L. Zbroniec

Fracture toughness of intermetallic compacts consolidated from nanocrystalline powders

Abstract Ball-milled and fully disordered intermetallic powders of Fe–45at.%Al (iron aluminide) and titanium trialuminide (Al 3 Ti) stabilized to cubic (L1 2 ) structure by alloying with 9 at.% Mn, with nanocrystalline (nanophase) grain size in the range of ∼10 and ∼3 nm (from X-ray diffraction, XRD), respectively, were successfully consolidated into nearly pore-free bulk compacts. Fe–45at.%Al powders were consolidated only by explosive shock wave compaction and titanium trialuminide powders were consolidated by hot pressing and explosive shock wave compaction. After shock consolidation a microcrystalline structure appeared in larger powder grains of the Fe–45Al compacts. Compacts were re-ordered after hot or shock consolidation. Vickers indentation fracture toughness of compacts was investigated. Fe–45Al compacts did not develop any corner cracks up to 2000 g indentation load, indicating some intrinsic fracture resistance. Cubic titanium trialuminide compacts developed corner cracks under the indentation load and their average measured fracture toughness was barely ∼2 MPa m 0.5 , i.e. even lower than the fracture toughness of bulk specimens of coarse-grained cubic titanium trialuminides (∼4–5 MPa m 0.5 ). The results demonstrate that refining the grain size towards the nanolevel is not sufficient to beneficially modify toughness of brittle intermetallics.

Formation of nanocrystalline cubic (L12) titanium trialuminide by controlled ball milling

Abstract Pre-alloyed, as-cast ingots of the Mn-modified, cubic (L1 2 ) titanium trialuminide (65 at% Al, 25.6 at% Ti and 94 at% Mn) were homogenized (1000 °C/100), crushed into a coarse-particle powdered material and subsequently ball milled for up to 386 h under shearing mode in a controlled ball movement mill. X-ray spectra of milled powders showed line broadening and decrease in intensity of Bragg peaks with increasing milling time. This is associated with the formation of nanocrystalline grains and lattice strains upon milling. Crystallite size calculated from peak broadening, remains relatively unchanged from 19 up to 100 h of milling (20–30 nm) and then drastically decreases reaching a saturation size of about 3 nm after 200 h of milling. Lattice strains are on the average less than 1%. Simultaneously, the ordered L1 2 crystal structure undergoes disordering which commences after approximately 40 h and terminates after 160 h of milling. The microstructure of powder particles undergoes a complex evolution. With increasing milling time the particles are formed which appear to contain a work-hardened core. Each such a particle is surrounded by a heavily deformed, hard outer layer containing nanometer grains. After 386 h of milling all the core/outer layer particles are transformed into uniform ‘no core’ ones, characterized by approximately 3 nm crystallite size (XRD measurements). The microhardness data for both outer layer in the powder particles with a core, and the ‘no core’ particles can be fitted by a Hall-Petch dependence on the inverse root of crystallite size: HV 0.01 = 431.7 + 387.5 d −0.5 (kg mm −2 ) where HV 0.01 is Vickers microhardness at 0.01 kg and d is crystallite size in nm. These results are discussed in view of the existing models of hardening of nanosized materials.

Fracture toughness and yield strength of boron-doped, high (Ti+Mn) L12 titanium trialuminides

Abstract Three batches of L1 2 -base titanium trialuminide alloys, one with low Ti content: 9 Mn–25 Ti, and the other two with high Ti content: 14 Mn–29 Ti and 18 Mn–32 Ti (at.%), referred to as 9 Mn, 14 Mn and 18 Mn, respectively, some doped with 0.004 to 0.66 at.% boron, were induction melted under high purity argon, homogenized and subsequently HIP-ed at 1250°C/2 h/180 MPa. Both Vickers microhardness and compressive 0.2% offset yield strength at room temperature increase primarily with increasing Ti concentration in the L1 2 matrix and secondarily with boron doping, attaining 550 MPa. The yield strength increment by boron doping for high Ti 14 Mn is much higher (∼1.74 MPa/0.01 at.% B) than that for low Ti 9 Mn (∼0.4 MPa/0.01 at.% B). At room temperature, a combination of high Ti concentration and boron doping increases chevron-notched beam (CNB) fracture toughness to ∼8 MPa m 1/2 (a 100% increase with respect to low Ti 9 Mn alloy) but fracture mode remains transgranular cleavage regardless of the composition. At 800–1000°C the toughness increase of high Ti 14 Mn is mainly determined by a high Ti concentration. For boron-free and boron-doped high Ti 14 Mn the transgranular cleavage/intergranular failure transition temperature approaches ∼600°C and at 1000°C the fraction of intergranular failure mode is suppressed to barely ∼45%.

Observations of crack tip process zones in cubic titanium trialuminide intermetallics

Despite the fact that the single-phase L12-ordered titanium trialuminides, derived from D022-ordered Al3Ti by alloying with fourth-period transition elements such as Cr, Mn, Fe, Co, Ni, Cu, and Zn have a cubic lattice structure, their room temperature fracture toughness remains quite low (4–5 MPa m1/2). In general, process zones developed at the crack tips determine the fracture toughness of a material. In this work the results of the crack tip fracture studies of cubic (L12) Al3Ti alloys stabilized with Mn are presented. The process zones at the crack tip in nearly stoichiometric single-phase L12 9Mn–25Ti (at.%) titanium trialuminides were not observed in most of the specimens studied. Occasionally, two types of process zones were observed: either small, heavily localized process-plastic zones accompanied by a crack tip “collapse”, or “pseudo-bifurcated” ones, reminiscent of those in brittle ceramics. Observations of the crack tip process zones in multiphase, high Ti (up to ∼33 at.%), B-doped trialuminides, exhibiting increased fracture toughness (∼7 MPa m1/2), show the presence of secondary microcracks in the zone ahead of the crack tip and adjacent to the propagating crack, and more plasticity at the crack tip.