E.M. Gutman

Mechanochemistry of Solid Surfaces

Dependence of Thermodynamics and Kinetics of Surface Chemical Reactions on Stressed State of Solids Mechanochemical Activity of Solids having Structural Defects Relationships between Local and Integral Mechanochemical Activity of Surfaces Kinetics of Mechanochemical Effect on Solids having Dislocations Chemomechanical Effect and Conjugate Mechanochemical Phenomena Some Applications of Mechanochemical Phenomena.

An investigation on the microstructure of an AM50 magnesium alloy

Abstract The microstructure and the dislocation arrangement in the die cast AM50 magnesium alloy as well as in the stressed states have been investigated using conventional transmission electron microscopy (TEM), high-resolution TEM and energy dispersive X-ray analysis. The microstructure of the die cast AM50 alloy is found to mainly consist of α-Mg, β-Mg17Al12 and Al8Mn5 phases. Two kinds of β-Mg17Al12 particles with different sizes have been found in the stressed AM50 magnesium alloy. Besides the normal β-Mg17Al12 particles with size around several micrometers as in the die-cast AM50 magnesium alloy, some finer β-Mg17Al12 particles in the stressed states of the AM50 alloy have also been found. The finer β-Mg17Al12 particles are found to be only about tens of nanometers with oval or rod-like morphology, which may be formed during deformation. Dislocation pile-ups have been found in the stressed AM50 alloy for the first time. The spacing between each parallel dislocation in the pile-ups is only several nanometers. The dislocations are confined in the slip planes and piled up against grain boundaries. Dislocations in the networks are found to increase with deformation of the alloy. Also, dislocation networks have been found in the β-Mg17Al12 and Al8Mn5 phases as well as in the matrix in the deformed AM50 magnesium alloy. However, the dislocation pile-ups are found to be almost identical from 1.3% deformation to rupture, which explains the stable tensile yield strength of the AM50 magnesium alloy during the deformation.

Corrosion behaviour of stressed magnesium alloys

Abstract Potentiodynamic polarisation and impedance measurements are used to examine the corrosion aspects of some Mg-based alloys, which were previously stressed in order to established the effect of mechanical deformation on surface electrochemical reactions. A first approach was made for the unstressed alloys. The electrochemical tests were carried out in a sodium borate buffer solution.

Microstructures and dislocations in the stressed AZ91D magnesium alloys

Abstract The microstructures and the dislocation arrangements in the die cast AZ91D magnesium alloy as well as the stressed alloys have been investigated using transmission electron microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray (EDX) analysis. Besides the dominant α-Mg and β-Mg17Al12 phases, Al8Mn5 and Mg5Al phases have also been found and studied in the alloy. Dislocation pile-ups have been found in the stressed AZ91D alloys for the first time. They are confined in the slip planes and piled up against the grain boundaries. The dislocation pile-ups increase with deformation till about 2.1% and then remain almost identical even deformed to higher degree. The dislocations pile-ups in the AZ91D magnesium alloy are found to be beneficial to the resistance to stress corrosion cracking of the alloy, and thereby beneficial to the mechanical properties of the alloy. Dislocation networks are found to increase with deformation in all cases. The dislocation networks have also been found in the β-Mg17Al12 phase as well as in the matrix in the deformed AZ91D magnesium alloys.

Corrosion fatigue of die-cast and extruded magnesium alloys

Abstract To study the fatigue life of die-cast and extruded AZ91D, AM50 and AZ31 magnesium alloys, corrosion fatigue tests were carried out using a rotating beam type fatigue machine. Corrosive environment (3.5% NaCl) significantly decreases fatigue life of alloys, especially for extruded alloys. The fatigue data at high stresses were analyzed using fitting equations. Extruded alloys show a higher sensitivity to the action of NaCl solution in comparison with die-cast alloys; however, their corrosion fatigue life is longer than that of die-cast alloys. Corrosion fatigue behavior of Mg alloys correlates with their mechanochemical behavior. The latter was studied by the polarization measurements of the dissolution rate of strained alloys. TEM observations confirmed that the maximums on the curves of Mg dissolution rate dependence on the strain in NaCl solution appeared at the work-hardening stages due to the creation and destruction of dislocation pile-ups.

Corrosion fatigue of extruded magnesium alloys

Abstract Corrosion fatigue tests were carried out on extruded AZ31 (3% Al, 1% Zn, 0.3% Mn, Mg—the rest), AM50 (5% Al, 0.4% Mn, Mg—the rest) and ZK60 (5% Zn, 0.5% Zr, Mg—the rest) Mg alloys in air, NaCl-based and borate solutions. N sol / N air ratios (the relative fatigue life) were used for the analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N sol and N air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life ( N sol / N air ∼10 −3 –10 −2 ) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys ( N sol / N air ∼10 −2 –10 −1 ). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys ( N sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50).

Development of semisolid casting for AZ91 and AM50 magnesium alloys

The feasibility of semisolid casting process with AZ91 and AM50 magnesium alloys, which are usually used for hot- and coldchamber die-casting, was investigated in this work. Two casting methods were examined: direct casting from the melting–stirring unit and billets casting. The billets had been precast with thixotropic properties, and were heated again by an induction furnace before the casting. The apparent viscosity of the semisolid magnesium slurry during continuous cooling was found as a function of the casting temperature. Viscosity tests show that the appropriate casting temperatures in the two-phase region were in the range of 595–575 C for AZ91 alloy and 620–614 C for AM50 alloy. At casting temperature of 585 C, AZ91 exhibited maximum density and the highest standard mechanical properties. AM50 alloy demonstrated the relative poor mechanical properties in the two-phase region. The toleration of heat treatment of semisolid AZ91 alloy with a 1.8-fold increase in the elongation-to-fracture was observed. No significant differences of the densities and the standard mechanical properties of the alloys between direct or billet castings were demonstrated. 2002 Elsevier Science Ltd. All rights reserved.

On the thermodynamic definition of surface stress

Surface energy and surface stress have been thoroughly discussed in the literature. However, the widely accepted definition of surface stress as the strain derivative of the total free surface energy divided by the surface area (Shuttleworth, Hening, etc.) is inconsistent with the correct definition of a stress tensor (given in continuum elasticity theory) as the strain derivative of the specific free energy. We analyse this anomaly in detail in order to prevent the erroneous use of the surface stress definition.

Influence of technological parameters of permanent mold casting and die casting on creep and strength of Mg alloy AZ91D

To increase the strength of cast magnesium alloys operating at elevated temperatures, it is most important to increase their creep resistance. However, the influence of Mg-alloys die casting and permanent mold casting parameters on creep resistance has not been practically studied in the past. It is shown in this work that such casting parameters as metal injection rate into the die, die (mold) temperature and liquid metal temperature exert essential influence on creep resistance and other mechanical properties of the magnesium alloy AZ91D. It is established that in the case of permanent mold cast alloy, creep magnitude is defined by intermetallic β-phase inclusions, whereas in the case of die casting it is defined by the extent of micro- and macro-porosity development in the alloy.

Mechanoelectrochemical behavior and creep corrosion of magnesium alloys

Abstract Both creep and corrosion resistances are significant problems in the application of magnesium alloys. The synergistic effect of corrosion and stress on the viscoelasticity of magnesium alloys named corrosion creep has been studied in die-cast AZ91D (Mg–9%Al–1%Zn) and AM50 (Mg–5% Al–0.4% Mn) alloys in air and in the borate buffer solution. The highest sensitivity to creep in the corrosive environment is observed in the alloy with the highest Al content. An electrochemical study under a tensile strain demonstrates a good correlation with corrosion creep tests.