Production of nanograined intermetallics using high-pressure torsion

Abstract

Formation of intermetallics is generally feasible at high temperatures when the lattice diffusion is fast enough to form the ordered phases. This study shows that nanograined intermetallics are formed at a low temperature as 573 K in Al-25 mol% Ni, Al-50 mol.% Ni and Al-50 mol% Ti powder mixtures through powder consolidation using high-pressure torsion (HPT). For the three compositions, the hardness gradually increases with straining but saturates to the levels as high as 550-920 Hv. In addition to the high hardness, the TiAl material exhibits high yield strength as ~3 GPa with good ductility as ~23%, when they are examined by micropillar compression tests. X-ray diffraction analysis and high-resolution transmission electron microscopy reveal that the significant increase in hardness and strength is due to the formation of nanograined intermetallics such as Al 3 Ni, Al 3 Ni 2 , TiAl 3 , TiAl 2 and TiAl with average grain sizes of 20-40 nm.

Effect of temperature on solid-state formation of bulk nanograined intermetallic Al3Ni during high-pressure torsion

“…Figure 6 shows dark-field images and corresponding SAED patterns for the samples processed by HPT for N = 3, 10, 25 and 50 at RT, 373, 473 and 573 K. It is noted that the dark-field images were taken by the diffracted beams indicated by the arrows in the SAED patterns, which include both diffractions for the elemental constituent and the superlattice diffractions for the Al 3 Ni intermetallics. Grain sizes are at the submicrometre or at the micrometre levels when processed at the early stages of straining (N = 3 and 10), but the grain sizes are reduced with increasing the number of revolutions and reach the nanometre level after processing for N = 50 at RT and 373 K, and after processing for N = 25 at 473 and 573 K. As shown in Figure 7, the steady-state grain size of 30-40 nm is attained irrespective of the HPT processing temperature, but the grain size at the early stages of straining increases significantly at 573 K. The results clearly show that it is possible to produce nanograined Al 3 Ni intermetallics through the application of HPT to powder mixtures of Al-Ni as in the earlier reports on the formation of other intermetallics in the Al-Ni-Ti systems [24][25][26][27].…”

“…Recently, a fabrication process of intermetallics from their elemental constituent powders was developed using HPT [24][25][26][27]. It was shown that the application of HPT to powder mixtures of Al-Ni, Ti-Al and Ni-Al-Ti at 573 K can result in the formation of nanograined intermetallics with the average grain sizes of 11-28 nm.…”

Hydrostatic Compression Behavior and High-Pressure Stabilized β-Phase in γ-Based Titanium Aluminide Intermetallics

“…In the present study, we investigate the influence of pressure on the formation of the ductile β-phase in γ-based TiAl alloys, which is not only of fundamental interest, but is also most relevant to modelling high-pressure deformation techniques, such as high-pressure torsion, in order to achieve severe plastic deformation [11][12][13][14][15] and high-pressure near-net-shape forging [16][17][18][19]. These deformation processes operate at pressures up to 7 GPa and forces exceeding 1 GN, respectively.…”

The Strength–Grain Size Relationship in Ultrafine-Grained Metals

“…1a. Effect of grain size on superplastic strain rate in Al alloys; [7] datum points for ECAP [11][12][13][14][15][16][17][18][19][20] are in black and for HPT [21][22][23][24][25][26][27][28][29] in red; the encircling ovals are in blue for ECAP and pink for HPT and the solid line corresponds to eq. (2) in the text.…”

“…[7,10] The experimental results shown in Fig. 1[a] are taken from various reports for Al alloys, [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] and in Fig. 1 [b] there are similar sets of data for a range of Mg alloys [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] where the upper solid lines in Figs 1[a] and [b] show the predicted normalized strain rates derived using eq.…”