hardening and softening in milled nanostructured feal on annealing

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Hardening and softening in milled nanostructured FeAl on annealing D.G. Morris, * I. Gutierrez-Urrutia and M.A. Mun ˜ oz-Morris Department of Physical Metallurgy, CENIM-CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, Spain Received 23 February 2007; accepted 9 May 2007 Available online 8 June 2007 Changes of hardness occurring in milled FeAl intermetallics on annealing are examined and compared with the behaviour of conventional disordered Fe-base materials. A hardness peak is seen after annealing to about 500–600 °C which is explained by the precipitation and growth of fine oxide particles. There is no evidence to support any role of the state of order nor the presence of partial dislocations in affecting the hardness. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Iron aluminides (based on FeAl); Hardness; Mechanical milling; Nanocrystalline materials; Grain size dependence The hardness of nanocrystalline materials, whether produced by milling methods, condensation, severe plas- tic deformation or other methods, is often related to the grain size by a Hall–Petch type of relationship with a wide variety of slopes observed, sometimes similar to those found for large-grained materials and sometimes with smaller or even negative slopes. The present study examines changes in hardness on annealing FeAl inter- metallics and analyses such changes which cannot be explained simply in terms of grain size dependency. Materials were prepared with compositions of Fe–40% Al and Fe–50% Al (at.%) by milling elemental powders in the desired ratio, under an inert atmosphere, using a planetary ball mill. The results obtained were very similar for both compositions, with the Fe–40Al mixture showing slightly faster dissolution of the Al in the Fe and a slower onset of order. After milling for longer times, especially after annealing, behaviour was essentially identical. Most of the data shown relate to the Fe–50Al material, with some relating to Fe–40Al in- cluded in the analyses. Powder mixtures were milled for 8 h, long after the disappearance of any trace (X-ray diffraction) of Al, at which point the FeAl appeared to be homogeneously mixed and partially ordered. Following milling, powder samples were annealed for 1 h at temperatures of up to 1000 °C. This was done after pre-compacting the pow- ders to green billets, loading in an inert atmosphere and hot pressing for a total time of about 30 min. Hard- ness was measured using a Vickers microhardness instrument with a 50 g load, taking care to measure within single powders and not over the remaining poros- ity in the compacts, and averaging the results of 10–20 measurements. Microstructures were mostly determined by transmission electron microscopy (TEM), with sup- porting evidence provided by X-ray diffraction and dif- ferential scanning calorimetry. The results are shown in Figure 1, where both hard- ness and grain size (determined from TEM photographs for materials annealed above 500 °C and by X-ray diffraction for loose powder and after annealing at 400 °C) are presented as a function of treatment temper- ature. Data shown for 1100 °C correspond to a range of values obtained on a similar FeAl alloy milled and con- solidated at that temperature [1,2]. The powder is seen to be rather hard, the hardness increasing upon anneal- ing in the 500–600 °C range, and the material softening slowly thereafter. The grain size is initially about 10 nm in the milled powder, changes only slightly up to about the hardness peak and gradually increases at higher temperatures. Several possible explanations may be offered to ex- plain the hardening observed at the intermediate temperatures: the onset of ordering, grain growth to some critical value, changes in dislocation type, for example from some partial dislocation to perfect dislocation configuration, some other effect. 1359-6462/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.05.009 * Corresponding author. E-mail: [email protected] Scripta Materialia 57 (2007) 369–372 www.elsevier.com/locate/scriptamat

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Page 1: Hardening and softening in milled nanostructured FeAl on annealing

Scripta Materialia 57 (2007) 369–372

www.elsevier.com/locate/scriptamat

Hardening and softening in milled nanostructuredFeAl on annealing

D.G. Morris,* I. Gutierrez-Urrutia and M.A. Munoz-Morris

Department of Physical Metallurgy, CENIM-CSIC, Avenida Gregorio del Amo 8, 28040 Madrid, Spain

Received 23 February 2007; accepted 9 May 2007Available online 8 June 2007

Changes of hardness occurring in milled FeAl intermetallics on annealing are examined and compared with the behaviour ofconventional disordered Fe-base materials. A hardness peak is seen after annealing to about 500–600 �C which is explained bythe precipitation and growth of fine oxide particles. There is no evidence to support any role of the state of order nor the presenceof partial dislocations in affecting the hardness.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Iron aluminides (based on FeAl); Hardness; Mechanical milling; Nanocrystalline materials; Grain size dependence

The hardness of nanocrystalline materials, whetherproduced by milling methods, condensation, severe plas-tic deformation or other methods, is often related to thegrain size by a Hall–Petch type of relationship with awide variety of slopes observed, sometimes similar tothose found for large-grained materials and sometimeswith smaller or even negative slopes. The present studyexamines changes in hardness on annealing FeAl inter-metallics and analyses such changes which cannot beexplained simply in terms of grain size dependency.

Materials were prepared with compositions ofFe–40% Al and Fe–50% Al (at.%) by milling elementalpowders in the desired ratio, under an inert atmosphere,using a planetary ball mill. The results obtained werevery similar for both compositions, with the Fe–40Almixture showing slightly faster dissolution of the Al inthe Fe and a slower onset of order. After milling forlonger times, especially after annealing, behaviour wasessentially identical. Most of the data shown relate tothe Fe–50Al material, with some relating to Fe–40Al in-cluded in the analyses.

Powder mixtures were milled for 8 h, long after thedisappearance of any trace (X-ray diffraction) of Al, atwhich point the FeAl appeared to be homogeneouslymixed and partially ordered. Following milling, powdersamples were annealed for 1 h at temperatures of up to1000 �C. This was done after pre-compacting the pow-ders to green billets, loading in an inert atmosphere

1359-6462/$ - see front matter � 2007 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2007.05.009

* Corresponding author. E-mail: [email protected]

and hot pressing for a total time of about 30 min. Hard-ness was measured using a Vickers microhardnessinstrument with a 50 g load, taking care to measurewithin single powders and not over the remaining poros-ity in the compacts, and averaging the results of 10–20measurements. Microstructures were mostly determinedby transmission electron microscopy (TEM), with sup-porting evidence provided by X-ray diffraction and dif-ferential scanning calorimetry.

The results are shown in Figure 1, where both hard-ness and grain size (determined from TEM photographsfor materials annealed above 500 �C and by X-raydiffraction for loose powder and after annealing at400 �C) are presented as a function of treatment temper-ature. Data shown for 1100 �C correspond to a range ofvalues obtained on a similar FeAl alloy milled and con-solidated at that temperature [1,2]. The powder is seento be rather hard, the hardness increasing upon anneal-ing in the 500–600 �C range, and the material softeningslowly thereafter. The grain size is initially about 10 nmin the milled powder, changes only slightly up to aboutthe hardness peak and gradually increases at highertemperatures.

Several possible explanations may be offered to ex-plain the hardening observed at the intermediatetemperatures:– the onset of ordering,– grain growth to some critical value,– changes in dislocation type, for example from some

partial dislocation to perfect dislocation configuration,– some other effect.

sevier Ltd. All rights reserved.

Page 2: Hardening and softening in milled nanostructured FeAl on annealing

Figure 2. Hall–Petch plots (hardness versus reciprocal square root ofgrain size) showing: (a) present data compared with reference data onFe–base alloys [10], and (b) comparison with a wide range of literaturedata on FeAl (see text for details).

Figure 1. Evolution of hardness and grain size on annealing milledFeAl powders.

370 D. G. Morris et al. / Scripta Materialia 57 (2007) 369–372

Each of these possibilities will be examined based onthe structural examinations carried out.

The state of order achieved after milling Fe–Al pow-ders has been examined many times [3–8] and has beenshown to be one of very low order, apparently in theform of nanoscale, partially ordered regions separatedby a matrix of disordered domain walls [5,6,8]. Suchstudies [5,6,8,9] have also shown that reordering to avery high state of order occurs at temperatures around200–300 �C, with internal strain relaxation occurring at300–400 �C, and recrystallization or grain growth athigher temperatures. As such, any role of ordering onhardness evolution (Fig. 1) can be eliminated as beingunimportant.

The possible role of grain size on hardness evolutionis examined in Figure 2. Figure 2a shows a Hall–Petchrepresentation of the present hardness data (as relatedto the reciprocal square root of the grain size), withtwo straight lines taken from Guduru et al. [10] superim-posed. These two lines summarize the general overallbehaviour of disordered Fe-based alloys for typicalcoarse grain sizes (the line of the steeper slope passingthrough the origin) and for milled, nanocrystalline mate-rials (the line of less steep slope). The present data areseen to start (the milled state) somewhat below thisnanocrystalline line and pass above it on annealing to500 �C and above. As the grain size coarsens at900 �C, the hardness data approach those expected formicrocrystalline Fe alloys. Figure 2b shows additionalhardness data from a wide range of studies of milledand annealed FeAl alloys [1,2,11–18]. To assist interpre-tation, these additional data are classified in severalgroups, representing (i) as-milled and disordered FeAl;(ii) FeAl milled and compacted at very high pressureand temperature; and (iii) conventional milled andhigh-temperature extruded, coarse grained yttria disper-sion-strengthened FeAl. (Note, again, that some of thedata refer to alloys with compositions near Fe–40% Aland others with compositions near Fe–50% Al, but thereis no systematic change in hardness due to this varia-tion.) The overall trend of data change is similar to that

seen for the present data – disordered materials tend tobe relatively soft, annealing leads to significant harden-ing until microstructure coarsening/grain growth be-comes very significant at very high temperatures(above 900 �C). The present data after annealing at700–900 �C can be seen to tend towards the hardnessdata of the coarse (micrometre-sized) yttria-strength-ened materials (ODS).

Dislocations were found in room temperature com-pression-deformed samples but only in those preparedat temperatures of 800–900 �C, i.e. where the grain sizewas greater than 50 nm. This is not evidence that otherdeformation mechanisms operated at room-temperaturein materials with finer grain sizes, but reflects simply theabsence of any residual dislocations after deformation.The dislocations seen inside the grains were often paireddislocations with a separation of about 4–10 nm, asillustrated in Figure 3, but it was not possible to imagethese with sufficient numbers of imaging vectors for acomplete Burgers vector analysis. The single disloca-tions could either be partial 1/2h11 1i dislocations,

Page 3: Hardening and softening in milled nanostructured FeAl on annealing

Figure 3. Weak beam transmission electron micrographs illustratingsingle and paired dislocations (arrowed) in samples prepared at 900 �Cand deformed at room temperature.

D. G. Morris et al. / Scripta Materialia 57 (2007) 369–372 371

separated from the partner that would constitute theusual h111i superdislocation, or perfect h100i disloca-tions [19]. Since the dislocations are sometimes paired(as h111i superdislocations), and strong Peierls forcestypically mean that the h1 00i dislocations are found

Figure 4. TEM photographs showing fine particles in materials previously anof particles are arrowed. (a) A dark field image taken, far from the Bragg cond

as straight segments, it may be argued that the disloca-tions seen are probably conventional h1 11i dislocations,full superdislocations or partial dislocations. The ab-sence of residual dislocations after deforming materialswith grain size of 20–50 nm, i.e. previously annealed to400–700 �C, where the hardness peak is found, meansthat it is not possible to analyse this peak in terms ofdeformation mechanisms involving dislocations, partialor perfect, or other types of mechanism.

Materials annealed at high temperature showed fineparticles distributed throughout the grains (Fig. 4). Thiswas especially clear for material annealed at 900 �C, butsimilar particles were also found in materials annealed at600–800 �C. Careful examination of material annealedto 500 �C also revealed the presence of a number ofparticles. The particles are presumed to be oxides, andperhaps also carbides, resulting from contamination ofthe powders during milling. Chemical analysis by energydispersion spectroscopy of X-rays in the TEM using anultra-thin window showed a weak oxygen peak for someof the larger particles in the 900 �C material, confirmingthe suggestion that the particles are probably oxides,although insensitivity of the analysis to carbon cannoteliminate the presence of carbides.

The evolution of particle size, and grain size, withtemperature is shown in Figure 5. While both parame-ters increase steadily at higher temperatures, there isno fixed proportionality of grain size and particle size,as would be expected if particles were responsible forpinning boundaries and defining grain size (by a Zenermechanism, for example). The implication is that bothparticle coarsening and grain growth are still indepen-dent processes, at least up to temperatures of 900–1000 �C.

A possible reason for the hardening peak observedat 500–600–700 �C is the precipitation and growth/

nealed at: (a) 900 �C, (b) 800 �C, (c) 700 �C, and (d) 600 �C. Examplesition, using a diffracted beam of the matrix; (b)–(d) bright field images.

Page 4: Hardening and softening in milled nanostructured FeAl on annealing

Figure 5. Evolution of particle and grain size on annealing at hightemperatures.

372 D. G. Morris et al. / Scripta Materialia 57 (2007) 369–372

coarsening of these oxide particles to a size of 1–3 nm.This hypothesis can be examined by calculating thestrengthening due to a 1–2% volume fraction of particles(measured from micrographs such as Fig. 4a) by assum-ing Orowan pinning of dislocations. This is presumablyan overestimate of the strengthening effect of the fineoxide particles, which may instead be sheared at a lowerstress level. Orowan strengthening (Dr) may be calcu-lated as:

Dr ¼ 2mGb lnf/=2bg=½ð1:18Þ4pðk� /Þ�;where m is the Taylor factor taken as 2.5 for the bccmaterial, the shear modulus G is taken as the energyfactor (75 GPa) [1,2], b is the Burgers vector(2.5 · 10�10 m), / is the particle size and k is the planarinterparticle separation, //

pfv, where fv is the volume

fraction of particle phase. Substituting suitable valuesfor the parameters, the strengthening due to 1–2% par-ticles of size 1.5–3 nm is about 600–470 MPa. Consider-ing that the corresponding hardness increment is threetimes larger, the hardening due to particles is about1.4–1.8 GPa. For annealing temperatures of about 600and 800–900 �C, where particles have sizes of about1.5 and 3 nm (Fig. 5), the additional hardening observedover the as-milled state (Fig. 1) or over the referenceHall–Petch line (Fig. 2a) is about 1–2 GPa, in excellentagreement with estimations.

The materials shown in Figure 2b with remarkablyhigh hardness (12–14 GPa) were obtained followingpowder compaction at very high temperature andpressure, and had the deliberate addition of nanosizedstrengthening particles. Using the same Orowanstrengthening calculation as above, a hardening incre-ment of 7–12 GPa is deduced, to be compared withthe observed excess hardening in Figure 2b of about6 GPa – again reasonable agreement, considering theapproximations of the assessments made.

In conclusion, the high hardness observed after mill-ing and annealing FeAl is seen to show a significant con-tribution of Hall–Petch type grain size strengthening for

grain sizes over the complete range examined (near10 nm to above 100 nm), and a significant hardening ex-cess after annealing at intermediate temperatures due tothe formation of fine oxide particles. These result fromcontamination with oxygen, and perhaps also carbon,during milling, which is believed to be present in solu-tion or as sub-nanometre particles in the milled state,and form as nanoscale particles on annealing at 500–900 �C. These results suggest that such impurity or par-ticle hardening should be considered for many suchmilled materials, where impurity ingress is likely, ashas been considered by other authors (e.g. [20]). Defor-mation mechanisms are confirmed to be controlled bystandard superdislocations for materials with grain sizesnear 50–100 nm, but it has not been possible to obtainevidence of controlling mechanisms for finer grain sizes.

I.G. would like to acknowledge financial supportthrough the award of a Juan de la Cierva postdoctoralfellowship.

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