Bimodal (mixed coarse and fine) grain structures, which have been observed in some Nb-containing thermomechanically-controlled rolled steel plates, adversely affect their mechanical properties by causing scatter in cleavage fracture stress values. It is known that bimodal grain structures can develop during reheating prior to rolling; however, no quantitative predictions of the level of bimodality or the critical reheat temperatures for formation have been reported. In this article, three high-strength low-alloy (HSLA) steel slabs with varying microalloying additions (Ti, Nb, and V) have been characterized in the as-continuously cast and reheated (to various temperatures in the range 1050°C to 1225°C) conditions to determine the link between their grain size distribution (and any bimodality observed) and the microalloy precipitate type, size, and distribution. The as-cast slabs showed inhomogeneous microalloying precipitate distributions with the separation between precipitate-rich and precipitate-poor regions being consistent with interdendritic segregation and hence, the secondary dendrite arm spacing (SDAS). The susceptibility of the slabs to the formation of bimodality, based on the steel chemical compositions and critical reheat temperature ranges has been identified, both experimentally and theoretically using ThermoCalc (Thermo-Calc Software, Stockholm, Sweden) modeling of precipitate stability in the solute-rich and the solute-depleted regions formed during casting.

AA5182 (Al-4.5 wt% Mg) can become susceptible to intergranular corrosion (IGC) with time at moderately elevated service temperatures owing to precipitation of Mg-rich β-phase at grain boundaries, which can lead to stress corrosion cracking (SCC). The IGC and SCC susceptibility of AA5182 was found to depend strongly on sensitisation heat treatments. AFM and TEM studies demonstrated that the degree of precipitation and thus susceptibility to attack for a boundary can be related to its crystallographic misorientation. Low angle boundaries (<20°) are most resistant to attack as they do not show β-phase precipitation. However, higher angle boundaries show highly variable precipitation and corrosion susceptibility: critical factors are the grain boundary plane and precipitate/matrix crystallographic relationship.

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a b s t r a c tAluminium alloy 2024 (AA2024) is extensively used as a structural material in the aircraft industry because of its good combination of strength and fatigue resistance. However, complex shaped components, particularly those made from sheet, are extremely difficult to form by traditional cold forming due to its low ductility at room temperature. A possible solution of this problem is to form sheet workpieces at elevated temperature. The aim of the work described in this paper is to determine the relationship between formability and temperature for AA2024 by conducting a series of tensile tests at elevated temperatures ranging from 350 to 493 • C. Ductility of AA2024 was found to increase gradually with increasing temperature up to 450 • C, followed by a sharp decrease with further increase in temperature. So-called cup tests confirmed that the formability of AA2024 is very high at a temperature of about 450 • C. Fracture surfaces and longitudinal sections of formed samples were examined by scanning electron microscope. It was found that fracture occurred in three different modes depending upon the temperature, and the sharp decrease in ductility when the temperature exceeds 450 • C was caused by softening of grain boundaries by solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles.