The Role of Light Metals in Nuclear Engineering

ALUMINIUM, magnesium and beryllium have been used extensively in research and power reactors chiefly because of their low neutron absorption properties. in this respect, beryllium is superior to magnesium and aluminium. While aluminium and magnesium are used as canning and structural materials, beryllium has been used mainly as reflector and moderator, although attempts are being made to employ it for canning also.The primary functions of fuel element cans in a reactorare : (a) to protect the fuel from corrosion and (h) to prevent the fission products from contaminating the coolant. Simult-aneously the cans should also act as extensible envelopes to accommodate dimensional changes occurring in the fuel due to irradiation and thermal fluctuations. In the case of finned fuel elements, the material should also possess sufficient creep resistance at operating temperatures to withstand the coolant pressure on the heat transfer fins. Other equally important requisites of the cans are good compatibility with fissile material and coolant media and adequate corrosion resistance

Deformation behaviour of pearlite in the copper-aluminium system

The room temperature mechanical properties of eutectoidal binary aluminium bronze (Cu + 11.8 wt % Al) have been studied as a function of interlamellar spacing in the pearlite. Specimens with different lamellar spacings were produced by isothermal transformations at various sub-critical temperatures. The proportional limit, 0.2% yield strength and fracture strength were found to be inversely related to the square root of the lamellar spacing. This is in accordance with the mechanism suggested by Ansell and Lenel for deformation of two-phase alloys, according to which the criterion for macroscopic yielding is the fracture of the non-deformable phase, caused by the stress-concentration due to pile-up of dislocations at the lamellar interfaces. Metallographic evidence supporting the suggested mechanism is also presented

Effect of thermal cycling on the mechanical properties of 350-grade maraging steel

The effects of retained austenite produced by thermal cycling on the mechanical properties of a precipitation-hardened 350-grade commercial maraging steel were examined. The presence of retained austenite caused decreases in the yield strength (YS) and ultimate tensile strength (UTS) and effected a significant increase in the tensile ductility. Increased impact toughness was also produced by this treatment. The mechanical stability of retained austenite was evaluated by tension and impact tests at subambient temperatures. A deformation-induced transformation of the austenite was manifested as load drops on the load-elongation plots at subzero temperatures. This transformation imparts excellent low-temperature ductility to the material. A wide range of strength, ductility, and toughness can be obtained by subjecting the steel to thermal cycling before the precipitation-hardening treatment

Precipitation hardening in 350 grade maraging steel

Evolution of microstructure in a 350 grade commercial maraging steel has been examined. In the earlier stages of aging, the strengthening phases are formed by the heterogeneous precipitation, and these phases have been identified as intermetallic compounds of the Ni3 (Ti, Mo) and Fe2Mo types. The kinetics of precipitation are studied in terms of the activation energy by carrying out isothermal hardness measurements of aged material. The mechanical properties in the peak-aged and overaged conditions were evaluated and the flow behavior examined. The overaging behavior of the steel has been studied and the formation of austenite of different morphologies identified. The crystallography of the austenite has been examined in detail. From the microstructural examination of peak-aged and deformed samples, it could be inferred that the dislocation-precipitate interaction is by precipitate shearing. Increased work hardening of the material in the overaged condition was suggestive of looping of precipitates by dislocations

Solid-state diffusion reaction and formation of intermetallic compounds in the nickel-zirconium system

Chemical diffusion studies in the nickel-zirconium system are investigated in the temperature range of 1046 to 1213 K employing diffusion couples of pure nickel and pure zirconium. Electron microprobe and X-ray diffraction studies have been employed to investigate the formation of different compounds and to study their layer growth kinetics in the diffusion zone. It is observed that growth of each phase is controlled by the process of volume diffusion as the layer growth obeys the parabolic law. The activation energies for interdiffusion in NiZr and NiZr2, which are the dominant phases in the diffusion zone, are 119.0 ±13.4 and 103.0 ±25.0 kJ/ mole, respectively. The formation and stability of compounds over the temperature range have been discussed on the basis of existing thermodynamic and kinetic data

Processing map for hot working of alpha-zirconium

The hot deformation characteristics of alpha-zirconium in the temperature range of 650°C to 850°C and in the strain-rate range of 10-3 to 102 s-1 are studied with the help of a power dissipation map developed on the basis of the Dynamic Materials Model. The processing map describes the variation of the efficiency of power dissipation (η=2m/m + 1) calculated on the basis of the strain-rate sensitivity parameter (m), which partitions power dissipation between thermal and microstructural means. The processing map reveals a domain of dynamic recrystallization in the range of 730°C to 850°C and 10-2 to 1-1 with its peak efficiency of 40 pct at 800°C and 0.1 s-1 which may be considered as optimum hot-working parameters. The characteristics of dynamic recrystallization are similar to those of static recrystallization regarding the sigmoidal variation of grain size (or hardness) with temperature, although the dynamic recrystallization temperature is much higher. When deformed at 650°C and 10-3 s-1 texture-induced dynamic recovery occurred, while at strain rates higher than 1 s-1, alpha-zirconium exhibits microstructural instabilities in the form of localized shear bands which are to be avoided in processing

Low temperature deformation behaviour of ASTM A-203D nuclear structural steel

The steel A-203D in martensitic, tempered martensitic and ferritic-pearlitic microstructural conditions, was deformed in tension at temperatures between 77 to 300 K and at a strain rate of 6.67×10-5 sec-1. The thermal component of the flow stress and the activation parameters were measured as a function of temperature. It was observed that the microstructure did not affect either the thermal part of the flow stress (σ*) or the activation parameters (V* and ΔH) and that its effect was felt only on the athermal component (σμ) of the flow stress. Further the relations of the activation parameters with stress, strain and temperature were found to be consistent with Dorn-Rajnak theory of Peierls mechanism of plastic deformation. In addition measurements of slow-bend and impact transition were also carried out for ferritic-pearlitic structure. Based on these observations, it is shown that the impact transition temperature of this structure can be emperically correlated with the thermal activation parameters

Zirconium alloys in nuclear technology

This paper describes the historical development of zirconium and its alloys as structural materials for nuclear reactors. The various problems encountered in the early stages of the development of zircaloys and their performance in reactors operating presently are described in detail. The development of Zr-2.5 % Nb alloys for pressure tube applications is discussed. The paper concludes with a detailed discussion on the development potential of zirconium alloys for high temperature applications and a brief account of the work carried out at Trombay in this field