the interface between γ and γ’ in ni-al alloys by hrem
TRANSCRIPT
The Interface between γγγγ and γγγγ’ in Ni-Al Alloys by HREM
Hector A. Calderon1, a, L. Calzado-Lopez2,b and T. Mori3,c 1Depto. de Ciencia de Materiales, ESFM-IPN, Ed. 9 UPALM Zacatenco D.F., Mexico
2Universidad de la Ciudad de Mexico. Mexico DF Mexico
3Materials Science Centre, University of Manchester, Manchester, M1 7HS, UK
Key words: NI-Based Superalloys, Interfaces, Electron Microscopy, Micromechanics.
Abstract. Ni Base superalloys owe their excellent mechanical properties to the presence of particles
of γ’ phase (Ni3Al with an L12 structure) in a γ matrix (Ni–Al solid solution with an fcc structure).
Besides Al, other elements are used to impart either a higher strengthening or improved corrosion
properties at high temperature. The interface between γ and γ’ becomes of absolute importance for
the resulting mechanical properties and technological application. Especially by considering the
consequences that diffusion driven coarsening brings about to the particle distribution either with or
without the influence of an applied stress or strain. In this work the interface between γ and γ’
phases is characterized by means of measurements on phase images obtained from high resolution
transmission electron microscopy images (HRTEM). Phase images represent the sample structure
much more accurately than typical HREM experimental images and allow correction of spherical
aberration and other residual aberrations. The investigation is performed by using a binary Ni-Al
alloy as well as technical Ni base superalloys (MC2 and MCNG). While a sharp interface is
developed during stress free coarsening in Ni-Al alloys, a wider volume needs to be considered
when alloying elements are introduced. Measurements of lattice spacings on phase images and
chemical composition from energy dispersive spectroscopy are used to show the interface
characteristics in the alloys under consideration. The interface in the binary Ni-Al alloy can be
described by micromechanics as a typical misfitting inclusion. In the technical alloys, the presence
of concentration gradients changes the expected lattice strains in a given volume around the
particles.
Introduction
Ni-based superalloys have a microstructure consisting of coherent particles (γ’ phase with a L12
structure) embedded in a solid solution matrix (γ phase with a fcc structure). Coarsening of γ’
particles is an important mechanism since it influences the high-temperature mechanical properties
of these materials. As a consequence it has been extensively investigated both experimentally and
through computer simulations. Coarsening of second phase particles involves a reduction of the
total free energy of the system. In fluids, the tendency for larger particles to grow at the expense of
smaller ones is driven by the reduction of surface energy. In the case of solids, the elastic strain
energy becomes part of the driving force [1]. Reduction of elastic energy promotes particle
alignment along elastically soft directions and changes of morphology as the precipitate volume
increases. In addition according to several authors, reduction of elastic energy produces the splitting
of large particles that reach a critical size [2,3]. Actually symmetric arrays of two, four, and eight γ’
particles have been observed during microstructural evolution of Ni alloys with low volume fraction
and interpreted in terms of splitting. According to models based on numerical simulation, there is a
finite distance between particles, called equilibrium distance, which minimizes the elastic strain
energy [2,4]. Such an equilibrium distance arises from the interaction elastic energy which is
attractive for long distances and repulsive for short distances [5,6]. An alternative mechanism for
the formation of particle arrays is based on particle migration and selective coalescence [7-9].
According to this mechanism, particle arrays are formed by attractive interaction. Once the particle
Solid State Phenomena Vols. 172-174 (2011) pp 254-259Online available since 2011/Jun/30 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.172-174.254
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separation is sufficiently small, coalescence can take place if both particles have an identical
translation order domain (TOD). Thus, particle groups can be formed either by splitting of larger
particles or by migration of individual particles. Interestingly splitting of large particles is not
supported by experimental evidence as the TODs in many particle pairs show that most of them do
not match. This suggests that particles migrate or coalesce and form arrays of several particles [8].
Despite such evidence simulations based on the phase field method have predicted particle splitting
[10] but also coalescence [11]. In this method, the interface is assumed to have a width and becomes
a tunable parameter. In this investigation, the nature of interfaces is investigated both structurally
and compositionally in binary Ni-Al alloys and the technical superalloys MC2 and MCNG. This is
done by using HRTEM techniques and measuring lattice spacings directly from aberration corrected
phase images as well as determining chemical compositions by direct techniques (energy dispersive
spectroscopy, EDS) as a function of position at the interface region.
Experimental Procedure
Binary Ni-12 at. % Al, MC2 and MCNG alloys are used for this investigation. Binary alloys have
been aged for 5 h at 1113 K and 1700 h at 923 K. MC2 and MCNG alloys have several different
alloying additions that are given in [11]. They are investigated in the standard condition (for details
of heat treatment see [11]). Sufficiently thin samples can be prepared by manually polishing the
samples with diamond impregnated paper and subsequent electrolytic polishing. HRTEM has been
performed in different microscopes including a CM300 and an image Cs corrected Titan (FEI®).
Approximately 20 experimental images (each one with a different defocus setting) are acquired to
determine the phase and the amplitude images by using the software TrueImage (FEI ®). The results
are further corrected to compensate for residual aberrations (focus, astigmatismus and coma).
Results and Discussion
Figure 1 shows representative images of γ’ particles after aging in the alloys under investigation.
Figs. 1a,b show typical arrays of particles that are formed by migration of particles and coalescence.
The particles in the array have different TODs and further coalescence is unlikely. Fig. 1b shows a
representative STEM image from the particle distribution in the alloy MC2 after aging without
stress (Standard Condition), the volume fraction is considerably high which most likely reduces
migration leaving coalescence to the randomly distributed TODs. Fig. 1c shows a STEM image of
the γ’ particles after coarsening under stress, the rafts develop as a function of creep time. The
interfaces that are shown below are taken from such particle distributions.
Figure 2 shows a phase image corresponding to the binary Ni-Al alloy, the image plane is
perpendicular to the crystallographic direction [001]. The contrast is relatively low but the γ’ region
can be recognized by the ordered pattern with a stronger intensity every two lattice planes. This
image can be quantitatively evaluated giving rise to the results in Figs. 2b and 2c. The measurement
of lattice spacings is performed parallel (direction a) and perpendicularly (direction c) to the
interface as a function of unit cell. The tendency is clearly shown in the above figures but averaging
the results for each unit cell (image columns) in the selected area produces clearer results as shown
in Fig. 2d. There is a continuous value for the averaged lattice parameter a, but a distinctive
discontinuity in the case of the parameter c due to the conditions imposed by strain continuity
around an inclusion. The lattice parameter mismatch (δ = ap-ap/am, m=matrix, p=particle) normally
used for characterization of a Ni alloy can be readily derived from the data in Fig. 2d as shown
elsewhere [13]. In this case there is no appreciable difference in the contrast at either side of the
interface suggesting that the involved volume tends to zero i.e., the interface tends to be a surface.
This is supported by the high long range order parameter of the Ni3Al compound and the fact that
order is kept up to the melting point. Without ruling out a given Al or Ni composition profile
outside the particle, the clear behavior of the γ’ particles as an inclusion, shows that the interface is
rather thin and judging only from experimental or processed HREM images, the interface is rather
sharp.
Solid State Phenomena Vols. 172-174 255
Fig. 1. γ’ particles in alloys
under investigation. (a,b) Ni-12
at.% at. Al after 5 h at 1113 K
and 1700 h at 923 K. (c)
MCNG alloy in the standard
condition. (d) MC2 alloy after
120 h at 1150 oC and under 150
MPa. Arrows indicate <001>,
the electron beam is traveling
along [ ]100 . Line in (d)
indicates direction for strain
measurement.
Figure 2. (a) Phase image after exit wave
reconstruction procedure. (b) Measurement of
lattice spacings a parallel to interface. (c)
Measurement of lattice spacings c perpendicularly
to the interface. (d) Averaged c spacings as a
function of position. Area for measurements is
sketched. Units = nm. The arrows indicate cube
directions.
0.2 µm
1 µm
(a)
(b)
(c)
(d)
(a)
(b)
(c)
(d)
a Spacings
c Spacings
256 Solid-Solid Phase Transformations in Inorganic Materials
Figure 3. Alloy MC2 in the standard condition. (a) Phase
image after exit wave reconstruction. (b) Area for
measurement of lattice spacings. (c) Representative
measurement of a spacings (in pixels) as a function of
location. (d) Relative variation of c and (e) a spacings
after averaging along a given column or cell in the phase
image.
The interface between γ and γ’ in the technical alloys under consideration is affected by the
alloying additions. While in the case of the binary alloy micromechanics can be directly applied to
describe the local strains i.e., via consideration of a misfitting inclusion, the situation is apparently
changed once alloying elements are added. Figure 3a shows the phase image obtained after exit
wave reconstruction for a representative interface in alloy MC2 in the standard condition. The area
for quantitative analysis is only a section and given in Fig. 3b. The corresponding measurement is
shown in Fig. 3c. In this case measurements perpendicular or parallel to the interface show similar
patterns with an approximately homogeneous lattice spacing and frequent peaked variations at
specific locations. Figs. 3d,e show averaged (along each column in the image) a and c parameters as
a function of position for the image in Fig. 3b, the values have been normalized to the average. This
is taken as a measure of strain as a function of position. Apparently the effect of alloying addition in
these alloys with a variety of alloying elements is to modify the strain field and the measurements
reflect the distribution of alloying elements around the interface. Such profiles for the different
alloying elements are likely to depend on aging condition up to certain degree which reflects on the
need to determine the conditions for stability represented by the standard condition.
(a)
(b)
(c)
(e)
(d)
Solid State Phenomena Vols. 172-174 257
Figure 4. Alloy MCNG in the standard condition. (a)
Phase image containing the γ−γ’ interface as it is shown
by the (b) intensity profiles from selected columns. (c)
Measurement of lattice parameter as a function of
position, peak high (in pixels) is equivalent to lattice
parameter. (d,e) strain (measured with c and a spacings,
respectively).
Figure 4 shows results regarding alloy MCNG. A representative phase image is given in Fig. 4a
where clear intensity maxima are seen for the lattice positions but the typical contrast differences in
the γ’ particle (due to ordering) are absent. This is an effect produced by the extremely thin sample
in use. However there are intensity differences that make possible to locate the interface region as is
shown by the intensity profiles in Fig 4b. The profiles are taken from rows as indicated by a
number. Apparently both the ordering in the L12 structure and the chemical element distribution
affect the blob intensity in the image. There is a characteristic intensity variation at the γ’-γ interface
but also the intensity varies inside the γ or the γ’ phase indicating a variation of the chemical
composition of the atomic columns since a considerable thickness variation is rather unlikely in the
reduced area under observation. Fig. 4 shows that the interface position is not unique and thus it is
not atomically flat. However ordering ends abruptly at each column in the image. Additionally the
distribution of the alloying elements produces a transition zone or volume characteristic of the
interface. On the other hand, measurements give similar results to those shown for the MC2 alloy,
but the lattice parameter differences are smaller giving rise to lower localized strains as can be
qualitatively seen in Fig. 4c and quantitatively in Figs 4d ,e. Interestingly the strains shown in Figs
3d,e and Fig. 4d,e are considerably different. While in the case of alloy MC2, relatively high local
strain fields can be found reaching a maximum of around 4-5%, those in alloy MCNG are much
lower and around 1-1.5%. This impacts the lattice resistance to plastic deformation and dislocation
motion but it will be discussed elsewhere.
(c)
(d)
(e)
(a)
(b)
258 Solid-Solid Phase Transformations in Inorganic Materials
The use of HREM allows characterization of the γ−γ’ interface up to a very fine scale. In the past,
HREM has been limited by the aberrations in the microscope lenses. Image delocalization and aber-
rations reduce precision in the measurement of lattice spacings. The use of aberration corrected mi-
croscopes and determination of the corresponding exit wave from reconstruction procedures allow
quantitative determination of strain fields at γ−γ’ interfaces. In the case of the binary Ni-Al alloy, the
strain field predicted from micromechanicas can be reproduced. As for the technical MC2 and
MCNG alloys, the interface region becomes a volume due to the distribution of alloying elements.
Such a distribution can be readily measured by spectroscopic techniques as shown in Fig. 5 where
the elemental composition is determined across an interface in the MC2 alloy (standard condition).
The profile has been measured from the region in the inset (see line). It clearly shows that the
chemical distribution of most elements extends over a distance few nanometers wide as observed by
HREM.
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[13] T. Mori to be published.
[14] Grant 58133 (CONACYT), CEMES-CNRS (France) and NCEM (USA) are gratefully
acknowledged.
Fig. 5. Concentration profiles of W, Ta, Ni, Co, Cr, Ti and Al in the MC2 alloy (standard
condition). The inset shows the region for the line scan.
100
200
300
0 50 100 150 Position (nm)
Inte
nsi
ty (
cou
nts
)
Solid State Phenomena Vols. 172-174 259
Solid-Solid Phase Transformations in Inorganic Materials 10.4028/www.scientific.net/SSP.172-174 The Interface between γ and γ’ in Ni-Al Alloys by HREM 10.4028/www.scientific.net/SSP.172-174.254