P. Tasnádi

Determination of the hardness and elastic modulus from continuous Vickers indentation testing

Continuous Vickers (Hv) indentation tests were performed on different materials (ion crystals, metals, ceramics, silica glass and plastic). Load-indentation depth curves were taken during the loading as well as during the unloading period by a computer controlled hydraulic mechanical testing machine (MTS 810). The indentation work measured both the loading and the unloading periods, and these were used for the evaluation of parameters characterizing the materials. It was found empirically that there were linear connections between the maximum load to the power 3/2 and the indentation work. These connections were used to relate the conventional hardness number, Hv, and Youngs modulus, E, with the work performed during loading and unloading. This work can be determined with great accuracy from the measurements. The values of the Youngs modulus and the Vickers hardness determined this way agree well with those obtained by conventional methods. On the basis of continuous indentation tests, materials can be easily classified into the isomechanical groups introduced by Ashby. For this classification the Hv/E ratio is generally used. As a substitute for Hv/E another parameter is recommended which can be determined easily from a single measurement.

Indentation test for the investigation of high-temperature plasticity of materials

High-temperature behaviour of materials has been investigated by indentation creep methods. Materials investigated belong to three typical groups of materials: glasses, pure metals and alloys. Indentation tests were performed with flat-ended cylindrical and hemispherical punches. It is shown that the hemispherical punch is also proper for the determination of the high-temperature creep characteristics of the materials investigated. It is also proved that there is a unique correspondence between the results obtained by the two indentation methods.

Indentation tests for the investigation of the plasticity of glasses

Various types of indentation tests are used to investigate the plasticity of glassy materials. It is shown that indentation creep tests performed with both flat ended cylindrical and hemispherical indenters are suitable for viscosity measurements in the viscosity range of 108–1011Pa·s. It is also shown that the activation energy of the flow process can be evaluated from the results of indentation measurements.

Threshold stress in dispersionally strengthened superplastic Al alloys

It is important for practical applications that some commercial alloys with stabilized finegrained structure should exhibit superplastic behaviour at high temperatures. In this paper the results of impression creep tests conducted on AlMgZn alloys are reported and the strain rate sensitivity and activation enthalpy were determined. The mechanical behaviour of the alloys as a function of the strain rate sensitivity can be divided into three regions. At low and high stresses the strain rate sensitivity parameter is low and the deformation process is not superplastic. Superplastic deformation takes place only at intermediate stresses. The microstructural interpretation of these processes involves, in general, the change of the micromechanisms controlling the different deformation processes. It was determined that by the supposition of a threshold stress depending strongly on temperature, the two regions due to low and intermediate stresses of the deformation can be described by the same constitutive equation.

Superplasticity of aluminium alloys grain-refined by zirconium

The superplasticity of aluminium alloys containing magnesium, oint, iron and manganese as well as zirconium as a grain-refining addition element was investigated by high-temperature tensile tests. The tool elongation and the strain rate sensitivity as a function of strain rate and temperature were determined. The activation enthalpy and activation volume were also determined in the superlastic region of the deformation. !n addition to the tensile tests metallo-graphic investigations were also made. The results obtained show that the superplasticity of the alloys investigated is increasing by the addition of iron but it is decreasing by the simultaneous addition of iron and manganese.

Evaluation of the true activation enthalpy of superplastic flow including a threshold stress

A new method is suggested for the evaluation of the true activation enthalpy for alloys where the strain rate of the superplastic flow varies with a power of an effective stress σe = σ-σo, where σ and σo are the applied stress and a threshold stress, respectively. Some earlier results concerning superplastic AlMgZnCu alloys containing chromium and in which a strongly temperature-dependent threshold stress can be revealed, are reanalysed. The results are in good agreement with the previous ones. It has been shown further that for the alloys investigated the true activation energy increases with increasing chromium content.

Changes of the mechanical properties during the crystallization of bio-active glass-ceramics

A bio-active glass-ceramic material containing hydroxy apatite was investigated. The crystallization of the material during continuous heating was found to take place in three steps. The kinetics of the crystallization processes as well as their dependence on the particle size of the powder sample was investigated. Changes in the microstructure during the crystallization were observed and the crystallized phases were identified. The local mechanical properties such as microhardness and probability of crack formation as well as their changes with advancing crystallization were determined. The changes in the microstructure were correlated with those of the mechanical properties.

Stress−strain curves of superplastic alloys

Force-elongation and stress-strain curves have been analysed for superplastically deformed alloys tested in uniaxial tension under constant cross-head velocity conditions. By considering instability criteria the curves can be divided into three characteristic stages. In the course of the first two stages the samples are work hardened while during the third stage no work hardening takes place, although 85 to 90% of the total deformation occurs at this stage. However, sometimes at the end of stage III a weak strain hardening appears again due to grain growth. From the analysis of stage III the strain-rate sensitivity can be determined in good agreement with other methods.