LOCALIZED LATTICE INSTABILITY RELATED TO THE NUCLEATION PROBLEM OF MARTENSITE

The generalized soft mode is not very convenient to explain the martensite nucleation process. It must be substituted by a localized soft mode approach. The starting point for this approach is the sensitivity of the mechanical stability to the strains. It is first demonstrated that calculations using second and third other elastic constants provide a good approximation to the strain producing the instability. The effects of some particular strain such as "Bain strain" or {110} in ß phase alloys are reviewed. The soft zones around some specific lattice defects like dislocations and free surfaces are then analyzed. It is shown that large lattice vibrations are localized in these soft zones. An attempt is made to explain the role of the soft zones and associate large vibration to the nucleation process

Phénomènes de relaxation et frottement intérieur dans les solides vitreux

Several mechanical relaxation phenomena are observed on glasses. The characteristics of these phenomena varry from one type of glass to another but some features seem to be common to all solids in the vitreous state. Hence, results about : — oxyde glasses, — organic glasses, — vitreous semiconductors, — metallic glasses, are recalled. Then, it is shown that there are a relaxation process about the glass transition (primary relaxation) and several relaxation processes more or less differenciated at lower temperature (secondary relaxations). The primary or α relaxation corresponds to a rapid increase of internal friction with temperature ; the Arrhenius law is not any more valid and the response of the material to stress can be non-linear. On the contrary, secondary relaxation processes are linear and obey to the Arrhenius law. Some of them can be interpreted in terms of movement of well defined structural elements (for exemple : jump of alcaly ions in silica-alcaly oxyde glasses) ; others are not clearly explained : for instance, just before the glass transition, a peak or an anomaly in the spectrum internal friction versus temperature more or less resolved from the increase of damping due to the α relaxation, is often observed. This feature seems to be associated to delayed elasticity. We propose that such a behavior corresponds to a second relaxation phenomenon (β relaxation) which is, as α relaxation, characteristic of the vitreous state. In order to have agreement between the structural models generally admitted for describing the vitreous state and the necessity to have two types of atomic arrangements for explaining the two relaxation phenomena α and β, we propose the following assumption : whatever the structural model considered, there are planar defects or boundaries in glasses. Then, the β relaxation could be due to thermally activated movements of atoms or molecules in the boundaries while the α relaxation should be the consequence of cooperative rearrangements of atoms or molecules between the boundaries. In a first step, the relaxation models well known in the case of crystalline solids are adapted to the case of glassy materials ; hence, experimental results can be interpreted at less qualitatively.Les solides vitreux présentent divers phénomènes de relaxation mécanique que l'on peut étudier par mesure de frottement intérieur. Les caractéristiques de ces phénomènes varient d'un type de solides vitreux à un autre. Néanmoins, on peut relever certains comportements qui semblent communs à l'ensemble des solides à l'état vitreux. Ainsi, le rappel de résultats concernant : — les verres minéraux, — les matériaux organiques vitreux, — les solides covalents vitreux, — les verres métalliques, fait apparaître que, dans tous les cas, il existe un processus de relaxation vers le domaine de transition vitreuse (relaxation primaire) et des processus de relaxation plus ou moins bien différenciés à plus basse température (relaxation secondaire). La relaxation primaire ou α qui se traduit généralement par une montée rapide du frottement intérieur avec la température est caractérisée par la non-validité de la loi d'Arrhénius et la non-linéarité vis-à-vis de la contrainte. Les phénomènes de relaxation secondaire, au contraire, sont linéaires et suivent la loi d'Arrhénius. Ils peuvent être nombreux et si certains peuvent être expliqués en envisageant le mouvement d'éléments structuraux bien définis (par exemple, saut des ions alcalins dans un verre silice-oxyde alcalin), d'autres donnent lieu à des anomalies de frottement intérieur dont l'interprétation n'est pas toujours clairement établie. Ainsi, un peu au-dessous du domaine de transition vitreuse, on observe souvent un pic ou une anomalie de frottement intérieur plus ou moins bien résolue de la montée de frottement intérieur lié à la relaxation α ; cette manifestation semble être associée à des effets d'élasticité retardée. Un tel comportement nous paraît correspondre à un 2e phénomène de relaxation propre à l'état vitreux (relaxation β). Pour concilier les données concernant les modèles structuraux proposés pour décrire l'état vitreux et la nécessité d'envisager 2 environnements atomiques différents pour rendre compte des phénomènes de relaxation α et β, nous proposons d'ajouter aux modèles de structure généralement admis l'hypothèse suivant laquelle existeraient des défauts de type interfaces. La relaxation β pourrait ainsi résulter du mouvement des atomes ou molécules au niveau des interfaces alors que la relaxation α serait la conséquence du mouvement coopératif des atomes ou des molécules entre les interfaces. Dans une première approche, l'adaptation des modèles de relaxation déjà connus dans le cas des solides cristallins aux solides vitreux nous permet de rendre compte, au moins qualitativement, des résultats expérimentaux

Influence du désancrage des dislocations sur l'atténuation des ondes ultrasonores dans l'aluminium

Ultrasonic attenuation changes have been observed during tensile test of polycristalline aluminium 99,999 %, the strain beeing always smaller than 0,006 %. The curve Δα = f(σ0) exhibits three parts wheen σ0 increases : (i) a first increase of Δα in attenuation is observed, (ii) then a constant value Δα 1 of Δα is obtained, (iii) above a stress σp attenuation increases strongly. The first increase of Δα has been interpreted in terms of breakaway of dislocations from pinning points such as impurity atoms (a simple expression is proposed to describe this increase of Δα), the second increase is attributed to dislocation multiplication. When Δα ≃ Δα 1 different effects have been observed : a tensile speed effect, a time effect when tensile test is stopped at a stress συ, and a « memory » effect associated to συ. These results are qualitatively interpreted in terms of migration of impurities towards dislocations.Nous observons la variation de l'atténuation Δα des ondes ultra-sonores dans un aluminium 99,999 % pendant l'application d'une contrainte croissante σ 0, la déformation n'excédant jamais 0,006 %. La courbe Δα = f(σ0) présente trois parties : quand σ0 croît Δα augmente tout d'abord, puis garde une valeur sensiblement constante Δα1, et enfin augmente à nouveau lorsque la contrainte dépasse une valeur notée σp. La première augmentation de Δα1 est attribuée au dépiégeage des dislocations à partir des impuretés (une relation est proposée pour décrire cette croissance de Δα), la seconde à la multiplication du nombre de boucles. Dans la partie où Δα ≃ Δα 1 nous mettons en évidence le rôle de la vitesse de mise en charge, du maintien à une contrainte quasi constante συ et un effet de « mémoire » attaché à cette contrainte. Nous interprétons qualitativement ces résultats en envisageant la migration des impuretés vers les dislocations

INTERNAL FRICTION MEASUREMENTS RELATED TO THE TWO WAY MEMORY EFFECT IN Cu-Zn-Al ALLOY EXHIBITING THERMOELASTIC MARTENSITIC TRANSFORMATION

The alloys with Thermoelastic Martensitic Transformation, like the Cu-Zn-Al, exhibit, after a thermomechanical treatment named "training", The Two Way Shape Memory Effect. We have compared internal friction measurements versus amplitude for a "trained" and a "virgin" sample. The obtained results are coherent with the existence in the "trained" sample of particular sets of dislocations. These dislocations generates orientated internal stresses, which favour particular martensite variants during the direct transformation

INTERNAL FRICTION OF SINGLE AND POLYVARIANT MARTENSITES OF Cu-Zn-Al

The alloys which exhibit a thermoelastic martensitic transformation are generally characterised by a high damping in the martensitic phase. In the case of 18R faulted martensites, two explanations can be proposed : the movement of the interfaces between the variants or the movement of the partial dislocations present at the edge of the faults. These movements can be both detected by electron microscopy. To cut between these two hypothesis, the internal friction of a single then polyvariant martensite has been measured. A very low internal friction is obtained in the martensite single variant whereas high internal friction is as usually obtained for the polyvariant one. This proves that the internal friction of martensite is probably due to the movement of interfaces between the variants

Intelligent structural materials and biomimetism [Matériaux intelligents structuraux et biomimétisme]

cited By 0International audienc

Mise en évidence de deux types d'ancrages sur les dislocations dans l'aluminium irradié puis déformé

The simultaneous study of the Bordoni relaxation and the modulus change measured at 20 K after neutron irradiation at 77 K and cold-working at 10 or 80 K shows the existence of two types of pinning points : one on the geometric kinks, another on dislocation segments lying along Peierls's valleys. The second type of pinning point is less stable than the first. In addition, for high irradiation doses, a wide maximum appears, which seems linked to the Bordoni relaxation.L'étude comparée de la relaxation de Bordoni et de l'anomalie de période mesurée à 20 K après irradiation aux neutrons à 77 K et écrouissage à 10 ou 80 K permet de mettre en évidence deux types de points d'ancrage : l'un sur les écrochements géométriques, l'autre sur les segments de dislocation parallèles aux vallées de Peierls, moins stable que le premier. En outre, pour les fortes doses d'irradiation, il apparaît un maximum diffus qui semble lié à la relaxation de Bordoni

ELECTRICAL EMISSION ASSOCIATED WITH THE MARTENSITIC BURST OF Fe - Ni ALLOY

Fast response optical reflectivity and electrical voltage measurements show that, in a Fe-32wt%. Ni alloy, the martensitic transformation begins by a single big burst within a few µs. When a d.c. current flows through the sample, the electrical signal linked to the burst presents a negative time-voltage integral and HF oscillations. The time-voltage integral is well explained by the bulk electromagnetic effects of the transformation. We have shown that a moving austenite-martensite interface must create electrical currents inside the sample and therefore explain the H.F. oscillations. For a burst, a computer-simulated signal has been obtained and agrees rather well with the experimental one

Microplasticity of MMC. Experimental results and modelling

The microplastic behavior of several MMC is investigated by means of tension and compression tests. This behavior is assymetric : the proportional limit is higher in tension than in compression but the work hardening rate is higher in compression. These differences are analysed in terms of maxium of the Tresca's shear stress at the interface (proportional limit) and of the emission of dislocation loops during the cooling (work hardening rate). On another hand, a model is proposed to calculate the value of the yield stress, describing the composite as a material composed of three phases : inclusion, unaffected matrix and matrix surrounding the inclusion having a gradient in the density of the thermally induced dilocations

QUALITATIVE ANALYSIS OF PLASMA CREATED BY SHOCK LASER

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