Materials Science and Technology

The effect of high-temperature deformation on the kinetics of precipitation for several commercial aluminium alloys has been investigated. It has been found that mobile dislocations introduced during high-temperature deformation are efficient sinks for vacancies and retard the ageing process in spite of the additional vacancies produced through deformation. The plentiful supply of dislocations accelerates the loss of coherency of intermediate precipitates. Cold deformation after the quench, but before ageing, refines the precipitate size, which retards loss of coherency and results in improved creep strength.

A range of plain carbon, carbon and manganese, and low-alloy cast steels have been tested in fatigue in bending with notches of radii from 0.125 to 25.4 mm. The number of cycles to produce a detectable fatigue crack correlated well with the range of stress-intensity factor divided by the square root of the notch-root radius, providing an adequate engineering method for design. Fatigue-crack propagation rates were also measured. Fatigue-crack propagation is anomalously high for fatigue cracks of 1.0 mm or less emanating from notches. This is thought to arise because of rapid fatigue propagation through the region of the Neuber particle.

Previous work has reported on initiation and propagation of fatigue cracks from machined notches in cast steels. That work is extended here to notches of 12.5 and 25.4 mm radii with sand-cast surfaces. The comparison of initiation lives of machined and cast notches allows calculation of the effective notch radius arising from the cast surfaces. This effective radius is, in all cases, significantly less than the macroscopic notch radius and is a practical and important factor in the design of steel castings facing fatigue service conditions.

Internal friction of high-purity zirconium-oxygen alloys has been measured with an inverted torsion pendulum apparatus having a very low background damping. A damping peak originating from low-strength anelastic relaxation has been detected at 410°C: 0.44 Hz. Increasing the oxygen content has produced a significant increase in the relaxation strength and a consequent increase in the magnitude of the damping peak. A mechanism involving stress-induced diffusion of oxygen atoms in the basal plane of the hcp lattice has been proposed as the only relaxation process capable of producing the observed damping behaviour. The diffusion coefficient based on the proposed diffusion mechanism and derived from the experimental data is D = 1.5 × 10⁻⁴ exp (−48 OOO/RT) m²s⁻¹.

Transmission electron microscopy has been used to determine the T-T-T curve for precipitation of Cr-rich intergranular carbides during direct ageing in one batch of low-C, high- Ti/(C + N)-ratio Alloy 800 annealed at 1050°C. T-T-S data were also determined using test solutions which conformed to ASTM A 262-70 (B) and (E) for the 120 h ferric sulphate-sulphuric acid and 24 h copper-copper sulphate-sulphuric acid tests, respectively, with samples annealed at 1050 and 950°C and directly aged. The present work demonstrates that the alloy can show sensitization to inter granular corrosion after appropriate heat-treatment. A suppressed sensitization response was observed in the alloy after annealing at 950°C with respect to material annealed at 1050°C. Metallography showed that this suppressed sensitization response corresponded with (i) an increased spatial density of TiC precipitates, and (ii) a decrease in grain size. This correspondence is readily explained in terms of the Cr-depleted zone theory of intergranular corrosion coupled with a self-healing model for the alloy. Transmission electron microscopy revealed the presence of M₂₃C₆ intergranular precipitates throughout the sensitization/self-healing régime.

To gain a better understanding of the mechanisms operating during the first stage of fatigue-crack propagation in aluminium alloys, large slip-band facets of an age-hardened Al-Zn-Mg alloy, produced by fatiguing notched single-crystal specimens, were studied in detail using optical and electron fractographic methods. The features observed on the fracture surfaces were found to be a function of (i) crack propagation velocity, (ii) orientation of the crystallographic deformation modes with reference to macroscopic crack-propagation direction, and (iii) moisture level in the test environment. The observations have led to the following conclusions: (1) crack propagation in crystallographically favoured slip directions is energetically favoured; (2) nucleation and propagation of secondary cracks ahead of the main crack front plays an important role during slipband cracking, propagation of secondary cracks occurring parallel to the slip band preferentially in crystallographically favoured slip directions; (3) moisture in the test environment introduces a cleavage component into the fracture process and suppresses secondary cracking.