This review surveys the present knowledge of the mechanisms of decomposition processes in Al-Zn-Mg alloys. The various experimental observations connected with the formation and properties of the different second phase particles are described and critically analysed. The effect of different heat treatments, compositions, impurities, etc on the different stages of the decomposition process is discussed in detail.

Decomposition processes in Al-Zn-Mg alloys

Microstructure formed by diffusional or martensitic transformation from fine-grained austenite of which grain size is smaller than 5 μm was studied. Grain refinement of austenite was established through two kinds of reversion processes; (1) cyclic transformation between martensite and austenite and (2) reverse transformation from tempered and cold-rolled lath martensite (or pearlite). In the process of (1), the fine austenite structures whose grain sizes of 5-10 μm are obtained. Refinement of austenite grain size results in the increase of hardness. In the process of (2), austenite grain size can be refined down to about 2 μm in low-carbon Mn steels by microalloying through pinning of austenite grain growth by alloy carbides. The ferrite grain size after continuous cooling transformation becomes finer as austenite grain size is refined. However, the grain size ratio of austenite and ferrite, d α /d γ , increases by refining austenite grain size. For the austenite of grain size smaller than 5 μm, the ferrite grain size becomes coarser than that of austenite for slow cooling. A similar trend in the change of ferrite grain size by refinement of austenite was recognized for isothermal pearlite transformation in eutectoid alloys. Thus, it is suggested that extensive accelerated cooling is important to obtain fine-grained ferrite by diffusional transformations from the fine-grained austenite. Packet and block sizes of lath martensite in low carbon steels are also refined by decreasing the austenite grain size. Several packets and blocks are formed even from the austenite matrix of 2 μm in grain size.

Phase Transformation from Fine-grained Austenite

This paper aims to investigate the corrosion behaviour of manganese phosphate coatings on AISI 4140 after various heat treatments. Prior to coating, quenching and tempering treatments were applied to the substrates prepared for this process. The structural and mechanical properties of nonheat-treated and heat-treated substrates were determined using hardness test, optical microscope, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The changes of hardness, microstructure and phase density of heat-treated substrates before the coating were also observed. Manganese phosphate coatings were applied to heat-treated substrates in a coating bath at a temperature of 90 °C, and electrochemical polarisation measurements of the phosphate-coated substrates were carried out. It was observed that the corrosion potential of manganese-phosphate-coated substrates, which followed the quenched treatments, were higher than that of the tempered substrates. The highest corrosion potential was obtained for the coatings on water-quenched (WQ) substrates. It was found that the heat treatment applied to the substrates yield an important improvement in the corrosion resistance of the coatings on substrates.

The corrosion behaviour of manganese phosphate coatings applied to AISI 4140 steel subjected to different heat treatments

The strengthening of AISI 4140 steel by cyclic heat treatment has been investigated. The applied cyclic heat treatment was consisted of both, temperature cycling with diffusional transformation and double quenching with diffusionless transformation. Both, temperature cycling and second quenching increase yield strength and Charpy-V toughness as measures of steel strength and toughness. The increase of yield strength and Charpy-V notch toughness are directly in relation of structure fineness. Much better mechanical properties are obtained by the appropriate cyclic heat treatment than by the quenching and tempering of steel AISI 4140.

A.K. Mallik

Papers

Grain refinement of steel by cyclic rapid heating

Effects of preprecipitation on the precipitation behavior of Al-Zn-Mg alloys