Hot Tension Research Articles

A theoretical investigation of the effect of material properties and cavity architecture/shape on ductile failure during the hot tension test

The effect of material properties and cavity architecture, shape, and orientation on ductile failure behavior during hot tension testing was established using a numerical analysis of the deformation of a representative “microspecimen.” The microspecimen consisted of two regions, or slices, one containing the cavities and the other comprising a uniform, cavity-free area. The cavities were assumed to be spherical or cylindrical and to form a simple cubic (sc), body-centered cubic (bcc), or face-centered cubic (fcc) network; tensile loading was taken to be parallel to either the cube edge, face diagonal, or body diagonal. By invoking load equilibrium, expressions describing the relation between the deformations in the uniform and cavity-containing regions were derived. The principal material-related coefficients in these equations were a geometry factor G, whose value depended on the specific cavity architecture and tensile loading direction, the strain-hardening and strain-rate sensitivity exponents n and m, and the parameter η, used to describe the (volumetric) cavity growth kinetics. For cyclindrical cavities, the pertinent void-growth parameter was deduced to be the area cavity growth rate ηA. Failure was predicted to be either retarded or accelerated when ηA is less than or greater than 2η/3, respectively. The simulations were used to quantify the microscopic strain localization kinetics and, thus, to identify those deformation regimes in which void growth vs void coalescence (i.e., “internal necking”) predominates during the ductile failure process. Model predictions of tensile elongation were validated by comparison with experimental measurements for cavitating materials found in the literature.

Intergranular fracture of gamma titanium aluminides under hot working conditions

A comparative study of the hot workability of a near gamma titanium aluminide alloy Ti-49.5Al-2.5Nb-1.1Mn in the cast and wrought conditions was performed. Tension tests conducted on coarse grain, cast material, and fine grain wrought material revealed a pronounced variation in both fracture/peak stress and ductility with temperature and strain rate. Brittle, intergranular fracture occurring at high strain rates was found to be controlled by wedge crack nucleation, whereas the ductile fracture observed at low strain rates was controlled by the growth of wedge cracks and cavities. Dynamic recrystallization was shown to be the main restorative mechanism to accommodate grain boundary sliding and thereby control the crack growth rates. The ductile-to-brittle (DB) transition was found to be determined by the critical values of a grain size-based stress intensity factor given by the product of the peak/fracture stress and the square root of grain size. A processing map for the near gamma titanium aluminides was constructed based on the comparative analysis of the hot tension and compression test results.

Influencia de los elementos residuales cobre, estaño, fósforo y arsénico en el agrietamiento de la superficie del acero inoxidable 18-8 durante la compresión a altas temperaturas

The effect of certain different concentrations of Cu, Sn, P and As on the surface cracking of 18-8 austenitic stainless steel hot compressed specimens has been studied, at 1,123 and 1,273 K, in an oxidizing atmosphere (air). A procedure for determining surface cracking has been established, and the cracking factor obtained in this way is correlated with the chemical composition of the materials at both temperatures. The cracking factors obtained at 1,273 K have been compared with the reduction of area drops obtained by hot tension tests at the same temperature.

Microstructure and modelling of the deformation behaviour of CoCr22Ni22W14 in hot-tensile- and creep tests

The present study describes the application of hot tension experiments in order to predict the creep and tensile behaviour of the highly loaded superalloy CoCr 22Ni 22W 14. The results of mechanical tests and transmission electron microscopy (TEM) investigations have been used as input data in a model which describes the high temperature plastic deformation. A common log ɛ - log σ plot of tensile and creep data shows a systematic shift of about 10% towards higher stresses in the case of tensile loading. In creep tests the dislocation densities reach their steady-state values at the points of minimal strain rates, whereas in tensile tests the dislocation densities increase at any given time until the tensile strength is reached at significantly higher strains. Under the condition of equal true stresses subgrain formation is more pronounced after tensile than after creep deformation. By including the experimentally determined dislocation densities, the constitutive model yields qualitatively correct predictions of the creep and tensile behaviour.

Deformation and Fracture Characteristics of a Gamma Titanium Alumtnide at High Temperatures

The hot workability of a γ titanium alumlnlde alloy was investigated using hot tension tests In the temperature range 900-1350°C and for deformation rates varying from 0.0001 to 5 s −1 . One series of tests was performed on cast + HIP'ped material and was designed to characterize fracture behavior during primary ingot break-down. Another series of tests was conducted on extruded and heat treated material In order to Investigate the fracture processes occurring during secondary processing. The deformation and fracture characteristics were analyzed to assess the influence of microstructural features as well as the relative volume fractions of the γ and α/α2 phases.

Restoration mechanism during hot tension of an Al-4. 5Cu-1. 5Mg alloy

Specimens of an Al-4. 5Cu-1. 5Mg alloy are deformed in hot tension at temperatures of 20°C, 300°C, 400°C and 480°C at strain rates of 0.00083–0.083 S −1. Immediately they are water quenched after the tension. The microstructural changes which occur during the hot tension have been investigated using true stress-true strain curves, X-ray photographs, optical and electron microscopy. It has been shown that the restoration mechanism consists of both dynamic recrystallization and dynamic recovery. The dynamic recrystallization is accelerated with the increase of deformation temperature and strain rate. The nucleation mechanism of dynamic recrystallization is discussed.

Analysis of Hot Tension Test Data To Obtain Stress-Strain Curves to High Strains

Abstract High strain rate hot tension tests have been carried out on AISI 1020 (Unified Numbering System [UNS] G10200) steel using a Gleeble® testing machine. Tests have been terminated at a series of strains and the specimen geometries measured. From these measurements, local areas, strains, and strain rates are derived and are used to obtain the local true stress-true strain curves. These are corrected for the change in strain rate with strain using an equation of state. The corrected curves are in good agreement with published curves on similar steels determined by compression or torsion testing. Under the present testing conditions, the maximum strain to which correction can be applied is ∼ 1.5. This is imposed by the onset of rapid local deformation heating. Up to this strain, no stress correction of the Bridgmann type for necking appears to be necessary.

Phenomene portevin-le-chatelier dans les alliages Fe-Co ordonnes et desordonnes

For temperatures below 730°C, Fe-Co-2V (Fe 49%-Co 49%-V 2%) has a B 2 type superstructure. By water-quenching, it is possible to vary the degree of the long-range order between 0 and 0.92. In the course of hot tension tests, serrations typical of the Portevin-le-Châtelier effect are observed. For stress rates between 10 −6 and 2 × 10 −4 sec −1, the temperatures are determined at which kinks appear and disappear for different degrees of order. The activation energy of the phenomenon which controls the appearance of the serrations is comparable to vacancy migration energy, while the energy which governs the disappearance of the phenomenon is similar to self-diffusion energy. This study allows us to deduce the variations in migration and vacancy-formation energies as a function of the degree of order; thus, we have been able to verify the parabolic nature of these variations and to confirm Girifalco's results: E m ( S) = E m (0) + AS 2 E f ( S) = E f (0) + aS 2 On the other hand, the experimental values of A and a are a great deal higher than the theory envisaged.