Two-dimensional mapping of micro-hardness increase on surface treated steel determined by photothermal deflection microscopy

An optical noncontact technique is presented that provides a two-dimensional map of the hardness of treated steel at the micrometer level. The photodeflection technique for determining the thermal diffusivity is shown to be a useful and rapid way to determine the hardness increase profile in two dimensions with only minor preparation of the sample (flat polish). This is possible due to the strong correlation found for this type of material between the inverse of the diffusivity and the hardness increment after treatment. The diffusivity retrieval is performed by a single measurement of the phase delay between the pump beam and the photodeflection signal thus allowing a rapid scanning of the surface. The surface scans of the hardness performed with this technique showed that anomalous regions can be identified that direct optical or scanning electron microscopy observation do not reveal.Fil: Crossa Archiopoli, Ulises. Universidad de Buenos Aires. Facultad de Ingeniería; ArgentinaFil: Mingolo, Nélida. Universidad de Buenos Aires. Facultad de Ingeniería; ArgentinaFil: Martinez, Oscar Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Belgrano. Facultad de Ciencias Exactas y Naturales; Argentin

Photothermal Microscopy

AbstractAn accessory for a commercial optical microscope was implemented to map the thermal diffusivity of the sample with high spatial resolution. The system is based in the photothermal technique recently developed by the group. It consists in the measurement with a probe laser of the surface curvature induced by the heat delivered by a modulated pump laser. A lock-in detection technique provides the signal amplitude and phase as a function of the modulation frequency. The accessory was mounted on the camera port of a metallographic microscope Olympus BX51. Results obtained on different samples are presented

Simple model of a glow discharge electron beam for materials processing, A

Includes bibliographical references (page 432).A simple semiempirical model of the electron beam generated by a pulsed cold cathode electron gun has been developed. The model describes analytically the observed self-focusing of the discharge and predicts the dynamical variation of the focal distance, in good agreement with experiments. This effect plays a major role in the determination of the effective duration of the energy pulse. The model was used to conduct simple calculations of energy thresholds for melting of solid materials, giving helpful insight on ranges of operation of this kind of electron gun for its application to material processing. A comparison with available experimental data for Mg70Zn30 samples is given

Investigation of laser annealing mechanisms in thin film coatings by photothermal microscopy

We study the evolution of the absorptance of amorphous metal oxide thin films when exposed to intense CW laser radiation measured using a photothermal microscope. The evolution of the absorptance is characterized by a nonexponential decay. Different models that incorporate linear and nonlinear absorption, free carrier absorption, and defect diffusion are used to fit the results, with constraints imposed on the fit parameters to scale with power and intensity. The model that best fits is that two types of interband defects are passivated independently, one by a one-photon process and the other one by a two-photon process.Fil: Zaldivar Escola, Facundo. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Mingolo, Nélida. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física; ArgentinaFil: Martínez, Oscar E.. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física; ArgentinaFil: Rocca, Jorge J.. State University of Colorado - Fort Collins; Estados UnidosFil: Menoni, Carmen S.. State University of Colorado - Fort Collins; Estados Unido

Photoacoustic characterization of phase transitions in amorphous metal alloys

In this work a pulsed laser photoacoustic technique is employed in amorphous alloys for the characterization of crystallization temperatures and the determination of the activation energies for the transitions. It is shown that the changes produced in the photoacoustic signal generated by low energy laser pulses (< 200 μJ), is a sensitive probe for detecting the crystallization of a metallic amorphous sample. A piezoelectric transducer attached to the sample by means of a glass substrate was used for the acoustic detection. The developed technique has the advantages of requiring a minimum amount of sample, with no special sample preparation or conditioning. In order to demonstrate the validity of the technique, it was applied to the study of the crystallization processes of the amorphous Mg-Zn alloy, comparing the result with respect to resistivity measurements

Photoacoustic characterization of phase transitions in amorphous metal alloys

In this work a pulsed laser photoacoustic technique is employed in amorphous alloys for the characterization of crystallization temperatures and the determination of the activation energies for the transitions. It is shown that the changes produced in the photoacoustic signal generated by low energy laser pulses (< 200 μJ), is a sensitive probe for detecting the crystallization of a metallic amorphous sample. A piezoelectric transducer attached to the sample by means of a glass substrate was used for the acoustic detection. The developed technique has the advantages of requiring a minimum amount of sample, with no special sample preparation or conditioning. In order to demonstrate the validity of the technique, it was applied to the study of the crystallization processes of the amorphous Mg-Zn alloy, comparing the result with respect to resistivity measurements.Facultad de Ciencias Exacta

Photoacoustic characterization of phase transitions in amorphous metal alloys

In this work a pulsed laser photoacoustic technique is employed in amorphous alloys for the characterization of crystallization temperatures and the determination of the activation energies for the transitions. It is shown that the changes produced in the photoacoustic signal generated by low energy laser pulses (< 200 μJ), is a sensitive probe for detecting the crystallization of a metallic amorphous sample. A piezoelectric transducer attached to the sample by means of a glass substrate was used for the acoustic detection. The developed technique has the advantages of requiring a minimum amount of sample, with no special sample preparation or conditioning. In order to demonstrate the validity of the technique, it was applied to the study of the crystallization processes of the amorphous Mg-Zn alloy, comparing the result with respect to resistivity measurements.Facultad de Ciencias Exacta

Nuevos criterios para modificar la capacidad de amorfización de aleaciones metálicas

Se demostrará en el presente trabajo que se puede modificar la capacidad de amorfización de una aleación por el agregado de partículas perturbadoras de la cristalización. Para incorporar las partículas a la aleación se selecciona un elemento con una fuerte afinidad con una fuerte afinidad con una de las componentes de la aleación de partida. De esta manera se generan asociaciones que constituyen las partículas perturbadoras. En nuestros experimentos se utilizó Mg2Sn y Mg2Pb como partículas perturbadoras con parámetros de red muy distintos a los de la fase Mg7Zn3. También se seleccionó el Mg2Sn para perturbar la fase β-Sn en la aleación Sn-Zn. En la parte experimental de este trabajo se describirán las técnicas de fabricación y caracterización de las fases amorfas y de las partículas perturbadoras. Se desarrollará una sistemática que permitirá determinar cuales son los mecanismos en juego y su importancia relativa (concentración y tamaño de partículas, velocidad de enfriamiento, corrimiento en la composición, temperatura de partida, etc.). Por medio de un modelo cinético se mostrará que las partículas perturbadoras modifican la velocidad de nucleación y que el efecto en la velocidad de crecimiento no es significativo. El modelo predice que el efecto será más importante cuanto mayor sea la fracción en volúmen de las mismas. Experimentalmente se mostró la existencia de una cantidad óptima en volúmen de dichas partículas. El efecto de las partículas perturbadoras es agregar un término de superficie a la energía de Gibbs del núcleo cristalino. Dicho término es proporcional a la fracción en volúmen de partículas, inversamente proporcional al radio de las partículas y creciente con la energía de superficie entre la fase de las partículas perturbadoras y la fase cristalina de la aleación que se pretende amorfizar. Para que el término de energía de superficie sea favorable es necesario seleccionar las partículas perturbadoras de modo que su estructura cristalina y parámetro de red sean muy distintos a los de la fase cristalina que se quiere obstaculizar. El agregado de partículas mejora la amorfización en forma apreciable cuando son agregadas a aleaciones que son buenas formadoras de vidrio según se desprende del modelo cinético presentado.Fil: Mingolo, Nélida. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Adaptive telescope for confocal photothermal microscopy of irregular surfaces

An adaptive telescope was developed to perform measurements using the confocal photothermal microscopy technique. The telescope automatically aligns to reinject the reflected probe beam as the measurement scans over irregular surfaces. The system uses DVD lenses that adapt to surfaces through controlled movements. Alignment algorithms are presented along with measurements on complex surfaces.Fil: Jan, Luis Emiliano. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Laboratorio de Haces Dirigidos; ArgentinaFil: Zaldivar Escola, Facundo. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Laboratorio de Haces Dirigidos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Mingolo, Nélida. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Física. Laboratorio de Haces Dirigidos; Argentin