Stephan Scheriau

Technical parameters affecting grain refinement by high pressure torsion

Abstract High pressure torsion is a well known and widespread processing technique for severe plastic deformation. The aim of high pressure torsion and other comparable techniques is to obtain ultrafine-grained or even nanocrystalline materials with enhanced mechanical and physical properties compared with their coarse-grained counterparts. Generally this refinement process is strongly influenced by processing parameters such as temperature or accumulated strain, but can also simply be affected by the entire experimental setup. Therefore, the benefits and limitations of the process with regard to grain refinement, homogeneity and specimen size, underlined with experimental results using different tools, will be discussed.

Advantages and Limitations of HPT: A Review

The improvements in the design of the HPT tools lead to a well defined torsion deformation and permits, therefore, a comparison with other SPD-techniques. The design of the tools, the advantages and disadvantages of HPT, as well as the limitation in the sample size are discussed.

Influence of external and internal length scale on the flow stress of copper

Abstract The flow stress of bulk specimens is known to depend on the microstructure. With a reduction of specimen dimensions into the micrometer and nanometer regime, specimen size-effects also influence the mechanical properties. We characterized the size-dependent flow stress of copper over more than three orders of magnitude, starting from several tens of micrometers down to a few tens of nanometers. For this purpose nanoindentation, micro-compression, and tensile testing experiments were performed. Additionally, different grain sizes were generated by severe plastic deformation. The observed increase in flow stress with reduced critical dimension is discussed with respect to the different stress states and microstructures present in the reported experiments. The mechanism controlling deformation changes from dislocation pile-up for critical dimensions > 1 μm via a transition regime (1 μm > critical dimensions > 100 nm) to dislocation nucleation for critical dimensions < 100 nm.

TEM Study of Local Disordering: A Structural Phase Change Induced by High-Pressure Torsion

Long-range ordered intermetallic alloys with L12 (Ni3Al, Cu3Au) and B2 (FeAl) structures were deformed by high-pressure torsion at room temperature up to high grades of deformation. Transmission electron microscopy shows that disordering caused by the deformation occurs on a very local scale within coarse grains along glide planes (Cu3Au, Ni3Al) and in the form of well defined local regions (Ni3Al, FeAl). The latter leads to a duplex structure consisting of an ordered coarse-grained structure and a disordered nanocrystalline structure. The different mechanisms that can lead to disordering during severe plastic deformation are discussed on the basis of the different ordering energies and on the basis of antiphase boundaries associated with gliding dislocations. The results indicate that in intermetallic alloys the formation of a nanocrystalline structure by severe plastic deformation is facilitated by the loss of order.

Broadband optoacoustic measurements of ultrasound attenuation in severely plastically deformed nickel

Laser optoacoustics and immersion techniques allowed a broadband ultrasound spectroscopy which was used for measuring the attenuation of severely plastically deformed nickel. A disk shaped specimen of nickel of about 33 mm diameter and 2.5 mm thickness was prepared by the high pressure torsion method. The produced equivalent shear strain linearly increased from a minimum at the center up to 1000% at the edge, gradually refining the grain size distribution down to 200 nm. The metal water interface was illuminated by 5 ns laser pulses, generating longitudinal ultrasound pulses with a pronounced compression phase and a smooth spectrum covering the range from 0.1 up to 150 MHz. The laser beam spot diameter was 6 mm, yielding a maximum power density below 15 MW/cm2. The ultrasound passed through the sample thickness and a 2 mm layer of coupling water. The pulse was detected by a 25 μm thick piezoelectric foil transducer with a diameter of the sensitive area of 2 mm. The transient signals were locally measured ...

Tailoring the Magnetic Properties of Ferritic Alloys by HPT

Industrial available FeSi, FeCo and FeNi alloys with an initial grain size of 20-50 m were subjected to Severe Plastic Deformation (SPD) up to strain levels where a saturation of the microstructural refinement is observed. For both SPD conditions, ambient temperature (293 K) and liquid nitrogen temperature (77 K), the microstructure of the severely deformed state is analysed by Back Scattered Electrons (BSE) micrographs captured in a SEM. Additionally, samples that were deformed at 77 K are examined in a Transmission Electron Microscope (TEM). The magnetic properties were characterised by means of SQUID-magnetography providing information about the magnetization behaviour of the material in the as-processed state. Depending on the SPD conditions the mean microstructural sizes in the steady state are about 100-150 nm and 30-80 nm at 293 K and 77 K, respectively. The small microstructural sizes influence significantly the magnetic properties of these ferritic alloys. The initial soft-magnetic behaviour of the coarse grained state shifts towards a hard-magnetic with decreasing crystallite size. For crystallite sizes smaller than ~80 nm the magnetic properties become again more soft-magnetic. The experiments show that very low coercitivity can be obtained by SPD if the grain size is smaller than ~50 nm.

Ultrafine Grain Structure Evolution in AA6085 Aluminium Alloy Processed by HPT at Increased Temperature

In this study ultrafine grain structure evolution during high pressure torsion (HPT) of commercial aluminium alloy AA6082 at increased temperature is presented. Two different initial structural states of the alloy were prepared by thermal treatment. The progress in structure refinement in dependence on the shear strain level strain was investigated by TEM of thin foils. The impact of different amount of strain (εef) introduced was analyzed with respect to the effect of increased temperature. The microhardness results measured across the deformed discs pointed out that some data scattering. The results of microstructure analyses showed that ultrafine grain (ufg) structure was already formed in deformed disc upon the first turn, regardless the initial structure of alloy, resulting from prior thermal treatment. The observed heterogeneity in ufg structure formation across the deformed disc was observed, supporting microhardness results scattering. By increasing the strain level (number of turns N-2,4,6), more effectively homogenized ufg structure was observed across the deformed discs. The effect of increased deformation temperature became evident and dynamic recrystalization modified locally ufg structure.. The retardation of new grains growth and higher thermal stability of ufg structure was observed, when two steps thermal treatment of alloy (quenching and ageing) was executed prior deformation. Strength measurements results yielded form tensile tests showed that the effect of structure strengthening was degraded by local recrystallization. The results of torque measurement versus the time showed that the torque required to deform the sample was increasing until the first turn and then kept stable or even decreased.

Structural Evolution in Aluminium Alloy AA6082 during HPT Deformation at Increased Temperature

In this study, ultrafine grain structure evolution during high pressure torsion (HPT) of commercial aluminium alloy AA6082 at an elevated temperature is presented. Two different initial structural states of the alloy were prepared by thermal treatment. The dependence of the progress of microstructure refinement on shear strain was investigated by TEM observation of thin foils. The impact of various amounts of strain (ɛef) was analysed with respect to the increased temperature of deformation. Microhardness data measured across the deformed discs show scatter. Observation of microstructure revealed that an ultrafine grain (UFG) structure formed in the deformed disc as early as the end of the first turn, regardless of the initial structure of alloy resulting from the prior thermal treatment. The heterogeneity of UFG deformed structure in the deformed discs is consistent with the scatter in microhardness values. By increasing the strain level through adding turns (N = 2, 4, 6), the UFG structure was homogenized in the deformed discs. The effect of the increase in deformation temperature became more evident and dynamic recrystallization modified the UFG structure locally. In the specimens prepared by two-stage thermal treatment (quenching and ageing) prior to torsion deformation, the growth of new grains was inhibited and the UFG microstructure was more stable. Tensile strength values suggest that strengthening was partially relaxed by local recrystallization. Torque vs. time plots reveal that the torque required to deform the sample was increasing until the completion of the first turn and then remained stable or even decreased slightly.

Ultrafine Structure Formation in Aluminium Alloy Processed by HPT and the Mechanical Properties Response

In this study ultrafine grain structure evolution during high pressure torsion (HPT) of commercial aluminium alloy AA6082 at increased temperature is presented. Two different initial structural states of the alloy were prepared by thermal treatment. The progress in structure refinement in dependence on the shear strain level strain was investigated by TEM of thin foils. The impact of different amount of strain (εef) introduced was analyzed with respect to the effect of increased temperature. The microhardness results measured across the deformed discs pointed out that some data scattering. The results of microstructure analyses showed that ultrafine grain (ufg) structure was already formed in deformed disc upon the first turn, regardless the initial structure of alloy, resulting from prior thermal treatment. The observed heterogeneity in ufg structure formation across the deformed disc was observed, supporting microhardness results scattering. By increasing the strain level (number of turns N -2,4,6), more effectively homogenized ufg structure was observed across the deformed discs. The effect of increased deformation temperature became evident and dynamic recrystalization modified locally ufg structure. The retardation of new grains growth and higher thermal stability of ufg structure was observed, when two steps thermal treatment of alloy (quenching and ageing) was executed prior deformation. Strength measurements results yielded from tensile tests showed that the effect of structure strengthening was degraded by local recrystallization. The results of torque measurement versus the time showed that the torque required to deform the sample was increasing until the first turn and then kept stable or even decreased.

Microstructure evolution and strength response of ultrafine grain medium carbon steel processed by high pressure torsion

High pressure torsion (HPT) deformation method at increased temperature of 400°C with varying strain was applied to refine microstructure of the medium carbon steel (AISI 1045). To investigate the deformation behaviour of the ferrite-pearlite two phase structure the different shear deformation and constant high hydrostatic pressure of 7 GPa were applied. The shear stress evolution during deformation and measurement of the torque were recorded and related to structure development and mechanical strength. In order to characterize microstructure development the transmission electron microscopy (TEM) was accomplished. By execution of the first turn the grain refinement was observed at deformed disc periphery. In the centre of the disc, no matter what strain was introduced, the ferrite grain structure showed moderately deformed features and pearlite colonies were preserved. With further equivalent strain increase, equilibrium between the refinement of coarse phases and new grains restoration processes led to saturation of the grain refinement process. Upon tensile properties testing, the yield strength and ultimate strength increased with increasing equivalent strain (eq) and only short region of strain hardening period prior failure appeared, regardless the strain applied. A small drop in the hardness across the disc was measured after execution of N= 4 and 6 turns, which may be related to formation of fine grain structure and structure recovery in disc centre.