Materials Science and Engineering

November 2022

DOI: https://doi.org/10.1007/s10853-022-07914-1

Sarshad Rommel, Hannah R. Leonard, Mingxuan Li, Thomas J. Watson, Tod Policandriotes, Mark Aindow

Abstract

The impact of the surface corrosion product layers on the pitting corrosion behavior of three Al–Cr–Mn–Co–Zr alloys has been studied using a combination of electron microscopy, X-ray and electrochemical techniques. These alloys contain an FCC Al matrix, different volume fractions of icosahedral quasicrystalline (I-phase) dispersoids and minority grain boundary phases. The I-phase dispersoids act as cathodic sites, and alloys with higher I-phase volume fractions have lower open circuit potentials and higher corrosion current densities before salt fog exposure. Upon ASTM salt fog exposure for 6 weeks, these trends reverse due to the development of a corrosion product bilayer. The bilayer consists of a dense outer layer with angular crystallites of bayerite, and an inner boehmite layer with a fibrillar morphology and a gradient structure with the highest density at the interface with the alloy. This bilayer was more well developed at higher I-phase volume fractions and after upset forging. The layer was found to improve the resistance of the alloys to pitting corrosion, with the pitting potentials increasing dramatically to values as high as 1432 mV (with respect to saturated calomel electrode) after the salt fog exposure. These values were significantly higher than those for the conventional high-strength Al alloys Al 6061 (- 126 mV) and Al 7075 (- 706 mV) tested under identical conditions.

November 2021

DOI: https://doi.org/10.1016/j.matchar.2021.111490

Hannah R. Leonard, Sarshad Rommel, Mingxuan Li, Matthew E. Krug, Thomas J. Watson, Tod Policandriotes, Mark Aindow

Abstract

The thermal stability and decomposition products of the quasicrystalline I-phase in a powder-processed Al-2.8Cr-1.51Mn-0.28Co-0.31Zr alloy (in at. %) have been studied using thermal analysis, electron microscopy studies on heat-treated samples, and in situ TEM heating studies. The alloy was consolidated through blind-die compaction and exhibited a nano-composite FCC Al matrix with ≈25% by volume of I-phase dispersoids. In samples heat treated ex situ at 300–450 ◦C, two transformations were observed. Initially, the I-phase decomposed by transforming to a ternary Al45(Cr,Mn)7 phase. This decomposition was accompanied by ejection of Co, which formed Al9Co2. After longer exposures, the Al45(Cr,Mn)7 transformed to an Al12(Cr,Mn) phase with a coarse irregular morphology. In situ TEM observations on sections through powder particles were used to identify the onset timesfor the initial decomposition process, and these were used together with activation energies determined by Kissinger analysis of isothermal DSC data to construct transformation curves. TEM measurements of the Co content from dispersoids in the samples heat treated ex situ were used to determine the extent of I-phase decomposition, and these data matched well with the trends expected from the curves. These observations provide a useful insight into the processing and operating conditions that might be used for such QC-reinforced Al alloys.

August 2021

DOI: https://doi.org/10.1016/j.matchar.2021.111239

Hannah R. Leonard, Sarshad Rommel, Mingxuan Li, Sriram Vijayan, Thomas J. Watson, Tod Policandriotes, Mark Aindow

Abstract

The thermal behavior of a powder-processed Al-2.5Cr-1.35Mn-0.25Co-0.31Zr alloy (in at.%) has been studied by comparing hardness and SEM data from as-consolidated and heat-treated samples of the bulk alloy with data from in situ TEM heating experiments on sections through individual particles of the as-atomized powder. The starting microstructure consisted of an Al matrix with I-phase quasicrystalline dispersoids, and the distribution of the I-phase depended on the powder particle size. Most of the powder (≈ 70% by volume) exhibited a cellular dendritic Al microstructure with minority phases at the cell boundaries and I-phase at the powder particle surfaces. Upon heat treatment, the consolidated alloy retained the initial hardness and microstructure up to about 400 ◦C. At this point the alloy hardened by about 6%, and then softened significantly at higher temperatures. The main microstructural change was the precipitation of a plate-like ternary Al11(Cr,Mn)2 phase within the cellular Al matrix. In situ TEM observations on powder particles with the cellular dendritic microstructure at temperatures of 400–450 ◦C revealed the kinetics of the precipitation for this ternary phase. The kinetic data gave Avrami exponents of approximately 3 in the nucleation and growth regime. The temperature dependence of the rate constants gave an activation energy of 277 kJ/mol, which is consistent with Cr diffusion in the Al matrix being the rate-limiting process for the precipitation of Al11(Cr,Mn)2. The onset of the precipitation at 350–400 ◦C should not restrict the application of such alloys, but it may limit the processing if the decomposition of the supersaturated solid solution is to be avoided.

December 2020

DOI: https://doi.org/10.1016/j.corsci.2020.108970

Sarshad Rommel, Hannah R. Leonard, Thomas J. Watson, Tod Policandriotes, Mark Aindow

Abstract

The corrosion behaviour of a powder-processed Al-Cr-Mn-Co-Zr alloy upon immersion in a 3.5 wt.% NaCl solution has been studied using a combination of electron microscopy and X-ray techniques. The alloy contains an FCC Al matrix, 25 vol.% of quasicrystalline I-phase dispersoids, and minority boundary phases. Corrosion proceeds by dissolution of the FCC Al matrix. This effect is enhanced at prior particle boundaries and at cathodic Al9Co2 precipitates, but even then the depth of attack is <2 μm after 1 week. A boehmite corrosion product layer forms across the surface, and this is anchored at the I-phase features inhibiting spallation.

June 2020

DOI: https://doi.org/10.1016/j.msea.2020.139487

Hannah R. Leonard, Sarshad Rommel, Mingxuan Li, Thomas J. Watson, Tod Policandriotes, Mark Aindow

Abstract

A series of three Al–Cr–Mn–Co–Zr alloys with total alloy contents of 3.58, 4.41 and 4.90 at.% has been produced by blind-die-compaction of gas-atomized nano-composite powders. The alloy microstructures consist of a face centered cubic (FCC) Al matrix with characteristic distributions of quasicrystalline I-phase dispersoids giving hardness values of up to 162 HV. The consolidated alloys were forged uniaxially to upsets of 30–90% at temperatures of 300–370 ˚C to investigate the effects of deformation processing on the microstructure and mechanical behavior. At low upsets the characteristic phase distribution was retained, whereas at high upsets a refined more homogeneous microstructure developed with a break-up of the coarser I-phase features. No I-phase decomposition was observed for any of the forging conditions studied. Tensile testing of samples cut perpendicular to the forging axis gave yield strengths of up to 480 MPa, and ultimate tensile strengths of up to 520 MPa, but have low ductilities at low upsets. At higher upsets, the samples exhibited moderately reduced strengths but much higher ductilities, with elongations to failure of up to 15%, and reductions in area of up to 50%. An analysis of the strengthening mechanisms indicates that solid solution strengthening by super-saturation of Cr and Mn in the FCC Al matrix may contribute significantly. The modest softening of the alloys at high upset thus probably corresponds to defect-mediated solute redistribution. The corresponding dramatic increase in ductility is attributed to closure of residual porosity from the consolidation process and to the elimination of weakly-bonded particle-particle interfaces that can act as sites for fracture initiation.

November 2019

DOI: https://doi.org/10.1016/j.matdes.2019.108094

Hannah R. Leonard, Sarshad Rommel, Thomas J. Watson, Tod Policandriotes, Mark Aindow

Abstract

The microstructures in gas-atomized powders and in blind-die-compacted material from three Al-Cr-Mn-Co-Zr alloys have been studied using X-ray diffraction and electron microscopy. The materials consist of a mixture of FCC Al and icosahedral quasicrystalline phases, but three different phase distributions are observed which depend on the powder particle size, and hence on the cooling rate. The finest particles contain a nano-composite mixture of equiaxed Al grains and fine (<200 nm) quasicrystalline dispersoids. Coarser particles exhibit large cellular dendritic Al grains with thin films of Co- and Cr/Mn-rich phases at the cell boundaries and coarser (up to 2 µm) quasicrystals at the particle surfaces. The largest particles contain coarse (up to 5 μm) radial quasicrystalline growths within the particle. The same microstructures were observed for each alloy, but the volume fractions of the microstructural types varied with the alloy content. These effects are explained on the basis of competing nucleation and growth phenomena during solidification of the atomized droplets. Since these microstructures are retained in the bulk material following blind-die-compaction, they could have a profound influence on the mechanical and other properties of materials made from such powders. The importance of such effects for alloy and process design are discussed.

March 2019

DOI: https://doi.org/10.1007/s10853-019-03562-0

Sriram Vijayan, Benjamin A. Bedard, Matthew A. Gleason, Hannah R. Leonard, Danielle L. Cote, Mark Aindow

Abstract

Gas atomization is the most common approach used to produce powders of metallic alloys, and the high cooling rates involved frequently lead to the formation of non-equilibrium microstructures and phases. The transformations that occur in the powders upon heating are of great interest but are challenging to study experimentally. Here we use a novel focused ion beam-based specimen preparation protocol to obtain cross sections through individual gas-atomized powder particles of three different aluminum alloys: solid solution-strengthened Al5056, precipitation-hardenable Al6061, and an Al–Cr–Mn–Co–Zr alloy which contains icosahedral quasicrystal dispersoids. In situ scanning transmission electron microscopy heating experiments were performed on these cross-sectional specimens to investigate the changes that occur in the metastable phases and non-equilibrium microstructures upon heating. The experiments reveal the details of a wide variety of thermally activated processes occurring in the particles including: solute redistribution to eliminate micro-segregation; dissolution, coarsening, transformation and decomposition of secondary phases; and precipitation within the aluminum matrix.

July 2017

DOI: https://doi.org/10.1007/s10853-017-1331-z

Yu Sun, Mark Aindow, Rainer J. Hebert, Terrence G Langdon, Enrique J. Lavernia

Abstract

High-pressure torsion (HPT) deformation of multiphase metallic systems produces a high density of interfaces and leads to atomic mixing between the constituent phases. Here we present a study of the interphase boundary structure, grain size evolution and intermetallic phase formation during HPT deformation of a nano-crystalline Al/Ti composite. High-resolution transmission electron microscopy was used to study the structural features of the interphase boundaries. The Al/Ti interphase boundaries were found to significantly promote the generation of dislocations during deformation. After HPT deformation to a shear strain of 87, the average grain sizes of Al and Ti are 22 and 31 nm, respectively. The chemical mixing between the Al and Ti phases was enhanced by defect-mediated short circuit diffusion and dislocation shuffle controlled plastic deformation at the interphase boundaries. The intermetallic phases formed during HPT deformation are associated with the strain energy stored by the high density of dislocations at the interphase boundaries.

June 2017

DOI: http://dx.doi.org/10.1016/j.corsci.2017.03.010

Thomas J. Watson, Mauricio A. Gordillo, Alexis T. Ernst, Benjamin A. Bedard, Mark Aindow

Abstract

The pitting corrosion resistance has been evaluated for a powder-processed Al-Cr-Mn-Co-Zr alloy which contains ≈35% by volume of an icosahedral quasi-crystalline phase and a little Al9Co2 in an Al matrix. ASTM standard salt fog exposure tests show that the alloy exhibits far lower corrosion pit densities and depths than commercial high-strength aerospace Al alloys under the same conditions. Electron microscopy data show that the salt fog exposure leads to the selective oxidation of the face-centered cubic Al matrix around the other phases, and to the development of a porous outer oxide scale.

October 2016

DOI: http://dx.doi.org/10.1016/j.scriptamat.2016.05.037

Thomas J. Watson, Mauricio A. Gordillo, Iuliana Cernatescu, Mark Aindow

Abstract

Nanocomposite powder particles of aluminum with dispersed icosahedral quasicrystals were produced by gas atomization from an Al-Cr-Mn-Co-Zr alloy. Bulk dispersion-strengthened material was obtained from the powder by blind-die compaction and forging. The material exhibited an attractive combination of room temperature mechanical properties with a dynamic elastic modulus of 90.5 GPa, a tensile yield strength of 690MPa with 6% elongation to failure, and a high cycle fatigue life of 109 cycles at 207 MPa applied stress. The material also exhibited significant potential for elevated temperature applications with a modulus of 75 GPa and yield strength of 400 MPa at 300 °C.

November 2015

DOI: http://dx.doi.org/10.1016/j.jallcom.2015.07.079

Yu Sun, James Haley, Kaustubh Kulkarni, Mark Aindow, Enrique J. Lavernia

Abstract

The synthesis of γ-TiAl from elemental metals via solid-state reactive diffusion processing routes involves multiple reaction steps with the formation of various intermediate intermetallic compounds, starting with TiAl3 because this phase is favored kinetically. To understand the processes by which the TiAl3 intermediate is eliminated during synthesis of γ-TiAl alloy via spark plasma sintering (SPS), the reaction between Ti and TiAl3 during SPS was studied with emphasis on the effects of the applied electric current and starting TiAl3 microstructure on the reaction kinetics and the underlying diffusion mechanisms. The intermediate intermetallic phases Ti3Al, TiAl and TiAl2 were formed between the Ti and TiAl3 upon SPS processing at 900 °C. The applied electric current did not alter the character of the phases formation in the Ti/TiAl3 system, but thermodynamic calculations suggest that the activation energy for the nucleation of TiAl2 is reduced significantly with an electric current flowing. Moreover, the kinetics of the reactions between Ti and TiAl3 were enhanced when the starting TiAl3 microstructure was refined. The electric field also had a more significant influence on the grain growth kinetics for TiAl2 and TiAl in powder blend compacts with refined microstructures.

May 2014

DOI: http://dx.doi.org/10.1007/s10853-014-8297-x

M. A. Gordillo, B. Bedard, T. J. Watson & M. Aindow

Abstract

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al–Ni–Co–Zr–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300–500 °C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22% by volume of Al19(Ni,Co)5Y3 plates surrounded by grains of FCC Al. Samples heat-treated at 350 °C and above also contain fine Al3Y and Al3Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 °C, and the Al19(Ni,Co)5Y3 plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al19(Ni,Co)5Y3 plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al19(Ni,Co)5Y3 inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.

February 2014

DOI: http://dx.doi.org/10.1007/s10853-014-8086-6

Mauricio A. Gordillo, Iuliana Cernatescu, Tai-Tsui Aindow, Thomas J. Watson, Mark Aindow

Abstract

There has been considerable interest in Al-rich Al–Mn–Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al–5Mn–2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al20Mn2Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al20Mn2Ce, and Al6Mn, with a small amount of Al12Mn and Al11Ce3. Upon heat-treatment at up to 450 °C, the Al20Mn2Ce and Al6Mn phases decompose to give a hard stable phase mixture with 72–73 % Al12Mn plus 13–14 % each of Al11Ce3 and FCC Al. Heat treatments at 500 °C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al3(Mn,Ce) phase, 9 % Al12Mn, 8 % Al6Mn, and 1 % Al11Ce3. The formation of large volume fractions of Al12Mn for heat-treatments at up to 450 °C suggests that the presence of Ce may stabilize this phase, and that more dilute Al–Mn–Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.

February 2013

DOI: http://dx.doi.org/10.1007/s10853-013-7185-0

Mauricio A. Gordillo, Lichun Zhang, Thomas J. Watson, Mark Aindow

Abstract

Devitrified Al—transition metal—rare earth alloys offer routes to obtain higher volume fractions of dispersed strengthening phases than conventional precipitation routes. Here, we report a study of the microstructure–property relationships of an Al–Ni–Co–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. Microstructural characterization by electron microscopy and serial section FIB tomography show that the alloy comprises an FCC Al matrix and 44 % by volume of elongated Al19(Ni,Co)5Y3 plates with the Al19Ni5Gd3 structure. The plates are aligned with the extrusion direction in the as-extruded alloy, and tensile data show a distinct anisotropy in yield strength and strain to failure. These data are consistent with the alloy acting more like a unidirectional short-fiber-reinforced metal–matrix composite than a conventional precipitation-hardened alloy. During axial upset forging, the ternary plates do not break up, but instead they rotate, until at large upset strains they lie perpendicular to their original orientation with corresponding changes in the tensile properties. The materials exhibit yield strengths of up to 713 MPa and tensile elongations of up to 5 %. Thus, such systems could form the basis for truly deformable high-strength low-density metal–matrix composites.

July 2005

DOI: http://dx.doi.org/10.1016/j.intermet.2004.11.004

Alexandre L. Vasiliev, Mark Aindow, Martin J. Blackburn, Thomas J. Watson

Abstract

A high-resolution transmission electron microscopy study has been performed on the phases present in an Al–8 at.% Ni–7 at.% Gd alloy produced from amorphous gas-atomized powder precursors by hot compaction and extrusion. The alloy had a fine fully devitrified microstructure consisting of: α-Al, binary Al3Gd, and two ternary phases; a rod-like τ1 phase and a plate-like τm/τμ phase. Upon annealing the former coarsened whereas the latter was eliminated. It is shown that these phases exhibit the Al19Ni5Gd3 and Al23Ni6Gd4 crystal structures, respectively, and not those reported elsewhere for the τ1 and τm/τμ phases.

September 2004

DOI: http://dx.doi.org/10.1016/j.scriptamat.2004.05.050

Neal J. Magdefrau, Alexandre L. Vasiliev, Mark Aindow, Martin J. Blackburn, Thomas J. Watson

Abstract

TEM and hardness testing have been used to monitor the effect of heat-treatment on a devitrified Al–Y–Gd–Ni–Fe–Co alloy. Coarsening of the multi-phase microstructure is sluggish even at high homologous temperatures. The hardness of the alloy is controlled by the size of rod-like particles with the orthorhombic Al19Ni5Gd3-type crystal structure.

April 2004

DOI: http://dx.doi.org/10.1016/j.intermet.2003.11.007

Alexandre L. Vasiliev, Mark Aindow, Martin J. Blackburn, Thomas J. Watson

Abstract

A transmission electron microscopy study has been performed on the phase distribution and morphology in a series of four Al-rich Al–Ni–Y alloys. The alloys were produced from amorphous gas-atomized powder precursors by hot compaction and extrusion. They had fine fully devitrified microstructures and in each case these were three-phase consisting of: α-Al, binary Al3Ni or Al3Y (depending on alloy composition), and a ternary phase. The same ternary phase was present in each alloy and this was found to correspond to Al19Ni5Y3. This phase has the orthorhombic Al19Ni5Gd3-type crystal structure but has not been reported previously in the Al–Ni–Y system. The presence of thin embedded slabs of the equilibrium Al23Ni6Y4 compound within the Al19Ni5Y3 particles indicated that this structure may be metastable, although it did not decompose even after extended annealing.