Serrated yielding in Cu-1 wt-%Cd alloy

Abstract

The influence of grain size and temperature on the serration patterns of the Portevin-Le Chatelier (PLC) effect and on the yield and flow stresses in a Cu-1 wt-%Cd alloy was investigated in the temperature range 150 to 360 ° C. Two kinds of serration patterns were observed in this alloy. Type I occurred at lower temperatures and its yield points are moderately spaced. Type II consists of regular jerky flow observed athigher temperatures. The Hall-Petch equation is obeyed over the temperature range in which jerky flow occurs. The Hall-Petch parameter k (?) is observed to show a local maximum in the temperature range where serrated flow is first observed. The PLC effect is associated with the solute- dislocation interactions, implying that k (?) contains a component associated with grain size dependent dislocation storage.     

Transport of large nuclear power plant components: experiences in mechanical design assessment

Abstract

In the course of decommissioning of power plants in Germany large nuclear components (steam generator, reactor pressure vessel) must be transported over public traffic routes to interim storage facilities, where they are dismantled or stored temporarily. Since it concerns surface contaminated objects or low specific activity materials, a safety evaluation considering the IAEA transport regulations mainly for industrial packages (type IP-2) is necessary. For these types of industrial packages the requirements from normal transport conditions are to be covered for the mechanical proof. For example, a free drop of the package from a defined height, in dependence of its mass, onto an unyielding target, and a stacking test are required. Since physical drop tests are impossible generally due to the singularity of such 'packages', a calculation has to be performed, preferably by a complex numerical analysis. The assessment of the loads takes place on the basis of local stress distributions, also with consideration of radiation induced brittleness of the material and with consideration of recent scientific investigation results. Large nuclear components have typically been transported in an unpackaged manner, so that the external shell of the component provides the packaging wall. The investigation must consider the entire component including all penetration areas such as manholes or nozzles. According to the present IAEA regulations the drop position is to be examined, which causes the maximum damage to the package. In the case of a transport under special arrangement a drop only in an attitude representing the usual handling position (administratively controlled) is necessary. If dose rate values of the package are higher than maximum allowable values for a public transport, then it is necessary that additional shielding construction units are attached to the large component.     

Tensile creep behaviour of NiAl-Cr(Zr) multiphase intermetallic alloy

Abstract

The microstructure of a multiphase NiAl-33.5Cr-0.5Zr intermetallic alloy was examined by SEM with energy dispersive spectroscopy and TEM. The tensile creep behaviour of the hot isostatically pressed NiAl-33.5Cr-0.5Zr alloy was studied. The results of the creep test indicated that all of the creep curves under the present test have similar characteristics: a short primary creep stage, a dominant tertiary creep stage, and nearly identical creep strains (~45%). The apparent stress exponent and the apparent activation energy were analysed and discussed. The mechanism of the creep deformation was also analysed by the observation of TEM.     

Effects of heat treatment processes on microstructure and properties of 2·25Cr–1Mo–0·25V HSLA steels

Abstract

In the present paper, the effects of the heat treatment processes with two conditioning treatments and four quenching–tempering processes on the mechanical properties of 2·25Cr–1Mo–0·25V high strength low alloyed (HSLA) steel are investigated. The results show that the conditioning treatments have obvious effects on the low temperature impact energy but little effect on the tensile strength. The elevation of the final austenitising temperature increases the strength, whereas it results in the decrease in the low temperature impact energy due to the coarse microstructure. The results of the fracture surfaces analysis further make sure that the fracture surfaces of tensile specimens all exhibit ductile characters with a lot of dimples. However, the fracture surfaces of impact specimens exhibit two typical fracture characters, i.e. the ductile and brittle fracture surface corresponding to the fine and coarse microstructures respectively. In addition, the elongation and reduction in area seem to be insensitive to the heat treatments. Meanwhile, the impact fracture mode is more sensitive to the grain size and not to the low temperature impact energy.     

New viscoplastic model to simulate hot isostatic pressing

Abstract

Most of the numerical simulation software on the hot isostatic pressing (hipping) process is based on viscoplastic modelling of densification, such as the well known Abouaf's model. These constitutive equations are generally obtained from a viscoplastic potential depending on Green's equivalent stress. An implicit coupling between isotropic and deviator parts is therefore only defined by an equivalent stress. A new viscoplastic formulation proposed by Stutz introduces an explicit coupling between isotropic and deviator parts of the stress state, allowing then more flexibility to take into account the experimental results achieved from isotropic, die compaction, and creep tests. This paper deals with the presentation of this new formulation which has been implemented in the finite elements software PreCAD, and the subsequent changes observed in numerical simulations. These simulations achieved with PreCAD software, are compared with an experiment on a complex part manufactured by CEA Grenoble.     

Design and Realization of a Non-Circular Cable Spool to Synthesize a Nonlinear Rotational Spring

In this paper we present a cable mechanism that realizes a nonlinear rotational spring from a linear translational spring. The spring is pulled by a cable wound around a non-circular spool, which is rigidly attached to the joint. The non-circular shape of the spool induces a nonlinear relationship between its angular position and the torque created by the tension of the cable. Depending on the shape of the spool, various torque–angle relationships can be realized. We show that for a given nonlinear torque–angle relationship, there is an explicit expression (closed-form solution) of the shape of the spool that synthesizes this function. First, we present the geometry of the problem. Then, we derive the methodology to calculate the shape of the spool to synthesize a prescribed torque–angle relationship. Finally, we verify the design methodology by experiments with three different spools realizing a constant force spring, an exponential softening spring and a cubic polynomial spring. We discuss the possible sources of errors between the theoretical and experimental results.     

Thermomechanical response of 50.7 at.-%Ni-Ti alloy in the pseudoelastic regime

Abstract

The thermomechanical response of 50.7 at.-%Ni-Ti superelastic alloy has been investigated for a range of applied strains and strain rates. Parameters of interest include the critical stresses for nucleation and completion (σ _ms and σ _mf respectively) of the stress induced martensite (SIM) transformation, as well as the magnitude of the endothermal and exothermal reactions as a function of the applied strain and rate of deformation. Novel techniques including infrared thermometry and laser extensometry have been utilised, and are found to be particularly suited to experimentation with nitinol. The transformation temperatures and values of entropy and enthalpy of formation have been determined using differential scanning calorimetry.     

Effect of local volume fraction of microporosity on tensile properties in Al–Si–Mg cast alloy

Abstract

Porosity in Al–Si–Mg cast alloys utilised in automotive parts affects directly products quality, i.e. mechanical properties. In this study, the effect of micropores on mechanical properties has been investigated by X-ray tomography from the viewpoint of clustering micropores. The local volume fraction (LVF) of porosity was introduced to analyse the effect of clustering micropores. The statistical Weibull method was also used in order to explain strength of the alloy tested. The fracture strain decreased drastically from 17 to 3% on an inverse parabolic relationship with increasing porosity. In the case of the specimens that contain the largest pore higher than 100 m m, the ultimate tensile strength decreases monotonically. It is found that the fracture surface passes through high LVF regions. The fracture strain obviously depends on the ratio of LVF higher than 10%. It is confirmed that the LVF, which represents unevenly distribution of micropores cluster, is one of important dominant factor for managing the mechanical properties in the Al–Si–Mg cast alloy.     

Structure evolution in annealed and hot deformed Al–V2 O5 composite

Abstract

Structural observations and hot deformation tests were carried out on mechanically alloyed Al-10 wt-%V₂ O₅ composite. Initial annealing experiments revealed a hardening of the material during the first stage of annealing. The material hardness increased from 114 HB for as extruded material to 167 HB after annealing at 873 K for 6 h. Differential scanning calorimetry tests conducted on as extruded material confirmed the development of an exothermic reaction during heating of the material within the temperature range 650–870 K. The amount of heat released was reduced with increasing annealing time at 873 K. Transmission electron microscopy (TEM) and X-ray analysis of annealed material revealed new intermetallic grains and very fine aluminium oxide particles, which resulted from the chemical reaction between the aluminium matrix and vanadium oxides. The development of voids in long aged specimens was found to be an undesirable effect of local specific volume reduction during the course of the chemical reaction that was not fully compensated by the local volume increase due to the growth of intermetallic particles. As a result, the material hardness was reduced in long time annealed specimens. The mechanical properties of as extruded and annealed specimens were investigated by means of hot compression testing within the temperature range 623–903 K. These tests revealed that the flow stress of as extruded material was reduced from 180 to 22 MPa when tested at 623 and 903 K, respectively. Annealed specimens exhibited higher flow stresses of 195 and 32 MPa at the same temperatures. The results indicate that the strength of the material can be effectively increased owing to a change of material structure as a result of the chemical reaction taking place during high temperature annealing.     

Development of process for thermomechanical treatment of ultrahigh strength steel prepared by electroslag refining

Abstract

Thermomechanical treatment (TMT) is the simultaneous use of work hardening, and grain refinement along with solid solution and precipitation strengthening. In this investigation, four alloys, with a base composition of 0·28%C, 1·0%Mn, 4·2%Cr, 1·0%Mo, 0·34%V, were prepared by electroslag refining (ESR) and by addition of small amounts of Ti and Nb and by increasing Cr and V to 4·8 and 0·48% respectively. In two of the alloys a yield strength in excess of 1550 MPa was obtained in the as cast quenched and tempered condition. Attempts were made to further increase the yield strength by thermomechanical treatment. The process parameters for thermomechanical treatment were optimised by adopting procedures such as calculation of stability of precipitates, hot compression test, determination of cooling rates in different coolants, and modelling of TTT and CCT diagrams. The process involved prerolling of the ESR ingot to a bar at 1200°C, followed by hot rolling in two passes starting from 950°C and finishing at 850°C with equal deformation of 25% in each pass to convert the bar into plates. These were immediately cooled in one of the cooling media: air, polymer–water solution (1 : 1·5) and oil. Yield strength in excess of 1750 MPa was obtained in oil cooled specimens of the alloy with titanium addition and that where Cr and V were increased. The niobium added specimen gave strengths, similar to that obtained for the base alloy, in spite of the fact that the as cast alloy had shown very high strengths, presumably because of the high soaking temperatures and grain growth. Air cooling gave the lowest strengths and oil cooling the highest.