TECHNICAL ARTICLE

Comprehensive Microstructural Characterization of c0

in a Nimonic Alloy Using Ultrasonic Velocity Measurements

G. V. S. Murthy • S. K. Das • B. Ravikumar

Received: 21 February 2014 / Revised: 1 May 2014 / Accepted: 27 May 2014 / Published online: 9 July 2014

� Springer Science+Business Media New York and ASM International 2014

Abstract Microstructure of a crystalline material is a key

factor in its technological applications as it determines a

wide variety of properties including mechanical strength,

toughness, corrosion resistance, and hardness. In Nimonic

alloys, precipitation takes place during processing and on

prolonged exposure to service at elevated temperatures.

Consequently, knowledge about the precipitation phases

and their influence on the various properties is of funda-

mental interest not only for its intrinsic study but also for

its technological significance. The purpose of the present

study is to explore the possibility of using ultrasonic

velocity measurement in a Nimonic alloy and characterize

the precipitation behavior on aging. It is concluded from

this study that ultrasonic velocity is more sensitive than the

conventional microstructural sensitive parameter like

hardness due to its dependence on the chemical composi-

tional variations.

Keywords Microstructure � Nickel alloys � Precipitation

phenomena

Introduction

The microstructure of a polycrystalline alloy material is a

key factor in its technological applications, as it determines

a wide variety of properties viz. strength, toughness, cor-

rosion resistance, and hardness, etc. Microstructural deg-

radation/formation of deleterious precipitates takes place

during processing and/on prolonged exposure to service at

elevated temperatures and their detrimental influence on

the various properties is well documented. Consequently,

knowledge about the precipitation behavior is of utmost

importance not only for its intrinsic interest but also for its

technological significance. Although microscopic exami-

nation is the best method for such measurement, it is time

consuming and destructive in nature. Therefore, develop-

ment of a suitable non-destructive tool which is fast and

efficient is of considerable importance. For several years,

Ultrasonic testing is being looked upon as an alternative

tool, which was till now better known for common appli-

cations like thickness gaging and flaw detection. There are

a number of non-destructive testing (NDT) techniques,

such as magnetic Barkhausen emission (MBE), x-ray dif-

fraction (XRD), and eddy current technique (ECT), but

these are all surface techniques only. Only ultrasonic

testing can be used for characterization of both surface, as

well as the full thickness of the materials, irrespective of

their electrical and magnetic properties.

The nickel-base superalloy Nimonic 263 was developed

and introduced for applications such as in the design of

combustion chamber, casing lining, exhaust ducting,

bearing housing, and such other components in aerospace

industry by Rolls Royce in 1960. These components are

fabricated from the plate/sheet of this alloy. Nimonic-263

was primarily intended to have properties of similar to the

weld ductility and fabrication characteristics of Nimonic 75

combined with the creep-rupture strength of Nimonic 80A

[1, 2]. Important to note here is that these alloys can be

age-hardened by the controlled precipitation of intragran-

ular sub-microscopic c0 phase, to enhance creep resistance

by acting as a barrier to dislocation movement. However,

the presence of carbon can lead to the formation of a series

of undesirable carbide phases: (a) Intragranularly occurring

primary carbides, nitrides, or carbonitrides of the general

G. V. S. Murthy (&) � S. K. Das � B. Ravikumar

Materials Science and Technology Division, National

Metallurgical Laboratory (CSIR), Jamshedpur 831 007, India

e-mail: gvs@nmlindia.org

123

Metallogr. Microstruct. Anal. (2014) 3:328–335

DOI 10.1007/s13632-014-0143-7

form M(C, N), where M is usually titanium, and (b) chro-

mium-rich grain boundary carbides like M7C3 and M23C6

[3, 4]. The standard heat treatment practice for Nimonic

263 is solution annealing at 1,423 K for 2 h followed by

aging at 1,273 K for 8 h [3]. In the optimum heat-treated

condition, the microstructure of the alloy shows a fine

discontinuous precipitation of carbides at the grain

boundaries and precipitation of c0 intermetallic in the

matrix [2]. The shape and size of c0 precipitates cannot be

resolved in an optical microscope. However, electron

microscopic studies show that the mean particle diameter is

about 18–20 nm [5]. The c0 phase that forms is metastable

in nature because of the high titanium to aluminum ratio,

the stable precipitate being g-Ni3Ti. On prolonged expo-

sure at temperatures in excess of 1,073 K, the c0 phase

gradually coarsens and acicular g begins to form [5]. The

primary objective of the solution treatment is to dissolve

the precipitated phases, mainly c0 and carbides, prior to

their controlled precipitation during subsequent aging

process. Aging treatments are primarily concerned with the

precipitation of the hardening phase, normally c0 in a form

to obtain the required mechanical properties. The changes

in the precipitation behavior are expected to affect the

elastic moduli (like change in the Young modulus due to

change in the local chemical compositions) of the material.

Change in Young modulus in turn affects the Ultrasonic

velocity through the empirical relationships [6]. Ultrasonic

velocity techniques have been attempted for characteriza-

tion of solutionizing treatment and precipitation behavior

in various alloy systems, such as aluminum alloys [7],

ferritic steel [8], maraging steel [9], nickel-base alloys [10–

13], and titanium alloys [14]. The changes in ultrasonic

velocity upon precipitation of intermetallic phases have

been attributed primarily to the change in the Young’s

modulus of the matrix of the alloy [11]. The objective of

the present study is to explore the possibility of using

ultrasonic measurements like longitudinal and shear wave

velocity to study the precipitation behavior in the nickel-

base superalloy Nimonic 263 for such characterization and

corroborate the same with, scanning electron microscopy

studies and hardness measurements.

Experimental

Table 1 shows the elemental composition of the alloy used in

the present study. Nimonic 263 specimens of dimensions

20 mm 9 20 mm 9 10 mm were solutionized at 1423 K for

1 h followed by water quenching. Aging of solution-treated

specimens was carried out for 1, 2, 4, 6, and 8 h at 923 K and 1,

2, 4, 6, 25, 50, and 75 h at 1,073 K followed by water

quenching.

A test coupon of the order of 5 mm in length, 5 mm in

width, and uniform thickness in the range of 2–5 mm was cut

from one corner of each of the specimens. These were pol-

ished in the conventional way as per the standard metallo-

graphic procedures and etched to reveal microstructure with

freshly prepared glycergia consisting of concentrated hydro-

chloric acid, concentrated nitric acid, and glycerol in the ratio

45:15:40. The microstructures were observed and recorded

using the scanning electron microscope along with an energy-

dispersive spectroscopy system.

Vicker’s hardness measurements were carried out using a

load of 30 kg. An average of five readings is reported here. A

maximum scatter of ±5 VHN was obtained in hardness

measurements in any specimen. Results of microstructure

analysis of precipitation behavior were used to corroborate the

ultrasonic velocity and hardness measurements.

The experimental set up used for the ultrasonic measure-

ments is shown in Fig. 1. A 35 MHz broadband pulser-

receiver and a 500 MHz digital oscilloscope were used for

carrying out the measurements. Ultrasonic velocities were

measured using 15 MHz longitudinal wave and 5 MHz shear

wave transducers. For post processing of the data, the RF

signals were digitized and the gated back wall echoes from the

oscilloscope were stored. A cross-correlation technique has

been used for precise ultrasonic velocity measurements [13].

The accuracy obtained in the time-of-flight measurement is

better than ±1 ns. This led to the maximum scatter of ±2.5

and ±1.5 m/s for ultrasonic longitudinal and shear wave

Fig. 1 Experimental set up used for ultrasonic velocity

measurements

Table 1 Chemical composition

of Nimonic 263Element C Co Cr Mo Ti Al Cu O N Ni

Wt% 0.06 19.6 20.1 6.0 2.0 0.4 0.001 0.0013 0.006 Bal.

Metallogr. Microstruct. Anal. (2014) 3:328–335 329

123

velocities, respectively. The density of the material was

assumed to be constant in the present study.

Results and Discussion

Figure 2 shows the microstructure of the solution-annealed

specimen at 1,423 K exhibiting a homogenous micro-

structure without any presence of precipitates in solution-

treated state. This was further confirmed by a very low

hardness value of 182Hv observed by hardness

measurement.

Effect of Aging at 923 K on Microstructural Changes

Aging for 1 h at 923 K did not result in the precipitation of

c0 intermetallic as kinetics of the precipitation were very

slow at this temperature. This was confirmed by the for-

mation of very fine precipitation of c0 intermetallic phase in

the specimen after 4 h of aging treatment. Figure 3(a) and

(b) shows the SEM micrographs corresponding to the

specimens aged at 923 K for 1 h, 923 K for 4 h, respec-

tively, to highlight occurrence of precipitates after 4 h

annealing time.

The hardness measurements indicated a continuous

increase of hardness up to the maximum duration of aging

(8 h) used in the present investigation.

Effect of Aging at 1,073 K on Microstructural Changes

Aging treatment at 1,073 K showed faster kinetics of c0

phase precipitation behavior compared to aging treatment

at 923 K. The SEM micrograph of the specimen aged for

6 h clearly shows extensive precipitation of c0 interme-

tallic phase, Fig. 4. In addition to the c0 phase precipita-

tion and its coarsening, formation of grain boundary

carbides was also observed in specimens aged for longer

durations. The results obtained are in line with the time

temperature transformation (TTT) diagram reported in

literature [15] for the precipitation of c0 intermetallic

phase in this alloy.

Sample Aged for 25 h

For the sample aged at 1,073 K for 25 h, the carbide pre-

cipitation has started and the microstructures observed

showed the extensive nucleation without much growth of

these precipitates. Figure 5 below shows a few of these

Fig. 2 SEM micrographs of Ni-263 specimen solution annealed at

1,423 K for 1 h

Fig. 3 SEM micrographs of Ni-263 specimens solution annealed at

1,423 K for 1 h followed by aging at (a) 923 K for 1 h exhibiting no

precipitation; (b) 923 K for 4 h exhibiting very fine precipitation

330 Metallogr. Microstruct. Anal. (2014) 3:328–335

123

representative microstructures from which it is clearly seen

the precipitates are uniformly distributed, but at the same

time are of very small dimensions.

Sample Aged at 50 h

The sample aged at 1,073 K for 50 h shows extensive

precipitation. The precipitation seems to be complete and

uniformly distributed in the matrix. This is very well in

agreement with the reported standard heat treatment for

Nimonic 263; which is solution annealing at 1,423 K for

2 h followed by aging at 1,273 K for 8 h [3]. In the opti-

mum heat-treated condition, the microstructure of the alloy

shows a fine discontinuous precipitation of carbides at the

grain boundaries and precipitation of c0 intermetallic in the

matrix [2]. The shape and size of c0 precipitates cannot be

resolved in an optical microscope. However, electron

microscopic studies show that the mean particle diameter is

about 18–20 nm [5]. The morphology and the composi-

tions of the different precipitates are different as can be

seen in the following microstructures. In what follows in

Fig. 6, we find that there are at least two distinct types of

precipitates present. They are (1) Ti-Mo rich precipitates

and (2) Carbides of Ti, Mo, and Cr. These can be either

grain boundary or otherwise.

Fig. 4 SEM micrographs of Ni-263 specimens solution annealed at

1,423 K for 1 h followed by aging at 1,073 K for 6 h showing

precipitation of c0

Fig. 5 SEM micrographs of Ni-263 specimens solution annealed at

1,423 K for 1 h followed by aging at 1,073 K for 25 h

Fig. 6 SEM micrographs of Ni-263 specimens solution annealed at

1,423 K for 1 h followed by aging at 1,073 K for 50 h, showing the

highlighted Ti–Mo rich precipitates which are also elongated and

away from the grain boundary. The EDAX compositional analysis of

this precipitate is Ti: 46.90; Cr: 7.07; Co: 4.64; Ni: 11.88; and Mo:

29.51

Fig. 7 SEM micrographs of Ni-263 specimens solution annealed at

1,423 K for 1 h followed by aging at 1,073 K for 50 h, showing

precipitate coarsening

Metallogr. Microstruct. Anal. (2014) 3:328–335 331

123

SEM micrographs of Ni-263 specimens solution

annealed at 1,423 K for 1 h followed by aging at 1,073 K

for 50 h also showed the carbide precipitation which are

along grain boundary and within the grain, as shown in

Fig. 7. Besides the elongated grains seen as above, some

finely dispersed precipitates were also observed (see

Fig. 7); these were assumed to be complex carbides of Ti,

Cr, Co Mo, and Ni.

In addition to all the above precipitates, there were

globular precipitates at some places as shown in Fig. 8

below. From the compositional analysis, it was inferred

that these are intermetallic precipitates containing Ni, Ti,

Mo, Cr, and Co.

Sample Aged at 75 h

The sample aged at 1,073 K for 75 h shows that the c0 was

transformed into the acicular g phase. This was evident

from the sharp decrease in hardness upon aging at 1,073 K

for 75 h and was attributed to the coarsening of fine

coherent c0 phase and its transformation to acicular g phase

as seen in Fig. 9. Even in this case also we have observed

at least two distinct types of precipitates. They are (1)

intermetallic of Ni, Cr, and Co-containing Ti and Al; and

(2) carbides of Ni and Co, Cr.

Upon aging at 1,073 K, hardness increased rapidly up to

6 h of time (277 HV), followed by a subtle increase up to

50 h (297 HV). Further increase in time to 75 h, a small

decrease in hardness (277 HV) was noted. The increase in

the hardness at both the temperatures is attributed to the

precipitation of coherent c0 intermetallic phase. The shorter

incubation period and faster increase in hardness during the

initial period of aging (up to 6 h) at 1,073 K compared to

those at 923 K was attributed to the faster kinetics of

precipitation at higher temperature (1,073 K). The decrease

in hardness upon aging at 1,073 K for 75 h was attributed

to the coarsening of fine coherent c0 phase and its trans-

formation to acicular g phase

Ultrasonic Velocity Characterization

Elastic waves propagating through an unbounded medium

can be expressed as plane waves [16]:

A ¼ A0 exp½iðkx� xtÞ� expð�axÞ; ð1Þ

where A is the amplitude, A0 is an initial amplitude at an

initial amplitude at x = 0 and t = 0, a is the attenuation

coefficient, and k is the propagation constant

k ¼ 2p=k ¼ 2pf=V ð2Þ

x is the angular frequency

x ¼ 2pf ð3Þ

k is the wavelength, f is the frequency, and V is the

velocity.

The velocity of sound V is determined by the moduli and

density q of the material supporting the sound wave by an

equation of the form

V ¼ ðM=qÞ1=2; ð4Þ

where M is a particular combination of the elastic moduli of

the material itself. The particular combination depends upon

the mode of motion being propagated. The mode in turn

depends upon the state of the material-gas, liquid, or solid-,

and in solids depends further upon the boundary conditions

and the stress direction relative to the direction of propagation.

Thus, there will be one longitudinal wave mode with velocity

Fig. 8 SEM micrographs of Ni-263 specimens, solution annealed at

1,423 K for 1 h followed by aging at 1,073 K for 50 h, showing

globular precipitates

Fig. 9 SEM micrographs of Ni-263 specimens solution annealed at

1,423 K for 1 h followed by aging at 1,073 K for 75 h, showing

carbide after the coarsening of c0

332 Metallogr. Microstruct. Anal. (2014) 3:328–335

123

Vl, where the direction of stress and strain is parallel to the

direction of propagation and two shear wave modes where the

shearing stress is perpendicular to the direction of propagation

and where shear in one mode is at 90� to shear in the other. For

isotropic materials, these two shear modes are indistinguish-

able (degenerate) and have the same velocity Vs. Shear

velocity is slower than longitudinal velocity, generally 50–

60% of the longitudinal. For isotropic materials, the Young’s

modulus (E), the shear modulus (G), and Poisson’s ratio (l)

are related to the longitudinal and shear velocities and to the

density by the equations:

l ¼ 1� Vs=Vlð Þ2h i

= 2� 2 Vs=Vlð Þ2h i

ð5Þ

E ¼ Vlð Þ2ð1þ lÞð1� 2lÞ=ð1� lÞ ð6Þ

G ¼ Vsð Þ2 ð7Þ

Thus to different degrees, elastic moduli, material micro-

structure, morphological conditions, and associated

mechanical properties can be characterized by ultrasonic

velocities. Table 2 lists the measured velocities, calculated

elastic moduli, and the hardness for each of the samples in

this study. By empirical considerations, it can be shown

that the longitudinal (VL) and shear (VT) wave velocity can

be represented respectively as [17]

vL ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiEð1� mÞ

q 1þ mð Þ 1� 2mð Þ

s

vT ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

E

2q 1þ mð Þ

s;

where E, and m are the Young’s modulus, and Poisson’s

ratio. The above relationship clearly indicates the

correlation ship between ultrasonic velocity and the elastic

moduli.

Figures 10 and 11 show the changes in the ultrasonic

velocity with aging temperature and time. Ultrasonic

velocities were found to be the minimum (VL = 5,960 m/s

and VS = 3,155 m/s) in the solution-annealed condition

and increased with aging at both the temperatures. Upon

aging at 923 K, velocities were found to increase contin-

uously up to the maximum duration of aging (8 h;

VL = 5,995 m/s and VS = 3,192 m/s) used in the present

investigation, whereas it exhibited saturation beyond 4 h of

aging at 1,073 K. The ultrasonic shear wave velocity

(1.17% change) was found to be more sensitive to the aging

treatment than the longitudinal wave velocity (0.59%

change). This is in line with those reported for precipitation

Table 2 Ultrasonic velocities, hardness, and elastic properties of Nimonic 263 in different heat treatment conditions

Heat treatment Sample

designation

Hardness

(Hv)

Longitudinal wave

velocity (m/s)

Shear wave

velocity (m/s)

Shear Modulus

(GPa)

Young’s Modulus

(GPa)

Poisson’s ratio

1,423 K for 1 h (SA*) S 182 5,960 3,155 83.22 217.25 0.3053

SA ? 923 K/1 h A1 183 5,981 3,170 84.01 219.21 0.3046

SA ? 923 K/2 h A2 193 5,980 3,175 84.27 219.74 0.3037

SA ? 923 K/4 h A3 209 5,992 3,187 84.88 221.17 0.3028

SA ? 923 K/6 h A4 215 5,993 3,185 84.81 221.04 0.3032

SA ? 923 K/8 h A5 236 5,995 3,192 85.18 221.83 0.3022

SA ? 1,073 K/1 h B1 224 5,984 3,181 84.75 220.74 0.3025

SA ? 1,073 K/2 h B2 241 5,982 3,179 84.38 219.99 0.3036

SA ? 1,073 K/4 h B3 266 5,987 3,183 84.70 220.74 0.3030

SA ? 1,073 K/6 h B4 277 5,988 3,180 84.54 220.41 0.3036

SA ? 1,073 K/25 h B5 285 5,987 3,182.6 84.68 220.68 0.3031

SA ? 1,073 K/50 h B6 297 5,986 3,182 84.65 220.60 0.3031

SA ? 1,073 K/75 h B7 277 5,985 3,181.1 84.60 220.48 0.3031

Fig. 10 Variation in ultrasonic longitudinal wave velocity and

hardness with aging at 923 and 1,073 K

Metallogr. Microstruct. Anal. (2014) 3:328–335 333

123

of intermetallic phases in other nickel-base superalloys [9–

12, 15] and maraging steel [8], where ultrasonic shear wave

velocity was found to change more compared to longitu-

dinal wave velocity, upon precipitation of intermetallic

phases. The maximum ultrasonic shear wave velocity in the

specimen aged at 923 K was found to be 3,192 m/s (upon

8 h of aging), which is higher than that observed for the

specimens aged at 1,073 K (3182 m/s). This was attributed

to the lesser amount of precipitation at 1,073 K due to

higher solubility of precipitate forming elements at higher

temperature. Further, even though the hardness started

increasing beyond 1 h of aging at 923 K, ultrasonic

velocities increased substantially upon aging for 1 h of

aging itself. Similarly, the ultrasonic velocities seem to

have attained saturation after aging for 4 h at 1,073 K,

whereas hardness exhibited continuous increasing trend up

to 50 h of aging. The coarsening of c0 phase and its

transformation to acicular g phase decreased the hardness

upon aging for 75 h at 1,073 K as discussed earlier; how-

ever, these phase transformation changes did not influence

the ultrasonic velocities.

From these results, it is evident that the ultrasonic

velocity is more sensitive to the initial stages of the

precipitation process, while hardness measurements do

not record any change in the hardness value. This is

attributed to the fact that the ultrasonic velocities are

sensitive to changes in the elastic moduli of the material,

which is affected by compositional variations of the bulk

material. On the other hand, hardness is a bulk property

that is influenced by the precipitation of coherent phases

which hinders the movement of dislocations. There are

several theories of precipitation hardening. According to

the coherent lattice theory [18], the alloy remains in a

supersaturated condition after solution treatment and

quenching, with the solute atoms distributed at random in

the lattice structure. The process of precipitation of

intermetallic phases upon aging the solution-annealed

alloy can be divided into two stages: (I) incubation per-

iod, when the excess solute atoms tend to migrate to

certain crystallographic planes, forming clusters or

embryos of the precipitate and (II) precipitation, when

these clusters form an intermediate crystal structure or

transitional lattice, maintaining registry (coherency) with

the lattice structure of the matrix. This phase will have

lattice parameters different from those of the solvent, and

as a result of the atom matching (coherency) there will be

considerable distortion of the matrix. This distortion

(strain free) extends over a distance more than the size of

a discrete (precipitate) particle. It is this distortion that

interferes with the movement of dislocations and accounts

for the increase in hardness and strength during aging.

The first stage of the precipitation (i.e., incubation) does

not influence the hardness; however, it drastically influ-

ences the elastic moduli of the material and that in turn

affects the ultrasonic velocities. The depletion of the

precipitate forming elements from the matrix leads to

increase in the modulus of the matrix due to the change in

the chemical composition. Hence, ultrasonic velocity is

sensitive to onset of compositional variations and hence

responds to early stages of microstructure changes in

these alloys. Further, as the coarsening of c0 phase and its

conversion to acicular g phase do not change the matrix

composition, therefore, they do not influence the ultra-

sonic velocities.

In Figs. 12 and 13, the variation of ultrasonic velocity

(both longitudinal and shear) with hardness is shown. It is

very interesting to note that the ultrasonic velocity response

is very fast and the slope is quite high indicating that from

Fig. 11 Variation in ultrasonic shear wave velocity and hardness

with aging at 923 and 1,073 K Fig. 12 Variation of ultrasonic velocity (longitudinal) with hardness

334 Metallogr. Microstruct. Anal. (2014) 3:328–335

123

the above considerations, ultrasonic velocity can be a more

reliable method for monitoring the precipitation behavior

with the aging time. Similar trends were observed in the

other temperature studied herein.

Conclusion

In the present study, the precipitation process in a Ni-based

super alloy Nimonic 263 has been studied using ultrasonic

velocity and microscopy. It has been observed that Ni 263

undergoes significant microstructural changes on aging at

1,073 K. For aging time up to 50 h, the formation of c0

phase is observed leading to the strengthening of the alloy.

This is shown by the increase in the hardness and ultrasonic

velocity. From the present study, it is also seen that that

ultrasonic measurements are more sensitive to the precipi-

tation process which is attributed to the compositional

changes and hence subtle changes in the elastic moduli. In

contrast, the influence of precipitation on hardness can be

felt/noted only after the precipitates attain a minimum size

to influence the movement of dislocations. These observa-

tions are corroborated with the electron microscopy studies.

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