Jerzy Bystrzycki

Cold-work induced phenomena in B2 FeAl intermetallics

For the last several years we have been conducting systematic studies of microstructural changes and mechanical behavior of B2 FeAl alloys, cold-worked by ball-milling and shock-wave loading. Fundamental physical phenomena induced in iron aluminides such as an accelerated chemical disordering, increase in the lattice parameter, non-magnetic/magnetic transformation and formation of nanocrystals in ball-milled powder particles are presented and discussed.

Grain boundary character distribution in B2 intermetallics

Abstract This paper reports the results of the experimental studies of the effect of chemical composition, texture and processing methods on the grain boundary character distribution evaluated by the electron back-scatter diffraction pattern (EBSD or EBSP) technique, in B2 FeAl and NiAl intermetallic compounds. An alternative method based on the grain boundary surface area for calculating the unbiased fraction of grain boundaries of a given character (ΣCSL) is proposed. It is shown that the proposed method gives different results than the classical method based on the counting of grain boundary segments. It is found for both FeAl and NiAl that the fraction of low-angle boundaries (Σ1 LABs) increases with increasing percentage of the and to a lesser extent the texture, up to the upper limit of ∼20%. In B2 FeAl the fraction of so-called “special” grain boundaries (SGBs) (Σ3-29) seems to be independent of the percentage of the texture. In B2 NiAl and NiAl+2wt.%HfC alloy the fraction of SGBs decreases continuously with increasing percentage of the texture. The texture does not have strong effect on LABs. Processing of B2 FeAl by shock-loading and subsequent annealing can increase the fraction of LABs to 90–97%. This effect is not observed in B2 NiAl. Instead, the premature abnormal grain growth occurs, accelerated by the accumulated shock strain energy. In B2 compounds the fraction of LABs seems to increase nearly linearly up to the limit of ∼20% with decreasing grain size from ∼400 to ∼100 μm. For grain sizes smaller than ∼100 μm the fraction of LABs seems to be independent of grain size. The fraction of SGBs does not exhibit any dependence on grain size.

The frictional component in microhardness testing of intermetallics

It has been recognized for quite a long time that Vickers (and Knoop) microhardness of many metallic and non-metallic materials becomes greater at lower loads (so-called indentation size effect or ISE). One group of investigators attributed this peculiar phenomenon to an operator factor, or simply speaking to an experimental error, rather than to intrinsic material properties. However, an alternative analysis takes a more physical approach in which two factors, namely, the surface energy contribution and the volume energy contribution are suggested. The former represents the work needed to create the new surface while the latter represents the work needed to produce the volume deformation. It has recently been shown that L1[sub 2] titanium and zirconium trialuminides (based on Al[sub 3]Ti and Al[sub 3]Zr intermetallics) exhibit a very strong ISE. In the present paper it is shown that the ISE is very characteristic for many other intermetallics and the PSR (proportional specimen resistance) model by Li et al. can be applied to explain the origin of ISE in intermetallics, as being fundamentally attributed to the frictional component of indentation testing.

Structure and hardness of explosively loaded FeAl intermetallic alloys

Abstract The effect of the explosive loading at the 4.5–11.0 GPa shock pressure range and a subsequent heat treatment on the structure and hardness of FeAl intermetallic alloys were studied. As-cast FeAl intermetallic alloys were shock-wave deformed using various cylindrical explosive assemblies and type of explosives. It was found that the microhardness of explosively loaded FeAl intermetallic alloys increases with increasing applied pressure and is considerably higher than that of their as-cast and homogenized or hot-worked counterparts. The TEM examinations revealed many straight line dislocations with sharp jogs indicative of cross-slipping in explosively-loaded iron-rich intermetallics. In the regions of the highest dislocation density a tendency to formation a dislocation cell substructure in Fe–39,43 and 46Al was observed. Anneals of intermetallics explosively-loaded above 5.9 GPa lead to their primary recrystallization. The cracks in all the FeAl-based intermetallic specimens that were either explosively loaded at 5.9 and 11.0 GPa of the applied explosive pressure or contained the highest Al content (near-stoichiometric) were revealed.

The microstructure and interface behaviour of Ni/NiAl composites produced by the explosive compaction of powders

Explosive compaction of Ni and NiAl powders was utilized for the processing of Ni/NiAl metal-matrix composites containing up to 57 vol% NiAl particulate. The microstructure, the Vickers microhardness and the Ni/NiAl interfacial bonding strength were studied. The resulting microstructure had a very low volume fraction of porosity (∼ 1 vol%) except for the melting zone formed in the upper portion of a cylindrical specimen. NiAl particles underwent welding during explosive compaction; this was particularly pronounced at the highest volume fractions of NiAl. The lowest microhardness of the Ni matrix was observed in the central portion of a cylindrical specimen. Other parts of the matrix were heavily cold-worked, indicating that a recrystallization had occurred in the centre. NiAl particles were also highly cold-worked regardless of their volume fraction in the composite. The Ni/NiAl interfacial bond strength, measured by the indentation debonding technique, was highest in a composite containing 57 vol% of NiAl particulate.

Environmental sensitivity and mechanical behavior of boron-doped Fe–45at.%Al intermetallic in the temperature range from 77 to 1000 K

Abstract The tensile properties and fracture behavior of a coarse-grained (grain size ∼420 μm) Fe–45at.%Al intermetallic doped with 0.05 at.% boron were examined at ambient temperature in air, argon and vacuum as well as in the 77–1000 K temperature range in liquid nitrogen, dry ice and air. Before testing the alloy was low temperature annealed (vacancy annealed) in order to remove all the retained vacancies. At ambient temperature ductility increases accordingly to decreasing water vapor (moisture) content in each environment. The mixed transgranular cleavage (TGC)+intergranular failure (IGF) mode in vacuum, which is associated with the highest elongation (∼6%), exhibits around 40% of IGF and the mixed fracture mode in argon, which is associated with the second highest elongation (∼3.2%), exhibits ∼15% of IGF. The TGC fracture mode in air is associated with the lowest elongation (∼1%). Elongation in the cryogenic temperature range from 77 to 213 K is very low being in the range from 0.6 to 2.8%, and is associated with a mixed transgranular+intergranular fracture mode. Elongation increases gradually from 300 to 800 K attaining a ductility peak at 800 K and then decreases rapidly with increasing temperature. At 600–800 K, the yield strength of Fe–45Al–0.05B exhibits anomalous temperature dependence with the yield strength peak at 800 K. The mode of fracture from 300 to 700 K is predominantly TGC and that at the ductility peak is ductile rupture with very deep dimples. At temperatures above 800 K the mode of fracture changes to a typical intergranular creep (fibrous) failure with numerous flat dimples (voids/cavities) at the grain boundary facets, which is associated with a tensile ductility drop. Fine particles (borides) are observed at the grain boundary facets, which assist the development of intergranular creep fracture.

Formation of Amorphous and Nanostructural Powder Particles from Amorphous Metallic Glass Ribbons Using Ball Milling and Electrical Discharge Milling

In this paper both electric discharge assisted milling [1, 2] and conventional mechanosynthesis techniques were applied to investigate the effects of milling conditions on the fracture and agglomeration of amorphous CoSiB ribbons produced by planar flow casting. The effect of spark energy on particle shape and size produced by discharge milling was studied. Conventional milling in inert atmosphere for extended periods generally leads to the formation of porous powder particle aggregates, each particle comprised of small amorphous or, after extended milling times, nanocrystalline elements. The mechanism of agglomeration was believed to originate from repeated fracture, deformation and cold welding of individual ribbon elements. In contrast to conventional milling, spark discharge milling was found to induce the formation of predominantly sub-micron single particles of amorphous powder. The morphology of individual particles varied from sub-micron irregular shaped particles to remelted particles, depending on selection of vibrational amplitude during discharge. For high vibrational amplitudes and high energy input a wider range of particles as produced. These included sub-micron particles, remelted particles and welded agglomerates, and nano-sized particles produced as a fume and collected during discharge milling under flowing argon. These results combined with observations that most re-melted particles produced by discharge milling were also amorphous confirmed that extremely high heating and cooling rates are associated with discharge milling of metals. They also confirm the potential of electrical discharge milling as a new route for the synthesis of ultrafine and nanosized powder particles from amorphous ribbon, for possible processing into 3-D shapes.

Synthesis of Fe-Al-Ti Based Intermetallics with the Use of Laser Engineered Net Shaping (LENS)

The Laser Engineered Net Shaping (LENS) technique was combined with direct synthesis to fabricate L21-ordered Fe-Al-Ti based intermetallic alloys. It was found that ternary Fe-Al-Ti alloys can be synthesized using the LENS technique from a feedstock composed of a pre-alloyed Fe-Al powder and elemental Ti powder. The obtained average compositions of the ternary alloys after the laser deposition and subsequent annealing were quite close to the nominal compositions, but the distributions of the elements in the annealed samples recorded over a large area were inhomogeneous. No traces of pure Ti were observed in the deposited alloys. Macroscopic cracking and porosity were observed in all investigated alloys. The amount of porosity in the samples was less than 1.2 vol. %. It seems that the porosity originates from the porous pre-alloyed Fe-Al powders. Single-phase (L21), two-phase (L21-C14) and multiphase (L21-A2-C14) Fe-Al-Ti intermetallic alloys were obtained from the direct laser synthesis and annealing process. The most prominent feature of the ternary Fe-Al-Ti intermetallics synthesized by the LENS method is their fine-grained structure. The grain size is in the range of 3–5 μm, indicating grain refinement effect through the highly rapid cooling of the LENS process. The Fe-Al-Ti alloys synthesized by LENS and annealed at 1000 °C in the single-phase B2 region were prone to an essential grain growth. In contrast, the alloys annealed at 1000 °C in the two-phase L21-C14 region exhibited almost constant grain size values after the high-temperature annealing.

Applications of cluster analysis in the quantitative estimation of similarities in geometrical characteristics of grains in polycrystalline materials

Abstract The article presents an application of the analysis of clusters for the quantitative evaluation of similarities in geometrical features of grains in polycrystalline aluminum. The cluster analysis is a method that can be used to analyze the results of measurements of a number of parameters specifying properties of given microstructural features. It is particularly attractive if employed to analyze the data acquired using systems for automatic image processing. The method enables thorough analysis of the shape of grains described by various shape factors. Possible other applications of the cluster analysis include studies of grain growth and phase transformations.

Hydriding properties of Mg- -Al- -Zn quasicrystal powder produced by mechanical alloying

Abstract The stable quasicrystal belonging to the Bergman class based on Mg–Al–Zn (Mg44Al15Zn41) was prepared by the mechanical alloying of elemental powders. The phase structure, chemical composition and hydriding properties of the obtained quasicrystal were investigated by XRD, SEM, EDS, DSC and the volumetric Sievert method. Our results have shown that the Mg44Al15Zn41 quasicrystal is unstable while hydriding above 200 °C and decomposes irreversibly into different Mg–Zn based intermetallic compounds. While being hydrided at 200 °C, where the quasicrystal is stable, Mg44Al15Zn41 decomposes mainly into the MgZn2 based intermetallic compound with MgH2 but above 300 °C, where the 2/1 approximant is stable, Mg44Al15Zn41 transforms mainly into the Frank-Kasper phase with MgH2.