Proceedings of Sino-Swedish Structural Materials Symposium 2007
Thermal Stability of Electrodeposited Nanocrystalline Niand Co-Based Materials Uta KLEMENT, Melina DA SILVA (Dept. of Materials and Manufacturing Technology, Chalmers University of Technology,412 96 Gothenburg, Sweden)
Abstract: The attractive properties associated with nanocrystalline materials are to a large extent a result of their high inter-crystalline volume fraction. However, the intrinsic instability of the nano-structured state may compromise the gain in properties by the occurrence of grain growth during exposure at elevated temperatures. Thermal stability is therefore a fundamental materials issue for nanocrystalline materials and grain growth is a complex phenomenon. To better understand the microstructural development upon annealing and to determine the mechanisms operative in the stabilization of Ni- and Co-based nanocrystalline electrodeposits, a wide range of advanced characterization techniques has been applied (transmission electron microscopy, electron backscatter diffraction in the scanning electron microscope, 3D atom probe, calorimetry, and X-ray diffraction). In pure materials like Ni and Co and in single-phase alloys like Ni-Fe, grain growth occurs at relatively low temperatures due to the lack of stabilizing additions; initial grain growth occurs abnormally and is followed by normal grain growth at higher temperatures. The formation of the first grown grains is described to occur by a sub-grain coalescence model similar to the one known from recrystallisation. Thermal stability of nanocrystalline materials can be enhanced significantly with addition of solutes. In a strongly segregating system like Co-P, the effect of solutes together with the allotropic phase transformation is investigated. Already in the as-prepared state, P is found in the grain boundaries and further segregation occurs upon annealing. Transmission electron microscopy and 3D atom probe reveal that precipitation takes place upon annealing. The P atoms in the grain boundaries are consumed during the formation of Co2P and COP precipitates and grain growth can take place. In addition to chemical and morphological influences, the texture development during grain growth is investigated in nanocrystalline Ni and Ni-Fe. The dominant orientation of the first grains changes from <4 1l>//ND to a faster growing/energetically more favourable <11 l>//ND orientation by twinning.The results are the basis for a model description in which it is stated that segregation leads to an improved thermal stability of the nanocrystalline structure by the combination of a thermodynamic effect (reduction in driving force for grain
growth) and a kinetic effect (reduction in grain boundary mobility). A texture with fast growing/energetically favourable orientations has to be avoided. Key words: nanocrystalline materia1s;grain growth; thermal stabi1ity;texture; Ni;Co;TEM; 3DAP; EBSD; DSC
nanocrystalline materials are attributed to an increased amount of structural defects resulting from the decreased grain size. However, from thermodynamic point of view, this high inter-crystalline volume fraction (e.g. grain boundaries, triple points, and quadruple nodes) also results in an intrinsic instability of the m i c r o ~ t r u c t u r e [ In ~ ~ order ~ ~ . to retain the attractive properties, especially during exposure to elevated temperatures, a high stability of the microstructure is required. Thermal stability is therefore a fundamental
Technological progress is often intimately linked with the development of novel materials with extraordinary combinations of physical and mechanical properties. A recent design strategy in materials science is to reduce the grain size to below 100 nrn"]. Such nano-structured materials have attracted significant attention in the last two decades since for example a dramatic strength and hardness increases, e.g. by factors of 5 to 10, could be a~hieved'~.~'. The improved mechanical properties of 173
Proceedings of Sino-Swedish Structural Materials Symposium 2007
materials issue for nanocrystalline materials and of great importance when it comes to general assessment of potential technological applications. Common for most of the nanocrystalline
3 V between the sample and a platinum electrode in a solution of 20% perchloric acid and 80% 2-butoxyethanol. Annealed Co - 3.2ut.% P samples had to be thinned with focus ion beam technique as a final step after electropolishing. For TAP analysis, a
electrodeposits investigated so far is the occurrence of abnormal grain growth upon annealing, that means
pulse rate of 10oO Hz, a pulse fraction of UPAJDC = 20% (2000Hz and 17.5%, respectively, for C o -
the formation of a bimodal microstructure with grown grains in a still nanocrystalline microstructure’”. Improving thermal stability in such nanocrystalline materials goes therefore hand in hand with circumventing the occurrence of abnormal grain growth. This paper is summarizing results from a
3.2ur.% P) and a temperature of -213 “c at the tip
were chosen as acquisition parameters. Electron backscatter diffraction measurements (EBSD) were performed in a LEO Supra 55VP field emission gun scanning electron microscope (SEM) at HKL Technologies NS, Hobro, Denmark. The
number of investigations which were performed with the aim to understand the mechanisms operative in the stabilization of Ni- and Co-based nanocrystalline electrodeposits and provides a model description of the grain growth behavior in such materials.
instrument was equipped with a HKL Channel 5 EBSD system and Nordlys I1 detector. All samples were prepared by conventional polishing with diamond paste followed by ion polishing for 1 h (2 ’ angle of incidence).
3 Results and Discussion
The nanocrystalline samples investigated within the scope of this study (Ni, Ni-Fe, Co-P, Co-W, and
3.1 Pure elements and single phase alloys In nanocrystalline electrodeposits made from pure elements, abnormal grain growth is very pronounced and occurs at relatively low temperatures. In Ni (10 and 20 nrn grain size), the first grown grains are observed to occur at about 80T, and are suggested to occur by a sub-grain coalescence mechanism‘81, whereas abnormal grain growth is reported to occur at 150 T in pure Co (initial grain size of 20 nrn)‘’’. The higher thermal stability of Co electrodeposits is supposed to be related to the different room temperature crystal structure as compared to Ni (hcp instead of fcc). This is based on the fact that the allotropic phase transformation (hcp + fcc), which is usually expected to occur at 420 “c in microcrystalline Co, is setting in much earlier, i.e. around 300 “c, and in connection with grain growth. Abnormally grown grains are identified to be of fcc-Co or hcp-Co structure, while the still nanocrystalline matrix is exclusively of hcp-Co structure“”. In Ni - 20ut. % Fe, a single-phase material with an initial grain size of 17 nn,the influence of Fe additions (alloying) and a possible ordering trans-
Co-W-P) have been produced in form of sheets by the pulsed electdeposition technique, and were provided by Integran Technologies Inc., Toronto, Canada. Calorimetric measurements were carried out with a Netsch DSC 200F3 instrument between 50 ‘z: and 595 T at different heating rates. X-ray diffraction measurements were performed with a commercial Co
KQ Philips X’Pert diffractometer. For TEM sample preparation, 3 mm disks were punched from the sheets and prepared by usual dimple grinding and ion milling. A Zeiss 912 Omega transmission electron microscope (TEM) with inbuilt energy filter, operating at 120 kV accelerating voltage. was applied for analytical work (electron spectroscopic imaging (ESI) and electron energy-loss spectroscopy (EELS)) and imaging of the inicrostructures. 3D atom probe (3DAP) was performed at the lnstitut fiir Materialphysik, University Gottingen, Germany. Samples were cut from the initial sheet material and afterwards mechanically polished to obtain square-shaped cross-sections. Needle-shaped samples were obtained by applying a direct voltage of 174
Proceedings of Sino-Swedish Structural Materials Symposium 2007
formation (Ni3Fe) is investigated. Conventional microcrystalline Ni-Fe of this composition is expected to have an ordered structure at room temperature, but this may not necessarily be the case for a nanocrystalline Ni-Fe alloy. X-ray diffraction and electron diffraction show that the material consists of the fcc Ni3Fe phase, but ordering could not be detected due to the similar scattering factors of Ni and Fe. However, TEM investigations show that the nanostructure is preserved up to 215OC where abnormal grain growth sets in, i.e. Fe-addition leads to a higher thermal stability compared to pure Ni'"]. Generally, the grain growth behavior is found to be quite similar to pure Ni (DSC and TEM measurements) even though growing grains are increasing in size more rapidly while the nanocrystalline matrix is not changing in grain size but contains grains which
Fig. 1 3DAP analysis: P-concentrationin as-prepared Co - 3.2at.% P
have changed in appearance'"'. 3.2 Supersaturated single phase material Alloying elements like P and W result in a substantial increase in thermal stability of nano-
Co - 3.2at.% P only COPprecipitates have been found by 3DAP"31.ESI-mapsshow that in both materials the P-rich precipitates are of nearly spherical or slightly elongated shape with a diameter of about 33 nm (Fig. 2). The main differences between the alloys are the precipitate size distribution (Co - 3.2at. % P having broader distribution) and precipitate volume number density (Co - 3.2at.% P contains nearly twice as many precipitates than Co - l . l a t % P). The volume fraction of precipitates is 3.0% in Co - l.lat.% P and 4.4% in Co - 3.2at. % P'I4'. 3.3 Grain growth model The state of lowest energy in an ideal metallic material is that obtained in a defect-free single crystal with an equilibrium number of vacancies. However, generally such a material does not have a particularly high hardness and/or strength. In case of nanocrystalline materials, increased strength is obtained by rqducing the grain size, i.e. increasing the number of grain boundaries (also reflected by the Hall-Petch relation). The total excess energy stored in the grain boundaries is considered to be the main driving force for grain growtht5.69'I. The velocity of grain boundary migration is given as the product of driving force and grain boundary m~bility"~'. Hence,
crystalline electrodeposits. A common characteristic of alloy systems like Ni-P, Co-P, and Co-W alloys is that the solubility in the host material (Ni or Co) is practically negligible. It is therefore not surprising that the distribution of solute atoms is already inhomogeneous in the as-prepared state and that they are segregated in the grain boundaries of Ni/Co as observed by 3DAPr12].In Co - l.lat.% P (14 nm grain size), the P-concentration in the Co grain boundaries reaches up to 10 at.% P, whereas in Co - 3.2at% P (12 nm grain size) enrichments with up to 25 at.% P are observed. Fig. 1 shows the P-concentration in as-prepared Co - 3.2at. % P in two perpendicular planes through the analyzed volume as measured with 3DAP. In this example a P-enrichment with more than 20 at. % P was located in the upper corner of the analyzed volume. Analysis of the Co-P system has revealed that annealing leads to further P-segregation to the grain boundaries and, when saturation is achieved, to precipitation. In Co - l.lat.% P, both COP and C02P precipitates have been observed, whereas in I75
Proceedmgs of Sino-Swedish Structural Materials Symposium 2007
grain growth can be hindered if at least one of these factors is reduced substantially. In Ni-P, Zener-drag (Ni3P precipitates) is responsible for the reduced grain boundary mobility and a reduction in grain boundary energy due to P-segregation is discussed"6. 17'; in the Ni-W system (W-solubility of up to 17.5at.%), thermal stability is attributed to the extremely low mobility of the W-atoms"". In strongly segregating systems such as Co-P, the segregation leads to a reduction of the grain boundary energy. resulting in a lower driving force for grain growth. Hence. in addition to the kinetic effect of solute drag, this thermodynamic effect is also contributing 10 the stabilization of the nanostructure. In fact, calculationdestimations show that the thermodynamic contribution is the dominant stabilizing effect in Co-P. Grain growth occurs when the impurity/solute grain boundary concentration is reduced after precipitation sets in. Grain boundary pinning by the precipitates (Zener drag) may hinder grain boundary migration to a certain extent, but is not as e ~ e c t i v e " ~ ~ . Hence, alloying, ordering, and allotropic phase transformation are all affecting the microstructural development upon annealing. Also the formation of second-phase precipitates increases thermal stability (e.g. formation of NilP in the Ni-P system). However, the most effective way to stabilize nanocrystalline electrodeposits seems to be impurities/ solute atoms (co-pll? 131 ~ i - 9 1 6 . 171, and Ni-W'18'). Irnpurities/solute atoms segregate to the grain boundaries of the host material and slow down boundary migration (solute drag) andor reduce the
Fig. 2 TEM bright field image of Co 3.2at% P annealed
for 15 min at 44OP: and corresponding ESI 2-window ratio maps of (b) P and (c) Co
solutes and the limited amount of impurities"' 'I. Fig. 3(a) - (d) describes schematically how impurities/ solutes affect the grain growth behaviour: Impurities /solutes are segregating to the grain boundaries rlpon annealing (a and b) and when saturation of the grain boundaries is achieved, precipitation occurs (c). When the impurities/solute atoms in the grain boundaries are consumed during the formation of precipitates, the grain boundaries are able to migrate
g a i n boundary energy. This effect may also be enhanced by dynamic segregation during abnormal grain growth, in which impurities/solutes are collected by the migrating boundaries when sweeping through the nanocrystalline matrix. The same mechanism is expected to be responsible for the high thermal stability in Co-W and Co-W-P observed in
DSC measurcrnentsltY1 (to be confirmed by TEM-investigations) whereas the low thermal stability of pure Ni and Co is in fact due to the lack of
and grain growth can take place (d). But a higher solute content is not necessarily 176
Proceedings of Sino-Swedish Structural Materials Symposium 2007 Temperature ['C] 100
-0 12 Co-1 lat %P
-0 14 460
Fig. 4:DSC curves of Co - l.lat.% P and Co 3.2at.% P (measured with a heating rate of 10°C/min). Fig. 3Grain growth model in presence of solutes: (a)
further grain growth occurs (i.e. the first grains grow further and new grains start to grow abnormally) and the dominant orientation of the abnormally growing grains changes from <4 11>//ND to <111>//ND'*']. Hence, <111>//ND grains are expected to have a mobility/energy advantage over the other orientations. However, also twinning is observed between the two
as-prepared state; (b) annealing results in segregation of solutes to grain boundaries of host material; (c) solute in the grain boundaries are consumed during formation of precipitates; (d) grain growth sets in
improving thermal stability. Comparison of DSC and TEM measurements of Co - l.lat.% P and Co - 3.2at.% P shows that Co - 3.2at.% P is somewhat less stable, i.e. grain growth occurs at a
dominant orientations (<41l>//ND + <1ll>//ND) and is one of the reasons for the orientation change.
4. Conclusion Ni- and Co-based nanocrystalline electrodeposits have been investigated to determine the underlying mechanisms for improving thermal stability in such materials. Alloying as well as phaseand ordering transformations are found to affect the grain growth behavior, but the addition of solutes has turned out to be the most efficient way to stabilize the nanostructure. However, the required solute content has to be determined and optimized in each case as the solutes are usually not homogeneously distributed in the grain boundaries of the host material. With respect to texture influences, a <11l>//ND texture (or an easy change into that orientation by for example twinning) has to be avoided as grains with this
slightly lower temperature, i.e. 400 "c (Fig. 4). This lower thermal stability of Co - 3.2at.% P can be explained by help of the 3DAP measurements. In fact, the measured Gibbsian P-excess in the grain boundaries of as-prepared Co - 3.2at.% P is with (5.3 2.6)x1Ol8m-' comparable with the excess value estimated for Co 1.lat. % P annealed for 1 h at
400"c"31. P enrichments with P-content of up to 25at.% are already detected in the as prepared state of
Co - 3.2at. % P (compare Fig.2). Hence, grain boundary saturation is achieved faster upon annealing and as a consequence precipitation as well as grain growth occurs earlier"2' l9]. 3.4 Texture influence Besides the morphological effects, also texture influences are investigated. In nanocrystalline Ni and Ni - 20at. % Fe electrodeposits, first grown grains have orientations close to <411> (spread of >lo") with respect to the sample normal direction (ND, equal to sheet normal direction) as observed by the SEM-based EBSD technique. Upon annealing,
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