T. Kruml

Dislocation structures in the bands of localised cyclic plastic strain in austenitic 316L and austenitic-ferritic duplex stainless steels

Abstract Dislocation structures in bands corresponding to cyclic strain localisation have been studied in two types of stainless steels, single phase austenitic 316L steel and two-phase austenitic-ferritic duplex steel. Dislocation structures are documented in thin foils oriented approximately perpendicular to the active slip plane of individual grains and parallel to the primary Burgers vector. Persistent slip bands, with the structure more or less reminiscent of the well-known ladder structure, were found in austenitic grains of both steels. These bands can be correlated with the distinct surface relief consisting of extrusions, intrusions and shallow surface cracks in austenitic phase. No clear features which can prove the existence of the persistent Lu¨ders bands in austenitic grains were found. The distribution of the wall and labyrinth structure embedded in the matrix structure in ferritic grains, which was proposed to be responsible for the localisation of the cyclic strain, however, does not correspond to the distribution of the distinct surface slip lines on the surface.

From dislocation cores to strength and work-hardening: A study of binary Ni3Al

Abstract A quantitative model for the peak temperature in work-hardening in L1 2 intermetallics is proposed. It is based on the competition between the exhaustion of mobile dislocations by the Kear Wilsdorf mechanism and the yielding of incomplete locks at high stress. The model is assessed by a set of experimental data measured in binary Ni 3 Al polycrystals of three different compositions. These include, in particular, the planar fault energies of the dislocation cores measured by weak-beam electron microscopy, combined with computer image simulations and macroscopic data about flow stress, work-hardening and mobile dislocation exhaustion rates. These parameters are measured as a function of alloy composition. The model also fits successfully data published for other L1 2 compounds.

Characterization of thermally activated dislocation mechanisms using transient tests

Abstract Two methods of repeated transient tests, namely relaxation and creep, are shown to be ideal techniques for the characterization of plastic deformation processes. They yield similar information about a microscopic activation volume, which is the signature of the operating dislocation mobility mechanism. Microstructural parameters are also obtained, the values of which are different in creep and stress relaxation. They characterize work-hardening during the transient and dislocation exhaustion rates respectively. The equations describing the transients and the assumptions involved are presented. Experimental results on Ni 3 Al polycrystals illustrate the possibilities of both tests and support the above assumptions. In particular, crystals which work-harden exhibit high dislocation exhaustion-rates, as shown by comparison of Ni 3 Al with TiAl, Ge and Cu. The respective contributions to the strain-rate of the mobile dislocation densities and velocities can also be estimated.

Fatigue softening of X10CrAl24 ferritic steel

Cyclic plastic response of X10CrAl24 ferritic stainless steel in a wide domain of plastic strain amplitude was measured. Cyclic hardening–softening curves show important cyclic softening during most of the fatigue life. Cyclic stress-strain curves have different slopes in the domain of low and high strain amplitudes. These two domains in cyclic stress–strain curve correspond to two types of internal structure as observed by transmission electron microscopy (TEM). The areas of condensed dislocation structure in the interior and the bands of extrusions and intrusions on the surface indicate the localization of cyclic strain. Fatigue softening was discussed in terms of properties of

Microstructure in 316LN stainless steel fatigued at low temperature

Abstract The internal structure of AISI 316LN austenitic stainless steel cyclically strained at liquid nitrogen temperature has been studied using transmission electron microscopy and electron diffraction. High amplitude cyclic straining promotes the transformation of austenite with face centred cubic (f.c.c.) structure into e-martensite with hexagonal close packed (h.c.p.) structure and α′-martensite with distorted base centred cubic (b.c.c.) structure. Thin plates containing e-martensite were identified in all grains. α′-martensite nucleates at the intersection of the plates in grains with two or more systems of plates and can grow in the bands. The orientation of transformed phases follows the Shoji–Nichiyama and Kurdjumov–Sachs relations. Mechanisms of low temperature cyclic straining are discussed.

The shape of extrusions and intrusions produced by cyclic straining

Abstract Detailed observations of the identical area on the surface of fatigued austenitic 316L steel and on its inverse copy via plastic replica were performed using atomic force microscopy (AFM) and high-resolution scanning electron microscopy. It was shown that the AFM technique has some limitations in observation and in quantitative evaluation of the surface relief and that the real geometry of extrusions and intrusions can be distorted when adopting AFM. Only the combination of the above experimental techniques allows assessing the true shape of extrusions and intrusions produced by cyclic straining.

Dislocation density in Ni3(Al,Hf)

Abstract The temperature dependence of dislocation density and its relationship with the anomalous growth of the flow stress are presented. A pronounced density increase of about one order of magnitude in a temperature range below the peak of anomalous flow stress growth has been clearly established experimentally. The correlation with different components of the applied stress is examined and it is shown that the dislocation density is related to the stress required for dislocation multiplication.

On the strengthening of Ni3Al by hafnium additions

Abstract To interpret the notable strengthening of Ni3Al due to Hf additions in the strength anomaly domain, the dislocation features of a 3at.% Hf compound are characterized. Since the general microstructure does not exhibit obvious differences from that observed in similar compounds, the superdislocation core is investigated to find reasons for this effect. Various weak beam conditions are tested which never yield more than 3 peaks for the intensity profiles. The latter are interpreted for the chosen g, ng conditions (with 3 < n < 6) after extensive computer image simulations. The different fault energies related to the core are determined and found to be γ111 = 300mJm−2, γ010 = 250mJm−2 at 300K while γCSF exhibits very high values (larger than or equal to 460mJm−2). This explains the peculiar dislocation images. A comparison of the flow stress—temperature plots with those corresponding to a binary and a 1at.% Ta compounds confirms that the shifts observed for the flow stress in the anomaly domain and those for the peak temperature can be correlated well with the γCSF values, but not with the antiphase boundary anisotropy ratio. γCSF appears to be the key parameter for dislocation locking in the strength anomaly domain. Other solid solution strengthening effects operate in addition, without hindering the influence of γCSF. This interpretation of the differences in mechanical properties agrees with previous studies on similar compounds, but it is shown to hold even when these differences are large. In addition it is strongly supported by data about dislocation exhaustion rates which are measured in the Hf, the Ta and the binary compounds through repeated load relaxation experiments at 575K. The high ability of superpartials to cross-slip in this large γCSF Hf compound also explains the rather large minimum dislocation character observed for dislocations lying on the octahedral plane.

Stress reduction experiments during constant-strain-rate tests in Cu and Ge

The technique of rapid reductions in applied stress (dip test) is occasionally performed in creep testing. In the work reported here, the technique was applied during a constant-strain-rate test. Both variants, the stress dip test and the strain dip test, give equivalent results. The specimen strain has to be measured in both cases; however, the strain dip technique is preferred since anelastic effects do not interfere. It is shown that the response of Cu and Ge single crystals to fast stress reductions, on the level of the internal stress and below it, is strongly material dependent, reflecting differences in microstructural processes involved in the plastic deformation. These processes are discussed.

Low-cycle fatigue properties of TiAl alloy with high Nb content

Abstract The low-cycle fatigue properties of TiAl alloy with 8 at.% of Nb with nearly fully lamellar microstructure were studied at room temperature and 750 °C and compared with those reported in literature for a previous class of TiAl alloys with 2 at.% of Nb. Hardening/softening curves, cyclic stress – strain curves (CSSC) and fatigue life curves were obtained at both temperatures. The material shows stable cyclic stress – strain response. Parameters of the CSSC, of the Manson – Coffin law and of the Basquin law were determined. Bilinear dependence is used to approximate the Manson – Coffin plot and the Basquin plot at 750 °C.