Serrated yielding in 9Cr–1Mo ferritic steel

Abstract

Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300–873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr–1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load–elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol^-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr–1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures.     

Temperature and surface potential correlations with serrated flow of low carbon steel

The Portevin–Le–Chatelier (PLC) effect manifests itself as serrated plastic flow in strain controlled tensile tests of dilute alloys. The effect causes most of the material qualities to suffer, including the ductility. Monitoring the effect is important since the induced loss of ductility can cause premature failures on structural materials under load. Surface potential fluctuations of AISI 1020 low carbon steel during its serrated flow was investigated by a combined set-up of an electrochemical cell and a slow strain tensile test. The steel was exposed to an aqueous electrolyte under a strain rate of 1.6 x 10⁻⁶/s, and the surface potential was monitored in situ during the creation, magnification, and annihilation phases of the serrated flow achieved by changing test temperatures stepwise between 50 and 85 °C. The serration amplitude showed strong correlations both with test temperatures and the surface potential. A rather large potential shift around 110 mV through active direction during the burst of serrations hinted that the slip bands, in addition to temperature and strain, could also be responsible for the charge release due to the dislocation domains carried in them. The study verifies a potential use of the set-up for monitoring the PLC events in aqueous environments where load cells, extensometers etc. are not utilizable.     

Serration and shear avalanches in a ZrCu based bulk metallic glass composite in different loading methods

In the current research, serrated flow is investigated under tensile and compressive loading in a ZrCu-based bulk metallic glass composite (BMGC) that is well known for its plastic deformability, which is higher than that of metallic glasses. Statistical analysis on serrations shows a complex, scale free process, in which shear bands are highly correlated. The distribution of the elastic-energy density stored in each serration event follows a power-law relationship, showing a randomly generated serrated event under both tension and compression tests. The plastic deformation in the temporal space is explored by a time-series analysis, which is consistent with the trajectory convergent evolution in critical dynamic behavior even in the low strain rate regime in both tests. The results demonstrate that the secondary phase in the BMGC can stabilize the shear band extension and facilitate the critical behavior in the low strain rate regime. This study provides a strong evidence of serrated flow phenomenon in BMGC under tension test, and offers a deep understanding of the correlation between serrations and shear banding in temporal space.     

Substitutional dynamic strain ageing in an iron — 1.1 at. % niobium alloy

The mechanical properties of an Fe-1.1 at. % Nb alloy have been studied in compression over the temperature range 300 to 1100°K. The substitutional niobium atoms were responsible for dynamic strain ageing which resulted in a small peak in the temperature dependence of the flow stress, negative strain rate sensitivity, and serrated stress strain curves.The serrations were preceded by a strain rate and temperature dependent critical strain. These dependencies were analysed using theories that have been successfully applied to substitutional strain ageing in fcc structures. The analysis showed that, unlike in fcc structures, the apparent activation energy for the onset of serrated flow increased at the faster strain rates; this was attributed to the vacancies produced by the plastic deformation rapidly annealing out due to the high temperatures involved at the faster strain rates.     

Effect of solute concentration on Portevin-Le Chatelier effect in Al-Cu alloys

The solute concentration and precipitated phase content in alloys were altered by solution treatment (ST) at different temperatures. A series of serrated load curves was obtained in tensile tests at a constant applied strain rate. The influences of solute concentration and precipitated phase content on dislocation movement were analyzed with tensile test results, and the micro-mechanism of the Portevin-Le Chatelier (PLC) effect is discussed in this paper. In the tests, when ST temperature was reduced from 500°C, the amplitude of serrated flow decreased. It reached the minimum when ST temperature was reduced to 300°C. On the other hand, the amplitude of serrated flow increased with ST temperature decreasing from 300 to 100°C. Experimental results show that when ST temperature is higher than 300°C, solute concentration is a governing factor to PLC effect; whereas when ST temperature is lower than 300°C, the precipitated phase content significantly affects the PLC effect. Translated from Acta Metallurgica Sinica, 2006, 42(12): 1248–1252 (in Chinese)     

Influence of heat treatment on creep of a Mn–N stabilised austenitic stainless steel

Creep at 700 °C/196 MPa and 900 or 925 °C/27.4 MPa of 21Cr–4Ni–9Mn austenitic stainless steel is determined as a function of the heat treatment. The heat treatment variation involves altering the solution heat treatment cooling rate from water quenching to cooling at 6 or 4 °C/min causing: serrated grain boundaries versus planar grain boundaries, coarser intergranular carbides, and discontinuous precipitation of grain boundary reaction zones. Water quenching causes improved creep resistance. Creep fracture and cracking is intergranular. Coarse intergranular carbides and grain boundary reaction zones cause premature void formation and cracking, this damage leading to an accelerating creep rate and lowering creep resistance of the more slowly cooled conditions. During creep, grain boundary serrations, which may otherwise contribute to improved creep, are eliminated. Determining the individual influence of grain boundary serrations on creep requires a detailed investigation of various heat treatment parameters to prevent concurrent formation of grain boundary reaction zones and serrations.     

Compressive creep of Fe3Al-type iron aluminide with Zr additions

High-temperature creep of a Fe₃Al-type iron aluminide alloyed by zirconium was studied in the temperature range 873–1073 K. The alloy contained (wt.%) 31.5% Al, 3.5% Cr, 0.25% Zr, 0.19% C (Fe balance). It was tested in two states: (i) as received after hot rolling and (ii) heat treated (1423 K/2 h/air). Creep tests were performed in compression at constant load with stepwise loading: in each step, the load was changed to a new value after steady state creep rate had been established. Stress exponent and activation energy of the creep rate were determined and possible creep mechanisms were discussed in terms of the threshold stress concept. A rapid fall of the stress exponent and of the threshold stress with the increasing temperature indicates that creep is impeded by the presence of precipitates only at temperature 873 K. The results were compared with the results of long-term creep tests in tension performed recently on the same alloy. __________ Translated from Problemy Prochnosti, No. 1, pp. 117–120, January–February, 2008.     

Unusual grain-size and strain-rate effects on the serrated flow in FeMnC twin-induced plasticity steels

The purpose of the paper is to clarify the serrated flow behavior and to explore the grain-size and strain-rate effects on serrations in coarse- and fine-grained FeMnC twin-induced plasticity (TWIP) steels at room temperature. Tensile tests were performed under extensometer-measured strain control, rather than under conventional cross-head displacement control, at strain rates ranging from 6 × 10^?6 to 6 × 10^?3 s^?1. The results indicate that both the coarse- and fine-grained steels show different types of serrations, which depend on strain rate, and demonstrate a nonmonotonic strain-rate sensitivity of the critical strain for the onset of serrations, i.e., a positive strain-rate sensitivity of the critical strain in the high strain-rate region typified by type A serrations, and a negative strain-rate sensitivity in the low strain-rate region typified by type C ones. As compared to the coarse-grained steel, the fine-grained steel increases the critical strains, showing an inverse grain-size effect. The findings suggest that the fine-grained FeMnC TWIP steel suppresses the serrated flow, possibly due to the enhanced dynamic recovery associated with the decreased planar slip length in the fine-grained low stacking-fault-energy steel.     

Investigation of serrated flow behaviour in Al–10Mg alloy

Abstract

In view of reported anomalies in the serrated flow behaviour of aged Al–8.6Mg alloy, characteristics of serrated flow were investigated in an Al–10Mg alloy after solution treatment as well as after aging. The material was prepared by melting and casting, and then it was extruded, solution treated, and aged at either 150 or 200°C. Strain rate sensitivity, types of serration, onset strain of serrated flow, magnitude of serrations, and frequency of serrations were studied as a function of aging and strain rate. It was found that the alloy exhibited all the usual features of serrated flow except one, i.e. the magnitude of serration increased in the overaged condition after decreasing up to peak aging.     

Serrated flow induced by twin boundary–slip band interactions in a FeNi-base austenitic alloy

Tensile tests on a FeNi-base austenitic alloy were conducted at room temperature (RT) and 400 °C, when serrated flow did not and did occur, respectively. Deformation microstructures such as topographies of slip bands (SBs), morphologies of twin boundaries (TBs) and arrangements of dislocations near TBs, as well as concentrations of strain were investigated. It is shown that TBs block SBs and induce remarkable stress accumulations at 400 °C. Effect of TB-density on the serrated flow was also investigated by comparative tensile tests on specimens with different TB densities at 400 °C. Details of tensile curves reveal that more TBs induce more pronounced serrations. Therefore, interaction between TBs and SBs is proposed to induce the serrated flow of the FeNi-base alloy at 400 °C.     

Smaller Deborah number inducing more serrated plastic flow of metallic glass

Spherical nano-indentations of Cu₄₆Zr₅₄ bulk metallic glass (BMG) model systems were performed using molecular dynamics (MD) computer simulations, focusing specifically on the physical origin of serrated plastic flow. The results demonstrate that there is a direct correlation between macroscopic flow serration and underlying irreversible rearrangement of atoms, which is strongly dependent on the loading (strain) rate and the temperature. The serrated plastic flow is, therefore, determined by the magnitude of such irreversible rearrangement that is inhomogeneous temporally. A dimensionless Deborah number is introduced to characterize the effects of strain rate and temperature on serrations. Our simulations are shown to compare favorably with the available experimental observations.     

X70管线钢的室温蠕变及其对流变应力的影响

利用MTS万能试验机研究了X70管线钢在不同应力加载速率和不同加载过程下的室温蠕变现象,以及室温蠕变对流变应力的影响.结果表明,X70管线钢有明显的室温蠕变现象存在,应力加载速率和加载过程对室温蠕变变形量有明显的影响,而且室温蠕变显著提高了材料的流变应力.并根据位错理论对实验结果进行了分析解释.     

Stress/strain/time relations for ice under uniaxial compression

Results of mechanical tests involving uniaxial compression of isotropic ice at ?5°C were analysed and interpreted. Constant load (CL) creep tests were made for applied stresses in the range 0.8 to 3.8 MPa, and “strength” tests under constant displacement rate (CD) were made for applied strain rates in the range 10^?7 to 10^?3 s^?1. Results from CL tests and CD tests corresponded closely, giving much the same information about failure strains, strength, creep rates, time to failure, stress/strain-rate relations, and suchlike. Empirical stress/strain-rate relations were developed for three distinct states of strain: (1) for the initial yield point, where axial strains are typically of the order of 0.1%, (2) for the ductile yield point, where axial strains are typically of the order of 1%, (3) for an axial strain of 10%. Stress/strain-rate relations and stress/strain relations for constant duration of CL loading were examined for load durations up to half an hour. The elapsed time up to the ductile yield point (～1% strain) was related to stress and to strain rate for CL tests and CD tests, and correspondence of the results was demonstrated both for interrelationship between CL and CD tests and for compatibility with the appropriate stress/strain-rate relations. The elapsed time up to the initial yield point was also considered. It was shown that CD stress/strain curves can be constructed from a suitable family of CL creep curves, and vice versa.The characteristics of CL creep curves and CD stress/strain curves were examined in some detail, considering relations between strain rates for certain identifiable points on creep curves, and relations between stresses for certain identifiable parts of stress/strain curves. Effective values for quasi-elastic moduli were considered. The strains for various critical points were compared with each other and with the strains at which rates of acoustic emissions reach maximum values.     

Study on dynamic strain aging phenomenon of 3004 aluminum alloy

3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10⁻⁵ to 5.56 × 10⁻³ s⁻¹) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, T_t, was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than T_t. According to the activation energy, it is believed that the process at the temperature region lower than T_t is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations.     

Effect of solute atom concentration and precipitates on serrated flow in Mg-3Nd-Zn alloy

Influence of solute atom concentration and precipitates on serrated flow, i.e., Portevin-Le Chatelier effect,was studied in Mg~(-3) Nd-Zn alloy by tensile test at 250?C with a strain rate of 1 × 10~(-3) s~(-1). Microstructure and tensile property of the Mg~(-3) Nd-Zn alloy in solution and aging conditions were also investigated.Results indicate that the serrated flow was weakened with aging time, and geometry of the serrations changed from sharp to rounded corner. Through analyzing the mechanism of the interactions between dislocations and solute atoms, it was identified that the precipitates did not only weaken the serrated flow due to the decrease in the concentration of solute atom, but also regulate the serration type by restraining the movement of dislocations during high temperature deformation.     

Superplasticity

Superplasticity is the phenomenon of extraordinary ductility exhibited by some alloys with extremely fine grain size, when deformed at elevated temperatures and in certain ranges of strain rate. To put the phenomenology on a proper basis, careful mechanical tests are necessary. These are divided into (i) primary creep tests, (ii) steady state deformation tests, and (iii) instability and fracture tests, all of which lead to identification of macroscopic parameters. At the same time, microstructural observations establish those characteristics that are pre-requisites for superplastic behaviour. Among the macroscopic characteristics to be explained by any theory is a proper form of the equation for the strain rate as a function of stress, grain size and temperature. It is commonly observed that the relationship between stress and strain rate at any temperature is a continuous one that has three distinct regions. The second region covers superplastic behaviour, and therefore receives maximum attention. Any satisfactory theory must also arrive at the dependence of the superplastic behaviour on the various microstructural characteristics. Theories presented so far for microstructural characteristics may be divided into two classes: (i) those that attempt to describe the macroscopic behaviour, and (ii) those that give atomic mechanisms for the processes leading to observable parameters. The former sometimes incorporate micromechanisms. The latter are broadly divided into those making use of dislocation creep, diffusional flow, grain boundary deformation and multimechanisms. The theories agree on the correct values of several parameters, but in matters that are of vital importance such as interphase grain boundary sliding or dislocation activity, there is violent disagreement. The various models are outlined bringing out their merits and faults. Work that must be done in the future is indicated.     

Grain Boundary Sliding at Serrated Grain Boundaries

A constitutive model is developed for grain boundary sliding (GBS) at serrated grain boundaries. Based on a previously developed GBS model, using the dynamics of grain boundary dislocation pile-up, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Thus, a geometric factor is introduced to account for the effects of serration wave length and amplitude on the GBS rate, as compared to the GBS rate at planar boundaries. By considering the role of grain boundary shear stress in stress balancing, the proposed model removes the singularity at planar boundaries which exists in the diffusion-controlled GBS model at serrated grain boundaries. The modified model describes very well the transient creep of complex Ni-base superalloys with and without grain boundary serrations and should be suitable for other engineering alloys (with the exception of columnar grained and single crystal alloys).     

Role of dynamic strain ageing in low cycle fatigue

Low cycle fatigue (LCF) at elevated temperatures is known to be influenced by time-dependent processes like creep, oxidation and metallurgical instabilities. Another time-dependent phenomenon namely, dynamic strain ageing (DSA) has been found to exert an influence on LCF behaviour at high temperatures. Research activities carried out in the present author’s laboratory with a view to understanding the effects of DSA on LCF are highlighted in this paper. Occurrence of DSA manifests during total strain-controlled fatigue tests in the form of serrated plastic flow in stress-strain hysteresis loops, increased cyclic work hardening and reduced plastic strain range. Further, DSA causes localization of plastic flow leading to enhanced planarity of slip and widely-spaced slip bands. Impingement of slip bands on grain boundaries causes increased grain boundary decohesion, leading to reduced fatigue life. The influence of prior microstructure such as second phase particles and grain size on the effects of DSA on LCF is also discussed.     

熔体发泡法泡沫铝的力学性能研究

对采用熔体发泡法制造的不同密度泡沫铝进行了准静态压缩试验、拉伸试验和弯曲试验。结果表明,泡沫铝的压缩特性曲线包括线弹性变形区、平台区和密实化区。试样的高宽比H/D明显影响压缩应力-应变曲线。当H/D较小时,平台应力曲线较平滑;当H/D较大时,平台应力曲线剧烈波动,呈显著的锯齿状。且在试样中间部位出现与加载轴线呈25°—45°的剪切带。拉伸和弯曲过程中,泡沫铝应力快速增加,当达到应力峰值即屈服点后急剧减小,在最终破断失效前,没有明显的屈服变形带。压缩坪应力Rpl、拉伸屈服应力RUTS和冷弯屈服应力Rf随密度的增加而增加。     

Aging Effect on Elapsed Time of Serrations in Al-Li Single Crystals

Elapsed time of serrations in Al-Li single crystals was investigated with two definitions: one is the time elapsed in one cycle of serrations, the other is the average elapsed time within a strain limit.Elapsed time of individual serration appears independent on stress drops during serrated flow. It primarily distributes within the limits of short elapsed time in peak aged condition compared with underaged and overaged conditions, corresponding to low level of the average elapsed time that appears indifferent to increasing strain. It seems that shearing behaviour of δ' particles could be responsible for the variations in elapsed time of serrations.