Department of Metallurgical & Materials Engineering

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Tensile samples of a modified 9Cr-1Mo steel after tempering at 823 and 983 K subsequent to a normalizing treatment at 1373 K (designated as 550T and 710T, respectively) were deformed in tension at temperatures in the range of 77 to 373 K both by constant initial strain-rate tests at ε + ̇ = 1.1 · 10-4/s and strain-rate change tests between 1.1 · 10-4 and 1.1 · 10-3/s at εtot = 0.05, 0.07, 0.1, 0.12, and 0.14. The 710T specimens showed an increase in ductility and a marginal increase in strength with decreasing test temperature, whereas the 550T specimen exhibited an increased flow stress with very low ductility. The thermal component of flow stress (τ*), the activation volume (υ*) and activation energy (ΔH) were strongly affected by the microstructure. A decrease in τ* observed in the 710T specimens, compared with that of pure iron, at temperatures below 200 K can be attributed to solid solution softening by the dissolved nitrogen atoms. Further, the variations in υ* and ΔH with stress and temperature supports the importance of the Peierls mechanism for plastic deformation, and the movement of dislocations by double kink formation was found to be strongly affected by free nitrogen atoms.

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Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques

Deformation behavior of a modified 9Cr-1Mo steel between 77 and 373 K

Samples of Zr-2.5 wt.% Nb alloy, obtained from pressure tube, were annealed in the temperature range 700-900 °C up to 100 h to investigate the grain growth behaviour. The grain size increases with annealing time (t) and temperature (T) according to the relationship d ∝ t0.21 exp((-387 kJ/mol)/RT). Room temperature hardness increases with the increasing annealing temperature and grain size, which is ascribed to the transformation of β-phase into martensitic α′ and the substructure developed during quenching. The grain sizes, produced by further cold working and annealing for short time (&lt; 1 h), and the corresponding hardness values, however, support the Hall-Petch type relationship in the limited cases.

Effect of cold working and annealing on grain size and hardness of a Zr-2.5Nb alloy

Tensile specimens of Type 316L stainless steel, containing boron and having grain sizes in the range 3.1 to 103.8 μm, were deformed at 24, 400 and 700°C at a constant strain rate of 1 × 10-4 s-1. The flow stress (σ) varies with strain (ε) and grain size (d) according to the Hall-Petch (H-P) type relationship σ(ε) = a εa + b εmd-0.5 with the H-P parameters (σ0 and k) interrelated through the constants a, n and b, m. The presence of boron affects the flow stress and changes the H-P parameters, depending on test temperature. The dislocation density increases with temperature due to dynamic strain ageing, which leads to some anomalous effects of grain size at 700°C. © 1997 Acta Metallurgica Inc.

Acta Materialia

Hall-Petch relationship and substructural evolution in boron containing type 316L stainless steel

Superplastic behaviour and concurrent microstructural evolution in microduplex alloys are reviewed to bring out the following less common trends reported in the literature. An anomaly exists about the shape of the log (stress)-log (strain rate) curves and the parameters of the constitutive relationship. Also, some microduplex alloys exhibit two values of activation energy for superplastic deformation with a lower value at T ≲ 0.8Tm. The ratio of the two activation energies is approximately equal to 0.55. The co-existing two phases and the α/α, α/β and β/β boundaries contribute differently to deformation and cavitation, but grain coarsening occurs in a concerted manner.

Materials Science Forum

Towards an integration of flow behaviour and microstructural evolution during superplastic deformation of microduplex alloys

Zircaloy, known to exhibit superplastic deformation in the temperature range of 800-850 °C, has been used for the forming of hemispherical domes. Conventional tensile test data was seen to satisfactorily predict the operating parameters in this alloy; these closely matched with that obtained by a simple bi-axial forming test. The cold work increases the formability and avoided apex cracking.

Superplastic forming of zircaloy-2 pressure tube material

Tensile testing of a modified 9Cr-1Mo steel in two microstructural conditions (710T -normalized at 1100°C, tempered at 710°C, and 550T - normalized at 1100°C tempered at 550°C) in the temperature range 25-450°C, under strain rates of 2.3 × 10-5-2.3 × 10-3 s-1, exhibited serrated flow curves, with serrations appearing almost at the onset of deformation and disappearing before ultimate strengths were attained. The serrated flow curves (characteristics of dynamic strain ageing) were accompanied by increased ultimate strengths, loss of ductility and negative strain-rate sensitivity, relative to the ambient temperature properties. However, the increase in ultimate strength and the reduction in ductility were much larger for 710T specimens, as compared to 550T ones. In the dynamic strain ageing regime, the work-hardenability of 710T specimens increased rapidly while that of 550T specimens remained practically unaffected. Based on the microstructural consideration and the observed activation energy of 45 kJ mol-1, it is proposed that serrations are initiated by a nitrogen atmosphere formation on the waiting dislocations by a pipe diffusion mechanism, and they disappear by diffusion to the precipitate sinks during deformation. Because the fine alloy carbide precipitates in 550T specimens are more effective sinks than those of 710T ones, they can cause much faster depletion of the atmosphere, resulting in a much smaller effect of dynamic strain ageing on the tensile properties of 550T specimens.

Journal of Materials Science

Effect of dynamic strain ageing on the tensile properties of a modified 9Cr-1Mo steel

Zirconium-2.5wt%Nb alloy, used as a pressure tube material of Pressurized Heavy Water Reactors, possess fine microstructure of elongated α-Zr grains with a grain-boundary network of β-Zr and a strong basal pole texture along the transverse (circumferrential) direction of the tube. Transversely oriented flat tensile specimens, machined from a commercial pressure tube were tested at 650-800°C by both monotonic and strain-rate cycling (2.68 × 10-5 - 5.3 × 10-3 s-1) tests in order to study the elevated temperature deformation of this material. It was observed that the alloy exhibited superplasticity in this temperature range, showing high ductility (elongation upto 900 pet.) and strain-rate sensitivity (m∼0.3-0.85). Microstructural studies of the deformed structure indicated the transformation of the starting α-β lamellar structure into a fine equiaxed grain-structure and the measured activation energies were found to be 123 kJ/mole at the higher end and 211 kJ/mole at the lower end of the test temperatures. Based on these results it appears that the superplastic behavior is controlled by grain-boundary sliding phenomenon accompanied by niobium diffusion along α-β boundaries at the lower end and lattice diffusion of β-zirconium at the higher end of the temperature range used in this investigation.

A study of superplasticity in a Zr-2.5wt%Nb pressure tube alloy

Scripta Materialia

Importance of the phase-proportion on superplastic ductility of the eutectic alloys

Al-3.37 wt pet Li alloy was deformed by differential strain rate and constant initial strain rate test techniques to investigate deformation and failure behavior over the strain rate range of 10-5 to 10-2 s-1 and the temperature range of 22 °C to 580 °C. Flow stress first increases then decreases with an increase in test temperature, whereas ductility shows a sigmoidal relationship with the test temperature. The maximum ductility of about 80 pet is obtained at intermediate strain rate and 550 °C. Failure is noted to occur by cavity interlinkage and crack formation. Strain rate sensitivity (m) and activation energy (Q) for deformation are determined to be 0.04 to 0.13 and 96.2 to 157.4 kJ/mol, respectively. Toward lower test temperatures, both the m and Q are found to have lower values. Deformation at high temperature is suggested to be controlled by dislocation climb. However, under non-steady-state conditions due to cavitation, m and Q both vary with strain.