Additive layer manufacturing of titanium matrix composites using the direct metal deposition laser process

Titanium Matrix Composites (TMC's) containing various volume fractions of (TiB+TiC) particles have been deposited from powder feedstocks consisting of a blend of pre-alloyed (Ti-6Al-4V+B4C) powders, using the direct metal deposition (DMD) laser process and the in-situ chemical reaction 5Ti+B4C→4TiB+TiC. Process optimization has allowed to obtain a homogeneous distribution of tiny TiB whiskers within the Ti-6Al-4V α/β matrix, with a full solubilization of C for low B4C contents (0.5 wt% and 1.5 wt%), and the formation of a small amount of globular TiC particles at higher B4C content (3%). Comparisons with Ti-6Al-4V DMD walls revealed a substantial grain refinement on TMC's due to enhanced grain nucleation on TiB whiskers, even for low B4C contents. Last, mechanical investigations indicated an increase of 10–15% of Vickers hardness, and a constant 10% increase of Young modulus on a large temperature range (20–600 °C) for all B4C conten

Atomic Species Associated with the Portevin–Le Chatelier Effect in Superalloy 718 Studied by Mechanical Spectroscopy

In many Ni-based superalloys, dynamic strain aging (DSA) generates an inhomogeneous plastic deformation resulting in jerky flow known as the Portevin--Le Chatelier (PLC) effect. This phenomenon has a deleterious effect on the mechanical properties and, at high temperature, is related to the diffusion of substitutional solute atoms toward the core of dislocations. However, the question about the nature of the atomic species responsible for the PLC effect at high temperature still remains open. The goal of the present work is to answer this important question; to this purpose, three different 718-type and a 625 superalloy were studied through a nonconventional approach by mechanical spectroscopy. The internal friction (IF) spectra of all the studied alloys show a relaxation peak P718 (at 885 K for 0.1 Hz) in the same temperature range, 700 K to 950 K, as the observed PLC effect. The activation parameters of this relaxation peak have been measured, Ea(P718){\thinspace}={\thinspace}2.68{\thinspace}{\textpm}{\thinspace}0.05 eV, $\tau$0{\thinspace}={\thinspace}2{\textperiodcentered}10-15 {\textpm} 1 s as well as its broadening factor $\beta${\thinspace}={\thinspace}1.1. Experiments on different alloys and the dependence of the relaxation strength on the amount of Mo attribute this relaxation to the stress-induced reorientation of Mo-Mo dipoles due to the short distance diffusion of one Mo atom by exchange with a vacancy. Then, it is concluded that Mo is the atomic species responsible for the high-temperature PLC effect in 718 superallo

Etude de la mobilité des dislocations dans le silicium monocristallin par frottement intérieur haute température

La mobilité des dislocations dans le silicium monocristallin a été étudiée par mesures isothermes de frottement intérieur. Trois pics de relaxation ont été mis en évidence à des températures croissantes, leur existence étant liée aux conditions de déformation. Le premier pic a été attribué à la migration de décrochements géométriques avec une énergie WM de 1,5 eV. Le second pic, d'énergie d'activation 1,95 eV, a été attribué à la création et à l'interaction des décrochements thermiques. Le dernier pic, d'énergie 2,4 eV, a été associé aux mouvements des décrochements thermiques dans le champ des atomes de dopant. L'énergie de formation des doubles décrochements déduite de ces essais (FDK) est de 0,9 eV

Etude de la mobilité des dislocations dans le silicium monocristallin par frottement intérieur haute température

Dislocation mobility in silicon single crystals has been studied by isothermal internal friction measurements. Three relaxation peaks have been observed at increasing temperatures, connected to the straining conditions. The first peak has been attributed to the migration of geometrical kinks, with activation energy WM of 1.5 eV. The second one, with an activation energy of 1.95 eV, has been connected to the nucleation and the interaction of thermal kinks. The last one, with an activation energy of 2.4 eV, is associated with the motion of double kinds in the field of the doping atoms. Following this results, the double kink formation energy, FDK, has been found equal to 0.9 eV.La mobilité des dislocations dans le silicium monocristallin a été étudiée par mesures isothermes de frottement intérieur. Trois pics de relaxation ont été mis en évidence à des températures croissantes, leur existence étant liée aux conditions de déformation. Le premier pic a été attribué à la migration de décrochements géométriques avec une énergie WM de 1,5 eV. Le second pic, d'énergie d'activation 1,95 eV, a été attribué à la création et à l'interaction des décrochements thermiques. Le dernier pic, d'énergie 2,4 eV, a été associé aux mouvements des décrochements thermiques dans le champ des atomes de dopant. L'énergie de formation des doubles décrochements déduite de ces essais (FDK) est de 0,9 eV

Thermoelastic Relaxation Due to Chemical Gradient in γ-γ' Ni3Al Materials

Internal friction measurements performed in two types of γ-γ' Ni3Al structures (an AM1 superalloy with periodic distribution of 1 µm and a non-periodic biphased material with grain size in the order of 100 µm) bring out a high amplitude relaxation peak located at about 10 Hz, independently of the measurernent temperature. The characteristics of the peak (amplitude, frequency position and temperature range observation) and experimental conditions (torsion tests) do not allow to interpretate this effect in terms of classic thermoelastic relaxation due to transverse or intercrystalline thermal currents. We propose a model of thermal relaxation under confined chemical gradient related to the elaboration of this kind of biphased materials ; the chemical potential appears as a new internal variable for thermomechanical equilibrium conditions and implies local temperature gradients under stress annihilated by delayed thermal currents. The expression of the relaxation time is a function of the chemical gradient and of the thermal diffusivity

Silicon Nitride and YMgSiAlON Glass Study by Mechanical Spectroscopy

Si3N4/TiN and YMgSiAlON/SiC composites have been studied by isothermal mechanical spectroscopy over a large frequency range. Internal friction curves obtained for the composite Si3N4-TiN(30%vol)-Al2O3,Y2O3(7.5%vol) exhibit a thermally activated pseudo peak superposed on a non purely exponential background. We can assume that this maximum is due to relaxations in the compressed intergranular glassy films. The apparent dynamical parameters deduced from its shift (11.2 eV and 3.10-45 s) are unusual as observed in amorphous materials. YMgSiAlON glass has a composition very similar to the one of the intergranular phase of Si3N4/TiN composite. The damping curves obtained for YMgSiAlON/SiC composites above the glass transition temperature are pure exponential backgrounds. An original analysis of these curves allows to determine an activation energy of about 3.6 eV. This value is more realistic for an elementary diffusion mechanism in a glass near Tg

Zener Relaxation in CuAl Single Crystals Studied by Isothermal Mechanical Spectroscopy

The Zener relaxation was studied in Cu-Al single crystals. All samples were cut with the same orientation and the Al concentrations were ranged between 3 and 19 at. % Al ( solid solution). For low temperature experiments ( < 550°K ), perfect Debye peaks are observed in all samples and the relaxation strength is proportional to the concentration square according with the pair reorientation Zener theory. At higher temperatures and for samples with higher Al concentration only, the internal friction peaks are broader. In the sarne time, the relaxation strength first increases and then decreases with the measurement temperature according to the short-range ordering LeClaire-Lomer theory with respectively the influence of only neighbouring and more distant atoms

Influence of temperature, strain ratio and strain rate on the cyclic stress-strain behaviour of a gamma-titanium-aluminide alloy

Influence de la température, du rapport de déformation et de la vitesse de déformation sur le comportement en fatigue oligocyclique de l'alliage TiAl. Cette étude porte sur l'influence de la température, du rapport de déformation et de la vitesse de déformation sur le comportement en fatigue oligocyclique d'un alliage de fonderie Ti-Al48-Cr2-Nb2 (% atomique) possédant une structure lamellaire. Les essais de fatigue sont réalisés à deux températures se situant de part et d'autre de la transition fragile ductile: 25 °C pour être dans la région fragile et 750 °C pour être dans la région ductile. Les essais sont effectues sous air en imposant l'amplitude de déformation totale, deux valeurs du rapport de déformation (R$_{\varepsilon}$ = $\varepsilon_{\min}$/$\varepsilon_{\max}$): –1 et 0, ainsi que deux vitesses de déformation: 10-3 s-1 10-5 s-1. Le comportement cyclique du matériau observé à 750 °C est différent de celui observé à 25 °C. En effet, à 25 °C, l'alliage consolide pendant toute la durée de vie, tandis qu'à750 °C l'amplitude de contrainte se stabilise rapidement. Une influence du rapport de déformation est remarquée, mais seulement à 750 °C. En effet, pour R$_{\varepsilon }$ = 0, le comportement du matériau présente un léger durcissement en début de cyclage suivi d'une stabilisation de l'amplitude de contrainte