Che-Yu Li

Load relaxation studies of a metallic glass

Experimental results of load relaxation studies of a commercial metallic glass as a function of temperature are reported. The data suggest that metallic glasses exhibit deformation behavior with flow laws similar to those governing plastic deformation in crystalline solids. The lack of appreciable work hardening in annealed material and the identification of an anelastic component are also indicated by the experimental observations. (GHT)

Comparison of load relaxation data of Type 316 austenitic stainless steel with Hart's deformation model

Load relaxation experiments have been performed on Type 316 stainless steel at temperatures up to 650°C. The resulting stress-plastic strain rate relationships below 500°C show the behavior of a plastic equation of state. The experimental results suggest that at lower temperatures the deformation is controlled by dislocation glide and at higher temperatures dislocation glide becomes less important. Harts phenomenological model based on a plastic equation of state is used to account for the experimental data.

Relationship between anelastic and nonlinear viscoplastic behavior of 316 stainless steel at low homologous temperature

At low homologous temperature the plastic strain rate seems to be controlled largely by dislocation glide friction. However, since a sizeable fraction of the applied stress σ is dissipated in overcoming the strong barriers due to dislocation tangles generated by strain hardening, only a portion of the applied stress is actually expended against the frictional resistance. A recent model for this process, proposed by Hart, includes the role of dislocation pile-ups at the strong barriers. The pile-ups provide a mechanism for producing the internal back stress that reflects the barrier penetration stress. They also appear in the deformation as a stored anelastic strain component. The resultant behavior at low temperature and high stress is similar to that proposed by Gupta and Li. The same model also predicts an anelastic behavior at low stress. Measurements at both high and low stress levels on 316 Stainless Steel have now shown that the predictions of the model are quantitatively consistent at both stress levels.

Deformation modelling of aluminum during repeated stress relaxation runs

A state variable model is capable of representing stress relaxation runs even in case of mainly elastic and anelastic loading. Apparent kink in log sigma versus log epsilon representation of stress relaxation event, after mainly elastic and anelastic loading, is due to change of the deformation mode from mainly anelastic to mainly plastic. The crossing of stress relaxation curves after an initial plastic loading and subsequent elastic and anelastic reloadings in commercially pure aluminum at room temperature, was explained in terms of thermally induced effects, the presence of which was experimentally confirmed.

Effect of copper on the strength of AISI 316 stainless steel

The load relaxation test was used to evaluate the elevated temperature (>0.4Tm) flow strength (as a function of strain rate and temperature) of a series of modified type AISI 316, austenitic stainless steels which had been given various thermomechanical pretreatments. Two classes of steels were investigated. The first contained only stabilizing additions; the second contained both stabilizing additions and copper. Under the same conditions the latter class of steels was found to have a much higher flow strength at all strain rates compared to the former class. Microstructural examinations showed that the high flow strength of the copper containing steels results from a larger number of finer matrix MC-type carbides which effectively pin dislocations. It is suggested that copper causes this finer distribution by increasing the stacking fault energy of the austenite lattice and thereby enhancing the nucleation rate of MC-type carbides on grain matrix dislocations.

The growth of grain boundary cavities under applied stress and internal pressure

Experiments were performed to investigate the growth of grain boundary cavities in nickel under internal pressure and applied stress. Under the experimental conditions the growth kinetics was found to be controlled by stress induced mass transfer. The growth rate was a nonlinear function of the applied stress and was found to depend on grain boundary orientation and grain boundary sliding.

Superplastic-like deformation in some solid solution alloys

Abstract The stress relaxation and tensile test data of Incoloy 800H and Al4.6%Mg are described at temperatures where the contribution of grain boundary sliding to flow is significant. It is shown that in the presence of grain boundary sliding the whole range of behavior, from ordinary creep to superplastic-like flow, can be exhibited in the same metals, showing that the strain rate sensitivity increases with decreasing grain size or increasing temperature in the chosen temperature range. The effect of solute hardening on the flow behaviour of solid solution alloys in the presence of grain boundary sliding is also discussed.

Effects of grain boundary sliding on the flow properties of Incoloy 800 H

Abstract The nature of grain boundary sliding (GBS) is investigated in Incoloy 800 H in terms of the effects of stress, temperature and grain size on the flow behavior obserced the load relaxation test. Flow behaviors are obtained for average grain sizes ranging from 6 to 225 μm at temperatures between 614 and 746°C. The flow behavior of large-grain-size material plotted as stress vs. strain rate in a doubly logarithmic scale, exhibits a sigmoidal shape which has been commonly associated with the effects of GBS on creep deformation. For the materials of smaller grain sizes the deformation properties tend toward those characteristic of structural superplasticity. It is shown that in Incoloy 800H there may exist, as a function of the grain size, a continuous scale of flow properties ranging from the normal creep to superplastic-like behavior.