Serration Amplitude Research Articles

Microstructure and mechanical properties of accumulative roll bonded aluminium alloy AA5754

The aluminium alloy AA5754 is used for many technical applications. In this work, the accumulative roll bonding process is applied to this alloy in order to investigate the potential of an ultrafine-grained structure on the mechanical properties of this Al-Mg alloy. Sheets from AA5754 (AlMg3) were successfully processed by accumulative roll bonding in order to obtain an ultrafine-grained microstructure. The ARB process was performed at 230 °C or 250 °C up to 7 or 8 cycles respectively. Thus the grain size decreased from 10 μm (initial state) to approximately 80 nm (ultrafine-grained state, normal direction). The microstructural evolution and the mechanical properties have been investigated by means of scanning electron microscopy, hardness measurements and tensile testing. After one ARB cycle the samples showed an increase in hardness by a factor of almost 2 in comparison to the as-received material. Further processing causes a linear increase of hardness with each additional cycle. Yield strength and tensile strength of the roll bonded specimens are highly increased in comparison to the as-received samples whereas the ductility declined. A considerable increase in ductility is obtained by heat treatment of the ARB specimens at 250 °C, but on the expense of a moderate decreased strength. The deformation behaviour is also influenced by the ultrafine-grained structure. The occurrence of the Portevin-Le Chatelier effect is manifested by serrated stress-strain curves. The amplitude of serrations increases with increasing number of ARB cycles but can be reduced by the appliance of a higher strain rate. Lüders strain only occurs at the as-received, i.e. not strained, samples.

The effect of grain boundary serration on creep resistance in a wrought nickel-based superalloy

Grain boundary serration, as a function of heat treatment parameters, and its effect on creep resistance have been investigated in a wrought nickel-based superalloy Nimonic 263. Grain boundary serration occurs in the absence of adjacent γ′ particles or M 23C 6 when a specimen is slow-cooled from solution treatment temperature. The average amplitude and wavelength of serrations can be controlled without a significant change in grain size and γ′ size by adjusting the heat treatment parameters of solution treatment time and cooling rate. Grain boundary serration leads to changes in M 23C 6 carbide characteristics: from granular to planar morphology, a lowered density, and the coherency pattern to two neighboring grains from consistent to zigzag. Creep resistance improvement due to serration is associated with a lower rate of cavitation and crack propagation through the modification of carbide characteristics, as well as grain boundary configuration.

Characterization of high temperature deformation behavior of INCONEL 617

INCONEL 617 (UNS N06617, also referred as Alloy 617) is an austenitic Ni–22Cr–12Co–9Mo alloy, which is a candidate structural material for next generation high temperature nuclear reactors. High temperature deformation behavior of Alloy 617 has been investigated as a function of strain rate ranging from 10 −3 to 10 −6 s −1 at 600 and 800 °C. Increase in the temperature and decrease in the strain rate decreased the flow stress of the material. Serrated flow was observed at both the test temperatures. The amplitude of serration increased with decrease in the strain rate and increase in the temperature. Microstructural analysis showed that deformation at slow strain rates increased both the grain size and volume fraction of second phase precipitation. Furthermore at slower strain rates, increase in the carbides size and a continuous network of carbides along the grain boundary could be observed. Scanning Kelvin Probe Force Microscopy (SKPFM) was performed on the tested specimen to observe variation in surface potential as a result of microstructural degradation during high temperature deformation at different strain rates. The second phase precipitates of the specimens deformed at higher strain rate (10 −3 s −1) showed more positive surface potentials than that of the specimens deformed at slower strain rate.

Effects of Surface Conditions on Rheological Properties and Phase Orientation of Sheared LCP Melts in Nanochannels by MD Studies

The rheological properties and phase orientation of liquid crystalline polymer (LCP) melts flowing in a nanochannel with different surface roughness are investigated by molecular dynamics (MD) simulations. Simulation results show the surface roughness has great impact on the rheological properties and phase orientation of LCP melts in the nanochannel (cross section is 12nm). As the amplitude of serrations increases, the shear viscosity increases nonlinearly and the value of orientational order parameter decreases. When the serration amplitude is larger than 1.1nm, a phase transition (from nematic to isotropic phase) of LCP melt happens, which makes flowing in nanochannels more difficult. On the other hand, the influence of serration period on the shear viscosity and orientational order parameter are found not so obvious. Findings in this study will be helpful for injection molding plastic products with nanofeatures.

PORTEVIN–LE CHÂTELIER INSTABILITY IN L10-TiAl SINGLE CRYSTAL

We studied the deformation behaviour of γ Ti -54.5% Al single crystal at temperatures ranging from 200°C to 700°C along [153] orientation, where the major operative slip system is [Formula: see text]. During this temperature regime, deformation microstructure consists of long heavily pinned screw dislocations. Serrated plastic flow has been observed at 500°C, the serration amplitude decreases with strain. The combination of the strain ageing and stress relaxation experimental results demonstrate for the first time the Portevin–Le Châtelier type instability in TiAl single crystal.

The Effect of Surface Features on Nanorheology of LCP Melts in Nanochannels by MD Simulation

The effects of wall surface features on the rheological properties and phase orientation of liquid crystalline polymer (LCP) melts flowing in a nanochannel have been first investigated by molecular dynamics (MD) simulations. The surfaces are modeled as rough atomic serrated walls whereby the roughness is characterized by the period and amplitude of serration. The molecular chains of LCPs are depicted by a newly developed molecular model named the GB-spring-bead model. Through simulating the phase formation of LCP melts, the new model was evaluated and the results have shown the new model is efficient and accurate to describe semi-flexible main-chain LCP molecules. MD simulations of the effect of wall surface features on the LCP shear flow were conducted and the results have revealed the surface features affect greatly the rheological properties and phase orientations of LCP melts in a nanochannel (the distance between the upper wall and the lower wall is 12.8nm). Findings in this study provide very useful information in the injection molding of plastic products with nanofeatures.

Study of surface conditions and shear flow of LCP melts in nanochannels through molecular dynamics simulation

The rheological properties and phase orientation of liquid crystalline polymer (LCP) melts flowing in a nanochannel with different surface roughness are investigated by molecular dynamics (MD) simulations. The molecular chains of LCPs are depicted by a newly developed molecular model named GB-spring-bead model, which has proved to be efficient and accurate in studying the phase transition behaviors of semi-flexible main chain LCPs [Yung KL, He L, Xu Y, Shen YW. Polymer 2005;46:11881 [1]]. The surfaces are modeled as rough atomic serrated walls whereby the roughness is characterized by the period and amplitude of serrations. Simulation results have shown that the surface roughness affects greatly the rheological properties and phase orientations of LCP melts in a nanochannel (the distance between the upper wall and the lower wall is 12.8 nm). As the amplitude of serration increases, the shear viscosity of LCP increases nonlinearly while its orientational order parameter decreases. When the serration amplitude reaches a certain magnitude, a phase transition (from nematic to isotropic phase) happens, which increases the viscosity of the nano LCP flow drastically. On the other hand, the influence of serration period on the shear viscosity and orientational order parameter is not so obvious relatively. Findings in this study provide very useful information in the injection molding of plastic products with nanofeatures.

Observations of serration characteristics and acoustic emission during serrated flow of an Al–Mg alloy

Study of serrated flow is important for the production of metal alloy parts with smooth surfaces and for fundamental understanding of microscale plastic deformation. In this experimental investigation of a rolled Al–Mg alloy, serrated flow was observed for the entire plastic flow region at three different strain rates. Strain and strain rate dependent serration amplitudes and widths were quantified. In addition, the acoustic emission (AE) signature from the plastic flow region was analyzed in detail and was related to the serrations in the macroscopic stress versus strain results. For all experiments, deformation bands on the surfaces of the specimens were found to be consistent with ‘Type B’ serrations. Video observations of the deformation bands were synchronized with the AE, and the deformation bands were found to coincide with more persistent AE bursts and anomalies in the serrations.

３００４アルミニウム合金の伸びに及ぼす結晶粒径の影響

In general, a fine grain structure may be produce an excellent elongation. However, the different tendency was observed with Al–Mg system alloys. In order to make clear a grain size dependence for elongation, a tensile test, a surface roughness measurement and a fractography were performed for 3004 aluminum alloy with various grain sizes from 18 to 121 μm. Then a total and a uniform elongation of 3004 aluminum alloy tensiled for a strain rate of 1.67 × 10−4 s−1 was plotted for grain size, a maximum elongation was obtained in 77 μm of mean grain size. The total and the uniform elongation was proportional to a reciprocal square root of grain size. There is no change a local elongation for the reciprocal square root of grain size. In fine grain region below the 77 μm of grain size, grain size dependence for elongation was indicated a inverse Hall-Petch type by the effects of a work hardening exponent and an amplitude of serration. In the meantime, coarse grain region over the 77 μm of grain size, the grain size dependence for elongation was indicated a Hall-Petch type by the effects of a three dimensional stress and a surface roughness on the tensile specimen. The same grain size dependence for elongation was observed in the strain rate of 1.67 × 10−3 and 1.67 × 10−5 s−1.

The anisotropy of the portevin-le chatelier effect in aluminum alloys

In this work, the effects of grain size on the tensile properties, serrated flow, Portevin-Le Châtelier (PLC) band evolution, dislocation density, and microstructure evolution of Fe–22Mn-0.6C steel were investigated using digital image correlation (DIC), X-ray diffraction (XRD), transmission electron microscope (TEM) and monotonic tensile tests. Results show that the yield strength, tensile strength, and work hardening value of Fe–22Mn-0.6C steel increase whereas its elongation decreases with decreasing grain size. The critical strain of serration decreases whereas the amplitude of serration increases with decreasing grain size. Moreover, the plateau of serrations is larger and that the strain concentration within PLC bands is more severe in fine-grained steel than in coarse-grained steel. The dynamic strain aging (DSA) behavior of the steel is enhanced by grain refinement. The speed of PLC band movement decreases faster in fine-grained steel than in coarse-grained steel, which results in a decrease in plasticity. XRD and TEM analyses show that the high dislocation density and unique twin substructure may be the main reasons for the strong DSA phenomenon observed in fine-grained steel.

Ａｌ―Ｍｇ系合金薄板の伸びおよびセレーションに及ぼす歪速度の影響

Effect of strain rate and Mg content on elongation and serration behavior was studied in pure aluminum and Al–Mg alloy sheets with Mg content up to 6.5%. Elongation increased monotonously with strain rate in pure aluminum, but Mg–containing alloys showed a minimum elongation at a strain rate of about 1 s−1. The elongation decrease with strain rate in a strain rate range below the minimum was much sharper and the elongation increase in a range above the minimum was less abrupt in alloys with Mg content of 4~6% than in alloys with Mg content of 2~3%. Consequently, although the alloys with high Mg content showed higher elongation in ordinary static tensile testing, the alloys with low Mg content exhibited higher elongation in dynamic condition which was considered closer to press forming of automobile body panels. Serration amplitude in load-displacement curves decreased with increasing strain rate and decreasing Mg content. The low-Mg alloys exhibited no serration in dynamic condition at a strain rate of about 1 s−1, while the high-Mg alloys still showed serration to a certain extent. The observed phenomena were discussed based on dynamic strain aging, dynamic recovery, and temperature rise during the testing by adiabatic deformation.

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 φ 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).

Lüders bands formation in a rapidly solidified Ni 3al alloy ribbon

The phenomenology of Lüders bands formation in a rapidly solidified Ni-20Al-12Cr-1.8Mo intermetallic alloy ribbon in the temperature range of 300-770 K is discussed. It was observed that strength and Lüders bands aspect on the specimen were irrespective of temperature. The flow characteristics in the Lüders region of the load-elongation curve were, however, very temperature sensitive. At low temperatures (<470 K), a flat plastic region with few instabilities was seen; but at higher temperatures (>470 K), a clear serrated behavior was manifested and the amplitude of serration increased with temperature. It is suggested that yielding occurs by dislocation generation at grain boundaries and that the stress required for dislocation generation (σ eff) is athermal. A temperature dependent stress originated by the dynamic pile-up of dislocations at grain boundaries (dynamic stress) is, however, introduced as rate controlling for Lüders front motion and responsible for serration appearance.

In-situ deformation of aluminium alloy polycrystals observed by high-voltage electron microscopy

In-situ deformation of aluminium alloy polycrystals has been investigated within a high-voltage transmission electron microscope in order to observe dislocation phenomena related to the PortevinLe Chatelier effect. The alloys examined are commercially pure aluminium, binary Al2 at.% Ag, Al2 at.% Zn, Al2 at.% Mg and a commercial alloy containing approximately 5 at.% Mg. All the materials tested exhibit flow-stress discontinuities during room temperature deformation, although the amplitude of serrations is substantially higher in the material containing Mg. In-situ deformation is correspondingly discontinuous in all cases but the co-ordination of dislocation motion in the material containing Mg is markedly higher. Deformation in all the material examined is characterised by the sudden activation and collective motion of multiple dislocations, but this is predominantly confined to a single-slip system in the case of alloys containing Mg. In contrast, alloys which do not contain Mg exhibit concurrent, dispersed slip on multiple intersecting slip systems characterised by extensive cross-slip.

Serrated flow and negative rate sensitivity in Al-Li base alloys

The characteristic behavior of dynamic strain aging, negative strain rate sensitivity, serrated flow and localized banding deformation observed in several Al-Li based alloys has been examined. It is concluded, based on the observation, that it is the negative strain rate sensitivity that determines the occurrence of jerky flow in the Al-Li base alloys examined in the present study. Dynamic strain aging of Li atoms itself can only result in a negative rate sensitivity, but does not directly get involved in the atomistic mechanism of serration. Both the characteristics of serration (amplitude, frequency, and onset) and the deformation bands are rationalized in terms of models in which the correlation between unstable jerky flow and negative rate sensitivity is established

Acoustic emission and deformation bands in Al-2.5% Mg and Cu-30% Zn

The acoustic emission (AE) generated during Luders and Portevin-Le Chatelier (PLC) deformation bands has been studied in Al-2.5% Mg and Cu-30% Zn. It has been observed that the cumulative AE detected during the Luders deformation increases at decreasing strain rates while Luders bands become shorter and are increasingly serrated, especially in AlMg. Once PLC bands develop, AE appears in a stepwise fashion. The acoustic activity concentrates mainly on the beginning of every new band, but preceding the first load drop. Once nucleated, the PLC bands propagate with relatively low AE. Some burst activity is recorded during the stress raise after the load drops, while a continuous type burst is emitted during the load drop. Total AE increases with decreasing strain rate during serrated flow, as during the Luders regime. The increase in total AE correlates very well either with the shortening of Luders bands or the increase in serration amplitude of PLC bands at decreasing imposed deformation rates. A close correspondence can be established between the phenomenology of AE and TEM and in situ HVTEM observations concerning dislocation dynamics and dislocation breeding in localized slip bands. The relatively low AE recorded at high imposed deformation rates when the bands (Luders or PLC) move smoothly can be ascribed to the multislip mechanism of coarse slip bands involved in the bands movement, which clearly may preclude the emission of large acoustic bursts. On the other hand, at low deformation rates stress relaxation and static ageing during discontinuous band movement immobilizes previously operative sources, so that activation of fresh sources become necessary in order to resume deformation. Dislocation movements in undeformed slip systems is presumably very fast so that a high level AE should be possible when fresh sources are put into operation. As a consequence, AE increases at decreasing strain rates. At increasing strain rates the available time for stress relaxation and pile up stabilization at the band front after load drops is progressively reduced. Previously operative sources can now be reactivated to a larger extent, decreasing the need for fresh sources activation. Reactivation of previously operative sources presumably occurs with low level AE due to the high dislocation density in the slip planes. In this way the strain per active plane increases with strain rate while acoustic activity decreases. The correspondence between stress amplitude of serrations and AE can be rationalized in the same terms. A complementary explanation in terms of the crystallographic microbands involved in the macro shear bands nucleation seems possible in the case of PLC bands. When the band is smooth at high strain rates the stress level at the band front is continuously high and the number of unsuccessful microbands needed to originate every localized shear band is presumably lower than at low strain rates when the stress level decreases after the load drops. AE is therefore higher at the lower strain rates than at the higher ones. Something essentially similar must occur when every new PLC band is nucleated. No stress concentrations nor mobile dislocations are available after a band traverses the entire gauge length and the deformation must be started by copious activation of new sources (microbands), with high emission preceding the first load drop. Once the band is formed the deformation mode involved reduces the acoustic activity during the band movement, so that a step-wise AE is observed.

A model of the mechanics of serrated flow in an amorphous alloy

A model is developed, of the mechanics of serrated flow in an amorphous alloy, with which it is possible to evaluate the serration amplitude (δσ) that would occur in a perfectly rigid machine, and to correct the applied extension rate ( u ̇ ) at which serrated flow occurs in a conventional testing machine. The model mechanics is based on two criteria for yielding: two independent physical processes of serrated flow and plastic sliding and ageing-time ( t)-dependent serrated flow. In agreement with the prediction from our model, the observed serration amplitude and the applied extension rate for compressive deformation of amorphous Pd 78Cu 6Si 16 depend on the rigidity of the machine used. We also present an analysis of inhomogeneous plastic deformation and serrated flow behavior for amorphous Pd 78Cu 6Si 16. At a commonly-used applied extension rate, the corrected serration amplitude attains 11% of the yield stress. The effective flow stress ( σ eff ) at room temperature is rate-dependent; its rate sensitivity (m) = ∂ log u ̇ /∂ log σ eff , and is 59, A power law of serration amplitude is derived as Δσ = C 1 t 0.46.

The deformation of commercial aluminum-magnesium alloys

The deformation and fracture of Al-Mg alloys have been investigated over a range of strain rates and temperatures. The rate and temperature-dependence of the deformation can be understood on the basis of the mobility of Mg atoms. When the Mg atoms can diffuse at a sufficient rate they interact with dislocations to produce serrated yielding and a zero or negative strain rate sensitivity. This greatly restricts the tensile ductility. Outside of this serrated flow regime enhanced ductilities are achieved due to the positive strain rate sensitivity stabilizing any inhomogeneities which develop. However, the final fracture is also influenced by the constituent particle concentration. Voiding occurs at particles and if the concentration of particles is sufficiently large the voids link up by local shear. Decreasing the constituent particle concentration, together with a reduction in the amplitude of serrations, results in larger tensile ductilities. At high temperatures, quite large strain rate sensitivities can be achieved but the ductility is finally limited by the propensity for voiding in these alloys.

準安定オーステナイトステンレス鋼における加工誘起マルテンサイト変態の組成および結晶粒度依存性

Composition and grain size dependencies of Md30, the parameter proposed by Angel for indicating austenite stability during deformation in metastable austenitic Fe-Ni-Cr stainless steels, were examined.The measurement of Md30 by changing composition, C, N, Si, Mn, Ni, Cr, and others, give the following empirical equation: Md30 (C) =551-462 (C%+N%) -9.2Si%-8.1Mn%-13.7Cr%-29.0 (Ni%+Cu%) -18.5Mo% -68.0Nb%. A great coefficient for Ni in this equation is quite different from that in the Angel's equation. With decreasing grain size of austenite, the amount of strain-induced martensite decreases, and Md30 is lowered. Consequently, MdGS30 which is Md30 modified by the grain size effect, can be expressed as MdGS30=Md30 -1.42 (ν-8.0); where ν is ASTM grain size number.The proof stress drops during deformation at lower temperatures because of the inducement of martensite at a small strain. This phenomenon easily occurs in the specimen with finer grain size. On the stress-strain curve the serrated region can be easily found in the deformation of the specimen with finer grain size. The amplitude of serration becomes too small to be observed in such cases as the specimen with coarser grain size, lower deformation temperature and stabilization of austenitic phase, even though a lot of martensite is induced.

Ti-2 at%Zr合金の多重降伏現象

Serrated yielding was clearly observed between 330 and 460°C at a strain rate of 2.22×10−4 sec−1 in Ti-2 at%Zr alloy, while in a commercially pure titanium it was not so clear. Two types of serrations were observed in the alloy. One type is the serrations with a regular periodicity and the other is the serrations with relatively large stress drops which occur intermittently. In cases where the strain increases, the strain rate decreases or the grain size decreases, the stress amplitude of the regular serrations increases. Examination of the temperature and strain rate dependence of the regular serrations suggests that serrated yielding in Ti-2 at%Zr alloy is probably dominated by the diffusion of both zirconium and interstitial impurity atoms. The regular serrations, therefore, must be closely related to the strong interaction between a dislocation and an interstitial impurity atom which is at the octahedral site distorted locally by a zirconium atom.