W. Blum

Geometric dynamic recrystallization in hot torsion of Al5Mg0.6Mn (AA5083)

Abstract Geometric dynamic recrystallization (GDRX) is a process in which a refined and nearly equiaxed grain structure is formed, because grain boundaries which have become serrated during formation of subgrains in the course of hot deformation recombine as serrations pinch off or as the grains thin down. GDRX was first found in aluminium and more recently in AlMg solid solution. In the present work the question was addressed whether GDRX occurs also in an industrial Al alloy (5083) containing particles. Specimens were deformed in torsion from 473 to 773 K at equivalent (surface) strain rates between 10−3 and 4 s−1 to strains up to 3.6. Under these conditions the egg tray model predicts that GDRX will occur. This is indeed found from observations of the grain structure with light and electron microscopy. The results indicate that GDRX occurs not only by recombination of opposite boundaries of the thinned grain but also by pinching off of serrations. The size of the GDRX grains is about two to three times the subgrain size. The close similarity to Al5Mg means that the particles in the alloy do not prevent the small-scale grain boundary migration which is necessary to form the serrations. Static recrystallization after hot deformation destroys the DRX structure, if the specimen is not cooled fast enough.

Comparative study of creep of the die-cast Mg-alloys AZ91, AS21, AS41, AM60 and AE42

The creep resistance of five high-purity die-cast Mg-Al-alloys was investigated in uniaxial compression at temperatures from 70 to 150 °C. The evolution of deformation resistance under creep conditions is described as a function of strain e. The work hardening in primary creep determines the times for creep by strains < 0.01. Among the investigated alloys AZ91 has the highest creep resistance with the exception of AS41 at stresses below 130 MPa. The microstructural origin of the creep resistance of Mg-Al-alloys is discussed in terms of work hardening by dislocations and hardening by creep-induced precipitation.

Long-range internal stresses in cell and subgrain structures of copper during deformation at constant stress

Long-range internal stresses in dislocation cell and subgrain structures were investigated experimentally. The transition of the dislocation structure from cells to subgrains was achieved by deforming copper polycrystals in compression creep tests at constant stress normalized by the shear modulus in the temperature range from 298 K to 633 K. The long-range internal stresses were investigated by two methods. The first one was the evaluation of characteristically asymmetric X-ray line profiles. The internal stresses are the result of the analysis of the X-ray line profiles. The second one was the measurement of local lattice parameters by convergent beam electron diffraction. The internal stresses can be determined from the changes in the local lattice parameters. The results obtained from both methods show that long-range internal stresses of the same type exist in the cell as well as in the subgrain structures.

Creep of Die‐Cast Light‐Weight Mg–Al‐base Alloy AZ91hp

Since a few years Mg alloys see a renaissance as light-weight materials for automobiles. In cars the Mg-alloys have to withstand elevated temperatures. Under these conditions the time dependence of the deformation of Mg-alloys must no longer be neglected. Therefore a basic research program was started in cooperation with AUDI AG, Ingolstadt, to understand and quantify the time dependent deformation behavior of the new high purity Mg-Al-base alloy AZ91. The results show that die-cast AZ91 has a good resistance to primary creep at temperatures up to 150 °C.

On the microstructural development of the tempered martensitic Cr-steel P 91 during long-term creep—a comparison of data

Abstract Microstructural data for creep of the tempered martensitic 9 wt.%CrMoV steel P 91 at 873 K were analyzed. It was found that the variation of the subgrain size with strain conforms to an exponential change from the initial to the stress dependent steady-state value. Analysis of the size distributions of precipitated particles confirmed the previous result that in P 91 there is a superposition of growth of particles with the dynamic precipitation of new particles mostly of type MX. From the distributions the numerical fractions of two types of particles, a predominating population of M 23 C 6 carbides and a population of fine particles representing mostly carbonitrides of type MX, and their sizes could be deduced. The variation of the creep rate with strain is in qualitative agreement with these microstructural changes.

Coarsening of the dislocation structure after stress reduction during creep of NaCl single crystals

The dislocation structure during compressive creep of NaCl single crystals has been investigated by means of the etch pit technique. The steady-state subgrain size L is about 27 times the average d...

Changes in Microstructure and Deformation Resistance during Creep of the Die‐Cast Mg–Al‐Base Alloy AZ91hp at Intermediate Temperatures up to 150 °C

Compression tests at constant stress and temperatures of 70 and 150°C were used to study the change of microstructure during creep of die-cast AZ91hp by light optical, scanning and transmission electron microscopy up to large strains without interference by fracture. At all stresses from 130 to 240 MPa there is a transition from work hardening (primary creep with decreasing creep rate e) to softening (tertiary creep with increasing e). The work hardening can be associated with increase in dislocation density as the dislocation structure approaches a stcady state subgrain structure and with precipitation of β-phase, which occurs in quite different forms in the primary α-phase and the surrounding eutectic regions. Twinning contributes to deformation at high stresses above 200 MPa. The pronounced softening in tertiary creep is associated with coarsening of β-phase. The instability of the phase structure has the consequence that the creep resistance of AZ91hp depends on thermomechanical history.

Stress dependence of the creep rate at constant dislocation structure

Stress reduction tests were performed during steady state compressive creep of pure aluminum at 523 K and of Al-5at.%Mg at 573 K in order to determine the dependence of the creep rate ϵdotr at constant (steady state) dislocation structure on (reduced) stress σr. Forward flow with ϵdotr > 0 was found in the whole range σr ⩾ 0, i.e. even after (nearly) full unloading, implying the existence of internal stresses in both materials. For aluminum the relation between log ϵdotr and σr consists of two branches with different stress sensitivities, indicating that the dominant mechanisms of dislocation motion are different for small and for large stress reductions. An interpretation is given in terms of the composite model, which assumes that the thermally activated glide in the (soft) subgrain interior and the recovery processes at (hard) subgrain boundaries are coupled by internal stresses. In the case of AlMg, on the contrary, the shape of the log ϵdotrvs. σr curves indicates that glide and recovery are not separable.

Dynamic grain growth a restoration mechanism in 99.999 Al

Aluminum and its alloys are noted for the high level of dynamic recovery (DRV) in ambient and high temperature deformation (T[sub D]). The preponderance of evidence is that they do not undergo dynamic recrystallization (DRX) except under special circumstances. However, the same alloys undergo static recrystallization (SRX) upon annealing (T[sub A]), which includes holding after hot deformation, even though static recovery (SRV) considerably reduces the dislocation density under circumstances where DRV has been so high as to inhibit DRX. Discontinuous recrystallization is inferred unless otherwise stated. The factors that influence these mechanisms are examined to see why DRX does not occur although SRX does and if DRX could be induced by augmented grain boundary (GB) mobility through an increase in purity which does not significantly increase DRV. Comments will then be made on the reports of DRX in 99.999 Al by Yamagata.

Subgrain structure during annealing and creep of the cast martensitic Cr-steel G-X12CrMoWVNbN 10-1-1

Quantitative investigations were performed using transmission electron microscopy in order to describe the evolution of the subgrain microstructure in the cast martensitic steel G-X12CrMoWVNbN 10-1-1 during annealing and creep at temperatures from 823 to 923 K. Annealing does not cause subgrain coarsening. During creep the subgrains grow dramatically when the stress is sufficiently low. The law of strain controlled growth of subgrains is consistent with that previously established for other martensitic Cr-steels.