arXiv subjects

Julian M. Rosalie

Publications and source records attributed to Julian M. Rosalie.

17 recordsLinked to original sources

arXiv2026

Precipitate size evolution in an ultrafine-grained magnesium-manganese alloy

Precipitate size evolution during room temperature high-pressure torsion (HPT) of a Mg-1.35wt.%Mn alloy was studied using scanning transmission electron microscopy (STEM) and Small-/Wide-angle X-ray scattering (SAXS/WAXS). The volume fraction of the nm-scale $α$-Mn particles increased with applied strain, however small angle X-ray scattering (SAXS) indicated that the majority of manganese remained in solution even after 10 HPT rotations, indicating that the reaction progress is still limited by the diffusivity of Mn. Analysis of the precipitate size distribution determined that the mean particle size did not increase over the course of HPT. This, in combination with the precipitate size distribution suggested that precipitate growth was subject to interfacial rather than diffusional control.

cond-mat.mtrl-sci↗

arXiv2025

Dynamic precipitation during high-pressure torsion of a magnesium-manganese alloy

An ultrafine grained magnesium alloy has been produced through room temperature high-pressure torsion (HPT) of solutionised Mg-1.35 wt.%Mn. Dynamic precipitation of nanometer-scale Mn particles occurred during deformation. These particles populated the grain boundaries, acting as pinning sites which allowed the alloy to develop a grain size of 140 nm after 0.5 rotations. Further HPT deformation resulted in a gradual increase in grain size with no increase in precipitate size. Despite the extensive deformation applied, the alloy did not develop a bimodal grain structure and retained a grain size of 230 nm after 10 complete rotations, demonstrating the stability and effectiveness of these pinning particles.

cond-mat.mtrl-sci↗

arXiv2023

The human factor: results of a small-angle scattering data analysis Round Robin

A Round Robin study has been carried out to estimate the impact of the human element in small-angle scattering data analysis. Four corrected datasets were provided to participants ready for analysis. All datasets were measured on samples containing spherical scatterers, with two datasets in dilute dispersions, and two from powders. Most of the 46 participants correctly identified the number of populations in the dilute dispersions, with half of the population mean entries within 1.5% and half of the population width entries within 40%, respectively. Due to the added complexity of the structure factor, much fewer people submitted answers on the powder datasets. For those that did, half of the entries for the means and widths were within 44% and 86% respectively. This Round Robin experiment highlights several causes for the discrepancies, for which solutions are proposed.

physics.data-an↗

arXiv2021

Spinodal decomposition stabilizes plastic flow in a nanocrystalline Cu-Ti alloy

A combination of high strength and reasonable ductility has been achieved in a copper-1.7 at.%titanium alloy deformed by high-pressure torsion. Grain refinement and a spinodal microstructure provided a hardness of 254 +/- 2 HV , yield strength of 800 MPa and elongation of 10%. The spinodal structure persisted during isothermal ageing, further increasing the yield strength to 890MPa while retaining an elongation of 7%. This work demonstrates the potential for spinodal microstructures to overcome the difficulties in retaining ductility in ultra-fine grained or nanocrystalline alloys, especially upon post-deformation heating where strain softening normally results in brittle behavior.

cond-mat.mtrl-sci↗

arXiv2018

Microstructural and texture evolution of Copper-(Chromium, Molybdenum, Tungsten) composites deformed by high-pressure-torsion

Cu-refractory metal composites containing Cr, Mo or W were subjected to severe plastic deformation using room temperature high-pressure torsion (HPT). A lamellar microstructure developed in each of the composites at equivalent strains of ~75. The refractory metals developed {hkl}<111> fibre textures with a slight tilt to the tangential direction. This texture was stronger and more clearly defined in Mo and W than in Cr. By applying additional HPT deformation to these samples, perpendicular to the original shear strain, it was found that the lamellar structure of Cu30Mo70 and Cu20W80 (wt.%) composites could be retained at high equivalent strains and the refractory layer thickness could be reduced to 20-50 nm in Cu20W80 and 10-20 nm in Cu30Mo70. Although neighbouring regions of the microstucture were aligned and there was evidence of local texture in both composites, the bulk texture of Cu30Mo70 became weaker during this second step of HPT deformation. This was attributed to the refractory metal lamellae being discontinuous and imperfectly aligned. This work shows that it is possible to form ultrafine composites of Cu-group VI refractory metals via high-pressure torsion, with namolamellar structures being possible where there is a sufficient volume fraction of Mo or W.

cond-mat.mtrl-sci↗

arXiv2017

The bi-layered precipitate phase zeta in the Al-Ag alloy system

The Al-Ag system is thought to be a well-understood model system used to study diffusional phase transformations in alloys. Here we report the existence of a new precipitate phase, zeta, in this classical system using scanning transmission electron microscopy (STEM). The zeta phase has a modulated structure composed of alternating bilayers enriched in Al or Ag. Our in situ annealing experiments reveal that the zeta phase is an intermediate precipitate phase between GP zones epsilon and gamma prime. First-principles calculations show that z is a local energy minimum state formed during Ag clustering in Al. The layered structure of zeta is analogous to the well-known Ag segregation at the precipitate-matrix interfaces when Ag is micro- alloyed in various aluminium alloys.

cond-mat.mtrl-sci↗

arXiv2017

On nanostructured molybdenum-copper composites produced by high pressure torsion

Nano-structured molybdenum-copper composites have been produced through severe plastic deformation of liquid-metal infiltrated Cu30Mo70 and Cu50Mo50 (wt.%) starting materials. Processing was carried out using high pressure torsion at room temperature with no subsequent sintering treatment, producing a porosity-free, ultrafine grained composite. Extensive deformation of the Cu50Mo50 composite via two-step high-pressure torsion produced equiaxed nanoscale grains of Mo and Cu with a grain size of 10-15 nm. Identical treatment of Cu30Mo70 produced a ultrafine, lamellar structure, comprised of Cu and Mo layers with thicknesses of ~5 nm and ~10-20 nm, respectively and an interlamellar spacing of 9 nm. This microstructure differs substantially from that of HPT-deformed Cu-Cr and Cu-W composites, in which the lamellar microstructure breaks down at high strains. The ultrafine-grained structure and absence of porosity resulted in composites with Vickers hardness values of 600 for Cu30Mo70 and 475 for Cu50Mo50. The ability to produce Cu30Mo70 nanocomposites with a combination of high strength, and a fine, oriented microstructure should be of interest for thermoelectric applications.

physics.app-ph↗

arXiv2014

Characterisation of precipitates formed in high-pressure torsion treated Mg-3.4at.%Zn alloy

Microstructural analysis of a Mg-Zn alloy deformed at room temperature by high-pressure torsion (HPT) indicates that fine-scale precipitation occurs even without post-deformation heat treatment. Small-angle X-ray scattering detects precipitates with radii between 2.5-20 nm after one rotation, with little increase in particle size or volume fraction after 20 rotations. High resolution electron micrographs identify grain boundary precipitates of monoclinic Mg$_4$Zn$_7$ phase after three rotations and MgZn$_2$ after 20 rotations.

cond-mat.mtrl-sci↗

arXiv2014

On chemical order and interfacial segregation in $γ^\prime$ (AlAg$_2$) precipitates

A detailed study has been carried out on $γ^\prime$ (AlAg$_2$) precipitates in Al-Ag and Al-Ag-Cu alloys to reconcile the conflicting reports on chemical ordering and stacking faults in this phase. High angle annular dark field scanning transmission electron microscopy and convergent beam electron diffraction show no indication of chemical ordering on alternate basal planes of $γ^\prime$ precipitates in alloys aged at 473 K for 2-23 h. Precipitates were visible as Ag-rich regions with 1-13 fcc$\rightarrow$hcp stacking faults, corresponding to $γ^\prime$ platelets with thicknesses ranging from 0.69-6.44 nm. There were no systematically absent thicknesses. Growth ledges with a riser height equal to the $c$-lattice parameter (0.46 nm) were directly observed for the first time. Genuine stacking faults within the precipitates were extremely rare and only observed in thicker precipitates. In precipitates with 1-3 stacking faults there was also substantial Ag in the surrounding fcc layers of the matrix, indicating that Ag strongly segregated to the broad, planar precipitate-matrix interfaces. This segregation is responsible for previous reports of stacking faults in $γ^\prime$ precipitates. The results indicate that the early stages of $γ^\prime$ precipitate growth are interfacially controlled.

cond-mat.mtrl-sci↗

arXiv2013

Form-free size distributions from complementary stereological TEM/SAXS on precipitates in a Mg-Zn alloy

This work describes a multidisciplinary research methodology for quantifying the size distribution of nanoscale precipitates in polycrystalline alloys. Complementary transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS) are employed in a study of precipitate growth in an isothermally aged Mg-Zn alloy. TEM is used to identify the precipitate phases as rod-shaped β' particles and to determine their radii and aspect ratio. Subsequently, SAXS data obtained from bulk quantities of the alloy is interpreted via a novel Monte-Carlo method to obtain accurate, form-free size distributions. Good agreement was obtained between particle radii distributions measured by both methods, exemplifying the applicability of this complementary methodology to study precipitation in textured alloys containing particles anisotropic with well-defined orientation-relationships to the matrix.

cond-mat.mtrl-sci↗

arXiv2012

The effect of precipitation on strength and ductility in a Mg-Zn-Y alloy

The effect of pre-ageing deformation on the size and distribution of beta-prime precipitates and subsequently on the resulting strength and ductility have been measured in a Mg-3.0at.%Zn-0.5at.%Y alloy. The alloy was extruded and then subjected to a T8 heat treatment comprised of a solution-treatment, cold-work and artificial ageing. Extrusion was used to introduce texture, ensuring that deformation occurred via slip rather than twinning. Samples were subjected to controlled uniaxial deformation and then isothermally aged to peak hardness. Precipitate length, diameter and number density were measured and evaluated in terms of the strength and ductility of the alloy. The nucleation of the beta-prime precipitates in peak-aged condition without pre-ageing deformation (i.e.T6 treatment) was poor, with only 0.5% volume fraction, compared to approximately 3.5% in T6 treated binary Mg-3.0at.%Zn alloy. The microstructure of the Mg-Zn-Y alloy was less refined, with larger diameter precipitates and lower beta-prime number densities compared to a binary Mg-3.0at.%Zn alloy. Deformation to 5% plastic strain increased the volume fraction of beta-prime precipitates to approximately 2.3% and refined the beta-prime precipitate length and diameter. The combination of these effects increased the yield strength after isothermal ageing from 217MPa (0% cold-work) to 287 MPa (5% cold-work). The yield stress increased linearly with reciprocal interparticle spacing on the basal and prismatic planes and the alloy showed similar strengthening against basal slip to Mg-Zn. The elongation increased linearly with particle spacing. The ductility of Mg-Zn-Y alloys was similar to that of Mg-Zn for equivalently spaced particles.

cond-mat.mtrl-sci↗

arXiv2012

The effect of size and distribution of rod-shaped β' precipitates on the strength and ductility of a Mg-Zn alloy

We report on a quantitative investigation into the effect of size and distribution of rod-shaped β' precipitates on strength and ductility of a Mg-Zn alloy. Despite precipitation strengthening being crucial for the practical application of magnesium alloys this study represents the first systematic examination of the effect of controlled deformation on the precipitate size distribution and the resulting strength and ductility of a magnesium alloy. Pre-ageing deformation was used to obtain various distributions of rod-shaped β' precipitates through heterogeneous nucleation. Alloys were extruded to obtain a texture so as to avoid formation of twins and thus to ensure that dislocations were the primary nucleation site. Pre-ageing strain refined precipitate length and diameter, with average length reduced from 440 nm to 60 nm and diameter from 14 nm to 9 nm. Interparticle spacings were measured from micrographs and indicated some inhomogeneity in the precipitate distribution. The yield stress of the alloy increased from 273 MPa to 309 MPa. The yield stress increased linearly as a function of reciprocal interparticle spacing, but at a lower rate than predicted for Orowan strengthening. Pre-ageing deformation also resulted in a significant loss of ductility (from 17% to 6% elongation). Both true strain at failure and uniform elongation showed a linear relationship with particle spacing, in agreement with models for the accumulation of dislocations around non-deforming obstacles. Samples subjected to 3% pre-ageing deformation showed a substantially increased ageing response compared to non-deformed material; however, additional deformation (to 5% strain) resulted in only modest changes in precipitate distribution and mechanical properties.

cond-mat.mtrl-sci↗

arXiv2012

Structural relationships among monoclinic and hexagonal phases and transition structures in Mg-Zn-Y alloys

Isothermal ageing of plastically deformed Mg-Zn-Y alloys resulted in precipitation along {10-12} twin boundaries. The bulky precipitates formed had structures similar to those recently reported for the rod-like β' precipitates, but afforded a more detailed study by high resolution TEM due to their larger size. The core of the precipitates often had the structure of monoclinic Mg4Zn7 phase, and had the orientation [0001]_Mg//[010]; {10-10}_Mg // (201) with either the matrix or the twin. On this Mg4Zn7 phase, hexagonal MgZn2 phase grew in two orientations, both with [010]_Mg4Zn7 // [11-20]_MgZn2. One of these orientations formed a known orientation relationship [0001]_Mg // [11-20]_MgZn2; {11-20}_Mg // (0001)_MgZn2 with the matrix. The part of the precipitate with the MgZn2 structure was usually in direct contact with the twin boundary. Both Mg4Zn7 and MgZn2 phases have layered structures that can be described with similar building blocks of icosahedrally coordinated atoms. The atomic positions of zinc atoms comprise the vertices of these icosahedra and form "thick" rhombic tiles. Orientation of these rhombuses remain unchanged across the interfaces between the two phases. Near the interface with MgZn2, transition structures formed in Mg4Zn7 phase, with Zn:Mg atom ratio between those of Mg4Zn7 and MgZn2 phases. In these transition structures, the unit cell of Mg4Zn7 phase is extended along [100] or [001] by half a unit cell length by continuation of the rhombic tiling. Structures of these extended unit cells are proposed.

cond-mat.mtrl-sci↗

arXiv2012

Orientation relationships between icosahedral clusters in hexagonal MgZn2 and monoclinic Mg4Zn7 phases in Mg-Zn(-Y) alloys

Intermetallic precipitates formed in heat-treated and aged Mg-Zn and Mg-Zn-Y alloys have been investigated via electron microscopy. Coarse spheroidal precipitates formed on deformation twin boundaries contained domains belonging to either the MgZn2 hexagonal Laves phase or the monoclinic Mg4Zn7 phase. Both phases are structurally related to the quasi-crystalline phase formed in Mg-Zn-Y alloys, containing icosahedrally coordinated zinc atoms arranged as a series of broad rhombohedral units. This rhombohedral arrangement was also visible in intragranular precipitates where local regions with the structures of hexagonal MgZn2 and Mg4Zn7 were found. The orientation adopted by the MgZn2 and Mg4Zn7 phases in twin-boundary and intragranular precipitates was such that the icosahedral clusters were aligned similarly. These results highlight the close structural similarities between the precipitates of the Mg-Zn-Y alloy system.

cond-mat.mtrl-sci↗

arXiv2012

Precipitate assemblies formed on dislocation loops in aluminium-silver alloys

A detailed study of the precipitation of the γ' (AlAg2) phase in undeformed aluminium-silver alloys has been conducted. Several previously unreported features were observed, including the formation of discrete three-dimensional assemblies comprised of 5-7 precipitates on faulted dislocation loops. The precipitates assemblies adopted a morphology approximating a tetrahedral bipyramidal assembly. The bounding defect of the stacking fault was found to control both the nucleation of additional precipitates which was responsible for the formation of the assembly structure and also the growth characteristics of individual precipitates with these assemblies.

cond-mat.mtrl-sci↗

arXiv2012

Precipitate assemblies formed on dislocation loops in aluminium-silver-copper alloys

The precipitation microstructure of the γ' (AlAg2) intermetallic phase has been examined in aluminium-silver-copper alloys. The microstructure developed in an Al-0.90at.%Ag-0.90at.%Cu alloy was significantly different from that reported for binary Al-Ag alloys. The orientation relationship between the matrix and precipitate was unchanged; however, the γ' phase formed as aggregates with a two-dimensional open assemblies. Each such assembly contained two variants of the γ' phase alternately arranged to form a faceted elliptical unit. The θ' (Al2Cu) phase formed on these assemblies after further ageing. The faceted elliptical assembly morphology has not been previously reported for the γ' precipitate. The change in precipitation behaviour was attributed to copper modifying the as-quenched defect structure of the matrix. This precipitation morphology clarifies earlier observations on the precipitate number density and mechanical properties of aluminium-silver-copper alloys.

cond-mat.mtrl-sci↗

arXiv2012

Nucleation and growth of the γ' (AlAg2) precipitate in Al-Ag(-Cu) alloys

Precipitation of the γ' (AlAg2) phase was investigated in Al-Ag(-Cu) alloys using high resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of silver to stacking faults, followed by thickening in steps corresponding to single unit cell ledges. In conjunction with gradual segregation of silver and aluminium into ordered layers this yielded γ' phase platelets with a thickness of either 2 or 3x the AlAg2 c-lattice parameter. Plates with a thickness of 2 c(AlAg2) would not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable silver segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.

cond-mat.mtrl-sci↗