Y.T. Pei

The evolution of microstructure in a laser clad TiB-Ti composite coating

Abstract The microstructure of a TiB/Ti composite coating, obtained by laser cladding a Ti–6Al–4V substrate with a Ti/TiB2 powder mixture, was scrutinized using transmission electron microscopy. TiB showed three different morphologies: fine needles (200 nm diameter, 15 μm length); plates (thickness 1 μm, short length 3 μm and long length 15 μm); and coarse needles (diameter 3 μm and length of 50 μm). All TiB is composed of both the stable B27 and the metastable Bf crystal structures. Intimate mixing of B27 and Bf is possible because B27(200) planes fit excellently on Bf(110) or ( 1 1 0 ) planes (with B27[010]//Bf[001]) that easily leads to stacking disorder. Aspects of disorder are quantitatively analyzed using high-resolution transmission electron microscopy (HRTEM). Throughout the fine needles, extensive stacking disorder occurs. In the plates a core of B27 (with relatively low stacking-fault (SF) density) is present, with faulted Bf on the outer surfaces that has a rough but fully faceted interface with the Ti matrix. The coarse needles consist predominantly of nearly defect-free B27. It is noteworthy about the coarse needles that they have a large core composed of Ti. On a much finer scale Ti is also dispersed in the plates and fine needles. It will be shown that the differences in microstructure among the three types of TiB morphologies provide important clues about the evolution of the TiB under the conditions of rapid growth.

SiCp/Ti6Al4V functionally graded materials produced by laser melt injection

Abstract With a well-controlled laser melt injection (LMI) process, for the first time the feasibility is demonstrated to produce SiC particles (SiCp) reinforced Ti6Al4V functionally graded materials (FGMs). SiCp are injected just behind the laser beam into the extended part of the laser melt pool that is formed at relatively high beam scanning velocities. The process allows for the minimization of the decomposition reaction between SiCp and Ti6Al4V melt, and also leads to FGMs of SiCp/Ti6Al4V instead of a homogeneous composite layer on Ti6Al4V substrates. An injection model is designed based on the temperature/viscosity field of the laser pool for a deeper understanding of the mechanism of formation of the FGMs with LMI. The model is based on finite element calculations of the temperature field in the melt pool, physical considerations of the LMI process and it is supported by experimental observations. Three types of reaction layers are observed around SiCp, namely a thin monocrystalline TiC layer, a cellular polycrystalline TiC layer and a thick mixed layer of TiC with Ti5Si3. Among them, only the monocrystalline TiC layer exhibits particular orientation relationships (ORs) to the SiCp lattice, i.e. (111)TiC∥(0001)SiC and 〈110〉 TiC ∥ 〈1 1 00〉 SiC or ( 111 ) TiC ∥( 10 1 2 ) SiC and 〈1 1 0〉 TiC ∥ 〈1 2 10〉 SiC . These two kinds of TiC reaction layers act as a barrier against the interfacial reaction and its swift formation during rapid cooling hinders the dissolution of SiCp in the Ti-melt.

LASER MELT INJECTION IN ALUMINUM ALLOYS: ON THE ROLE OF THE OXIDE SKIN

In this paper the method of laser melt injection of SiC particles into an aluminum substrate is investigated both experimentally and theoretically. An extremely small operational parameter window was found for successful injection processing. It is shown that the final injection depth of the particles is controlled mainly by the temperature of the melt pool rather than by the particle velocity. A theoretical model that takes into account the wetting behavior and the particle penetration processes is developed on the basis of the observed particle velocity, thickness and area fraction of oxide skin that partially covers the surface of the heated aluminum melt pool. The model reveals the role of the oxide skin: it is relatively strong at low temperature and acts as a severe barrier for the injection process. It was found that preheating the aluminum substrate results in a higher temperature of the melt pool and partial dissolution of the oxide skin, through which the injected particles are able to penetrate.

Five-Fold Branched Si Particles in Laser Clad AlSi Functionally Graded Materials

Abstract Many five-fold branched Si particles (Si p ) were observed in Al–40 wt% Si functionally graded materials produced by a single-step laser cladding process on cast Al-alloy substrate. In this paper the five-fold twinning and growth features of Si p are scrutinized with orientation imaging microscopy and electron microscopic examination. It is a more in depth study of formation of the Si particles in functionally graded materials as published in our previous paper [Pei, Y. T. and De Hosson, J. Th. M., Acta mater. , 2000, 48 , 2617]. These Si particles have grown from twinned decahedron nuclei consisting of five tetrahedrons that share a common 110 axis. The twin plane re-entrant edge (TPRE) mechanism explains both the branch growth in the radial direction and the elongation of Si p along their common 110 axis. Subsequent twinning within the twinned tetrahedrons provides additional re-entrant grooves on their top faces, which are important for the rapid elongation and consequently for the continuous growth of the branched particle. The 7.5° mismatch that arises by putting together five tetrahedrons around a common 110 axis is accommodated by small-angle grain boundaries (SAGBs). The SAGBs may disturb the progress of growth steps, which causes the particles to branch. The most remarkable facts of the study are that the five-fold branched silicon particles are much bigger (25∼40 μm) than the nanometer sizes previously reported in the literature and the 7.5° mismatch is accommodated mainly by multiple SAGBs. The examples of a single SAGB reported before are just a special case of the SAGB mechanism.

In-situ microscopy investigation of failure mechanisms in Al/SiCp metal matrix composite produced by laser embedding

Laser surface treatments are suitable techniques for improving the mechanical, tribological and chemical properties of metal surfaces. In the laser injection process the laser beam interacts primarily with the substrate and to a lesser extent with the particles, which are simultaneously injected into the melt pool produced by the absorbed laser power density. Metal Matrix composite (MMC) layers with interesting properties and very good connection to the metal substrate may be prepared by a selection of suitable combination of metallic substrate and ceramic particles. In this study Al/SiC MMC layers prepared by laser injection of SiC particles into Al substrate were mechanically tested to reveal the weakest structural components from a mechanical point of view. The objective is to obtain accurate information about the initiation and propagation of cracks in such heterogeneous structures by microscopic inspections of the sample surface during the loading. A combination of techniques was used to characterize the nucleation and progress of fracture: i.e., in-situ microscopy observations during testing and by conventional fractographic methods after failure.

Breakdown of the Coulomb friction law in TiC/a-C:H nanocomposite coatings

Advanced TiC∕a‐C:H nanocomposite coatings have been produced via reactive deposition in a closed-field unbalanced magnetron sputtering system (Hauzer HTC-1000 or HTC 1200). In this paper, we report on the tribological behavior of TiC∕a‐C:H nanocomposite coatings in which ultralow friction is tailored with superior wear resistance, two properties often difficult to achieve simultaneously. Tribotests have been performed at room temperature with a ball-on-disk configuration. In situ monitoring of the wear depth of the coated disk together with the wear height of the ball counterpart at nanometer scale reveals that the self-lubricating effects are induced by the formation of transfer films on the surface of the ball counterpart. A remarkable finding is a breakdown of the Coulomb friction law in the TiC∕a‐C:H nanocomposite coatings. In addition, the coefficient of friction of TiC∕a‐C:H nanocomposite coatings decreases with decreasing relative humidity. A superior wear resistance of the coated disk at a level o...

Nanoscale deformation mechanism of TiC/a-C nanocomposite thin films

This paper concentrates on the deformation behavior of amorphous diamondlike carbon composite materials. Combined nanoindentation and ex situ cross-sectional transmission electron microscopy investigations are carried out on TiC/a-C nanocomposite films, with and without multilayered structures deposited by pulse dc magnetron sputtering. It is shown that by controlling the distribution of nanocrystallites forming nanoscale multilayers, the system can be used as a “microstructural ruler” that is able to distinguish various deformation patterns, which can be hardly detected otherwise in a homogeneous structure. It is shown that rearrangement of nanocrystallites and displacement of a-C matrix occur at length scales from tens of nanometer down to 1 nm. At submicrometer scale homogeneous nucleation of multiple shear bands has been observed within the nanocomposites. The multilayered structure in the TiC/a-C nanocomposite film contributes to an enhanced toughness.

Microstructure and Properties of TiB/Ti-6Al-4V Coatings Produced With Laser Treatments

TiB/Ti-6Al-4V metal-matrix composite (MMC) layers were produced on Ti-6Al-4V substrates by laser cladding. A TiB2/Ti powder mixture was used as a precursor to obtain a dispersion of TiB needles in the Ti alloy matrix, with the aid of an exothermic reaction between TiB2 and Ti. A eutectic microstructure was obtained that consisted of an extremely homogeneous dispersion of TiB eutectic needles in the Ti alloy matrix, having a volume fraction as high as 0.33. Also, an equilibrium-like microstructure was found, consisting of a dispersion of both primary and eutectic TiB needles inside the Ti alloy matrix. An analysis of the geometry of the layers was performed and proved successful in determining the percentage of B. Further, it correctly predicted the variation of atomic B content as a function of laser power. The relative wear resistance coefficient, defined as the wear coefficient of the uncoated matrix divided by that of coating, shows an improvement by a factor as high as 1500 for the eutectic microstructure.

Tunable self-organization of nanocomposite multilayers

In this letter we report the controlled growth and microstructural evolution of self-assembled nanocomposite multilayers that are induced by surface ion-impingement. The nanoscale structures together with chemical composition, especially at the growing front, have been investigated with high-resolution transmission electron microscopy. Concurrent ion impingement of growing films produces an amorphous capping layer 3 nm in thickness where spatially modulated phase separation is initiated. It is shown that the modulation of multilayers as controlled by the self-organization of nanocrystallites below the capping layer, can be tuned through the entire film.

In situ compression study of taper-free metallic glass nanopillars

Because tapering leads to inevitable artifacts in the analyses of compression experiments on micrometer sized pillars, in this study taper-free nanosized pillars of Zr-based metallic glass of Zr61.8Cu18Ni10.2Al10 composition with diameter ranging from 600 to 90 nm were fabricated. These pillars were compressed in situ in a transmission electron microscope as a function of pillar diameter. Under compression each pillar of large diameter exhibits predominant inhomogeneous and intermittent plastic flow characterized by shear banding (SB) events. However, pillars around 150 nm in diameter and below show homogeneous deformation during compression without SB.