Oxidation Mechanism of Ni—20Cr Foils and Its Relation to the Oxide-Scale Microstructure

The oxidation behavior of Ni—20Cr foils of 100- and 200-m thickness wasstudied in air between 500 and 900°C. Simultaneously, the morphology,microstructure, and composition of the oxide layers were determined byscanning and transmission electron microscopies. Depending on thetemperature, the oxide layer differed significantly. The scale formedat all temperatures was complex, with an outer NiO layer having columnargrains, and an inner layer of equiaxedNiCr₂O₄+NiO+Cr₂O₃ grains. At low temperatures (500 and 600°C),the chromium content was insufficient to form a continuousCr₂O₃ layer, while such a continuous layer formed at theinner interface at oxidation temperatures of 700 to 900°C. At 600°C,internal oxidation of chromium occurred in the substrate. The oxidationmechanism is described taking into account these morphologies and theoxidation kinetics. The observation of no significant differences betweenthe oxidation behavior of thin strips and thick materials is related to thelimited exposure times of the study.     

Etude des phases ferroelastiques de Pb3V2O8 : Determination de la maille cristalline de la phase α ferroelectrique basse temperature

The cell parameters of the ferroelastic and ferroelectric phase of Pb₃V₂O₈ (T < 262 K) have been determined by very accurate X-ray powder diffractometry ; at 77 K, $\text{a}\text{= 7.460 (1)}\text{A}\text{?}$, $\text{b}\text{= 6.191 (1)}\text{A}\text{?}$, $\text{c}\text{= 9.348 (1)}\text{A}\text{?}$ and $\text{β = 116.63 (1)}\text{A}\text{?}$. This monoclinic cell is characterized by Z = 2 and the space group is A2. Thermal expansion tensors have been measured versus temperature for the three phases α (T < 262K), β (262K < T < 360 K) and γ (T > 360 K). The intermediate phase β is characterized by an important thermal expansion anisotropy and its molecular volume is higher than that of the two other phases α and γ.     

Comparison of Oxidation-Growth Stresses in NiO Film Measured by Deflection and Calculated Using Creep Analysis or Finite-Element Modeling

An attempt was made to determine the strain and the stresses generated by the growth of an oxide film using several approaches: an experimental one by means of deflection tests and modeling using either a recently developed creep analysis or a finite-element simulation. A new deflection apparatus was developed and NiO growth studied during the early stages of oxidation of a Ni₈₀Cr₂₀ alloy at 900°C, since many microstructural, kinetics and mechanical data are available for this system. The comparison of experiments and modeling indicate that the oxide layers are mostly subjected to compressive stresses when NiO is growing and the stress level and evolution clearly show that viscoplastic strain occurs in both the substrate and the oxide during oxidation. The comparison between the two modeling approaches with experiment leads to good agreement and suggests that the compressive-growth stresses derive from the lateral expansion of the fraction of new oxide that is formed within the oxide layer.     

Calorimetric study of lead oxide Pb3O4 phase transition

The heat capacity of Pb₃O₄ has been measured in the range 80 – 280 K. No anomaly has been observed at the temperature of the crystallographic transition (170 K), between the tetragonal and orthorhombic phases. However a deviation related to a pretransitional effect occurs in the range 150 – 220 K. The results are compared to those obtained for other physical properties.