Brian McKay

TEM investigation of heterogeneous nucleation mechanisms in Al–Si alloys

Abstract Heterogeneous nucleation was studied in an Al 80 Ni 10 Cu 8 Si 2 glass containing hexagonal TiB 2 particles within the glassy matrix. Distinct orientation relationships were found for different phases on the TiB 2 faces. On basal faces copious nucleation of α-Al occurred with the close-packed planes and directions of Al and TiB 2 parallel, while on prism faces copious nucleation of tetragonal Al 2 Cu was observed. Holding for 30 min at 1173 K prior to melt-spinning resulted in the formation of TiSi 2 on the basal faces instead of Al. The implications of the observed nucleation mechanism are discussed with respect to industrial casting practice, in particular to Si poisoning.

Heterogeneous nucleation in an Al-Ni-Si alloy studied using a metallic glass technique

Abstract Heterogeneous nucleation of α-Al and other phases was investigated in an Al 70 Ni 13 Si 17 (in at.%) alloy, using a novel metallic glass technique. The glass was formed by melt spinning and its devitrification/crystallisation behaviour during isothermal and anisothermal anneals was investigated using differential scanning calorimetry and X-ray diffractometry. The results obtained indicated suitability of the above alloy system for the nucleation study. TiB 2 grain refiner particles were subsequently added to the melt and the melt-spun ribbons produced were examined using transmission electron microscopy. No α-Al nucleation was detected on the boride particles, while it was systematically observed on hexagonal dendrites present in the glassy matrix, which suggests that Si poisoning of the borides is occurring. The high Si content appears to suppress the formation of Al 3 Ti layers on the surface of TiB 2 particles, thus reducing their potency to nucleate α-Al.

Study of Mechanical Properties of an LM24 Composite Alloy Reinforced with Cu-CNT Nanofillers, Processed Using Ultrasonic Cavitation

This study investigates the effect of Cu-Carbon Nanotube (Cu-CNT´s) composite powders on the mechanical properties of an Al-Si9.5-Cu4-Fe1.3 wt.% (LM24) aluminium matrix composite (AMC). Carbon nanotubes (CNT’s) can exhibit exceptional mechanical properties, e.g. stiffness up to 1000 GPa and strength in the order of 100 GPa. In recent years there has been significant scientific interest in improving properties in conventional alloys, via fabricating CNT metal matrix composites in order to attempt to harness their extraordinary attributes. In this study mechanically alloyed Cu-CNTS powders were added to molten LM24. The melt was processed using ultrasonic cavitation and subsequently high pressure die casting to form as-cast tensile specimens. SEM results indicate that CNT’s can be successfully introduced into the melt using this method. Compared to the unreinforced alloy, the CNT additions resulted in an increment (~20±10 MPa) to both ultimate tensile strength and yield strength, with a corresponding decline (~1±0.5l %) in elongation. This observed increase in strengthening may be attributed to the CNT’s pinning and hindering both grain boundary and dislocation migration during applied loading. Interestingly, no significant difference in properties were found with an increase in the CNT content (from 0.05 to 0.1 wt.%) potentially indicating a saturation limit.

Manufacture of CNTs-Al Powder Precursors for Casting of CNTs-Al Matrix Composites

CNTs-Al matrix composites are considered to be promising heat dissipating materials because thermal conductivity can potentially be improved whilst their density is reduced. Although casting has many advantages in the fabrication of large, complex components, this process cannot be easily employed when manufacturing CNTs-Al composites. In order to produce CNTs-Al matrix composites by casting a CNTs-Al powder precursor was manufactured using mechanical milling and electroless plating processes. Aluminium powder with CNTs of 10 wt.% and 20 wt.% were mixed and ball milled using a horizontal mill. After milling for 3 hrs., the milled powder exhibits a flattened morphology with a band-type distribution of CNT clusters observed within the aluminium particles. Prolonged milling of up to 24 hrs. introduces an equiaxed particle shape for the milled powders with a uniform distribution of CNTs within the aluminium particles. However, as milling time increases, the CNTs become fractured by ball-to-ball collisions. There was no reaction evident between the aluminium and the CNTs at milling times up to 24 hrs. In order to improve the wettability between the CNTs-Al powder precursor and Al melt during the casting process, electroless Ni plating was performed. The processing time for the Ni-plating affects the uniformity of the coating layer. For a uniform coating condition, the average thickness of the coating was ~1.87 μm, with no evidence of gaps between the milled powders and coatings observed.

Effect of Mn on Fe containing phase formation in high purity aluminium

Abstract In this study, the effect of varying Mn additions on Fe phase formation in high purity Al and its corresponding effect on the resulting mechanical properties have been investigated. Thermodynamic simulations have shown that in the Al–Fe–Mn ternary system two intermetallic phases (namely Al6(Fe,Mn) and Al13Fe4) form. Findings indicated that a relatively high amount (>1 wt-%) of Mn was required to achieve Al6(FeMn) phase formation which was congruent with experimental results. Both the Al–Fe and Al–Fe–Mn phases observed displayed fibre/platelet type morphologies and were found to exist at α-Al grain/cell boundaries. Results indicate that the Fe phases coarsen with increasing Mn content. In the Al–Fe system the Mn addition improves yield strength (YS) and ultimate tensile strength (UTS) but reduces elongation beyond the reduction resulting from the Fe addition. The further decrease in elongation with Mn was attributed to the increase in volume fraction of the intermetallic phases.

Preliminary Investigation on the Grain Refinement Behavior of ZrB2 Particles in Mg-Al Alloys

This paper investigates the effect of ZrB2 particles on the grain refinement of an Mg-Al alloy. A comparison, with respect to grain size, of the effect of ZrB2 particles formed in situ and commercially obtained synthetic ZrB2 particles, which are subsequently added to the melt, was made. In situ ZrB2 particles were formed by reacting conventional Al-Ti-B and Al-Zr alloys. Samples were taken in accordance with the TP1 test procedure and the resulting grain size of the primary Mg measured using the intersect method. An SEM equipped with EDS was employed to elucidate the effect of the Zr. Results show that the ZrB2 successfully grain refines the Mg-Al alloy resulting in ultimate grain sizes of 100 and 60 μm for the in situ and synthetic ZrB2 particles respectively. Mg-Al alloys can be successfully grain refined using ZrB2 heterogeneous particles and the resultant effect should be beneficial in improving the mechanical properties of the alloy.

The Role of Fe on the Grain Refinement of High Purity Aluminium

The role of Fe on the grain refinement of high purity aluminium (HPAl) was investigated after adding commercial Al-5Ti-1B grain refiner rod. Experimental results show that with a 0.08% Fe addition, the grain structure at chill zone of the HPAl sample changes from coarse to fine equiaxed grains. More importantly, the grain size observed at the centre of the HPAl sample decreased from 500±50µm to 206±30µm. The improvement has been attributed to the interfacial segregation and solute concentration of the Fe at the solid-liquid interface.

Modelling the Precipitation of Al3X Dispersoids in Aluminium Alloys and their Effect on Recrystallization

A combined model is presented that predicts the non-uniform distribution of Al3X dispersoid particles in commercial aluminium alloys containing zirconium and scandium and uses these predictions as inputs to a simple recrystallization model. The recrystallization model relies on knowledge of the stored energy in the sub-structure after deformation and this has been measured using electron backscattered diffraction (EBSD) techniques. The recrystallization model is based on the concept that partial recrystallization results from the non-uniform distribution of dispersoid particles due to their precipitation from a segregated cast structure. The model has been used to devise an improved homogenization treatment for AA7050, which uses an isothermal hold during heat up to maximize dispersoid nucleation. It has also been applied to predict the effect of scandium additions on recrystallization, investigate the factors that control the through thickness variation in recrystallized fraction, and interpret the results of experiments where the effect of strain rate have been studied.