Hans-Joachim Klaar

Evolution of dislocation structures and deformation behavior of iron at different temperatures: Part I. strain hardening curves and cellular structure

The deformation behavior of iron has been investigated at different temperatures by means of tension tests. There exist two temperature ranges for deformation. In the low-temperature range(T < 293 K), the flow stress σ, the work-hardening rate θ at ε = 0.06, and the yield stress σy decrease with increasing temperature, but in the higher temperature range(T ≥: 293 K), σ and θ at the same strain increase while σy decreases more slowly. The change of dislocation density, with temperature, atε = 0.06 exhibits the same tendency as that of the flow stress. The strainhardening rates decrease almost linearly with increasing stress up to necking in the low-temperature range, except the initial strain range. At the higher temperature range, the hardening rates decrease linearly with stress only at the early stage of deformation, but above certain strains, the decreases become more gradual; that is, the G-cr curves deviate from the linear region. The evolution of dislocation structure has also been observed by transmission electron microscopy (TEM). The results show that a substructural transition takes place in the nonlinear range of G-cr curves. In the linear decreasing region of strain-hardening curves, the deformation is controlled by the uniformly distributed dislocations or cell multiplication prevails. However, in the nonlinear region of G-cr curves, cell multiplication seems to be balanced by cell annihilation.

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α2-Ti3Al, and ξ-Ti5(Si, Al)3. The microstructure can be described as a duplex structure(i.e., lamellar γ/α2 regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti5(Si, Al)3 particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti5(Si, Al)3 particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.

Evolution of dislocation structures and deformation behavior of iron at different temperatures: Part II. dislocation density and theoretical analysis

The evolution of dislocation density in iron deformed at 173 K and at room temperature has been examined by transmission electron microscopy (TEM). At room temperature, the dislocation density in the cell walls increases as the deformation progresses up to large strains, whereas in cell interiors, the density evolves toward a saturation value. A linear relationship exists between the flow stress and the square root of total dislocation density both at 173 K and room temperature. The dependence of deformation behavior on the evolution of dislocation structures is discussed in terms of a model considering the dislocation distribution during deformation. Comparison of the calculated result using this model with the experimental curve at room temperature gives excellent agreement. The changes of deformation behaviors at different temperatures can be described by the effect of temperature on the evolution of dislocation distribution.

Orientation relationship between Al6Mn precipitates and the Al matrix during continuous recrystallization in Al-1.3%Mn

Annealing of cold-rolled supersaturated Al–1.3u2005wt%u2005Mn leads to heavy precipitation of fine particles on the as-deformed microstructure. Depending on the crystallographic orientation of the deformed matrix grains, particles with different morphologies, namely spherical, rhomboidal and plate-like, have been observed. This variation in morphology could be traced back to differences in local misorientation caused by different dislocation substructures in different matrix orientations. Microstructural investigations and selected-area diffraction (SAD) analysis in a transmission electron microscopy (TEM) study were employed to characterize the various particles and to determine their orientation relationship to the Al matrix. Independent of the particle morphology, most Mn precipitates were identified as Al6Mn; only occasionally was Al12Mn observed. Upon consideration of the various crystal symmetries, two different orientation relationships were discerned, which can be written as (110)m||(111)p, [001]m||[2¯11]p and (110)m||(111)p, [11¯2]m||[12¯1]p. These orientation relationships are discussed with respect to the existence of several low misfits of lattice spacings and compared to literature data.

Investigation of the reaction zone between TiAl and Mo

Pure Mo was incorporated in TiAl matrixvia two different routes: (1) hot pressing of alternately sandwiched Ti-Al sheets and Mo foils; and (2) coextrusion and heat treatment of Ti-Al green compact and Mo rod. The reaction zone between TiAl and Mo is found to contain two intermetallic phases: β-(Mo,Ti)Al andp-(Mo,Ti)3Al. The β-p boundary is incoherent, whereas the TiAl-β andp-Mo boundaries are semicoherent. The reaction zone grows with increasing heat-treatment time in a parabolic form. The incorporated Mo exhibits lower hardness than the TiAl matrix, implying that ductilizing and toughening of TiAl by introducing Mo as a ductile reinforcement are possible.