Solute Atom Atmosphere Research Articles

Dynamic strain ageing of AISI 316L during cyclic loading at 300 °C: Mechanism, evolution, and its effects

Under the influence of dynamic strain ageing (DSA), at 300°C 316L stainless steel exhibits: primary cyclic hardening due to a significant increase in dislocation density and the incidence of profuse point defects, then cyclic softening due to dislocation re-arrangement, and finally an almost stabilised response stage or secondary cyclic hardening. Flow stress serration is examined in every cycle along with a comprehensive microstructural investigation in order to study DSA and its relationship with cyclic deformation response. During primary cyclic hardening, stress serrations are initially pronounced, before gradually disappearing. There is then almost no incidence of stress serration during cyclic softening, in particular for low strain amplitude tests. However, serrated flow stress reoccurs towards the end of fatigue life. An analysis of potential dislocation locking mechanisms shows that Suzuki atmospheres of solute atoms forming in faulted areas of partial dislocations are responsible for DSA during the first two cyclic response stages. Upon further loading, Snoek atmospheres form to further strengthen the locking effect of solute atoms on dislocations, resulting in the re-occurrence of stress serration. Snoek atmospheres and the formation of a corduroy structure are responsible for secondary cyclic hardening. Finally, the influence of hydrostatic pressure on the mobility of vacancies induces a slight difference in DSA characteristics with respect to the loading direction, i.e. tensile direction or compressive direction transients.

Influence of straining and ageing on the room temperature mechanical properties of dual phase steel

In this work, changes in mechanical properties in dual phase steel containing 20% martensite volume fractions were observed at various ageing temperatures. For example, Δ Y (increase in yield strength due to strain ageing), YS and UTS exhibit maximum values at ageing temperature of 100 °C for the pre-strains of 2 and 4%. This is due to the formation of solute atom atmospheres around dislocation. When the ageing temperature increased to 200 °C, yield strength decreased due to overageing resulted from tempering that starts in martensite phase.

Static strain ageing behaviour of dual phase steels

In this work an investigation was conducted into the cold deformation ageing susceptibility of a carbon steel and a microalloyed steel, both with dual phase micro-structure. Ageing experiments after different prestrains were carried out at temperatures ranging from 25 to 250 °C. It was found that yield strength (YS) and tensile strength (UTS) of the steels with different dual phase micro-structures exhibit maximum values at ageing temperature of 100 °C after different prestrains. It is assumed that the first rise is based on the formation of solute atom atmospheres around dislocations and the further strengthening in the second step is caused by the low-temperature carbide precipitation in ferrite. When the ageing temperature increased to 150, 200 or 250 °C, YS decreased due to tempering effect in martensite. It was also found that the ageing of the microalloyed steel occurred more slowly than that of the carbon steel. The slow occurrence of ageing was clearly observed at temperatures of 100, 150, 200 and 250 °C and was attributed to the chemical composition of the steels.

Study of an anomalous serrated yielding phenomenon in 3004 aluminum alloy

The serrated yielding in metals and alloys is an important and interesting physical phenomenon. No. 3004 aluminum alloy has been subjected to tension test in a range of strain rates 5.56×10-5s-1—5.56×10-3s-1 and temperatures 223K—773K to investigate the ordinary rules of serrated yielding phenomenon. The micro-mechanism and physical essence of serrated yielding of the material has been studied by means of activation energy, internal friction and microstructure. The results show that the serrated yielding phenomenon is associated with the dynamic strain aging process. In the serrated yielding temperature region, an anomalous serrated yielding phenomenon was found. Within this temperature region, a critical transition temperature Tt exists. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than Tt, respectively. The former is the normal serrated yielding and the latter is called “anomalous”. It is believed that the process at the temperature region lower than Tt is controlled by the interaction between Mg solute atom atmosphere and dislocations. In the positive coefficient region, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and leads to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations.

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.

Kinetic Monte Carlo method for dislocation migration in the presence of solute

We present a kinetic Monte Carlo method for simulating dislocation motion in alloys within the framework of the kink model. The model considers the glide of a dislocation in a static, three-dimensional solute atom atmosphere. It includes both a description of the short-range interaction between a dislocation core and the solute and long-range solute-dislocation interactions arising from the interplay of the solute misfit and the dislocation stress field. Double-kink nucleation rates are calculated using a first-passage-time analysis that accounts for the subcritical annihilation of embryonic double kinks as well as the presence of solutes. We explicitly consider the case of the motion of a -oriented screw dislocation on a {l_brace}011{r_brace}-slip plane in body-centered-cubic Mo-based alloys. Simulations yield dislocation velocity as a function of stress, temperature, and solute concentration. The dislocation velocity results are shown to be consistent with existing experimental data and, in some cases, analytical models. Application of this model depends upon the validity of the kink model and the availability of fundamental properties (i.e., single-kink energy, Peierls stress, secondary Peierls barrier to kink migration, single-kink mobility, solute-kink interaction energies, solute misfit), which can be obtained from first-principles calculations and/or molecular-dynamics simulations.

Effect of dynamic strain aging on high temperature properties of austenitic stainless steel

Abstract 18-8 type austenitic stainless steel has been subjected to tension test at a range of strain rates (5×10−4–5×10−2 s−1) and temperatures ( 298–973 K ) to investigate the effect of temperature and strain rate on its mechanical properties. It was found that the serrated flow behavior of this material, one of the evident features of dynamic strain aging (DSA) phenomenon can be divided into two sections according to the temperatures, i.e. the range of 523–673 K and 723–873 K at the strain rate of 5×10−4 s−1. Within these two temperature ranges, one can find some similar rules, for example, the critical strain ec for the onset of serrations reduces with increasing temperature. By calculating the activation energy of the DSA process of the material, it is found that the DSA at lower temperature range is caused by the interaction between (C, Ni) solute atom atmosphere and dislocation, and the DSA at higher temperature range is caused by the interaction between (C, Cr) solute atom atmosphere and dislocations. In order to further investigate the influence of DSA on high temperature strength of the material, the static tensile tests at temperatures of 573, 723, and 873 K, creep tests were also carried out, respectively, after various DSA pre-treatments and cold-working processes. The results show that the high temperature short-time strength and 873 K creep rupture strength of the specimens increase with increasing pre-strain temperature and pre-strain, and the strengthening effect of DSA is higher than that of cold-working at the same pre-strain.

Unstable Motion of an Edge Dislocation in a Solute Atom Atmosphere Studied by Kinetic Monte Carlo Simulations

AbstractThe discontinuous yielding of a model material, which contains an edge dislocation moving in the atmosphere of solute atoms, is studied by Kinetic Monte Carlo (KMC) simulations. The stress-strain curves for a constant strain rate were obtained at different temperatures. The dislocation moves discontinuously, producing three types of serrated yielding behavior at intermediate temperatures for different imposed strain rates. Positive dependence of flow stress on temperature and negative strain rate sensitivity were observed in the regime of discontinuous motion. The present model, though highly simplified and not taking into account the collective behaviors of dislocations in real materials, does exhibit some of the basic features observed in experiments.

Unstable Motion of an Edge Dislocation in a Solute Atom Atmosphere Studied by Kinetic Monte Carlo Simulations

Unstable Motion of an Edge Dislocation in a Solute Atom Atmosphere Studied by Kinetic Monte Carlo Simulations

Creep behavior of an AZ91 magnesium alloy reinforced with alumina fibers

Creep tests were conducted at elevated temperatures on an AZ91 alloy reinforced with 20 vol pct Al2O3 fibers. When the creep data are interpreted by incorporating a threshold stress into the analysis, it is shown that the true stress exponent, n, is ∼3 at the lower stress levels and increases to >3 at the higher stresses. The true activation energy for creep is close to the value anticipated for interdiffusion of aluminum in magnesium. This behavior is interpreted in terms of a viscous glide process with n =3 and a breakaway of the dislocations from their solute atom atmospheres at the higher stress levels. The threshold stresses in this composite appear to arise from an attractive interaction between mobile dislocations in the matrix alloy and Mg17Al12 precipitates. The experimental results reveal several important similarities between the creep behavior of this magnesium-based composite and the well-documented creep properties of aluminum-based composites.

High-temperature deformation in a Ta–W alloy

High-temperature deformation was investigated in a Ta–2.5 wt% W commercially available tantalum alloy, at temperatures in the range of 1523–1723 K and at a stress range extending from 35 MPa to 210 MPa. The experimental data, which cover several orders of magnitude of strain rate, show that the stress dependence of creep rate is high and that the temperature dependence of creep rate is higher than that for self-diffusion in tantalum. An analysis of the experimental data indicates that a threshold stress for creep exists, and that the temperature dependence of the threshold stress is much stronger than that attributable to the shear modulus. By considering the effect of the threshold stress and its temperature dependence on creep plots, it is demonstrated that the true creep characteristics of Ta–2.5 wt% W are consistent with those reported for solid-solution alloys at high stresses. In particular, the creep behavior of the alloy exhibits a transition from a region controlled by viscous glide to a high-stress region related to the breakaway of dislocations from solute-atom atmospheres. An examination of creep substructure in Ta–2.5 wt% W reveals the presence of interaction between moving dislocations and dispersion particles. It is suggested that such an interaction provides the most likely source of the threshold stress for creep in the alloy.

Creep behavior of an Al-6061 metal matrix composite reinforced with alumina particulates

Creep tests were conducted on an Al-6061 matrix alloy reinforced with 20 vol.% of irregularly shaped Al 2O 3 particulates. The composite was fabricated using an ingot metallurgy technique and the creep properties were determined at temperatures from 623 to 773 K. The results show high values for both the apparent stress exponent (up to > 10) and the apparent activation energy for creep (∼ 200–275 kJ/mol) but it is demonstrated, by incorporating a threshold stress into the analysis, that the true stress exponent is close to 3 and the true activation energy is close to the value for diffusion of Mg in the Al matrix. The results suggest that creep is controlled by the viscous glide of dislocations in the Al-6061 matrix alloy. Very fast creep rates are observed at the highest stress levels owing to the breakaway of dislocations from their solute atom atmospheres. Direct comparison shows that the creep resistance of this composite is less than in an Al-6061 alloy reinforced with 20 vol.% of Al 2O 3 microspheres. This difference is attributed to the creation of additional precipitates in the microsphere-reinforced composite because of an interfacial reaction between the matrix alloy and the reinforcement.

An examination of creep data for an Al-Mg composite

In a recent investigation of the creep behavior of an Al-4 wt pct Mg composite reinforced with 10 vol pct of SiC particles, henceforth designated Al-4 pct Mg-10 vol pct SiC(p), Pandey et al. obtained a plot of the steady-state creep rate, {dot {epsilon}}, vs the applied stress, {sigma}, over the temperature range from 573 to 673 K. The present analysis leads to two conclusions. First, the experimental data reported by Pandey et al. for an Al-4 pct Mg-10 vol pct SiC(p) composite divides into two distinct creep regimes at all testing temperatures, with a transition with increasing stress from region I to region II at a strain rate of {approximately}3 {times} 10{sup {minus}8} s{sup {minus}1}. Second, a detailed examination of the data at the higher stresses in region II does not support a constant structure model for creep, as proposed by Pandey et al., but rather it leads to a true stress exponent ({approximately}3.0) and a true activation energy for creep ({approximately}125 kJ mol{sup {minus}1}) which are consistent with control by the viscous glide of dislocations dragging Mg solute atom atmospheres within the Al-Mg matrix.

Ti-15V-3Cr-3Sn-3Al合金の溶接部における時効促進

Weld joint is locally deformed plastically and simultaneously restored at a high temperature after welding, and subsequently cooled to room temperature. Its singular thermomechanical hysteresis of the weld joint may affect aging behavior. Ti-15V-3Cr-3Sn-3Al alloys have showed aging enhancement in the weld joint. In the present study, we clarify the aging enhancement mechansim of this alloy on the base of the thermomechanical processing. A distinctive microstructure has existed in the weld joint after the welding and the subsequent cooling. Fringe-type microstructures of plate and bulk configuration may be created by local plastic deformation due to thermal stress after welding and by freezing of a solute atom atmosphere in the vicinity of edge dislocation component on cooling. Its distinctive microstructures have promoted the aging because of preferential sites for α precipitation. From the present results, a thermomechanical treatment has been proposed to obtain a high strength of this alloy by more short aging time.

Nonlinear mechanical relaxation associated with dislocation—point defect interaction

The concept of nonlinear anelasticity is proposed to denote that property of solids in virtue of which mechanical relaxation is acompanied by nonlinear relationship between stress and strain, so that a strong amplitude effect appears even under low stress levels. Seven internal friction peaks (vs. temperature) were observed in cold-worked and partially annealed aluminium containing copper or magnesium solute atoms. These peaks exhibit nonlinear anelastic behaviour manifested by the appearance of normal and anomalous amplitude effect within the temperature range of these peaks. The origin of these seven peaks was attributed to the short-ranged or long-ranged diffusion of the solute atoms (single or complex) caused by their interaction with dislocation kinks. A unified “dislocation kink—solute atom atmosphere” model in correlation with a “following, dragging, breaking away” relaxation process was suggested for these seven peaks so that the specific explanations for each peak are self-consistent and support each other.

Viscous glide in Ti3Al-base intermetallics

Recently, Mendiratta and Lipsitt investigated the creep behaviour of nearly stoichiometric Ti 3 Al (Ti-16 wt % Al) and Ti 3 Al + 10 wt % Nb (Ti-16 wt% Al + 10 wt % Nb) over the temperature range 550 to 825°C and at stresses between 69 to 312 MPa. In this communication, a suggestion is presented to explain the results of Mendiratta and Lipsett. It is proposed that the rate-controlling deformation mechanism in the low stress region (n=2 to 3) for the Ti 3 Al and Ti 3 Al + 10 wt% Nb alloys is due to the viscous glide of dislocations controlled by draging of solute atom atmospheres and not grain boundary sliding

Creep and ductility in an Al-Cu solid-solution alloy

High-temperature creep was investigated in an Al-3 wt pct Cu alloy at temperatures in the range of 773 to 853 K and at a normalized shear stress range extending from 10-5 to 7 × 10-4. The results show the presence of three distinct regions. In region I (low stresses), the stress exponent is 4.5 and the activation energy is 155 kJ/mole. In region II (intermediate stresses), the stress exponent is 3.2 and the activation energy is 151 kJ/mole. In region III (high stresses), the stress exponent is 4.5 and the activation energy is 205 kJ/mole. Creep curves obtained in the three regions exhibit a normal primary stage, but the extent of the stage is less pronounced in region II than in regions I and III. The creep characteristics in regions I and II, along with the values of the transition stresses between the two regions, are in conformity with the prediction of the deformation criterion for solid-solution alloys. While the advent of region III (high stresses) correlates well with dislocation breakaway from a solute-atom atmosphere, the creep characteristics in this region are not entirely consistent with any of the existing high-stress creep mechanisms. The plot of elongation to fracturevs initial strain rate at 853 K exhibits two peaks at strain rates of 1 × 10-4 and 6 × 10-4 s-1. The first peak (1 × 10-4 s-1) is attributed to the variation of the stress exponent for creep in the alloy with strain rate, and the second peak (6 × 10-4 s-1) appears to reflect the effect of solute drag on dislocation velocity.

Creep transitions in an Al-Zn alloy

The creep behavior of an Al-10 at. pct Zn alloy was investigated at temperatures in the range 573 to 800 K and in a normalized shear stress range, τ/G, extending from 10•6 to 10•3, where τ is the applied shear stress andG is the shear modulus. The results reveal the presence of four deformation regions: region I (very low stresses), region II (low stresses), region III (intermediate stresses), and region IV (high stresses). In regions II and III, the creep characteristics including the stress exponent, the shape of the creep curve, and the nature of creep transients after stress reductions are consistent with dislocation climb and viscous glide, respectively. The experimental transition stresses between region II (climb) and region III (glide) were compared with those predicted from the deformation criterion for solid-solution alloys, and it is demonstrated that the correlation between experiment and prediction becomes satisfactory when corrections concerning the appropriate diffusion coefficients for describing glide-controlled creep and climb-controlled creep, the stacking fault energy of the alloy, and the contributions of Suzuki interaction and Fisher interaction to solute atom drag forces are incorporated in the analysis. Experimental data in region IV are not sufficient to unambiguously identify the rate process, but calculations show that the values of the transition stress between region III and region IV agree reasonably well with those of the critical stress for dislocation breakaway from a solute-atom atmosphere. The characteristics of deformation in region I are still under investigation.

LOW FREQUENCY INTERNAL FRICTION IN ALUMINIUM-COPPER ALLOYS IN THE PROCESS OF PLASTIC DEFORMATION

Low frequency internal friction in the process of plastic deformation of Al-Cu and Al-Mg alloys was measured on a small tensile test machine and a series of internal friction jumps was observed on the internal friction-strain curve. A study was made on the effect of temperature of measurement and some metallurgical factors on these internal friction jumps. Experimental results showed that the range of the temperature of measurement within which these internal friction jumps appeared corresponds tc that for the appearance of the serrated stress-strain curves of aluminium-copper alloys. This indicates that these internal friction jumps on the internal friction-strain curves are associated with the process of strain aging occured in the process of plastic deformation, i.e., they are originated from the alternate anchoring of dislocations by atmospheres of solute atoms and their subsequent escaping in the course of plastic deformation.

FURTHER EXPERIMENTS ON THE DISLOCATION INTERNAL FRICTION PEAKS WITH ANOMALOUS AMPLITUDE EFFECTIN Al-0.5% Cu ALLOY AND THE DISLOCATION KINK ATMOSPHERE MODEL

用低频扭摆进一步研究了在Al-0.5％Cu合金中观察到反常位错内耗峯的条件。结果指出,对于完全退火的试样来说,需要有适当的冷加工量,但是对于高温淬火试样则不需要冷加工。用位错气团模型定性地解释了过去所观测到的表现反常振幅效应的时效内耗峯和温度内耗峯;同时指出,简单的气团模型在作定量的解释时,遇到了下述的困难:(i)在测量内耗所用的交变应力的作用下,位错线所能够拖着气团移动的距离太短。(ii)为了气团能够被位错拖着移动,组成气团的溶质原子必须具有比通常大很多个数量级的扩散系数。(iii)根据气团模型,从理论上计算出来的使位错拖着气团以临界速度而移动时,所需的临界应力比观测值大几百倍。提出了溶质原子沿着位错弯结而扩散的气团模型,这个改进模型能够初步解决上述困难,并能定性地解释所观测的结果。这个模型所依据的基本假设是,要观测到反常内耗现象,位错线上必须具有一定数目的弯结。要得到这种弯结,可以对于退火试样进行适量的冷加工,或者把试样从高温淬火。带着弯结的位错线能够通过弯结的沿边运动而实现垂直于位错线方向的移动。可以假定,气团只在弯结两端的直位错段处形成,在弯结本身上并不形成气团。在弯结的沿边振动的过程中,聚集在弯结两端的溶质原子可以沿着位错管道进行来回的短程扩散。已知沿着位错管道的扩散具有比在正常晶体点阵中扩散时大得多的扩散系数。