Dislocation Exhaustion Research Articles

Experimental study of Ni3Al slip traces by atomic force microscopy: an evidence of mobile dislocation exhaustion

Slip markings produced on the surfaces of Ni 3 (Al, Ta) single crystals, plastically deformed at various temperatures in the flow stress anomaly domain, were examined by atomic force microscopy (AFM). A dominant feature is that for all investigated temperatures, the slip traces are rectilinear and correspond to the primary octahedral glide plane. In addition, their lengths drastically decrease when the temperature is raised. This latter result is interpreted as a strong increase of the exhaustion rate of mobile dislocations with increasing temperatures. The consequences of these results in the understanding of Ni 3 Al flow stress anomaly are discussed.

Dislocation multiplication, exhaustion and mechanical behaviour for Ge singlecrystals

Dislocation multiplication and exhaustion processes in germanium arerevisited in the light of new data from mechanical test transients. They aredocumented by transmission electron microscope (TEM) observations anddislocation velocity measurements by the etch pit technique. The transient testsconsist of stress relaxations, creep tests and stress dip tests performedalong the monotonic curve. The relaxation and creep curves exhibit anon-logarithmic dependence on time, unlike in other classes of materials.Dislocation multiplication is evidenced by these transient curves before the upperyield point. In addition, after the lower yield point, dislocation exhaustion takesplace at the end of the transient. Some evidence of strain localization isalso provided by strain dip tests and transmission electron micrographs.

From dislocation cores to strength and work-hardening: a study of binary Ni 3Al

A quantitative model for the peak temperature in work-hardening in L1 2 intermetallics is proposed. It is based on the competition between the exhaustion of mobile dislocations by the Kear Wilsdorf mechanism and the yielding of incomplete locks at high stress. The model is assessed by a set of experimental data measured in binary Ni 3Al polycrystals of three different compositions. These include, in particular, the planar fault energies of the dislocation cores measured by weak-beam electron microscopy, combined with computer image simulations and macroscopic data about flow stress, work-hardening and mobile dislocation exhaustion rates. These parameters are measured as a function of alloy composition. The model also fits successfully data published for other L1 2 compounds.

Characterization of thermally activated dislocation mechanisms using transient tests

Two methods of repeated transient tests, namely relaxation and creep, are shown to be ideal techniques for the characterization of plastic deformation processes. They yield similar information about a microscopic activation volume, which is the signature of the operating dislocation mobility mechanism. Microstructural parameters are also obtained, the values of which are different in creep and stress relaxation. They characterize work-hardening during the transient and dislocation exhaustion rates respectively. The equations describing the transients and the assumptions involved are presented. Experimental results on Ni 3Al polycrystals illustrate the possibilities of both tests and support the above assumptions. In particular, crystals which work-harden exhibit high dislocation exhaustion-rates, as shown by comparison of Ni 3Al with TiAl, Ge and Cu. The respective contributions to the strain-rate of the mobile dislocation densities and velocities can also be estimated.

Is plastic flow always controlled by dislocation mobility? An answer from in situ transmission electron microscopy straining tests.

In situ transmission electron microscopy (TEM) straining experiments are used to illustrate in two extreme cases the possible role of dislocation nucleation and exhaustion as a controlling factor in plastic flow. In the first example (FeAl intermetallic compounds), a thermally activated dislocation exhaustion is responsible for an anomalous stress-temperature dependence and an associated small strain rate sensitivity, the latter being evidenced during in situ experiments through unstable localized slip. The second example (heavily drawn pearlite) shows specific dislocation loop nucleation processes that may account for the Hall-Petch law breakdown characteristic of fine scale nanostructures.

Dynamic observation of Al thin films plastically strained in a TEM

Aluminum thin films deposited onto oxidized silicon substrates were deformed using the difference in thermal expansion coefficients between the two materials. Stresses in the Al films were measured by the wafer curvature method. The stress-temperature behavior is explained by cross-sectional TEM studies which revealed grain growth and dislocation exhaustion. In order to track down finer dislocation mechanisms, in situ cross-sectional samples were heated from room temperature up to 450°C in a TEM. Dynamic observations demonstrated that the Al/SiO x –Si interface acts as a sink for dislocations and grain boundaries.

Characterising thermally activated dislocation mechanisms

Various methods based on mechanical tests have been developed through the years for investigating dislocation mobility. The measured experimental quantities are the strain-rate sensitivity of the stress or thermodynamic parameters, such as the activation volume and energy, which are considered as characteristic parameters of dislocation mobility. However, these parameters are generally determined by assuming that, during the mechanical test, the dislocation microstructure remains constant, a condition which is not frequently satisfied. We show that a careful examination of the transient test kinetics, such as repeated load relaxations and repeated creep tests performed on the Ni 3Al intermetallic compound, can provide useful information concerning microstructural evolutions, in particular on the mobile dislocation densities and the internal stress variations. Correlations are found between dislocation exhaustion rates and work hardening coefficients.

The role of thermal activation in the strength anomaly of Ni3Al

Using two transient mechanical tests which are described (repeated load relaxations and repeated creep tests) some microscopic parameters of the deformation processes in Ni3Al compounds such as the activation volume V of the dislocation velocity and the exhaustion rate of mobile dislocations are determined. V values in the range of 80 to 350 b3 (Ni75Al25 polycrystal at 300 K) that dislocation mobility is thermally activated, while large values of the exhaustion parameter correlate well with the work-hardening coefficient. This new information about quantitative aspects of dislocation exhaustion are supported by the observation of a yield point at reloading subsequent to the relaxation transient. The variation of the amplitude of this yield point as a function of temperature is similar to the variation of the exhaustion parameter. Both curves peak at 500 K which is also the peak temperature of the work-hardening coefficient. The variation with temperature of this latter parameter is discussed in the light of the present results.

On the strengthening of Ni3Al by hafnium additions

Abstract To interpret the notable strengthening of Ni3Al due to Hf additions in the strength anomaly domain, the dislocation features of a 3at.% Hf compound are characterized. Since the general microstructure does not exhibit obvious differences from that observed in similar compounds, the superdislocation core is investigated to find reasons for this effect. Various weak beam conditions are tested which never yield more than 3 peaks for the intensity profiles. The latter are interpreted for the chosen g, ng conditions (with 3 < n < 6) after extensive computer image simulations. The different fault energies related to the core are determined and found to be γ111 = 300mJm−2, γ010 = 250mJm−2 at 300K while γCSF exhibits very high values (larger than or equal to 460mJm−2). This explains the peculiar dislocation images. A comparison of the flow stress—temperature plots with those corresponding to a binary and a 1at.% Ta compounds confirms that the shifts observed for the flow stress in the anomaly domain and those for the peak temperature can be correlated well with the γCSF values, but not with the antiphase boundary anisotropy ratio. γCSF appears to be the key parameter for dislocation locking in the strength anomaly domain. Other solid solution strengthening effects operate in addition, without hindering the influence of γCSF. This interpretation of the differences in mechanical properties agrees with previous studies on similar compounds, but it is shown to hold even when these differences are large. In addition it is strongly supported by data about dislocation exhaustion rates which are measured in the Hf, the Ta and the binary compounds through repeated load relaxation experiments at 575K. The high ability of superpartials to cross-slip in this large γCSF Hf compound also explains the rather large minimum dislocation character observed for dislocations lying on the octahedral plane.

On the Stress Anomaly of γ-TiAl

ABSTRACTA 3D mesoscopic simulation of dislocation behaviour is adapted to the case of γ-TiAl. It shows that, in the temperature range of the stress anomaly, ordinary dislocation motion essentially proceeds through a series of pinning and unzipping processes on screw dislocations. Pinning points are cross-slip generated jogs, whose density increases with temperature. Unzipping of cusps restores the screw character of the dislocation. The balance between pinning and unzipping becomes increasingly difficult as temperature raises, and results in dislocation exhaustion. All these features agree with the so-called “Local Pinning Unzipping” mechanism.

Dislocation Exhaustion During Plastic Deformation

AbstractRepeated stress-relaxation experiments are used to characterize the deformation parameters in 3 types of single crystals (Cu, Ni3Al and Ge) in which different dislocation mechanisms are known to operate. Mobile dislocation exhaustion rates and the amplitude of the yield point at reloading after stress-relaxation are measured. These two parameters are discussed in terms of the work-hardening coefficient in monotonic tests.

Dislocation Exhaustion During Plastic Deformation

Dislocation Exhaustion During Plastic Deformation

To the theory of logarithmical low-temperature creeping at the expence of the dislocation exhaustion

To the theory of logarithmical low-temperature creeping at the expence of the dislocation exhaustion

Experimental Study of Mobile Dislocation Densities and Velocities through Transient Mechanical Tests

AbstractThe use of transient mechanical tests such as repeated load relaxations and repeated creep tests provide some insight on the respective contribution of mobile dislocation densities and velocities to the plastic strain-rate and also on the hardening mechanisms. Improvements of the second technique are reported as well as results on Ni3Al polycrystals. Other materials (e.g. Cu, TiAl) are considered for comparison. Correlations are found between dislocation exhaustion rates, workhardening coefficients and the amplitude of yield point at reloading after the relaxations.

Microplastic Relaxations of Single and Polycrystalline Molybdenum

The microplasticity of high-purity molybdenum single crystals and of Mo polycrystals of technical purity has been investigated by relaxation step tests in uniaxial compression. A new model for the evaluation of relaxation tests in the microplastic range of b.c.c metals is presented which takes into account the decrease of the mobile dislocation density due to exhaustion of non-screw dislocations. The model allows an independent determination of the activation volume and of the microstructure parameters controlling dislocation exhaustion. The results indicate that in the high-purity single crystals the deformation rate is controlled by interactions of non-screw dislocations with the grown-in network. In the polycrystals additional interactions with impurity atoms seem to occur. In the single crystals the activity and subsequent exhaustion of two different glide systems was observed, followed by a gradual onset of screw dislocation motion. Die Mikroplastizität von hochreinen Molybdän-Einkristallen und von Mo Polykristallen technischer Reinheit wurde durch Stufen-Relaxationsversuche in uniaxialer Kompression untersucht. Es wird ein neues Modell für die Auswertung von Relaxationsversuchen im mikroplastischen Bereich kubisch-raumzentrierter Metalle präsentiert, welches die Abnahme der mobilen Versetzungsdichte infolge von Erschöpfung der Nicht-Schraubenversetzungen berücksichtigt. Das Modell ermöglicht die getrennte Bestimmung des Aktivierungsvolumens und der Mikrostrukturparameter. Die Ergebnisse deuten darauf hin, daß die Verformungsrate in den hochreinen Einkristallen durch Wechselwirkungen mit dem eingewachsenen Versetzungsnetzwerk bestimmt ist. In den Polykristallen dürften zusätzliche Wechselwirkungen mit Verunreinigungen eine Rolle spielen. In den Einkristallen wurde die Betätigung und anschließende Erschöpfung zweier verschiedener Gleitsysteme beobachtet. Danach erfolgte ein allmähliches Einsetzen der Schraubenversetzungsbewegung.

Serrated flow associated with strength anomalies: a new type of plastic instability?

Abstract After a short review of the association of strength anomalies and plastic instabilities in various materials, the aim of the present paper is to give a general and simple interpretation for most of these flow stress instabilities. It is based on a linear stability analysis of the steady state resulting from a balance between dislocation exhaustion and multiplication, which also accounts for the other characteristics of strength anomalies. The influence of additional unlocking of dislocations is discussed. This approach is illustrated by Monte Carlo simulations of unstable plastic flow in L1 2 ordered alloys.

Role of Interfaces in Deformation and Fracture of Ordered Intermetallics

While sub- and grain-boundaries are the primary dislocation sources in Ll{sub 2} alloys, yield and flow stresses are strongly influenced by the multiplication and exhaustion of mobile dislocations from the secondary sources. The concept of enhanced microplasticity at grain boundaries due to chemical disordering is well supported by theoretical modeling, but no conclusive direct evidence exist for Ni{sub 3}Al bicrystals. The strong plastic anisotropy reported in TiAl PST (polysynthetically twinned) crystals is attributed in part to localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Calculations of work of adhesion suggest that, intrinsically, interfacial cracking is more likely to initiate on {gamma}/{gamma}-type interfaces than on the {alpha}{sub 2}/{gamma} boundary. 70 refs, 5 tabs, 5 figs.

Direct Measurement of Dislocation Exhaustion Rates During Plastic Deformation of Ni3Al Compounds

ABSTRACTTwo methods of repeated transients (stress relaxation and creep) which can be performed during straining at a constant strain rate are recalled together with the way the results are interpreted. The repeated creep method has been adapted to Ni3A1 compounds for which the amount of creep strain during the second creep could not be detected so far. Because of a small stress increase between creeps, it is possible to measure microscopic activation volumes, dislocation exhaustion rates and strain hardening coefficients. Results in Ni3A1 polycrystals and Ni3(AI,Hf) single crystals are presented. Orders of magnitude of dislocation exhaustion rates are given. They are particularly high in Ni3A1 as compared with other materials. They compare well with the work hardening coefficient and with total dislocation densities measured by electron microscopy.

Effect of Off-Stoichiometry on the Deformation Behavior of Ni3AI Binary Polycrystals

ABSTRACTPolycrystals of binary Ni3Al with different compositions are deformed in compression between 300K and 1300K. They exhibit a strength anomaly with peak temperatures between 800K and 950K. Below the peak, the aluminium rich compounds are the strongest ones while the nickel rich compounds are the weakest ones. As the aluminium content increases, the complex stacking fault energy, measured through weak beam experiments, increases as well. This confirms the cross slip mechanism of mobile dislocation exhaustion through Kear-Wilsdorf lock formation. A ‘ductile-to-brittle transition’ is observed at 1000K. Fracture occurs along grain boundaries, the strength of which decreases as environmental effects are enhanced at high temperatures.

Deformation microstructures in Ni3(Al,Hf)

The dislocation structures resulting from compression tests on Ni-3 (Al, I-If) single crystals are studied as a function of the deformation temperature between 77 and 900 K. Mechanical parameters are also reported, such as the 0.2% offset stress, the work-hardening coefficient, the microscopic activation volume and the dislocation exhaustion rate. The two latter parameters are measured using repeated load relaxation experiments. It is shown that the microstructural features are qualitatively similar to those reported for other Ni3Al compounds: Kear Wilsdorf locks are observed in the temperature domain which corresponds to the strength anomaly, the peak temperature seems to be associated with the occurrence of cube slip. Therefore, the spectacular strengthening due to I-If additions is discussed in terms of other parameters of the dislocation structure such as the complex stacking fault energy. (C) 1997 Elsevier Science S.A.