Verena Maier

Microstructure-dependent deformation behaviour of bcc-metals – indentation size effect and strain rate sensitivity

In this work, the indentation size effect and the influence of the microstructure on the time-dependent deformation behaviour of body-centred cubic (bcc) metals are studied by performing nanoindentation strain rate jump tests at room temperature. During these experiments, the strain rate is abruptly changed, and from the resulting hardness difference the local strain rate sensitivity has been derived. Single-crystalline materials exhibit a strong indentation size effect; ultrafine-grained metals have nearly a depth-independent hardness. Tungsten as a bcc metal shows the opposite behaviour as generally found for face-centered cubic metals. While for UFG-W only slightly enhanced strain rate sensitivity was observed, SX-W exhibits a pronounced influence of the strain rate on the resulting hardness at room temperature. This is due to the effects of the high lattice friction of bcc metals at low temperatures, where the thermally activated motion of screw dislocations is the dominating deformation mechanisms, which causes the enhanced strain rate sensitivity. For the SX-materials, it was found that the degree of the indentation size effect directly correlates with the homologous testing temperature and thus, the material specific parameter of the critical temperature Tc. However, for the resultant strain rate sensitivity no depth-dependent change was found.

Tailored Heat Treated Accumulative Roll Bonded Aluminum Blanks: Microstructure and Mechanical Behavior

Aluminum alloy AA6016 was accumulative roll bonded up to eight cycles and investigated regarding formability by bending tests. Due to the limited bendability of accumulative roll bonding (ARB) processed materials, a tailored laser heat treatment was performed along the bending edge before forming. This tailored laser heat treatment causes a local recrystallization and recovery of the bending samples at the deformation zone, which locally increases ductility and allows higher bending angles achievable with lower forming forces. Between the recrystallized heat treated zone and the unaffected ultrafine-grained (UFG) base material, a gradient in grain size with a bimodal region is formed. This observed microstructural profile is confirmed by local mechanical testing measuring the hardness and strain rate sensitivity by nanoindentation techniques.

Nanomechanical behaviour of Al-Ti layered composites produced by accumulative roll bonding

In this study Ti-foils were roll bonded together with commercially pure aluminium AA1050. The laminates were produced by using two 1 mm thick AA1050 sheets at the outer side of the stack combined with 100 μm thick Ti-foils as an intermediate layer for each accumulative roll bonding process. The samples were rolled up to 4 ARB cycles. Subsequently the sheets were post-process heat treated at 180°C, 400°C or 600°C, respectively, for 24 hours. The local mechanical behaviour of the Al/Ti intermetallic interfaces have been investigated using nanoindentation experiments. A strong dependence between annealing-temperature, – time and deformation grade is detected. While a heat treatment at 180°C only leads to a weak bonding between Al and Ti with a preservation of the UFG structure, temperatures up to 600°C are causing a complete recrystallisation of the microstructure and formation of diffusion layers with different Al and Ti concentrations.

Thermally Activated Deformation Behavior of ufg-Au: Environmental Issues During Long-Term and High-Temperature Nanoindentation Testing

Abstract For testing time-dependent material properties by nanoindentation, in particular for long-term creep or relaxation experiments, thermal drift influences on the displacement signal are of prime concern. To address this at room and elevated temperatures, we tested fused quartz at various contact depths at room temperature and ultra-fine grained (ufg) Au at various temperatures. We found that the raw data for fused quartz are strongly affected by thermal drift, but corrected by use of dynamic stiffness measurements all the datasets collapse. The situation for the ufg Au shows again that the data are only useful with drift correction, but with this applied it turns out that there is a significant change of elastic and plastic properties when exceeding 200°C, which is also reflected by an increasing strain rate sensitivity.

Influence of Nanoparticle Reinforcement on the Mechanical Properties of Ultrafine-Grained Aluminium Produced by ARB

Dispersed nanoparticles are introduced from stabilized suspensions during the accumulative roll bonding process in aluminium AA1050A by air gun spraying up to a final volume fraction of 0.1 % after eight cycles. Additional strengthening caused by particle insertion is observed and strongly depends on the suspension medium and stabilizing agent as both influence interfacial bonding of the particles to the matrix. The particle insertion furthermore results in reduced peel strength of the sheets irrespective of particle material and size caused by a reduction of effective metal to metal bonding area during rolling through the presence of the particles.

Multicomponent materials from machining chips compacted by equal-channel angular pressing

Al and Mg machining chip blends were compacted by equal-channel angular pressing with back pressure. By varying the weight fraction of the constituent materials, temperature and processing route, as well as employing subsequent heat treatment, the microstructure and the mechanical properties of the compact were varied. The width of the interdiffusion zone and the formation of intermetallic phases near the interfaces between the two metals were studied by energy-dispersive X-ray spectroscopy and nanoindentation. It was shown that substantial improvement of mechanical properties, such as an increase of strength, strain-hardening capability and ductility, can be obtained. This is achieved by changing the processing parameters of equal-channel angular pressing and the annealing temperature, as well as by optimising the weight fraction of the constituent metals.

Strain-Rate Sensitivity (SRS) of Nickel by Instrumented Indentation

For materials which exhibit a power-law relationship between stress and strain rate, it is theoretically possible to evaluate the exponent (m) which governs the relationship by means of instrumented indentation. However, in practice, tests at small strain rates take so long that the results can easily be dominated by thermal drift. A new test method is developed in which several constant strain rates are examined within a single indentation test by switching strain rates as the indenter continues to move into the material. Switching strain rates within a single test overcomes the problem of long testing times by examining large strain rates first and transitioning to smaller strain rates as the test proceeds. The new method is used to test a sample of fine-grained nickel sold by NIST as a standard reference material for Vickers hardness. The strain-rate sensitivity of this sample is measured to be m = 0.021. This value is in good agreement with values obtained by others on fine-grained nickel using both instrumented indentation and uniaxial creep testing.

Formability of Ultrafine-Grained AA6016 Sheets Processed by Accumulative Roll Bonding

Aluminium alloy AA6016 was accumulative roll bonded up to eight cycles in order to produce an ultrafine-grained microstructure. The formability of these sheets was investigated by means of bending tests. Furthermore the influence of a local laser heat treatment at the bending edge is observed. The strength of the UFG samples is increased by a factor of around two compared to the conventionally grained T4 condition which also results in up to 50 % higher punch forces needed for bending of ARB processed samples. An anisotropic bending behaviour is observed. By applying a tailored laser heat treatment along the bending edge prior to the bending tests a local recrystallization and recovery at the deformation zone of the samples is achieved. Thus, ductility is increased locally whereby bending to an angle of 80° is possible with lower forming forces compared to the non-heat treated specimens. The results are compared to previous studies on mechanical properties and formability investigations of ARB processed AA6016.