Band 93, Heft 9

Finite element simulations were conducted to examine spherical indentation in the elastic – plastic transition regime. Various elastic-perfectly plastic materials were studied by varying the ratio of the elastic modulus to the yield stress in the range 25 – 1000. Special attention was given to the influence of residual stress and friction on the indentation load – displacement behavior and development of the plastic zone. A new method for measuring yield strengths from indentation load – displacement curves is proposed, and a recently developed experimental method for measuring biaxial residual stresses by nanoindentation methods is assessed. In the appropriate limits, the simulations show good agreement with the theoretical descriptions of spherical indentation given by Hertz and Tabor.

The precise knowledge of materials mechanical properties is always a core issue in every technical application. Through a developed finite elements method (FEM) continuous simulation of the nanoindentation, the applied force course versus the penetration depth is adequately simulated during the loading and unloading phases of this test, and the corresponding material stress – strain curves, as well as the universal hardness, are stepwise defined. Furthermore, the actual tip geometries of various indenters are approached and through equivalent magnitudes described. The results show that the defined material elastoplastic deformation characteristics are independent of the indenter type, as Vickers or Berkovich, since the existing indenter tip form deviations from their ideal geometry are considered. Furthermore, using the developed FEM-based nanoindentation simulation, the influence of the indenter tip geometry on the defined constitutive laws and the universal hardness is sufficiently elucidated. Various materials stress – strain curves and universal hardness courses versus the indentation depth, determined by means of the developed procedure, are presented.

Conventional methods for analysing nanoindentation load–displacement curves to obtain mechanical properties, such as Young’s modulus or hardness, require careful calibration of the indenter tip end shape and can overestimate hardness in cases where significant pile-up occurs. This paper introduces alternative methods for analysis, which are not so susceptible to such problems. Results from TiN and nickel suggest that the slopes method recently introduced by Oliver is an improvement on the other existing analysis methods.

Nanohardness investigations have been carried out on the plastically deformed regions around a quadrant of a 2 N Vickers microindentations in a cleaved single crystal of MgO (100) using a Hysitron machine. In all about four hundred 1 mN nanoindents were made and for each indentation an indenter load versus indenter displacement plot was obtained. Each of such plots was automatically analysed with the Hysitron machine to yield information about the nanohardness and elastic modulus. The data obtained showed that the nanohardness and elastic modulus values varied systematically with the crystallographic direction. The maximum value of nanohardness occurred at about 29 ± 1° relative to the 〈100〉 direction although higher values of hardness could possibly occur along the 〈110〉 directions if there were no cracking. Further investigations are required for understanding the variation of Young’s modulus with crystallographic orientation. Independent atomic force microscopy studies of the nanoindents are planned.

This paper outlines the content of a draft standard “Determination of hardness and Young’s modulus by instrumented indentation testing” and discusses two important aspects of the instrumented indentation testing method – namely the accuracy requirements and the strategy for measurement of coating-only properties when the indentation response is a composite of substrate and coating responses. This draft shall supplement the future ISO 14577 on the indentation test, addressing the peculiarities of test pieces with thin layers. Reduction in test uncertainty by more reproducible and more accurately calibrated instruments is already opening the way for the application of the nanoindentation test to the characterisation of the mechanical properties of ultra-thin coatings.

A systematic search for metal– indium systems which show a potentially high remelting temperature in diffusion-soldered bonds was done by investigation of phase formation and reaction kinetics in M– In systems (M = Pt, Pd, Mn). Phase equilibration experiments M–In (M = Pd, Mn) were done with powder samples prepared from pure elements, heated in evacuated silica capsules between 200 and 700 °C for 15 min to 570 h, and studied by X-ray diffraction analysis. Diffusion couples M/In (M = Pt, Pd, Mn) were prepared from the foils or wires of pure elements and heated in a bonding furnace or in evacuated silica tubes at 250 – 700 °C for 30 min to 96 h. The couples were cross-sectioned and studied by scanning electron microscopy with energy-dispersive X-ray analysis. The rates of M– In reactions can be semiquantitatively ordered in the sequence Pt > Pd≫Mn ( – In). The activation energy of the well-defined reaction of Mn– In powder samples was calculated to be 56 ± 10 kJ/mol. The activation energies calculated from the layer growth in diffusion couples show a large uncertainty, they are 39 kJ/mol (Pt/In), 23 kJ/mol (Pd/In) and 41 kJ/mol (Mn/In). Pt – In and Pd– In are very promising candidates for diffusion soldering of high-temperature stable bonds.

A systematic search for metal – tin systems which show a potentially high remelting temperature in diffusion-soldered bonds was done by investigation of phase formation and reaction kinetics in M–Sn systems (M = Zr, Hf, Nb, Ta, Mo). Phase equilibration experiments were done with powder samples prepared from pure elements, heated in evacuated silica capsules between 300 and 1000 °C for 1 h to 200 days, and studied by X-ray diffraction analysis. Diffusion couples M/Sn (M = Zr, Nb, Ta, Mo) were prepared from the foils of pure elements and heated in a bonding furnace or in evacuated silica tubes at 300 – 700 °C for 6 – 95 h. The couples were cross-sectioned and studied by scanning electron microscopy with energy-dispersive X-ray analysis. The rates of M–Sn reactions can be semiquantitatively ordered in the sequence Zr > Nb > Hf > Ta≫Mo (– Sn). The activation energy of the better defined reactions Zr – Sn, Nb–Sn, Hf–Sn are in the range of 67 – 93 (± 15) kJ/mol. A Ta–Sn phase diagram was constructed with similarities to the V–Sn system. Only Zr –Sn is a potential candidate for diffusion soldering of high-temperature stable bonds, Mo is suggested as an effective diffusion barrier against the attack of liquid tin.

The excess volume and density of liquid Al –Sr alloys at 1323, 1423 and 1523 K have been evaluated from the enthalpy of mixing and the excess entropy, using the solution model based on the free-volume theory. It has been found that the excess volume of liquid Al –Sr alloys exhibits a minimum at x Sr ≈ 0.31, well consistent with the thermodynamic results that both the enthalpy of mixing and excess entropy exhibit minima at x Sr ≈ 0.35. By contrast, the density of liquid alloys shows a maximum at x Sr ≈ 0.31. The excess volume and density, in combination with the thermodynamic properties and the phase diagram, suggest the presence of complexes of the type Al 2 Sr in liquid Al –Sr alloys. The excess volume and the density of liquid alloys slightly decrease with increasing temperature.

The strength differential (SD) effect as a function of plastic strain in SiC P reinforced Al –Cu –Mg alloy matrix composites with varying reinforcement sizes was investigated. Matrix alloy was also produced in a similar manner for comparison and designated as Control. All the materials exhibited significantly negative SD at very low plastic strains. The magnitude of the negative SD decreased and eventually became positive with increasing plastic strain. All the metal matrix composites exhibited a higher negative SD below and a higher positive SD above 0.35% plastic strain as compared to Control. The observed results were attributed to the nature, magnitude and distribution of thermal residual stresses in the matrix of MMCs and work hardening during deformation.

The hot working characteristics of the Mg-11.5Li-1.5Al-0.15Zr (wt.%) alloy have been studied using hot-compression testing in the temperature range 200– 400 °C and strain rate range 0.001– 100 s –1 . On the basis of these data, a processing map for this material has been developed. The map exhibits a single domain with a peak efficiency of 53% occurring at 350 °C and 0.001 s – 1 . On the basis of microstructural features, grain size variations and ductility variations, the domain is interpreted to represent dynamic recrystallization (DRX). Unlike in Zr-free alloy which exhibits only superplasticity, the Al 3 Zr particles in the Zr-containing alloy help in nucleating DRX and in refining the grain size. The material exhibits flow instabilities at temperatures higher than 350 °C and strain rates higher than 10 s – 1 .

Effect of different modes of rolling deformation and its role in the development of crystallographic texture has been studied for pure Cu, and Cu(Cr) and Cu(P) alloys with a view to obtain a very fine grained microstructure and very weak texture after annealing. Results indicate that textures developed after unidirectional rolling, two-step cross-rolling and multi-step cross-rolling are different. It has been shown that a transition of Cu type to the brass type is possible by changing the deformation mode during cold rolling.

Effect of annealing on textures resulting from different modes of rolling deformation has been studied in pure Cu, Cu(Cr) and Cu(P) alloys with a view to obtain a very fine-grained microstructure and very weak texture. The annealing textures of differently rolled materials are dependent on the mode of rolling employed prior to heat treatment. A good combination of uniform fine-grained microstructure with weak texture has been achieved by annealing the multi-step cross-rolled materials.