J. Bonneville

Microindentation of Al-Cu-Fe icosahedral quasicrystal

Reference CRPP-ARTICLE-1999-033doi:10.1016/S1359-6462(99)00242-0View record in Web of Science Record created on 2008-04-16, modified on 2017-05-12

The mechanical properties of single phase γ Ti47Al51Mn2 polycrystals

Abstract The mechanical properties of polycrystalline samples of the single phase γ-Ti 47 Al 51 Mn 2 alloy have been studied during compression tests in a wide range of temperatures (120–1270 K). The flow stress and the work hardening rate are measured during imposed strain rate tests, while the strain rate sensitivity of the stress is examined using both strain rate jumps and stress relaxation experiments. From the temperature, strain and stress dependence of these parameters, it is shown that the investigated temperature domain can be divided into three regimes corresponding to different deformation mechanisms. The results are compared to the data available in the literature and are found to be in good agreement with the dislocation structures and dislocation motion mechanisms that we have previously reported.

Repeated load transients for measuring the effective activation volume in A γ TiAl alloy

The strain-rate changes and the load relaxation experiments are two experimental techniques commonly used for measuring the strain-rate sensitivity (SRS) of the flow stress of metals and alloys during deformation tests performed at constant strain-rate. Within the framework of thermal activation the SRS is usually related to an activation volume (V{sub eff}) which, in units of an atomic volume, is representative of the number of atoms that are involved in the dislocation movements to overcome the local obstacles in their slip planes. Since for each thermally activated process V{sub eff} has typical values and a specific variation with stress, its knowledge is of prime importance and can be used as a guide to identify the microscopic mechanism that governs plastic flow. This paper determines V{sub eff} for a {gamma} TiAl alloy.

Mechanical loss spectrum of Ni3(Al,Ta) single crystals

Abstract Mechanical loss measurements were performed by means of torsion pendula on polycrystalline and single crystalline specimens of a Ni 3 (Al,1%Ta) alloy with the L1 2 ordered structure. Measured internal friction spectra are characterized by the occurrence of two main phenomena: a well-defined internal friction peak at a temperature of ∼950 K, for a frequency of 1 Hz and an internal friction background, which exponentially increases at higher temperatures. The internal friction peak with an activation energy of 2.97±0.1 eV was interpreted as a point defect relaxation peak induced by the presence of Al–Al elastic dipoles on octahedral planes, while the internal friction background was found to well reflect the decreasing (resp. increasing) mobility of superdislocations on the primary octahedral (resp. cube) planes below (resp. above) the peak temperature in flow stress.

Strain-amplitude-dependent mechanical loss at intermediate temperatures in a Ni3(Al, Ta) single crystal

Dislocation dynamics in a Ni-3(Al, Ta) single-crystal alloy have been studied by means of mechanical spectroscopy at intermediate temperatures, that is in the temperature range of the flow stress anomaly. The mechanical loss of specimens predeformed at 300 K was found to be strongly dependent on temperature, pre-deformation level and heating rate. Thermal cycling (heating-cooling) and ageing experiments were also performed, which provided additional information about the annealing time dependence of the mechanical loss, elastic modulus of the material and plastic deformation of the specimens. The obtained results can be explained by a combination of two phenomena simultaneously occurring as the temperature is increased from 300 to about 500 K exhaustion of the mobile dislocation parts (superkinks) and pinning of the screw dislocation segments via cross-slip from the (111) onto the (010) planes.

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

Interpretation of the measurement of activation parameters in L12 alloys

Abstract Trying to connect microscopic interactions with macroscopic mechanical measurements is one of the goals of plasticity studies. The measurement of relaxation kinetics has been shown to provide the determination of the so called activation area, i.e. the area swept by a dislocation during the elementary glide process. It is shown that all the methods used so far neglect a relevant physical quantity: the (plastic) strain rate gep0 at the beginning of the relaxation test. A careful theoretical analysis is therefore given. One interesting result is that both the strain rate sensitivity, or equivalently the activation area, and the initial (plastic) strain rate gep0 can be unambiguously determined in current relaxation tests. The method is applied to the case Ni3Al,X single crystals of various compositions. The initial (plastic) strain rate geinp 0 is found to depend markedly on the plastic deformation preceding the relaxation test and to be strongly influenced by temperature. A discussion on the transition between the elastic and plastic deformation of these alloys is given.

Mechanical properties of Al-Li-Cu icosahedral quasicrystals

Abstract Poly-quasicrystalline samples of an icosahedral Al-Li-Cu alloy have been deformed in compression at a constant engineering strain rate, at temperatures ranging from 300 to 773 K ( T m ≅ 896 K). The samples are very brittle below about 500 K but rather ductile above this temperature, with an upper yield stress decreasing linearly with increasing temperature, from 225 MPa at about 473 K to 22 MPa at 773 K. Load relaxation behavior and activation parameter values suggest that, in this material, plastic deformation at high-temperature is controlled by a thermally activated process that involves atomic diffusion.

Characteristics of plastic flow in Ni3(Al,Hf) single crystals

Abstract Characteristic parameters of plastic deformation have been measured for Ni 3 (Al,Hf) single crystalline specimens deformed in compression over a wide range of temperatures (293–1100 K). These parameters are: (1) the resolved proof stress ( τ 0.2% ) which is measured at 0.2% offset stress; (2) the corresponding work-hardening rate ( θ 0.2% ); (3) the microscopic activation volume ( V 0.2% ); and (4) the variation in the density of mobile dislocations (Δϱ m /ϱ m 0.02% ) during transient tests such as load relaxations. The two last parameters were determined with a technique of repeated load relaxations. It is observed that not only τ 0.02% exhibits an anomalous behaviour with temperature but also θ 0.02% and Δϱ m /ϱ m 0.02%. A comparison of V 0.02% (τ) with similar measurements in Ni 3 Al shows that alloying effects alter the deformation mechanism.

Temperature dependence of dislocation microstructure in Ni3(Al,Hf)

Abstract Transmission electron microscopy observations were performed on Ni 74.8 Al 21.9 Hf 3.3 specimens deformed at different temperatures (room temperature to 983 K). The dissociation width of superdislocations has been examined as a function of the deformation temperature in these samples. Evolution of the dissociation width was also investigated during in-situ heating experiment up to 1000 K. The temperature dependence of the antiphase boundary energy ( γ APB ) was deduced from these observations. The importance of taking into account the temperature dependence of elastic constants is highlighted and it is shown that γ APB decreases with increasing temperature.