Kyoung Il Moon

A study of the microstructure of nanocrystalline Al–Ti alloys synthesized by ball milling in a hydrogen atmosphere and hot extrusion

Abstract Nanocrystalline Al–Ti alloy powders were produced by reactive ball milling (RBM) in a hydrogen atmosphere and its microstructure consisted of nano-sized Al and nano-sized TiH 2 . Thermal analysis of as-milled powders showed that the decomposition of TiH 2 and the subsequent formation of Al 3 Ti occurred at 370–480°C. The powder was consolidated by hot extrusion at 500°C. The grain size of as-extruded specimens was about 50–100 nm. The hardness of Al-5 at.%Ti specimens synthesized by RBM and subsequent hot extrusion was 25–75% higher than that of Al-8 wt.%Ti alloys produced by mechanical alloying (MA) in Ar atmosphere and hot extrusion. Room temperature and high temperature (300, 400, 500°C) tensile strength of RBM Al-5 at.%Ti alloys were superior to those of MA Al-8 wt.%Ti alloys. The strength in these alloys appeared to be related to a large extent to the very fine grain size. The ductility of RBM alloys decreased with grain refinement. It is possible that the deterioration in ductility of nanocomposite Al–Ti alloys has to be attributed to the increase of the interface area between Al and Al 3 Ti and its high energy. SEM fractograph showed that fracture progressed intergranularly.

Microstructure and mechanical properties of nanocrystalline Al–Ti alloys consolidated by plasma activated sintering

Abstract Nanocrystalline Al–Ti alloys were produced by reactive ball milling and subsequently plasma activated sintered (PAS). The consolidation behavior and microstructure of the nanocrystalline alloys were studied as a function of sintering pressure and temperature with constant holding time of 60 s. This showed that the pressure and temperature had large effects on the final density and grain size. The PAS consolidated Al–Ti alloys exhibited full density (99% of theoretical density) while retaining a grain size of the order of 50–100 nm at a temperature of 500°C with a pressure of 75 MPa. This alloy showed a much more uniform grain distribution and finer grain size compared with Al–Ti alloys manufactured by conventional consolidation methods. At room temperature, the compressive yield strength (about 692 MPa) of PAS alloy was much higher than that of hot pressed Al–Ti alloys. The higher yield strength is considered to be due to the effect of grain refinement strengthening. At all temperatures above 300°C, the PAS alloys exhibited plastic strain beyond 60%.

Consolidation behavior of nanocrystalline Al–5at.%Ti alloys synthesized by cryogenic milling

Abstract Nanocrystalline powders of aluminum with titanium addition of 5 atomic percentage (Al–5at.%Ti) were prepared by cryogenic milling (CM) at −85°C. The mean particle and average grain sizes of powders prepared by cryogenic milling were 6 μm and 16 nm, respectively and those of powders produced by room temperature milling (RM) were 19 μm and 21 nm, respectively. Since dynamic recovery was suppressed and fracture was promoted during CM, the particle and grain sizes of Al–5at.%Ti powders were effectively reduced by CM. The powders synthesized by CM were consolidated to full density by vacuum hot pressing (VHP). No serious grain growth was detected because the consolidation of nanocrystalline powders was possible at low temperature for short time. In this study, the smallest grain size, 34 nm, was observed in the specimen VHPed at 390°C for 10 min with the pressure of 500 MPa. As a result, CM powder exhibits better sinterability than RM powder revealing CM powder reached the full density at 390°C while RM powder reached the full density at 450°C on the same consolidation conditions. During the consolidation of nanocrystalline Al–5at.%Ti powder by VHP, pure Al region was formed at a triple junction, which was previously pored region, of the powder particles. The length of the pure Al region was a few μm and the grain size in this region was 100 nm. It is considered that the pure Al region was formed by relatively small Al particles with energetically enhanced surface and existing between the large particles during consolidation.

The effect of ternary addition on the formation and the thermal stability of L12 Al3Zr alloy with nanocrystalline structure by mechanical alloying

Abstract We have tried to produce nanocrystalline L1 1 Al+25 at.% Zr+12.5 at.% M (M=Cu, Ni, Mn) alloys by planetary ball milling of elemental powders. Systematic studies on ternary addition on the formation and the thermal stability of metastable L1 2 phase are also given in this study. L1 2 Al 3 Zr alloy was effectively prepared by mechanical alloying of elemental powders of Al and Zr for 3 h. In the case of ternary addition the L1 2 phase was formed after 6-h milling for Cu and 3-h milling for Ni. In Mn addition, the L1 2 phase was formed after 3 h and transformed into an amorphous phase after 6 h. The as-milled L1 2 Al 3 Zr and L1 2 Al 5 Zr 2 Cu alloy powder had nanocrystalline structures with grain sizes less than 10 nm. L1 2 Al 3 Zr alloy transformed into stable D0 23 phase at 625°C. L1 2 phase has been stabilized significantly by ternary addition up to over 900°C for Cu and 850°C for Ni, respectively. Amorphous Mn added to Al 3 Zr alloy crystallized to L1 2 phase at 770°C and the crystallized L1 2 phase was stable up to over 900°C. The nanocrystalline structure was maintained after 20-min heat treatment at 900°C.

Consolidation of nanocrystalline Al–5 at.% Ti alloy powders by ultra high-pressure hot pressing

Abstract Consolidation behavior of nanocrystalline Al–5 at.% Ti powders has been investigated by using ultra high-pressure hot pressing method. Nanocrystalline Al–5 at.% Ti compacts with full density were successfully processed by hot pressing for 250 s at 120°C under 4.8 GPa with a grain size less than 50 nm. It is considered that each grain in as-compacted materials maintained random orientation with respect to its neighboring grains. The consolidation temperature, 120°C, is about 300–400°C lower than conventional one. Abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of the absolute melting temperature of Al. Some grains grew up over 500 nm in these specimens. Rockwell hardness and Vickers micro-hardness values of the specimen prepared by the proper conditions were 105.2 HR B and 243.7 H V , respectively. This hardness value was one of the highest one ever obtained in Al–5 at.% Ti alloys.

Study of the microstructure of nanocrystalline Al-5 at.% Ti compacts prepared by reactive ball milling and ultra-high-pressure hot pressing

Abstract The consolidation behavior of nanocrystalline Al–5 at.% Ti powders was investigated using an ultra-high-pressure hot pressing method. Nanocrystalline Al–5 at.% Ti compacts with full density were successfully synthesized by hot pressing for 250 s at 120°C under 4.8 GPa and the grain size was less than 50 nm. It is considered that each grain in the as-compacted materials maintained a random orientation with respect to neighboring grains. The consolidation temperature, 120°C, is about 300–400°C lower than the conventional temperature. Abnormal grain growth was observed in specimens prepared at temperatures over 300°C, which is more than one-half the absolute melting temperature of Al. Some grains grew more than 500 nm in these specimens. Rockwell hardness and Vickers micro-hardness values of specimens prepared using the appropriate conditions were 105.2 HR B and 243.7 H V , respectively. This hardness value is one of the highest ever obtained in Al–5 at.% Ti alloys.

Compressive deformation behaviour of nanocrystalline Al-5 at.% Ti alloys prepared by reactive ball milling in H2 and ultra high-pressure hot pressing

Abstract Bulk nanocrystalline Al-5 at.% Ti alloys have been prepared by using an ultra high-pressure hot pressing (UHP-HP) method and their mechanical properties have been investigated through compression tests at room temperature and high temperatures (300, 400 and 500°C). TiH 2 and Al 3 Ti acted as effective dispersoids to prevent grain growth during the consolidation process. A full density was reached within 250 s by UHP-HP at 120°C under 4.8 GPa, in the specimen A120 and its grain size was 500 nm in this specimen. The mechanical properties and the deformation behaviours of the A120 and A300 specimens were very different in the compression tests. The compressive stress of the A120 specimen was 1010 MPa and that of the A300 specimen was 467 MPa at room temperature. However, the strength of the former decreased greatly with increasing testing temperature. While the former specimens had a nanocomposite type microstructure, the latter showed very small change in ductility and strength with temperature. The optimum properties have been obtained in the specimens prepared by UHP-HP at 300°C as they have high ductilities and high strength values at both room temperature and high temperatures.

The effect of the third-element addition on the fatigue properties of mechanically alloyed Al-Ti alloys

Abstract Al–Ti–X (X=B, V, Zr, Ce) alloys were prepared by mechanical alloying and their fatigue properties were examined at room temperature, 300°C and 400°C. MA Al–Ti alloys showed fatigue strengths competitive with those of conventional precipitation hardened Al alloys (Al 7075, Al 2024) and the addition of third elements further improved the fatigue strength of MA alloys. As the temperature was increased to 300°C and 400°C, a decrease in fatigue strength was observed but the general trend in fatigue properties was the same as that at room temperature. The fatigue strength of Al–Ti–X alloys, except for the Ce-added alloy, was enhanced compared with Al–Ti alloy because the third element addition reduced the lattice mismatch between Al and Al 3 Ti effectively and thus maintained the fine particle size of dispersoids. Al–Ti–V alloy showed the smallest precipitate size and the best room and high temperature fatigue strengths. The fatigue ratio of MA Al–Ti alloys was about 0.4 and improved slightly with temperature. The fracture mode of MA Al–Ti alloys was thought to be intergranular failures, in some cases involving also the interface between the matrix and the dispersoids.

The effect of Cu and Zn on the phase stability of L12 Al3Hf intermetallic compound synthesized by mechanical alloying

Abstract Elemental powders of Al and Hf were mechanically alloyed to produce L1 2 Al 3 Hf powder with a nanocrystalline structure. The effect of the addition of ternary elements on the mechanical alloying behavior and the thermal stability of L1 2 phase was investigated. The start and finish temperatures of the phase transformation from L1 2 to D0 23 varied as a function of time and temperature in binary and ternary alloys. L1 2 Al 3 Hf alloy was readily prepared by mechanical alloying for 6 h. It is thought that the homogeneous distribution of Al and Hf and following formation of the L1 2 phase are related with the attainment of the ultra-fine grain size of the powder. However, L1 2 phase was formed in the (Al+12.5 at.% M) 3 Hf (M=Cu, Zn) alloys after 10 hours milling. The delayed formation of L1 2 phase in the ternary alloys was related to the retardation of the microstructural evolution or grain size refinement. The start temperature of the L1 2 to D0 23 phase transformation in the binary Al 3 Hf alloy was only about 650 K, but it was increased to about 970 and 1170 K with the addition of 10 at.% Cu and 12.5 at.% Zn, respectively. Their grain size was less than 20 nm after 20 min heat treatment at each temperature. The start temperature of the phase transformation was not significantly affected by annealing time but the finish temperature of the transformation was decreased significantly with increasing annealing time. It is worth noting that the results in this study will be presented as useful data for the various consolidation processes of L1 2 nanocrystalline Al 3 Hf powder.

Tensile properties of nitride dispersed Al–Ti alloy synthesized by reactive ball milling in N2 gas

Abstract Nitride dispersed Al–Ti alloys have been prepared by reactive ball milling (RBM) of elemental powders of Al and Ti in nitrogen gas. The particle size and the grain size were effectively reduced by the hard and brittle TiN and the dissolved nitrogen atoms. The as-milled powder was consolidated by hot extrusion at 500°C. The grain size of the as-extruded specimen was about 50–100 nm, and TiN with a grain size of 10 nm existed all over the specimen. UTS of Al–10 wt.%(Ti+TiN) alloy produced by two-step RBM in N 2 and hot extrusion was lower than that of Al–5 at.%Ti alloy prepared by RBM in H 2 and hot extrusion, although the hardness of the former was higher than that of the latter. The ductilities of Al–10 wt.%(Ti+TiN) were very poor and even less than those of specimens prepared by RBM in H 2 . The dispersoids existing at the grain boundary appeared to restrict deformation through the grain boundary and promote the formation of micro-voids at the grain boundaries. Thus, fracture easily occurred intergranularly in this alloy and the ductility was poor at all tested temperatures.