Colleen Bettles

Creep-resistant magnesium alloy

A die castable magnesium based alloy comprising, by weight, between about 3 and 10% aluminum, between about 0.5 and 2.5% calcium, up to about 1.5% silicon, up to about 0.7% zinc, and the remainder being magnesium. The alloy has been found to exhibit more favorable castability and creep resistance than comparative magnesium based alloys.

Refinement of precipitate distributions in an age-hardenable Mg–Sn alloy through microalloying

It has been known for some time that trace additions (<∼0.1 at.%) of selected elements to some age-hardenable alloys can have a disproportionate effect on the hardening response. This is now known to be related to the formation of heterogeneities (clusters or precipitates) on the matrix lattice at which the nucleation of hardening precipitates may be enhanced leading to a refinement in the particle distribution. In this work, qualitative thermo-kinetic criteria for choosing microalloying additions for precipitation-hardenable alloys are outlined and applied to a model Mg–Sn alloy. Additions of Na and Inu2009+u2009Li are chosen, and an acceleration and enhancement of the ageing process is observed for both sets of additions at 200°C. The microstructures are examined using transmission electron microscopy and the microalloying additions are shown not to affect the identity or crystallography of the Mg2Sn phase formed, although a substantial refinement of the distributions is observed. In the case of Na additions, the number density of particles is increased by two orders of magnitude, resulting in hardening increment increases of 270%. The Inu2009+u2009Li additions lead to increases of approximately one order of magnitude in number density and 150% increases in the hardening increment. The generality of the observed effects and the resulting hardening responses are discussed.

Effect of recrystallisation on Al3Zr dispersoid behaviour in thick plates of aluminium alloy AA7150

The ageing responses of surface/centre layer materials from a commercially produced AA7150 thick plate have been studied after both water quenching (WQ) and air cooling (AC). The results show that the higher degree of recrystallisation near the plate surface decreases its age hardening response in AC condition. Quench-induced phases have been found to precipitate preferentially on Al3Zr dispersoids in recrystallised grains. Microstructural observations show that Al3Zr dispersoids in recrystallised grains exhibit the same crystal structure and orientation as those in adjacent subgrains.

Microwave Sintering of Titanium

Effective sintering of titanium requires the use of a high sintering temperature (≥1200 °C), preferably in high vacuum (< 10-2 Pa). This confines the heating and cooling rates to ~ 4 °C/min because of the limited thermal shock resistance of ceramic tube furnaces. Consequently, it leads to lengthy sintering cycles (10-12 hr). This work presents an assessment of microwave (MW) sintering of titanium. Titanium powders in the size ranges of <20 µm, 45-63 µm, and 100-150 µm were used to make green samples with compaction pressures ranging from 200-800 MPa. Sintering was carried out at 1200 °C for 2 hr in a 3 kW MW furnace with a 2.45 GHz multimode cavity under a vacuum of 2-6×10-3 Pa. The characteristics of MW heating of green titanium samples in vacuum are described in terms of the heating rate, vacuum fluctuations, and sparking discharge. The actual MW heating rate achieved from 350 °C to 1200 °C was 34 °C/min. The attendant densities are comparable to those attained by conventional vacuum sintering. Cross-sectional examinations revealed a fairly uniform pore distribution in MW-sintered samples made from either the coarse or fine titanium powder.

Effects of Impurity Elements on Green Strength of Powder Compacts

The green strength of a powder compact results from the mechanical interlocking of the irregularities on the particle surfaces. During compaction, particle rearrangement, plastic deformation and particularly surface deformation of powders occur. Titanium powder is susceptible to interstitial element contamination, which may lead to solid solution strengthening of the particles and/or the formation of non-metallic compounds on the surface. However, the influence of these various impurities, namely oxygen and nitrogen, on the green strength has not been investigated. This work investigates and quantitatively evaluates the factors influencing the green strength of the powder compacts. The indirect tensile test was applied for the determination of the green strength of the powder compacts, and test results were compared to that of a more conventional 3-point bending test. The substantial dependence of green strength on both the amount of impurity element in the core of the powder particles and the compaction pressure is demonstrated. The effect of the surface condition of the powder particles on green strength is also reported.

A uniaxial tensile stage with tracking capabilities for micro X-ray diffraction applications

First results are presented for a uniaxial tensile stage designed to operate on a scanning micro X-ray diffraction synchrotron beamline. The new tensile stage allows experiments at typical loading cycles used in standard engineering stress–strain tests. Several key features have been implemented to support in situ loading experiments at the intragranular length scale. The physical size and weight of the load cell were minimized to maintain the correct working distance for the X-ray focusing optics and to avoid overloading the high-resolution raster scan translation stages. A high-magnification optical microscope and image correlation code were implemented to enable automated online tracking capabilities during macroscopic elongation of the sample. Preliminary in situ tensile loading experiments conducted on beamline 12.3.2 at the Advanced Light Source using a polycrystalline commercial-purity Ti test piece showed that the elastic–plastic response of individual grains could be measured with submicrometre spatial resolution. The experiments highlight the unique instrumentation capabilities of the tensile stage for direct measurement of deviatoric strain and observation of dislocation patterning on an intragranular length scale as a function of applied load.

The Ageing Behavior of Titanium Alloy Ti-10V-2Fe-3Al

A good combination of high strength and hardenability makes the alloy Ti 10V-2Fe-3Al a prime candidate for applications in the aerospace arena. However, these properties are very dependent on a post-forming heat treatment. The overall objective of this work is to determine the effect of prior deformation on the aging behaviour. In this particular study, the influence of the heat treatment, either solution and/or aging, on the microstructures, and consequently on the mechanical properties, without introduced strain is reported. Various solution heat treatments have been conducted, either in the β phase or in the (α+β) phase field, followed by rapid quenching or slow cooling, and aging treatments at different temperatures (250, 350, 400, 500C°) above and below the ω-transus temperature. Vickers hardness indentations were used to follow the precipitation hardening behaviour, and mechanical properties were determined using a shear punch test. The aging response is dependent not only on the presence of the athermal ω phase but also on the proximity of the aging temperature to the ω-transus. Most treatments showed an unusual initial softening behaviour prior to age hardening, however this appears to be related again to the composition and fraction of the β phase retained after solution treatment.

Development of Magnesium-Rare Earth Die-Casting Alloys

An overview of the development of a high-performance Mg–RE based alloy, HP2+, is presented, which has a good combination of die-castability and mechanical properties at ambient and elevated temperatures. The original alloy, HP2, was a die-casing version of the sand-cast alloy SC1 developed for powertrain applications. However, HP2 tended to crack substantially, leading to unusable castings due to its high Nd content. It was found that the solidification path of Mg–RE alloys can be engineered to reduce the propensity to hot tearing by changing the mixture of RE elements towards La-rich, which leads to an increase in the amount of eutectic and a reduction of the solidification range. Precipitate-forming RE elements, such as Nd or Y, were optimized for HP2+ to meet the requirement for high temperature creep resistance. Whilst some challenges remain with the commercial application of HP2+, the learnings from the alloy design process can be applied to other alloy development programs.

On the Role of C Addition on α Precipitation in a β Titanium Alloy

The onset of α nucleation in a carbon containing β-titanium alloy has been investigated by coupling atom probe tomography (APT) with transmission electron microscopy. The analysis of the APT results indicates that in addition to ω precipitates that can act as potential α nucleation sites, carbon atoms tend to form clusters within the β-matrix, which in turn give rise to additional nucleation sites for α, resulting in finer scale α precipitates due to increased nucleation density.

Evolution of Twin-Related Variants of α Phase in a Ti-V-Cu Alloy

The precipitation of a-phase has been investigated in a concentrated b-alloy of the Ti-V-Cu system. a-precipitates in geometrically coupled forms were developed in the alloy when subject to isothermal ageing at 500°C. High-resolution transmission electron microscopy (HRTEM) revealed that a-phase embryos tend to nucleate in a symmetrical manner directly from an early-stage solute-partitioned diffusional product. The a-precipitates so developed constitute twin-related variants characterized by a twin plane lying on one of the {0111}a planes. The results are discussed with respect to the role of Cu on the formation of heterogeneous nucleation sites for a-phase.