al alloy

Comprehensive knowledge on the hot deformation behavior and microstructure evolution of metastable titanium alloys is critical for process optimization and microstructure regulation. The isothermal compression of β-quenched metastable titanium alloys (Ti-10 V-2Fe-3Al) was experimented at different temperatures (600 ℃−800 ℃) with the strain rate of 10−3 s−1 in present work. The stress-strain curves show that the flow stress reduces with increasing temperature, and the characteristics of the stress-strain curve at low temperature (600 ℃) are clearly distinct from those at other high temperatures. During the isothermal compression process at 600 °C, the deformation went through four stages: initial linear hardening (stage Ⅰ), Discontinuous yielding phenomenon (DYP, stage Ⅱ) followed by work hardening (stage Ⅲ) and final flow softening (stage Ⅳ). Further observations of the microstructure throughout each deformation stage revealed that the precipitation of αGB and the proliferation of dislocation lead to initial linear hardening behavior. The softening of deformation behavior during the discontinuous yield stage is originated from the fragmentation of αGB and the generation of a large amount of moving dislocations. Almost all β phase transformed into lamellar α phase as the deformation degree increased, which is the main cause of work hardening after discontinuous yield. The major causes of final flow softening are the formation of deformation bands and spheroidization of α phase. The present work analyses the correlation between the deformation behavior and microstructure features on the deformation mechanisms of metastable β titanium alloys and reveals the potential mechanism of metastable β alloy under thermal deformation and provides more comprehensive information for the optimization of the process parameters of hot deformation.

Experimental and constitutive modelling studies of type 316L stainless steel based on internal stress under low cycle fatigue and creep-fatigue interaction

Low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behaviour of 316L stainless steel at 550 °C were investigated using experimental and modelling approaches. Results demonstrate that friction and back stress contribute to the initial hardening stage, whereas the former is primarily responsible for the subsequent softening stage. Besides, friction stress also accounts for the non-masing feature and varied softening rate. Increasing holding time induces a rise followed by a decline in friction stress, while the back stress decreases steadily to a saturated value. Moreover, a short tensile holding duration leads to planar structures and higher dislocation density. However, a longer tensile holding time promotes dislocation annihilation and subgrains growth, and elongates dislocation cells. Finally, an improved viscoplastic constitutive model was proposed based on the internal stress analysis. The kinematic hardening rule was improved by considering the cycle-dependent plastic modulus and stress relaxation, and the non-masing and variable softening rate were captured by modifying the isotropic hardening rule. In addition, holding time-related factors were also coupled into the model. The proposed model is robust for describing the cyclic features under both LCF and CFI.

Nano-phase transformation of composite precipitates in multicomponent Al-Mg-Si(-Sc) alloys

• Sc-atoms may distribute at the β"-precipitate/Al-matrix interface in an Al-Mg-Si-Sc alloy. • Sc-atoms can enter the B'-substructures and/or locate at the precipitate/Al interfaces. • Sc-atoms can greatly improve the stability of transitional β"/B'/β′ composite precipitates. • Sc-addition exhibits a much better thermal stability than that without Sc. Sc-addition can significantly enhance the performance of the micro-alloyed Al-Mg-Si-Sc alloys. However, the mechanisms by which the Sc element modifies the microstructure of the alloys are still unknown in many cases. Here, using atomic-scale transmission electron microscopy and atomic-resolution spectroscopy, we have revealed the microstructural differences between two age-hardened Al-0.5Mg-0.4Si (wt.%) alloys with and without Sc-addition. The first significant effect of Sc-addition on the precipitation microstructure of the Al-Mg-Si-Sc alloy is that Sc-atoms may distribute at the β"-precipitate/Al-matrix interface and therefore accelerate aging kinetics at the initial stage of hardening. The second significant effect of Sc-addition is that in the transition from the β"-hardened peak-age stage to the β′-hardened late stage, Sc-atoms can greatly improve the stability of transitional β"/B'/β′ composite precipitates by entering the B'-substructures and/or locating at the precipitate/Al interfaces. As such Sc-atoms effectively suppress β" to β' transformation and cross-sectional coarsening of both β" and composite precipitates, leading to much finer precipitate needles with smaller diameter but much larger length, as compared with those precipitate needles formed in the alloy without Sc-addition. Hence, the alloy with Sc-addition exhibits a much better thermal stability than that without Sc.

Ultrafast transfer strength of reinforced concrete sleepers by Using complex additives

The influence of superplasticizers-polycarboxylates and their complex additives withaccelerants - electrolytes and dispersion of calcium hydroxylates on the early strength of concretefor concrete of concrete sleepers has been studied. It was found that the use of superplasticizerspolycarboxylates can improve the early strength of the concrete after heat treatment, but eachadditive must be checked for consistency with the cement used. Approx Complex additives withaccelerators in conditions of natural hardening ensure an increase in the early strength of concrete,but some of them, which accelerate natural hardening, can reduce the strength after WWTP and mustbe checked before use. Electron-microscopic examinations of the structure of cement stone withadditives were carried out and it was found that the additive of only policarboxylate does not changethe structure of hydration products. Complex additives lead to formation of additional number ofcrystalline hydrates of AFm- and/or AFt-phases. The scheme of development of the cement hardeningprocess and the initial stage of cement hardening without or with additives has been developed. Bymeans of which the highest accelerating effect of the complex addition of polycarboxylate andcalcium hydroxylate dispersions was established by experimental investigations, Dispersion particlesincrease surface area where crystallization (condensation) of cement hydration products takes placeand ensures faster filling of spaces between cement particles (mineral additives) with them, fillers)with establishment of lances with electro-heterogeneous contacts. As a result of potentiodynamic andmicroscopic investigations it was established that the tested additives in the dosages do not causecorrosion effect on the steel reinforcement bars. Transmission strength values of 32 MPa for sleepersafter 24 years of hardening under low-heat regime and after 2 days of natural hardening have beenachieved. The formula for economic efficiency of using additives to reduce the energy intensity ofproduction of concrete sleepers has been proposed.

Characteristic stepwise strain hardening behaviour induced by slip and twinning of large-deformed copper single crystals: crystal plasticity modelling and simulation

ABSTRACT The micromechanism-based stepwise strain hardening behaviour of copper single crystals significantly depends on the competition and collaboration between dislocation slip and deformation twinning. A crystal plasticity-based model of the strain hardening characteristics associated with slip and twinning interactions is proposed that focuses on modelling the stepwise strain hardening behaviour. An accumulated twin volume fraction is incorporated into the evolution of slip resistance to model slip–twinning (S-T) interactions at small strains. Then, the following stress fluctuation stage caused by the crystal reorientation is related to the twinning–twinning (T-T) interactions. Upon deformation to a large strain, a saturated hardening law including the slip–slip (S-S) interactions in the twinned region and the accumulated slip are presented. The effects of the initial crystal orientation and twinning on distinguished hardening stages are investigated. The stepwise strain hardening model reveals the dominating hardening mechanism at the initial hardening stage, which has been identified for orientation-dependent copper single crystals by investigating the slip and twinning increments. The stress fluctuation of a copper single crystal with a particular grain orientation can be described by introducing the evolution of the twin volume fraction and its saturation value, which allows an in-depth understanding of the evolution of large plastic deformation.

Microstructure and tensile properties of magnesium nanocomposites fabricated using magnesium chips and carbon black

In this study, carbon black (0, 0.01, 0.03 and 0.08 wt%) and AZ31 (Mg–3Al–1Zn) magnesium chips were used to fabricate carbon black-reinforced magnesium matrix composites with extrusion or a combination of extrusion and high-ratio differential speed rolling. After hot pressing at 693 K and extrusion at 623 K with an extrusion ratio of 22, the magnesium chips coated with carbon black were soundly bonded into a bulk composite material. The grain sizes of the extruded materials were similar with a size of 48.2–51.5 µm despite the difference in the amount of carbon black. The yield strength and ultimate tensile strength increased from 177 to 191 MPa and from 240 to 265 MPa, respectively, as a result of the addition of 0.01% carbon black; however, a further increase in the strength was marginal with additional carbon black. The same trend was observed in the strain hardening behavior. The tensile elongation increased by to the addition of 0.01% carbon black (from 15.8% to 17.4%) due to the increased work hardening effect, but decreased with additional carbon black due to its agglomeration and poor dispersion at higher concentration. After high-ratio differential speed rolling (HRDSR) on the extruded materials and subsequent annealing, the AZ31 and AZ31 composites had a similar fine grain size of 16.3–17.9 µm. The annealed HRDSR composites showed the best mechanical properties at a higher content of carbon black (0.03%) compared to that (0.01%) for the extruded composites. This resulted from the enhanced dispersion effect of the carbon black due to the high shear flow induced during the HRDSR process. The extruded composites exhibited the three distinct hardening stages (stage II, stage III and stage IV), while the annealed HRDSR composites mainly displayed the stage III hardening. The addition of carbon black increased the strain hardening rate at all the strain hardening stages in both of the extruded and annealed HRDSR materials. At the initial hardening stage, the strain hardening rates of the extruded composites were higher than those of the annealed HRDSR composites, but this became reversed at the later stage of hardening. Possible explanations for this observation were discussed. The strength analysis suggests that dislocation-carbon black interaction by Orowan strengthening and dislocation generation due to a difference in thermal expansion between matrix and carbon black are the major strengthening mechanisms.

Extremely rapid age hardening behavior in solution treated Ti-5Al-2Fe-3Mo

To grasp age hardening and phase transformation behaviors in a β rich α+β type titanium alloy, Ti-5Al-2Fe-3Mo, during aging at 300-500°C after the solution treatment at high α+β temperature. Vickers hardness and microstructure changes during aging were closely investigated using XDR and TEM/EDS. Vickers hardness rapidly increased with increasing holding time and reached about 440HV by aging at 450°C for only 5 min. It further increased to 510HV in 8h of aging time. The initial stage of age hardening is extremely fast compared to that in other conventional α+β and β type titanium alloys. After aging for only 5min, extremely fine acicular products of about 2 to 10 nm in width were formed in the transformed β phase. TEM/EDS analysis revealed that all substitutional alloying elements, Al, Fe and Mo, homogeneously distributed after the aging, indicating that the transformation is diffusionless as far as substitutional elements are concerned just like martensite transformation although it has time dependency. To explain the mechanism of this unique phase transformation having features of isothermal martensite transformation, we propose bainitic transformation where interstitials such as O diffuse without conspicuous diffusion of substitutional elements.

Influence of Mn addition on cyclic deformation behaviour of bainitic rail steels

The relationship between the Mn content and the cyclic deformation behaviour in bainitic rail steels is not yet clearly understood. The aim of this paper is, therefore, to study the cyclic plastic response of bainitic rail steels with different Mn contents. In order to meet these goals, low-cycle fatigue tests under fully-reversed strain-controlled conditions are performed at strain amplitudes in the range of 0.6 to 1.0%. The microstructure is analysed by TEM, SEM and XRD, and fracture surfaces are examined by SEM. The results show that the cyclic stress-strain response is significantly affected by the Mn addition. The steel without Mn initially strain hardens and then strain softens until final failure, while the steels with Mn content have an initial stage of strain hardening followed by saturation. Concerning the fatigue response, the addition of Mn is detrimental under higher strain amplitudes but, on the contrary, leads to longer lives under lower strain amplitudes. The fatigue response is explained by the different microstructure morphologies associated with the Mn alloying element.

The effect of dried paper-mill sludge on cement hydration

In the paper, the impact of paper-mill sludge dried at 75 °C (PS) on cement hydration is being analysed. The used specimens were made of CEM I 42.5 R cement and the added PS replaced 0%, 5%, 10% and 15% by mass of cement. The results of calorimetric measurements of the mixtures and the compressive strength were analysed (W/(C + PS) = 0.35). SEM and XRD tests were carried out after 3, 14, 28 and 56 days of curing W/(C + PS) = 1. PS was found to extend the induction period of the cement hydration and delay the time of the secondary heat release effect, especially in specimens with the highest content of the PS. The total heat release (after 96 h) measurement results showed that the highest total heat evolved from specimens without the PS. The total liberated heat gradually increased with the increase in the PS content in the cement paste mixture. In the specimens with 15% of PS, the total heat release decreased by 20%, as compared to the control specimen without the additive. XRD test results revealed that the standard minerals listed below were formed in cement stone: ettringite, calcite, portlandite, and calcium aluminium silicate hydrate. SEM tests revealed significant changes in cement stone microstructure caused by the increase in the PS in cement paste. The compressive strength of specimens with PS waste was found to be considerably lower in the initial stage of hardening; however, when samples hardening time is increased up to 56 days, the difference between the values of compressive strength for specimens 5% of PS and the control specimens decreases.

Plastic intermittency during cyclic loading: From dislocation patterning to microcrack initiation

In metallic materials subjected to cyclic loading, strain hardening as well as fatigue crack initiation have been linked for a long time with the evolution of dislocation patterns and structures. In particular, the development of low-energy dislocation configurations such as persistent slip bands (PSBs) is considered as a precursor to crack initiation. However, the associated scenarios have been elaborated mainly from postmortem observations capturing only static pictures of dislocation patterns, while the dynamics of the problem has been somewhat overlooked. Here we analyze collective dislocation dynamics during cycling loading of aluminum using acoustic emission (AE). A strong link is revealed between dislocation patterning, cyclic hardening/softening, and the intermittency of plasticity: Plastic intermittency and dislocation avalanches rapidly decay during the initial hardening stage, in conjunction with the reduction of an internal length scale characterizing the dislocation structure. However, in nonannealed samples, a transient softening stage ensues, associated with a brutal reorganization of this structure. These initial stages of cyclic deformation illustrate the competition between two phenomena: collective dislocation dynamics, governed by long-ranged elastic interactions among dislocations, and the emergence of a self-organizing network controlled by short-range interactions and progressively inhibiting collective effects. Later on, the emergence of PSBs is accompanied by a reincrease of the AE intermittent activity. We propose that the associated AE bursts may be the signature of collective and coordinated dislocation motions along PSBs leading to the formation of incipient microcracks.

Investigation of magnesium oxychloride cement at the initial hardening stage

The paper investigates the process of variation of magnesium oxychloride cement deformations at the initial hardening stage depending on the activity of magnesium oxide powder which is determined by the parameters of the source material burning. Investigation is focused on magnesium cements obtained from pure magnesium hydroxide. Source materials were burnt at various temperatures with the purpose to obtain magnesium oxide powder with different activity. Regular content of hydrated phases was determined in hardened magnesium cement prepared on the basis of binders with different activity. The study reveals the influence of magnesium oxide powder activity on the process of deformation occurrence in hardened magnesium cement and its tendency to crack formation.

Investigation of magnesium oxychloride cement at the initial hardening stage

Investigation of magnesium oxychloride cement at the initial hardening stage

Predicting the strength and maturity of hardening concrete

When predicting the growth of strength and maturity of hardening concrete, the role of the temperature factorshould be taken into account. Taking into account the temperature factor requires at the initial stage of hardening of concrete low positive temperatures and low speed of heating concrete. The level of maximum heating of concrete will depend not only on the exothermy of the cement, but also on the amplitude of the fluctuations in the ambient temperature. The contribution to the cracking of concrete of various types of cements from the point of view of heat release during their hydration is analyzed. The existing models for predicting the character of the set of strength and maturity of concrete on the basis of data on heat dissipation during hydration of cement in concrete structures are analyzed. It is shown that laboratory tests are insufficient to evaluate the nature of heat generation. To effectively predict the physical and mechanical properties, it is necessary to conduct temperature-time monitoring during the concrete hardening in the design to take into account the influence of external factors.

Cyclic deformation behaviors of a high strength carbide-free bainitic steel

The cyclic deformation behaviors of low-temperature bainite, lower bainite and upper bainite obtained on a high-strength carbide-free bainitic steel were examined through low-cycle fatigue testing. The relationship between the bainitic microstructure and fatigue behavior was systematically studied using scanning electron microscopy, transmission electron microscopy, atom probe technology, and electron back-scattered diffraction analyses. The results show that low-cycle fatigue undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. The low-temperature bainite exhibits a long fatigue life under the total strain amplitudes, because of its high strength and larger high-angle misorientation distribution of the packets of bainitic ferrite plates. The low-temperature bainite also presents bilinearity in the Coffin–Manson plots. Coordinate deformation and high-uniform elongation lead to prolonged fatigue life at low plastic strain amplitude, whereas phase component primarily affects the fatigue life at high plastic strain amplitude. Highly stable film-like retained austenite is beneficial to arresting fatigue crack propagation. Blocky type retained austenite easily transforms to martensite, resulting in high compatible deformation capability. The hardening ability of the low-temperature bainite is higher than that of the upper bainite, which is attributed to its high pre-existent density of dislocations transformed from movable to immovable during initial hardening stage.

Effect of microstructure characteristics on tetracalcium phosphate–nanomonetite cement in vitro cytotoxicity

MC3T3E1 murine pre-osteoblastic cells were used to evaluate the cytotoxicity of tetracalcium phosphate (TTCP)–nanomonetite (DCPA) cement. The starting cement powder mixture was prepared by the in situ reaction between TTCP and a diluted solution of orthophosphoric acid in a planetary ball mill. The cements in the form of pressed cement powder mixture discs differ from each other by the method of pre-treatment and degree of the transformation of cement components in phosphate-buffered saline (PBS). For the evaluation of TTCP–DCPA cement to be non-cytotoxic, it was sufficient to apply the short-time soaking in PBS solution, regardless of whether the cement components were completely transformed or not. If the texture motif and hydroxyapatite particle morphology were properly developed during the initial stage of hardening, the cement cytotoxicity or osteoblast proliferation were insignificantly influenced by the soaking time or the texture stability during cell cultivation, but the lattice ordering enhanced cell proliferation. Results showed that the surface texture and the hydroxyapatite particle morphology are crucial for in vitro cement cytotoxicity evaluation.

Microstructure formation in porous calcium phosphate-chitosan bone cements

We have studied the microstructure formation in composite cement materials consisting of amorphous calcium phosphate and chitosan and reinforced with sodium alginate granules, their compressive strength, and the kinetics of their dissolution in an isotonic solution. Hardening in air allowed us to obtain a cement stone with a strength of up to 18 MPa and limiting strain of 6–8%. During hardening in simulated body fluid, the maximum strength decreased to 0.8 MPa, and the strain increased to 25–30%, which was due to the dissolution of the granules and the increase in the elasticity of the chitosan framework. The calcium ion release to the isotonic solution varied nonmonotonically, with a maximum in the initial stage of hardening. The cements are intended for the fabrication of porous matrices in bone tissue regeneration.

The Influence of Based on Polyalcohols Additive on the Cement Paste Shrinkage

The influence of a shrink-resistant “Estrifan Additive RCL” additives on the cement paste shrinkage was researched. The cement paste shrinkage is reduced when we use this additive. When the content of additives is more than 2 % there is expansion in the initial stage of hardening. The extension period is longer and the value of expansion deformation is higher, the additive dosage is higher. The presence of superplasticizer Muraplast FК48 enhances the initial expansion effect and shrinkage decrease by reducing the rate of evaporation of water. On the contrary, in a separate action superplasticizer increases the cement paste shrinkage.

Identifying the Effect of 475°C Embrittlement on the Cyclic Stress-Strain Response of Duplex Stainless Steel by Means of the Change in the Yield Stress Distribution

Duplex Stainless Steel is prone to the 475°C embrittlement, which leads to a change of the material properties. In this study the change in the cyclic stress-strain behavior was analyzed in room temperature fatigue tests in a condition without embrittlement and after annealing at 475°C for various times (different degrees of embrittlement), giving different degrees of pre-embrittlement. In the cyclic deformation curves a stronger softening with increasing degree of the embrittlement can be seen after an initial stage of cyclic hardening. In order to characterize the separation of the cyclic plastic deformation between the ferritic and austenitic phase of Duplex Stainless Steel the yield stress distribution function of the statistical treatment of the Masing model was used. For an automatic calculation of these individual yield stress distribution functions suitable mathematical distributions were selected and applied. The development of the distributions with the degree of embrittlement and the number of loading cycles reflect the microstructural changes in both phases.

Cyclic deformation behavior of 316LN under non-proportional loading based on the analysis on the hysteresis loop and microstructure

The evolution of the effective and internal stresses of 316LN under total strain controlled non-proportional loadings is presented by partitioning the hysteresis loop, and the microstructure evolution was studied by transmission electron microscopy observation on specimens loaded with different cycles. Particular attention was paid to the two different contributions of the internal stress, intergranular and intragranular components, which were studied by taking into account quantitatively the heterogeneous dislocation structures. As compared with uniaxial loadings, the dislocation structures were observed with higher heterogeneity and density under non-proportional loadings, especially at the beginning of cycling. The dislocation rearrangement from planar structure to heterogeneous distribution occurred during the initial hardening stage led to not only an increase in the intragranular component which dominates the initial cyclic hardening, but also a decrease in the intergranular one which induces the following cyclic softening. Moreover, the hardening with the loading non-proportionality was also determined mostly by the intragranular component.