Adhesion along metal-polymer interfaces during plastic deformation

In this paper a numerical study is presented that concentrates on the influence of the interface roughness that develops during plastic deformation of a metal, on the work of adhesion and on the change of interface energy upon contact with a glassy polymer. The polymer coating is described with a constitutive law that mimics the behavior of Poly-Ethylene Terephthalate. It includes an elastic part, a yield stress, softening and hardening with increasing strains. For the interface between the metal and the polymer a mixed-mode (mode I and II) stress-separation law is applied that defines the interface energy and an interaction length scale. At the onset of deformation the surface of the substrate has a self-affine roughness characterized by the so-called Hurst exponent, a correlation length and an rms roughness amplitude, that evolves as a function of increasing strain. The findings are the following: the interface energy decreases until the strain at yield of the polymer coating. Interestingly, after yielding as the polymer starts to soften macroscopically, the decreasing average stress levels result in partial recovery of the interface energy at the interface. At higher strains, when macroscopic hardening develops the recovery of the interface stops and the interface energy decreases. The effect of coating thickness is discussed as well as the physical relevance of various model parameters

Failure of WCp/Ti-6Al-4V layer prepared by laser melt injection

Metal Matrix Composite layers consisting of WC particles (similar to 80 pin diameter) incorporated into a Ti-alloy matrix by the so-called Laser Melt Injection process were mechanically tested. Standard tensile tests as well as in-situ scanning electron microscope observations of tensile stressed surfaces were performed. Crack initiation and crack propagation processes were observed at initial and final stages of failure. Cracks initiate always in the ceramic particles. Intergranular brittle fracture of the central part of the WC particles or brittle decohesion along the WC/W2C interface form the initial failure. The crack propagates further inside the Ti matrix through brittle fracture of individual TiC dendrites and induces new cracks in the ceramic particles in the front of the main crack tip, Ductile fracture of the metal matrix, that creates the resulting fracture surface, is the final stage of the failure process. Internal tensile stresses formed during the laser processing are superimposed on the external tensile stress, which causes that local failure of the MMC layer starts at a relatively low external stress. The toughness of the Ti matrix makes the final failure process ductile.</p

Microstructure characterization of laser melt injected WC particles in Ti-6Al-4V

The Laser Melt Injection (LMI) process is explored to create a protective Metal Matrix Composite (MMC) layer consisting of 80 mum sized WC particles embedded in the top layer of a Ti-6Al-4V alloy. In particular the influences of the main process parameters on the laser track dimensions and microstructure are examined. Typical dimensions of a single laser track are a width of 1.8 mm and a depth of 0.7 mm. The volume fraction of the homogeneous distributed WC particles is about 25 - 30%. Important is that by overlapping the single laser tracks larger surface areas may be treated.In the Ti-alloy melt pool matrix TiC and W dendrites are present. The bonding between the ceramic particles and metal matrix is realized by reactions that occurred during the laser process. Around the WC particles a W2C layer was detected. followed by a TiC layer. Occasionally a crystal orientation relation between WC, W2C and TiC is observed, depending on the WC interface. When the surface of the WC particle is close to (0001) the preferred growing direction [0001] can be retained while the misfit at the WC/W2C interface is minimized.</p

Microstructure characterization of laser melt injected WC particles in Ti-6Al-4V

The Laser Melt Injection (LMI) process is explored to create a protective Metal Matrix Composite (MMC) layer consisting of 80 mum sized WC particles embedded in the top layer of a Ti-6Al-4V alloy. In particular the influences of the main process parameters on the laser track dimensions and microstructure are examined. Typical dimensions of a single laser track are a width of 1.8 mm and a depth of 0.7 mm. The volume fraction of the homogeneous distributed WC particles is about 25 - 30%. Important is that by overlapping the single laser tracks larger surface areas may be treated. In the Ti-alloy melt pool matrix TiC and W dendrites are present. The bonding between the ceramic particles and metal matrix is realized by reactions that occurred during the laser process. Around the WC particles a W2C layer was detected. followed by a TiC layer. Occasionally a crystal orientation relation between WC, W2C and TiC is observed, depending on the WC interface. When the surface of the WC particle is close to (0001) the preferred growing direction [0001] can be retained while the misfit at the WC/W2C interface is minimized

Failure of WCp/Ti-6Al-4V layer prepared by laser melt injection

Metal Matrix Composite layers consisting of WC particles (similar to 80 pin diameter) incorporated into a Ti-alloy matrix by the so-called Laser Melt Injection process were mechanically tested. Standard tensile tests as well as in-situ scanning electron microscope observations of tensile stressed surfaces were performed. Crack initiation and crack propagation processes were observed at initial and final stages of failure. Cracks initiate always in the ceramic particles. Intergranular brittle fracture of the central part of the WC particles or brittle decohesion along the WC/W2C interface form the initial failure. The crack propagates further inside the Ti matrix through brittle fracture of individual TiC dendrites and induces new cracks in the ceramic particles in the front of the main crack tip, Ductile fracture of the metal matrix, that creates the resulting fracture surface, is the final stage of the failure process. Internal tensile stresses formed during the laser processing are superimposed on the external tensile stress, which causes that local failure of the MMC layer starts at a relatively low external stress. The toughness of the Ti matrix makes the final failure process ductile

Modification of metal/oxide interfaces by dissolution of Sb in oxide precipitates containing metal matrices

The influence of dissolution of a segregating element (Sb) in a metal matrix in which oxide precipitates are present on the precipitate morphology and the interface structure is studied using HRTEM. The influence on Mn3O4 precipitates in Ag is distinct: (i) the initial precipitates, sharply facetted by solely {111}, are changed into a globular shape with sometimes also short {220} and (002) facets, (ii) a partly reduction of Mn3O4 into MnO occurs for a part of the precipitates. Further Sb appeared to prevent Oswald ripening of the precipitates. The influence on MnO and MgO precipitates in Cu is more subtle: only a small but significant increase of the facet lengths of the {200} (and {220}) relative to the {111} occurs. The influence of Sb can be explained by a large decrease of the energy of steps at Ag/Mn3O4 interfaces and by a stronger tendency for segregation of Sb to {200} and {220} than to {111} facets of the Ag/Mn3O4, Cu/MnO and Cu/MgO interfaces.</p