E. Rybacki

Texture induced plastic anisotropy of NiAl polycrystals

Abstract Textured stoichiometric NiAl polycrystals have been deformed in compression at room temperature under 0.4 GPa confining pressure at a strain rate of about 10 −4 s −1 . The strength of the samples depends on the preferred orientation with respect to the compression axis, with 〈100〉 being much harder than those with 〈110〉 and 〈111〉. Similarly, the frequency of intercrystalline microcracking shows the same tendency. The orientation distribution of the microcracks indicates that they mainly result from internal stresses released after unloading. The plastic anisotropy at elevated strains correlates with the Taylor energy calculated. The deviation from axially symmetric deformation is due to the tendency of minimizing the Taylor energy.

Influence of texture and hydrostatic pressure on the room temperature compression of NiAl polycrystals

Abstract Compression of NiAl polycrystals was carried out at room temperature under atmospheric as well as 0.4 GPa confining pressure at constant strain rate. The stress–strain curves show that the strength as well as the work-hardening rate is generally higher when deformed under hydrostatic pressure. Moreover, there is a strong plastic anisotropy, that is, samples having a 〈100〉 crystallographic preferred orientation parallel to the compression axis are much harder than those having 〈110〉 and 〈111〉. Similarly, the frequency of microcracking shows the same tendency. It seems that the microcracks in the samples deformed under confining pressure mainly result from internal stresses released after unloading. The strength of the differently textured samples at elevated strains correlates with the Taylor energy calculated. These calculations also explain for certain textures the deviation from a homogeneous shape change during compression towards plane–strain deformation. The textural changes produced by deformation are simulated with the Taylor model.

Grain refinement and texture formation in torsion deformed NiAl

Abstract Torsion at elevated temperatures and pressures was used to severely deform the intermetallic compound NiAl. The microstructure and texture as a function of shear strain were investigated by orientation imaging microscopy and diffraction of synchrotron radiation, respectively. The results show that at high shear strains a steady state grain structure and texture develops by continuous dynamic recrystallisation. The lowest grain size achieved is in the micron range, the main texture component is {110}<100>. It is concluded that high-strain torsion may open new possibilities in terms of grain refinement, texture formation and ductilisation of NiAl.

Microstructure Development During High Strain Torsion of NiAl

The microstructure development was investigated in torsion deformed NiAl. High strain torsion of solid bars was done with a Paterson rock deformation machine at temperatures between 700 K and 1300 K under a confining pressure of 400 MPa. The maximum shear strains and shear strain rates applied were 19 and 2.2 × 10 -4 s -1 , respectively. The shear stress-shear strain curves are characterized by a peak at low shear strains, which is followed by softening and a steady state at high shear strains. Increasing shear strain leads to grain refinement, with the average grain size decreasing with temperature. Moreover, a steady state grain aspect ratio and inclination of the grain long axis with respect to the shear plane is observed. With increasing shear strain, the fraction of low angle grain boundaries goes over a maximum and approaches a steady state of about 20-40%. The development of the microstructure is characterized by two different temperature regimes. Up to 1000 K, continuous dynamic recrystallization characterized by limited grain growth takes place, leading to a transformation of low into high angle grain boundaries. At temperatures above 1000 K, discontinuous dynamic recrystallisation occurs by massive grain growth. The results are qualitatively discussed on the basis of models dealing with dynamic recrystallization.

Texture Formation and Swift Effect in High Strain Torsion of NiAl

Texture formation and Swift effect were investigated in torsion deformed NiAl. High-strain torsion of solid bars was done with a Paterson rock deformation machine at temperatures between 700 K and 1300 K under a confining pressure of 400 MPa. The maximum shear strains and shear strain rates applied were 19×10 -4 s -1 and 2.2 ×10 -4 s -1 , respectively. Textures were measured by diffraction of neutrons, electrons, and synchrotron radiation. The textures consist of an oblique cube and Goss component, the intensity of which depends on the initial texture and deformation temperature. The axial lengthening and shortening observed are related to the Goss and the oblique cube components, respectively. There is qualitative agreement between experiment and simulation at low temperature and low shear strains. With increasing temperature, continuous and discontinuous dynamic recrystallization take place, strongly influencing the development of texture and Swift effect.

Shear Texture Formation during High-Strain Torsion of Titanium Aluminides

Ti-47Al-4(Cr, Nb, Mn, B) samples with different initial grain structures and fibre textures were deformed in torsion at 1000 °C under hydrostatic pressure of 400 MPa in a Paterson type rock deformation machine at a maximum constant strain rate of 2*10-4 s-1. The microstructure was investigated by transmission electron microscopy (TEM). Local texture measurements as a function of shear strain were done with high-energy synchrotron radiation. During shearing due to dynamic recrystallization the initially lamellar structure breaks down into a fine-grained globular structure with a g grain size in the order of 5 µm. The shear texture developed consists of <110> and {110} inclined to the shear direction and shear plane, respectively. The microstructure and texture development is discussed.

Dynamic Recrystallization of Torsion Deformed NiAl

Polycrystalline samples of NiAl have been deformed in torsion in the temperature range 800K – 1300K. Deformation is accompanied by dynamic recrystallization, which with increasing temperature changes from continuous to discontinuous mode. Emphasis was put on the occurrence of continuous dynamic recrystallization, which will be discussed with respect to texture and microstructure.

Texture Evolution of HPT-Processed Ni50Mn29Ga21

The texture of polycrystalline Ni50Mn29Ga21 alloys fabricated by high pressure torsion (HPT) was investigated with high-energy synchrotron radiation. HPT was performed at temperatures between 873K and 1173K under a hydrostatic pressure of 400 MPa. During HPT above 973K the initial cyclic fibre texture changes to a strong cube and a weak F component. Below 973K a strong rotated cube and weak F and C components develop. Additionally, electron backscatter diffraction reveals that samples deformed at low temperature do not completely transform to martensite giving rise to residual austenite.