M. Cai

Evolution of surface morphology in TiNiCu shape memory thin films

A transition of filmsurface morphology between wrinkling and surface relief during heating/cooling is reported for a sputtered TiNiCu thin filmshape memoryalloy. The mechanisms for this transition are discussed based on film stress evolution. During annealingsurface wrinkling occurs to relieve compressive stress in the film, while the surface relief morphology occurs during cooling due to the martensitic transformation, which relieves tensile stress.

An electronic criterion for the intrinsic embrittlement of structural intermetallic compounds

Development of intermetallics for high-temperature structural applications has long been impeded by their brittleness. To overcome the brittleness, efforts have been devoted to understand the atomic bonding nature of intermetallics. However, there are no simple theoretical or experimental means for investigating the electronic origin of the brittleness. We find a simple experimental method, which employs the Kelvin-probe technique to measure the electron work function. The typical intermetallics in three alloy systems (Ni–Al, Fe–Al, and Ti–Al) were chosen as examples. An electronic criterion was therefore proposed for judging intrinsic brittleness. This criterion could provide some principles for improving the ductility of intermetallics and is also expected to be extendable to other materials (e.g., quasicrystalline alloys and metallic glasses).

Determination of strain localization in aluminum alloys using laser-induced photoelectron emission

Uniaxial tensile deformation of oxidized aluminum produces low work-function patches of fresh metal which can be probed by measurements of photoelectron emission during exposure to ultraviolet light. We report measurements of photoelectron emission during uniaxial testing of polycrystalline Al(1200), Al–Mn(3003), Al–Mg(5052), and Al–Mg–Si(6061) alloys where the broad face of the gauge section is exposed to pulsed excimer laser radiation (248nm). We show that strain localization alters the distribution of fresh surface metal produced by subsequent deformation. The transition from more homogenous deformation to the principally localized deformation associated with shear bands is associated with a discontinuity in the growth rate of photoelectron intensities versus time. At this transition, the rate of fresh metal production along the illuminated portion of gauge section decreases. In all four materials, the strain at the discontinuity is somewhat below the strain given by the Considere criterion, consistent...

A combined study of surface roughness in polycrystalline aluminium during uniaxial deformation using laser-induced photoemission and confocal microscopy

A new technique integrating measurements obtained by photo-stimulated electron (PSE) emission and scanning laser confocal microscopy (SLCM) has been developed to characterize the deformation of commercially pure aluminium in uniaxial plastic strain. Real time, in situ PSE signals provide details about the evolution and propagation rates of surface events during the deformation process. High-resolution SLCM measurements yield details describing the relative magnitude and spatial distribution of the surface features. During homogeneous deformation, uniform generation of surface area produces a monotonic increase in PSE intensity whereas necking induces a saturation condition. Analysis of the surface area created by the deformation revealed that the rate of generation correlates well with the PSE intensity. The consistencies in the data acquired with these two techniques signify that they yield complementary information and that the combination provides essential details about the deformation process in a material with low hardness such as an aluminium alloy.

Variation of surface morphology and electronic behavior under dynamic tensile conditions

Mechanical deformation can lead to various surface changes such as surface electronic behavior. The effect of deformation on work function (WF) has been studied extensively, but the mechanism responsible for such effect has been unclear. In this study, we measured the variations in the WF typically of an Al–Mn alloy with respect to deformation under dynamic tensile conditions using a scanning Kelvin probe. Furthermore, we also investigate the contribution of surface morphology, represented by roughness, caused by different deformation conditions to the WF. It was demonstrated that the WF depends strongly on both deformation and deformation speed, whereas roughness depends on deformation but hardly deformation speed. The present study therefore suggests a significant role of dislocation mechanism in determining the WF although surface morphology or roughness associated with deformation also affects the WF.

Simultaneous measurements of photoemission and morphology of various Al alloys during mechanical deformation

We report simultaneous measurements of strain and photoelectron emission from high purity Al (1350), Al–Mg (5052), Al–Mn (3003), Al–Cu (2024), and Al–Mg–Si (6061) alloys under uniaxial tension due to pulsed excimer laser radiation (248nm). The emission of low-energy photoelectrons is sensitive to deformation-induced changes in surface morphology, including the formation of slip lines and slip bands. Alloy composition and surface treatment significantly influence the photoemission during deformation. Surface oxide enhances the signal-to-noise level during photoemission measurement. In the early stage of deformation (strain ⩽0.04), photoemission intensity increases gradually in a nonlinear fashion. While subsequent photoemission increases almost linearly with strain until failure in samples with thin oxide layer (∼31A), there are two linear segments of photoemission for the samples with oxide of 45A. The onset of strain localization corresponds to the intersection point of two linear segments, usually at a ...

The effect of thermal oxidation on laser-induced photoelectron emission during tensile deformation of polycrystalline aluminum

Many metals emit electrons when exposed to UV radiation (photon energies 4 to 8 eV). Deformation can significantly affect the intensity of these emissions. In the case of reactive metals, these emissions are also altered by the presence of surface oxides. We have characterized the effect of thermal oxides on laser-induced photoelectron emission from commercially pure polycrystalline aluminum with a view toward using these emissions as a probe of deformation processes. The thickness of oxides produced by a range of annealing treatments in air was determined by x-ray photoelectron spectroscopy. Time-of-flight measurements on photoelectrons from these surfaces under 248 nm irradiation (5 eV photons) show two peaks: a fast peak which we attributed to electrons from metallic aluminum, and a slower peak, which may be due to electrons from interface states. Surface oxide films of sufficient thickness attenuate both peaks. We show that the sensitivity of the photoelectron signals to deformation varies with therma...

Laser-induced photoemission as a probe of slip band formation in single crystal and polycrystalline aluminum during uniaxial deformation

We report measurements of laser-induced photoelectron emission (LIPEE) from single crystal aluminum (99.999%) and high purity polycrystalline aluminum (>99.9%) during uniaxial tensile deformation. A 248-nm excimer laser (5-eV photon energy) was used as a light source. Deformation was performed on a tensile stage in ultra-high vacuum at an initial strain rate of 1 × 10−3 s−1. Photoelectron intensities are sensitive to changes in surface morphology accompanying deformation, including slip line and band formation. In the single crystal material, LIPEE intensity initially increases linearly with strain followed by a monotonically decreasing slope at larger strain. In the polycrystalline material, LIPEE intensities increase linearly with strain in two segments. Slip bands on the deformed surfaces were characterized by atomic force microscopy (AFM).

Tribochemical wear of single crystal aluminum in NaCl solution studied by atomic force microscopy

We report a systematic study of chemically enhanced wear of single crystal aluminum surfaces in aqueous solutions using an environmentally equipped atomic force microscope (AFM). The experiments were conducted by using a standard Si3N4 AFM tip to apply a localized force on a polished, single crystal aluminum (110) surface. Most measurements were performed in 0.5 M NaCl solution. We show the effect of applied force, number of scans, chemical solution, and temperature on the chemical−mechanical wear of aluminum on the nanometer scale. Aggressive chemical environments significantly enhance the wear of aluminum relative to scanning in dry air. Quantitative measurements show that the wear volume increases in proportion to the square root of force and the number of scans (or time). Arrhenius plots of wear volume versus temperature are consistent with an activation energy of 31 kJ/mol for scanning in 0.5 M NaCl. The wear of the AFM tip and the aluminum substrate is explained in terms of the synergistic surface c...