Experimental and numerical assessment of fracture toughness of dual-phase austempered ductile iron

The effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices.     

Segregation and Near-Surface Diffusion for Undoped and Cr-Doped CoO

Work function measurements were used to monitor the re-equilibration kinetics for the system CoO-Cr₂O₃ involving undoped CoO, Cr-doped CoO, CoCr₂O₄ spinel phase, and undoped Cr₂O₃. The activation energy of chemical diffusion for the near-surface layer was determined for these materials. Combined work function, X-ray, and ESCA studies indicate that Cr segregates in CoO-Cr_zO₃ solid solutions, which leads to the formation of a new "surface phase" of spinel-like structure.     

OPTIMIZATION OF CORNSTARCH/XANTHAN GUM CONTENT FOR THICKENING OF COCOA SYRUPS

ABSTRACT

Blended cornstarch–xanthan gum systems were optimized for thickening of cocoa syrups. For this purpose the sensory (whole and partial), textural (force of penetration, adhesiveness and stringiness) and rheological properties (flow curves as well as viscosity/time and viscosity/temperature relationships) of cocoa syrups were studied. Flow curves were described by rheological model of Casson, which accounted syrups under study for non‐Newtonian, pseudoplastic and thixotropic fluids. Also models of Weltman and Arrhenius were applied for comparison of obtained data. The areas of thixotropy hysteresis loops were also calculated. Obtained data (sensory, textural and rheological) were used as a base for mathematical calculations. The results were presented in the tables to enable their easier understanding. As a result, optimization of thickeners was achieved – the best ranges of cornstarch (0.45–0.49%) as well as xanthan gum (0.15–0.17%) concentrations in cocoa syrups were selected.

PRACTICAL APPLICATION

The result of this study could be used as a guide for thickening of cocoa syrups by the application of cornstarch–xanthan gum combination, chemically nonmodified thickeners. Mathematical method tried in this study for optimization of thickeners addition could be useful for optimization of the other thickeners and additives employed for different sauces, syrups, dressings, ketchups, mayonnaises, etc.

     

Evaluation of water embrittlement on ‘dual phase’ ADI

Austempered ductile iron (ADI) suffers an embrittlement phenomenon when loaded in contact with water and other liquids. This phenomenon causes noticeable drops in elongation, ultimate tensile strength and fracture toughness of ADI of different strength grades. This paper studies the susceptibility to embrittlement of a new kind of austempered ductile iron called dual phase ADI (DPADI), which shows a matrix composed by the typical ausferrite present in ADI mixed with allotriomorphic ferrite. The new material is obtained by means of specific heat treatments that involve a partial austenitising stage. The susceptibility of DPADI to this type of embrittlement was evaluated by carrying out tensile tests in dry and wet conditions. Fracture surfaces were observed using scanning electron microscopy. The results showed a gradual decrease of the degree of embrittlement caused by contact with water as the ferrite content in the matrix increases.