J.L. Martin

A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures

Abstract The microstructures of two tungsten carbide–cobalt (WC–Co) coatings, deposited using high velocity oxy-fuel (HVOF) thermal spraying method in different conditions, are studied. They are compared with that of the WC–Co powder grains injected in the flame, in an attempt to understand the transformations that occur during deposition. For this purpose, various imaging and analytical techniques in electron microscopy are used, in addition to global characterization methods such as X-ray diffraction and fluorescence. These methods reveal that the coatings are made of distinct islands, elongated along the substrate direction, which exhibit a nano-crystalline matrix containing tungsten, cobalt and carbon. The fraction of WC grains in the coating is smaller than that in the powder and fluctuates throughout the coating. A net loss in carbon is evidenced in the coatings as compared to the powder grains. New phases, W 2 C and W, appear in specific locations in the microstructure in relation with the local composition of the matrix. Very little metallic cobalt is retained. The extent of the transformation is related to the spraying conditions. Some processes that account for the change in microstructure and composition during spraying are proposed.

Characterisation of TiN thin films using the bulge test and the nanoindentation technique

Abstract In-plane mechanical properties of titanium nitride (TiN x ) thin films have been investigated by performing bulge test experiments on square membranes of side of approximately 2 a =4 mm. A layer of about 1 μ m thickness of TiN x ( x =0.84–1.3) was deposited onto an n-type Si(100) wafer using radio frequency magnetron sputtering. Prior to TiN x deposition, free-standing low-stress LPCVD silicon nitride (SiN y ) thin films were fabricated by means of standard micromachining techniques. The edges of windows were aligned with the [110] directions of underlying silicon wafer in order to make perfect squares bounded by (111) planes. The bulge test was first conducted on the silicon nitride films to determine its proper residual stress and Youngs modulus, being σ i =227±15 MPa and E =225±10 GPa, respectively. Then, the composite membrane made of TiN x together with underlying SiN y was bulged and the related load-displacement variation was measured. Finally, using a simple rule of mixtures formula, the elastic mechanical properties of TiN x coatings were determined and compared to those obtained during nanoindentation measurements. Both the Youngs modulus and residual stress showed increasing values with increasing bias voltage, nitrogen-to-titanium ratio and coating density. The effect of substrate temperature below 600°C was found to be less significant compared to other parameters. These results are presented and discussed in terms of coating porosity, microstructure and chemical composition determined by means of electron probe microscopy.

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.

Microstructural and analytical study of thermally sprayed WC-Co coatings in connection with their wear resistance

The microstructure of High Velocity Oxy-Fuel (HVOF) thermally sprayed WC-Co coatings was quantitatively evaluated in an attempt to describe the transformations that take place during thermal spraying. Slurry erosion tests were performed to measure the weight loss as a function of flow velocity and incident angle. It was found that an optimum amount of transformation is required to ensure a maximum of erosion resistance.

Portevin-Le Chatelier effect in biaxially strained Al-Fe-Si foils

Abstract Instability patterns linked to negative strain rate sensitivity of the flow stress are observed in Al–Fe–Si foils during biaxial bulge testing. They are interpreted as resulting from the Portevin–Le Chatelier effect. A linear stability analysis supporting this interpretation is provided.

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

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.

Mechanical strength of the binary compound Ni3Al

Abstract The mechanical properties of binary Ni 76.6 Al 23.4 single crystalline specimens have been studied in compression tests over a wide range of temperatures (293–1100 K). The resolved proof stress ( τ 0.2% ) and the corresponding work-hardening rate ( θ 0.2% ) have been measured as a function of temperature. A technique of repeated stress relaxations has been used to investigate the variation in the density of mobile dislocations that occurs during such transient tests. These experiments have been complemented with strain-rate changes for characterising the strain-rate sensitivity of the flow stress. The high hardening rates and their variation with temperature seem to correlate well with the values of the mobile dislocation exhaustion rates. The strain rate sensitivity of the stress exhibits negative values between 293 and 600 K, below the stress anomaly domain. These mechanical parameters are discussed in terms of the available dislocation mechanisms.

Characterising thermally activated dislocation mechanisms

Reference CRPP-ARTICLE-2001-029View record in Web of Science Record created on 2008-04-16, modified on 2017-05-12