Dynamic Recovery Process Research Articles

Physical interpretation of creep in single crystal René N4TM based on a phenomenological model

Creep and rafting in Ni-based single crystal René N4 is described using reported phenomenological models after modifying the creep model to represent the rafting induced hardening of the structure against creep and the greater propensity for shearing of γ′ as the structure rafts and dislocation density in the channels and γ/γ′ interface builds up. The proposed model can describe the creep strain evolution at 1144K over the stress range 241–413MPa. The rafting model is shown to provide an adequate representation of horizontal and vertical channel width evolution at 1144K and 1255K. The variation of the physical parameters in the creep model viz., back stress, dislocation density and shear rate is rationalized scientifically thereby substantiating the physical basis of the model. The development of back stress in the channels is explained mechanistically through the deposition of dislocation segments at γ/γ′ interface by the Orowan loops expanding in the channel and the recovery of the interfacial dislocation networks. The evolution in Orowan stress consequent to rafting and its effect on dislocation activity are elucidated. The evolution of the kinetics for dynamic and static recovery processes which accompany creep strain accumulation has also been explained mechanistically.

The Microstructural, Textural, and Mechanical Properties of Extruded and Equal Channel Angularly Pressed Mg-Li-Zn Alloys

The microstructural and textural evolution of the Mg-6Li-1Zn (LZ61), Mg-8Li-1Zn (LZ81), and Mg-12Li-1Zn (LZ121) alloys were investigated in the as-extruded condition and after being equal channel angularly pressed (ECAPed) for one, two, and four passes. The shear punch testing technique was employed to evaluate the room-temperature mechanical properties of the extruded and ECAPed materials. Microstructural analysis revealed that the grain refinement in both LZ61 and LZ121 alloys could be achieved after multipass ECAP through the continuous dynamic recovery and recrystallization process. For the LZ81 alloy, however, the occurrence of Li loss in the four passes of ECAP condition partly offsets the grain refining effect of the ECAP process by increasing grain size and volume fraction of the α phase. Textural studies in both LZ61 and LZ81 alloys indicated that the developed fiber texture after extrusion could be replaced by a typical ECAP texture, where the basal planes are mainly inclined about 45 deg to the extrusion axis. The increased volume fraction of the β phase in LZ81 significantly affected the α-phase texture by decreasing the intensity of the maximum orientations of the basal and prismatic planes in all deformation conditions, compared with the LZ61 alloy. It was also observed that the abnormal grain growth might be promoted by the strong texture developed in the extruded LZ121 alloy. This texture became more randomized when the number of ECAP passes increased. The SPT results showed that the shear yield stress, ultimate shear strength and normalized displacement in all studied alloys were improved through the grain refinement strengthening caused by ECAP. It was also established that increasing Li content decreased the shear strength and enhanced the shear elongation in all deformation conditions.

Improved strength of Ni and Zn-doped Sn–2.0Ag–0.5Cu lead-free solder alloys under controlled processing parameters

The Sn–Ag–Cu (SAC) solders with low Ag or Cu content have been identified as promising candidates to replace the traditional Sn–Pb solder for flip-chip interconnects. The changes in microstructure, microhardness and mechanical properties associated with the alloying of Ni and Zn to the Sn–2.0Ag–0.5Cu(SAC205) solders were explored in this work. Furthermore, correlations between the mechanical behavior and controlled processing parameters such as temperature, strain and strain rate were analyzed and established. Microstructure analysis showed that additions of Ni and Zn to SAC(205) alloy not only suppressed the formation of brittle Ag3Sn IMC phase but also refined the large β-Sn dendrites, and promoted the formation of new (Cu,Ni)6Sn5 and Cu5Zn8 intermetallic compounds (IMCs) in the SAC(205) solder. Besides, the IMC particles were uniformly distributed in the β-Sn matrix. Accordingly, the mechanical properties, the microhardness and total elongation of Ni and Zn-doped SAC(205) solders were significantly improved. Both the yield strength (YS) and ultimate tensile strength (UTS) were observed to increase with increasing strain rates and decreasing temperature. This behavior was attributed to the competing effects of work hardening and dynamic recovery processes. The Zn-doped solder consistently displayed the highest mechanical properties and ductility due to the formation of hard Cu5Zn8 IMC particles and the refinement of β-Sn grain size, which could provide more obstacles for dislocation pile up in the adjacent grains.

Advanced β-Solidifying Titanium Aluminides – Development Status and Perspectives

ABSTRACTAfter almost three decades of intensive fundamental research and development activities intermetallic titanium aluminides based on the -TiAl phase have found applications in automotive and aircraft engine industries. The advantages of this class of innovative high-temperature materials are their low density as well as their good strength and creep properties up to 750°C. A drawback, however, is their limited ductility at room temperature, which is reflected by a low plastic strain at fracture. This behavior can be attributed to a limited dislocation movement along with microstructural inhomogeneity. Advanced TiAl alloys, such as β-solidifying TNM™ alloys, are complex multi-phase materials which can be processed by ingot or powder metallurgy as well as precision casting methods. Each production process leads to specific microstructures which can be altered and optimized by thermo-mechanical processing and/or subsequent heat-treatments. The background of these heat-treatments is at least twofold, i.e. concurrent increase of ductility at room temperature and creep strength at elevated temperature. In order to achieve this goal the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict phase fraction diagrams of engineering TiAl alloys. After experimental verification, these phase diagrams provided the base for the development of heat treatments to adjust balanced mechanical properties. To determine the influence of deformation and kinetic aspects, sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing and subsequent multi-step heat-treatments. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. Summarizing all results a consistent picture regarding microstructure formation and its impact on mechanical properties in TNM alloys can be given.

Un nuevo modelo fenomenológico y diferencial para predecir la respuesta mecánica de materiales metálicos policristalinos sometidos a deformación en caliente

This paper presents a new phenomenological and differential model (that use differential equations) to predict the flow stress of a metallic polycrystalline material under hot working. The model, called MCC, depends on six parameters and uses two internal variables to consider the strain hardening, dynamic recovery and dynamic recrystallization processes that occur under hot working. The experimental validation of the MCC model has been carried out by means of stress-strain curves from torsion tests at high temperature (900 oC a 1200 oC) and moderate high strain rate (0.005 s -1 to 5 s -1 ) in a high nitrogen steel. The results reveal the very good agreement between experimental and predicted stresses. Furthermore, the Garofalo a-parameter and the strain to reach 50 % of recrystallized volume fraction have been employed as a control check being a first step to the physical interpretation of variables and parameters of the MCC model.

Quantifying the Dynamic Effects of Service Recovery on Customer Satisfaction

This study examines two issues which have challenged prior experimental or survey research: (1) whether the time-varying effects of service recovery on customer satisfaction may follow a long decay or short decay and (2) why and what service recovery efforts have a higher and quicker buildup, with respect to the significance and timing of recovering customer satisfaction losses due to service failures. The authors do so with a real-world data set from China’s mobile phone markets. The authors developed multivariate time-series model to simulate the dynamic service recovery process and implemented Bayesian estimation to resolve overparameterization problem. The empirical results surprisingly reveal that apology-based service recovery efforts are the least effective in salvaging customer satisfaction, with the shortest decay and lowest buildup intensity. In contrast, quality improvement is the most effective, with the highest buildup and longest decay but slowest buildup toward the peak impact point. Compensation has moderate and stable impact overtime. Communications' impact on customer satisfaction builds up the quickest, though with mild endurance and magnitude. Also, the decomposition models enable managers to monitor how many percentages of customer satisfaction gains are originated from which types of service rescue efforts.

Development of Processing Maps for Coiled Tubing Steel

Hot compression test of coiled tubing steel is performed on Gleeble3500 at 1123K—1373K and strain rate from 0.001 to 5s-1. The hot deformation behavior of coiled tubing steel was characterized using processing map developed on the basis of the dynamic materials model. The processing map gives the flow instable region and the flow stable region in which the process of dynamic recovery (DRV) and the dynamic recrystallization (DRX) occur. In the processing map, the variation of the efficiency of the power dissipation is plotted as a function of temperature and the strain rate. According to the processing map, the coiled tubing steel is rolled by The Thermo-Mechanical-Control-Process (TMCP), and finally it is obtained that the yield strength and tensile strength of coiled tubing steel are 565MPa and 685MPa respectively, and the elongation percentage is 32.1%.

Influence of temperature and strain rate on tensile properties of single walled carbon nanotubes reinforced Sn–Ag–Cu lead free solder alloy composites

► SAC387 alloy and SAC387 alloy with two different wt.% of SWCNT were investigated. ► The addition of 0.5 wt.% of SWCNT was found to be beneficial for mechanical properties. ► Both the yield and ultimate tensile strengths decreased with increase in temperature. ► Both the yield and ultimate tensile strengths increased with increase in strain rates. The effect of temperature and strain rate on the tensile properties of Sn–3.8Ag–0.7Cu (SAC387) alloy as well as SAC387 alloy reinforced with two different weight percentages (wt.%) of SWCNT was investigated. It was found that addition of 0.05 wt.% SWCNT to SAC387 alloy results in an increase in yield and ultimate tensile strengths without affecting the total elongation at all temperatures and strain rates studied, although the increase in strength values at 75 °C was marginal. Further increasing the SWCNT to 0.1 wt.% did not result in any significant improvements in the strengths as compared to the composite containing 0.05 wt.% SWCNT. Temperature and strain rate were observed to have a significant effect on the strengths in both SAC387 alloy as well as the composites. Both the yield and ultimate tensile strengths were observed to decrease with increase in temperature and increase with increase in strain rates in SAC387 alloy as well as the composites. This behaviour was attributed to the competing effects of work hardening and dynamic recovery processes and the effect of temperature and strain rates on these processes.

Mechanisms of grain boundary softening and strain-rate sensitivity in deformation of ultrafine-grained metals at high temperatures

Two-dimensional dislocation dynamics and diffusion kinetics simulations are employed to study the different mechanisms of plastic deformation of ultrafine-grained (UFG) metals at different temperatures. Besides conventional plastic deformation by dislocation glide within the grains, we also consider grain boundary (GB)-mediated deformation and recovery mechanisms based on the absorption of dislocations into GBs. The material is modeled as an elastic continuum that contains a defect microstructure consisting of a pre-existing dislocation population, dislocation sources and GBs. The mechanical response of the material to an external load is calculated with this model over a wide range of temperatures. We find that at low homologous temperatures, the model material behaves in agreement with the classical Hall–Petch law. At high homologous temperatures, however, a pronounced GB softening and, moreover, a high strain-rate sensitivity of the model material is found. Qualitatively, these numerical results agree well with experimental results known from the literature. Thus, we conclude that dynamic recovery processes at GBs and GB diffusion are the rate-limiting processes during plastic deformation of UFG metals.

In situ investigation of the fast microstructure evolution during electropulse treatment of cold drawn NiTi wires

Microstructural changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in shape memory alloy technology, since they are responsible for setting the austenite shape and functional properties of the heat-treated alloy. In this work, microstructural evolution during non-conventional electropulse heat treatment of thin NiTi filaments was investigated in a unique high-speed in situ synchrotron X-ray diffraction experiment with simultaneous evaluation of the tensile force and electrical resistivity of the treated wire. The in situ results provide direct experimental evidence on the evolution of the internal stress and density of defects during fast heating from 20°C to ∼700°C. This evidence is used to characterize a sequence of dynamic recovery and recrystallization processes responsible for the microstructure and superelastic functional property changes during the electropulse treatments.

Cyclic behavior and microstructural stability of ultrafine-grained AA6060 under strain-controlled fatigue

The strain-controlled fatigue behavior of AA6060, a precipitation hardening aluminum alloy, was investigated in ultrafinegrained (UFG) conditions after severe plastic deformation (SPD) by equal-channel angular pressing (ECAP). Two as-processed conditions, representing different stages of strain hardening and grain refinement as well as a ductility-optimized condition, achieved by a combined ECAP and aging treatment were considered. Low-voltage scanning transmission electron microscopy on samples stopped at characteristic stages of the fatigue process was applied to investigate the microstructural development. The as-processed as well as the optimized condition showed cyclic softening, which was found to be dependent on the amount of prestrain induced by ECAP processing. This is linked to dynamic recovery processes in the microstructure, indicated by a clearer distinction of grain boundaries and a reduction of dislocations in the grain interior. For all applied plastic strain amplitudes, ranging from Δεpl/2=1×10−3 to 5×10−3, the fatigue life of the ductility-optimized condition did not reach that of the severely work-hardened counterpart. For explaining this unexpected result, the differing (size-dependent) effectiveness of precipitates for the pinning of dislocations during cyclic loading was considered.

In situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy

Hot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a β-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc β-phase recrystallizes dynamically, much faster than the hcp α-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the β-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the excellent deformation behavior of the material, while the presence of two phases effectively suppresses grain growth.

In Situ Measurements of Nonlinear and Nonequilibrium Dynamics in Shallow, Unconsolidated Sediments

Abstract We present results and in situ measurements from a field experiment in which a large seismic shaker truck is used to induce nonlinear and nonequilibrium dynamics in shallow, unconsolidated sediments. An array of accelerometers was deployed adjacent to the shaker truck to record strong ground motions exceeding 1 g . We determined high-strain Rayleigh-wave dispersion across the array in the band from 5 to 60 Hz. Rayleigh-wave phase velocities at frequencies above ∼10 Hz are wave amplitude-dependent and a function of the driving-force amplitude; as driving force was increased phase velocity decreased, consistent with nonlinear dynamics. We demonstrate the existence of a temporary, nonequilibrium state occurring in the near-surface soils during and after the induced nonlinear behavior. Nonlinear conditioning is demonstrated by measuring changes in Rayleigh-wave phase velocity for input signals with the same applied driving-force amplitude. A logarithmic slow dynamic recovery process is observed by analyzing the temporal variation in velocity of the noise field produced by the shaker truck when sitting idle. Measurements from weak-motion seismic surveys taken before and after shaking show compelling evidence that induced nonlinear behavior in the shallow sediments is reversible. We demonstrate that an active source, field-based approach has the potential to expand our knowledge of how sediments respond to strong ground motions, provide additional insight into the poorly understood slow dynamic recovery process, and possibly even lead to a new, site-specific, and noninvasive technique for characterizing the nonlinear properties of sedimentary deposits.

Dynamic recovery and its orientation dependence in face-centered cubic crystals

The orientation dependence of dynamic recovery in face-centered cubic crystals is investigated in relation to the critical annihilation distance of screw dislocations. The critical conditions for the onset of stage III are discussed in terms of a cross-slip mechanism that is locally modified by the interaction with a neighboring attractive screw dislocation. Two orientation domains are defined. From [ 1 ¯ 1 1 ] to the central region of the standard stereographic triangle, the attractive interaction stress enforces obtuse cross-slip, which inhibits dynamic recovery. The critical parameters for dynamic recovery are then derived using simple scaling laws. For other orientations, the interaction stress enhances cross-slip, which is treated using Escaig’s model. Numerical results are obtained using a single adjustable parameter. They are fully consistent with experimental data in copper and silver at 300 K. The discussion emphasizes the need for more precise information from atomic-scale modeling of the dynamic recovery process.

Stability of Friction Stir Welds at Superplastic Forming Temperatures

Friction stir welding is a solid state joining process which enables most metals and alloys to be welded without fusion temperatures occurring. The centre of the weld is comprised of dynamically recrystallized material which is beneficial for superplastic deformation. The high temperatures involved with conventional fusion welding techniques disrupt the delicate microstructure of superplastic materials. Superplasticity is heavily reliant on a small grained equiaxed matrix structure pinned by a fine distribution of hard second phase particles to inhibit grain growth during forming of the material. During the superplastic forming operation, the heavily strained structure present in the parent material undergoes a transformation from a banded structure comprising of very long, thin grains to fine equiaxed grains through various static and dynamic recrystallization mechanisms. During forming of friction stir welded materials the stability of the weld region has been investigated. Grain growth is more apparent in the strain hardening of AA5083 due to the relatively small amounts of strengthening precipitates. This material statically recrystallizes during the preheat stage of the superplastic forming process, the grains then begin to grow during a dynamic recovery process becoming far too large to allow superplastic deformation. AA2004 is specially designed superplastic forming (SPF) alloy which contains a large amount of Zr for grain stability. This alloying element is preferentially distributed along the grain boundaries which prevents grain growth during SPF. During the forming process the AA2004 dynamically recrystallizes; disruption to the parent material structure causes discontinuous dynamic recrystallization that results in a heterogeneous structure and makes the material prone to abnormal grain growth. The weld regions of FSWs in AA5083 and AA4004 have been shown to exhibit AGG in weld nugget and shoulder influenced regions. The stability of the superplastic material is reliant on their strengthening precipitates. The Zr in the AA2004 is a much more effective precipitate for maintaining stability.

High temperature plastic deformation behavior of non-oriented electrical steel

High temperature plastic deformation behavior of non-orientated electrical steel was investigated by Gleeble 1500 thermo-mechanical simulator at strain rate of 0.01−10 s−1 and high temperature of 500–1 200 °C. The stress level factor (a), stress exponent (n), structural factor (A) and activation energy (Q) of high temperature plastic deformation process of non-orientated electrical steel in different temperature ranges were calculated by the Arrhenius model. The results show that, with dynamic elevation of deformation temperature, phase transformation from α-Fe to γ-Fe takes place simultaneously during plastic deformation, dynamic recovery and dynamic recrystallization process, leading to an irregular change of the steady flow stress. For high temperature plastic deformation between 500 and 800°C, the calculated values of a, n, A, and Q are 0.039 0 MPa−1, 7.93, 1.9×1018 s−1, and 334.8 kJ/mol, respectively, and for high temperature plastic deformation between 1 050 and 1 200 °C, the calculated values of a, n, A, and Q are 0.125 8 MPa−1, 5.29, 1.0×1028 s−1, and 769.9 kJ/mol, respectively.

Formability of Ultrafine-Grain Mg Alloy AZ31B at Warm Temperatures

Warm formability of ultrafine-grain (UFG) AZ31B Mg alloy in the range of 150 °C to 311 °C is investigated. Refinement of grain size significantly improves the formability primarily through increasing strain rate sensitivity of flow stress and increasing diffuse quasi-stable flow, as compared with the coarse-grain AZ31B alloy. Strain rate sensitivity increases with increasing temperatures, and seems to be connected with dynamic recovery and grain boundary processes. For fine-grain sizes, twinning is inhibited. At 200 °C and strain rates below 2 × 10−4 s−1, the UFG AZ31B alloy demonstrates a high rate of strain hardening up to a true strain of 0.6, in addition to its high strain rate sensitivity, which is responsible for its enhanced formability. Examination of the microstructure shows that there is competition between dynamic grain growth and grain refinement during deformation at modest warm forming temperatures, the former causing hardening and the latter causing softening. Increasing the test strain rate and starting with larger grain size initiates deformation twinning during forming, and assists in the refinement of the coarser grains followed by recrystallization.

Characterisation of dynamic recovery during hot deformation of spray formed Al–Li alloy (UL40) using processing map approach

Processing maps, also known as power dissipation maps, for spray formed Al–Li alloy (UL40), encompassing a wide range of hot working temperature (375–575°C) and strain rate (3 × 10−4–101 s−1) have been developed. The constant true strain rate compression tests were carried out under isothermal conditions to generate the stress–strain data required for computing the efficiency of power dissipation η. The maps exhibit three distinct regimes. The two high efficiency (>55%) regimes occur at the low strain rates (<10−3 s&minus1) and at the high strain rates (>1 s−1), and the other with moderate efficiency values (20–23%) occurs at the intermediate strain rate (10−1 s−1). The latter one has been interpreted to be associated with dynamic recovery (DRV) process while the other two regimes have been found to be associated with cracking processes. The DRV regime in the map has been characterised in terms of stress–strain response, microstructure and tensile ductility. The kinetic rate analysis carried out in the DRV regime suggested that the thermally activated cross-slip is the rate controlling mechanism for the DRV process. The effect of Li addition on the optimum deformation condition of DRV in Al has also been discussed qualitatively.

Characteristics of the tensile flow behavior of Ti–46Al–9Nb sheet material – Analysis of thermally activated processes of plastic deformation

Flow behavior, strain hardening and activation parameters, i.e. activation volume, stress exponents and normalized free enthalpy of activation, of Ti–46Al–9Nb sheet with near-gamma microstructure have been investigated in tension tests between 700 and 1000 °C. The dependence of yield stress on temperature and strain rate, the course of the strain hardening curves and the values of activation parameters show that thermally activated dislocation mechanisms are mainly involved in the tensile deformation process of the investigated material. At constant temperature the value of the activation volume depends both on plastic strain and strain rate. The activation volume generally decreases with increasing strain. The decrease is particularly well observable for higher strain rates, thus indicating a growing role of thermally activated climb mechanisms governing the process of dynamic recovery. The activation volume calculated for a constant plastic strain (2% in case of this study) is a function of temperature and strain rate. At lower deformation rates, or alternatively at higher temperatures, the activation volume increases. Such behavior indicates a decrease in dislocation density due to the onset of dynamic recrystallization. The analysis of stress exponents and the obtained free enthalpy of activation confirm that different thermally activated processes are acting during deformation under the tensile test conditions studied.

Isolate-specific effects of ultraviolet radiation on photosynthesis, growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium Microcoleus chthonoplastes

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12-15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 mumol photon m(-2) s(-1)) = P; PAR + UV-A (8 W m(-2)) = PA; PAR + UV-A + UV-B (0.4 W m(-2)) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F (v)/F (m)) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F (v)/F (m) during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F (v)/F (m )sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.