Mechanical behaviour and damping properties of Ni modified Cu-Zn-Al shape memory alloys

(a) Damping capacity of the unmodified and Ni modified CueZneAl alloys at 1 Hz test frequency; (b) Damping capacity of the unmodified and Ni modified CueZneAl alloys at 2 Hz test frequency; [r]

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Original Article

Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure PMB 704, Nigeria

a r t i c l e i n f o

Article history:

Received 26 February 2018

Received in revised form

18 May 2018

Accepted 25 May 2018

Available online 1 June 2018

Keywords:

CuZnAl alloys

Shape memory capacity

Micro-alloying

Mechanical behaviour

Damping properties

Structural characterization

a b s t r a c t

The microstructure, mechanical behaviour and damping properties of Cue18Zne7AlexNi alloys (where

x¼ 0.1, 0.2, 0.3 and 0.4) were investigated The CueZneAl alloys were produced by casting and then subjected to a homogenization e cold rolling e annealing treatment scheme Optical-, scanning electron-microscopy and X-ray diffraction analysis were utilized for structural characterization of the alloys, while tensile test, fracture toughness, and hardness measurement were used to assess the me-chanical properties The results show that all the alloy compositions consisted of the predominating CuZn phase Sharp edged elongated grain structures were observed in the unmodified and the 0.4% Ni modified CuZnAl alloys, while the 0.1, 0.2 and 0.3 %Ni modified CuZnAl alloy compositions, had more of granular/curved/round grain edges and smaller grain widths The hardness of the unmodified CuZnAl alloy (294.5 ± 2.08 VHN) was lower than that of the Ni modified CuZnAl ones with an increase in hardness ranging between 23.5 and 38.4% The tensile strength, the percentage elongation (10.7e14.3%) and the fracture toughness of the 0.1, 0.2 and 0.3% Ni modified CuZnAl alloys were observed to be higher than those of the unmodified and the 0.4 %Ni modified CuZnAl alloys The 0.2% Ni modified CuZnAl alloy had the highest damping capacity among all compositions under investigation, while the 0.4% Ni modified one showed the least capacity to serve as a damping material

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

There is a growing attention to the problems created by

me-chanical, structural and noise vibrations in the environment on the

account of industrial processes, seismic events, excavation,

con-struction, mining, and exploration activities[1e3] These vibration

sources can generate diverse effects ranging from mild discomfort

and general machinery inefﬁciencies to collapse of structures, loss

of investments, lives and properties[3,4] In order to mitigate the

potential hazards which could arise from such vibrations, there is

growing interest in the development of damping materials for

vi-bration control in engineering structures and systems [4e6]

Damping materials possess the inherent capacity of attenuating

vibrations by dissipating the energy absorbed during the vibration

to a safe mode such as heat, usually by hysteretic actions[7] There

is a wide range of engineering materials with damping ability

among which shape memory alloys (SMAs) have been found extremely useful[8,9]

Shape memory alloys are known principally for their shape memory effect and pseudoelastic properties, but have also been observed to possess high damping capacitye which makes them attractive for the design of vibration control devices[10,11] The high damping capacity observed in SMAs has been attributed to the high internal friction occurring during martensitic transformation, which manifests in the loss of energy by the movement between the martensite variant interfaces and the parent martensite habit planes [12,13] These remarkable damping properties have been observed principally in NiTi and Cu based SMAs CuZnAl based SMAs are however the focus of this research because of its relatively cheaper processing cost and its relatively superior strain recovery compared to other Cu based SMAs[14]

The damping properties of CuZnAl based SMAs have been the subject of several investigations[15,16] Nai-chao[17]showed that the damping performance of CuZnAl SMAs is dependent on whether it is in martensitic or austenitic state The area enclosed by the hysteretic loope which is a measure of the damping capacity e was observed to be larger in the martensitic state than in the

* Corresponding author.

E-mail address: kalanemek@yahoo.co.uk (K.K Alaneme).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.05.002

Journal of Science: Advanced Materials and Devices 3 (2018) 371e379

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austenitic state The former was thus better suited for vibration

control application Wu et al.[18], studied the damping

charac-teristics of the inherent and intrinsic internal friction of CuZnAl

SMAs containing varied Zn content It was reported that the

damping capacity was sensitive to the Zn composition of the alloys

Compositions containing 7.5e8.5 wt.% Zn were found to have

higher damping capacities compared to compositions containing

7 wt.% Zn (Cu7Zn11Al), because theg03 martensite phase they are

characterized which possesses a 2H type structure with abundant

movable twin boundaries Cimpoesu et al.[19]studied the effect of

stress on damping capacity of CuZnAl SMAs It was observed that

the internal friction peak in these SMAs, is inﬂuenced by the

tem-perature and deformation Also lean amounts of elements like iron,

lead, and nickel present in the CuZnAl alloy, helped improve the

damping capacity of the alloy The role of these elements (iron, lead

and nickel) when used independently as additions to the CuZnAl

alloy was however not covered by the study

It is recalled that the use of elements such as Fe, B, Ni, Ti, among

others as micro-alloying additions in Cu based SMAs, have been

reported to modify the grain structure; and consequently, improve

the toughness and the overall mechanical performance of the SMAs

[14] Ni in particular, has been reported to help raise the

trans-formation temperature of Cu based alloys, but its impact on

me-chanical and damping behaviour when used as micro-alloying

addition in CuZnAl alloys has not received comprehensive

reportage in literature This happens to be the focus of the present

study, which aims at assessing the potentials of Ni modiﬁed CuZnAl

SMAs for structural and damping applications where there is a

growing quest for functional and cheap alternatives to NiTi based

SMAs

2 Experimental

2.1 Materials preparation

Conventional liquid metallurgy route was utilized for the

pro-duction of the Ni modiﬁed CuZnAl alloys following procedures in

accordance with Alaneme[20] Five compositions of the alloys were

produced containing: Cu18Zn7Al as base alloy composition, 0.1, 0.2,

0.3 and 0.4 wt.% Ni additions to four different heats of the

Cu18Z-n7Al alloy e for the production of the four Ni modiﬁed alloy

compositions A crucible furnace was used for the melting

opera-tion, and the melts cast into cast iron metallic moulds The chemical

compositions of the ﬁve CuZnAl based alloys produced were

determined by SEM-EDS analysis, and are presented with their

respective sample designations in Table 1 A homogenization

treatment performed at 800C for 4 h followed with air cooling,

was undertaken to improve the chemical and microstructural

ho-mogeneity in the alloys produced They were subsequently cold

deformed to 10% reduction of the original thickness, using a

mini-ature cold rolling machine; and thereafter, annealed at 450C for

1 h followed by air cooling to eliminate cold deformation induced

internal stresses developed in the samples The samples for

me-chanical and damping tests, microstructural and phase analysis,

were then machined to standard speciﬁcations before a ﬁnal

annealing at 400C for 2 h followed by water quenching at room temperature, was performed to eliminate machining stresses on the samples

2.2 Microstructure and phase characterization The structures of the produced alloys were characterized using optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffractometry (XRD) in accordance with Alaneme et al.[21] The microstructures of the CuZnAl alloys produced were studied using a Zeiss optical microscope The samples were prepared to metallographic finish using a series of grinding and polishing processes The mirrorfinish surface produced on each sample was etched by swabbing for 10e20 s using a solution containing 5 g ferric chloride, 10 ml HCl, and 95 ml ethanol, after which micro-structural analysis was performed on the samples The optical mi-croscopy analysis was complemented with detailed microstructural and compositional studies using a TESCAN VEGA3 thermionic emission scanning electron microscope system with accessories for energy dispersive spectroscopy (EDS) The SEM analysis entailed the use of back scattered electron (BSE) and secondary electron (SE) modes imaging for assessing the phase and the grain distribution, while SEM-EDS analysis was used for the determination of the chemical compositions of the CuZnAl alloys XRD analysis was finally utilised for the phase characterization of the CuZnAl alloys produced The samples for the analysis were prepared following standard procedures The crystalline phases present and their peak intensities were determined using a PANanalytical Empyrean diffractometer with PIXCEL detector and Fefiltered Co-Karadiation source was used for the analysis The analysis was performed from diffraction 2qangle spectral range of 0to 120while the phases were identified using the X'Pert Highscore plus software The crystal structures of the phases identified were determined by analyzing the pattern of the diffracting crystal planes (hkl) for the entire range of the 2qdiffraction angles[22]

2.3 Mechanical testing

A Vickers hardness scale was used for the evaluation of the hardness of the alloys, using a hardness testing machine The samples were prepared withﬁne ﬁnished plane parallel surface, while the testing procedure was in accordance with the ASTM

E92-17 standard[23] The hardness test was performed using a 30 kgf load for a dwell time of 10 s The hardness indentation was repeated for a minimum of ﬁve times and readings within the margin of 2% were taken for the determination of the average hardness values

The tensile testing was performed on the as-produced CuZnAl alloys using a universal testing machine The samples for the test were machined to tensile test speciﬁcation of 5 mm diameter and

30 mm gauge length The test samples were mounted on the testing platform and pulled in tension to fracture at a strain rate

of 103/s The samples preparation, testing procedure and data analysis were performed following the recommendations of ASTM E8/E8M-15a[24]standard Three repeated tests were per-formed for each CuZnAl alloy composition produced to guarantee the reliability and to assure the reproducibility of the test results The ultimate tensile strength and strain to fracture were evalu-ated from the stressestrain curves developed from the test conducted

The fracture toughness values of the CuZnAl alloys were eval-uated using the circumferential notch tensile (CNT) testing approach in accordance with Alaneme [25] The CuZnAl alloy samples were machined to the test speciﬁcations: gauge length of

27 mm, gauge diameter of 6 mm (D), notch diameter of 4.2 mm (d),

Table 1

Chemical composition of the unmodiﬁed and Ni modiﬁed CuZnAl alloys.

K.K Alaneme, S Umar / Journal of Science: Advanced Materials and Devices 3 (2018) 371e379 372

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and notch angle of 60 The samples were mounted on the testing

stage and subjected to tensile loading to fracture using a universal

testing machine operated at a strain rate of 103/s The fracture

loads (Pf) obtained from the CNT samples' loade extension plots

were used to evaluate the fracture toughness of the alloys based on

the relation[26]:

KIC¼ Pf

D32=

1:72

D d

1:27

(1)

where D and d are the specimen diameter and the diameter of the

notched section, respectively The results were validated for plane

strain condition required for valid fracture toughness

determina-tion using the reladetermina-tions in accordance with Nath and Das[27]:

D

K1C

sy

2

(2)

Three repeated tests were performed for each CueZneAl alloy

composition to ensure that generated results are consistent and

hence reliable

2.4 Damping behaviour

The temperature dependence of the damping properties of the

CuZnAl alloys were assessed on a dynamic mechanical analyzer,

using the three-point bending deformation mode in accordance

with the ASTME756-05[28]standard Rectangular bar samples with

dimensions of 40 mm 5 mm 0.9 mm, were prepared for the

damping properties tests For the measurements of the temperature

dependent damping properties, the test conditions were set to strain

amplitude (ε) of 10mm, vibration frequency ( f ) of 1 and 2 Hz,

tem-perature range (t) from room temtem-perature to 250C, and heating rate

(T0) of 5C/min The loss modulus (E00) and the storage modulus (E0)

were determined from the test, and the damping capacity measured

from the loss tangent (tan d), using the relation[29]:

tand¼E

00

3 Results and discussion

3.1 Microstructure and phase analysis of the CueZneAl alloys

3.1.1 Optical microscopy

The optical micrographs of the unmodiﬁed and Ni modiﬁed

CuZnAl alloys are presented in Fig 1 Fig 1a shows the optical

micrograph of the unmodiﬁed CuZnAl alloy, which is observed to

contain sharp edged directionally solidiﬁed grains The elongated

grain feature is very common in CuZnAl shape memory alloys

within the composition range considered in this research, and

speciﬁcally matches the structural features reported in[30e32]

The CuZn Al alloy composition modiﬁed with 0.1% Ni (Fig 1b),

shows a signiﬁcantly modiﬁed grain morphology from the sharp

edged feature observed for the unmodiﬁed CuZnAl alloy to mostly

granular/polygonal shaped grain structure The round shaped

grains appear smaller in size compared to those in the unmodiﬁed

CuZnAl alloy For the 0.2% Ni modiﬁed CuZnAl alloy (Fig 1c), the

grain structure consists ofﬁner and sparsely distributed elongated

grains The 0.3% Ni modiﬁed CuZnAl alloy (Fig 1d), also shows a

directionally solidiﬁed grain structure, but with some curve/round

edged grain features The 0.4% Ni modiﬁed CuZnAl alloy (Fig 1e),

equally contains dominantly elongated grains although the longi-tudinal lengths of the grains are seen slightly shorter than those observed for the unmodified CuZnAl alloy (Fig 1a).Table 2shows the average grain width (transversal axis thickness) of the un-modified and Ni modified CuZnAl alloys It is observed that the average grain width decreases from 7.9 mm for the unmodified CuZnAl alloy to as low as 3.8mm for the 0.2% Ni modified CuZnAl alloy, which corresponds to approximately 52% decrease in the grain width The increase in the Ni content above 0.2%, is observed

to result in a marginal grain refinement and in the grain width coarsening for the 0.3% and 0.4% Ni modified CuZnAl alloys, respectively Grain width increase as high as 22.7% was observed with the 0.4% Ni addition in comparison to the base CuZnAl alloy The analyses fromFig 1andTable 2show that the amount of Ni used as micro-alloying addition in CuZnAl alloy significantly affects its solidification patterns Grain modification was more pronounced and distinct for the 0.1% Ni modified CuZnAl alloy where a dramatic transformation from sharp edged elongated grain structure to a predominantly granular structure was observed The 0.2% Ni addition resulted in thefinest size and least predominant presence

of the needle-like structures and the 0.3% Ni addition only induces slight changes in the grain edge morphology, while the 0.4% Ni modiﬁed CuZnAl alloy composition shows changes in both grain growth and in grain edge morphology

3.1.2 SEM observations Representative secondary electron mode images of the un-modiﬁed and selected modiﬁed CueZneAl alloys are presented in Fig 2 They all demonstrate profoundly identical structure features

as observed in the corresponding optical micrographs of the investigated alloys (Fig 1) As it is seen inFig 2a, the unmodified alloy microstructure consists of predominantly elongated grains with sharp edges In Fig 2b, the micrograph of the 0.2% Ni modified CuZnAl alloy shows predominantly a finer structure with

a few needle-like precipitated features, while inFig 2c the 0.4% Ni modified CuZnAl alloy is seen to consist of slightly elongated larger size grains compared to the unmodified one These results confirm the observations from the optical microscopy investiga-tion that the presence of Ni in the CuZnAl alloys significantly alters their solidification patterns with essentially varied grain morphology This sort of influence of Ni as micro-alloy addition, has also been reported for several other micro-alloying elements [14,33,34] Fig 3 with representative EDS spectra confirms the presence of Cu, Zn, Al and Cu, Zn, Al, Ni for the unmodified and the 0.4% Ni modified CuZnAl alloy, respectively

3.1.3 Analysis of X-ray diffraction The X-ray diffraction analysis of the unmodified and Ni modified CuZnAl alloys is presented in Fig 4 As it is observed from the patterns, all samples consist essentially of CuZn phase despite the differences in grain morphology (seeFigs 1 and 2) There are slight shifts in the peak positions for each of the alloy compositions (although less than 2) which is a common feature in alloys with slightly altered alloying composition (due to the difference in lat-tice parameters) The analysis of the pattern of the diffracting crystal planes for the entire 2q diffraction angles and the lattice parameters confirm the CuZn phase to be thebe phase, which from studies has been shown to be martensitic and displays shape memory capacity[35] There are very small peaks of CuAl and NiAl phases observed from the XRD scan for the Ni modified composi-tions These peaks are, however, not well resolved in the plots because of the small intensities The predominance of the CueZn peaks in the difractograms as analyzed is in agreement with the EDS results presented inFig 3

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Fig 1 Optical micrographs of (a) unmodiﬁed CueZneAl alloy, and (b) 0.1 wt.% Ni-, (c) 0.2 wt.% Ni-, (d) 0.3 wt.% Ni-, and (e) 0.4 wt.% Ni-modiﬁed CueZneAl alloys.

Table 2

Average lath martensite transverse axis thickness in the CuZnAl alloys produced.

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3.2 Mechanical behaviour

3.2.1 Hardness

The hardness values of the unmodiﬁed and Ni modiﬁed CuZnAl

alloys are presented inFig 5 It is observed that the hardness values of

the CuZnAl alloys with Ni addition, are basically greater than that of

the unmodiﬁed CuZnAl alloy The maximum increase is obtained in

the 0.2% Ni modiﬁed CuZnAl alloy, which corresponds to a 38.4%

in-crease in hardness The hardness, however, is found dein-creased with

the further increase of the Ni addition, albeit not congruently with Ni

concentration The improved hardness observed in the Ni modiﬁed

CuZnAl alloys, is attributed to the presence of Ni as micro-alloying

addition, which resulted in the reduction of the elongated grain

width (i.e grain thickness) and the modiﬁcation of grain edge

morphology The 0.2% Ni modiﬁed CuZnAl alloy has the highest

hardness value, as it is seen inFig 1andTable 2, this sample also

contains theﬁnest size grain structure with the smallest grain width,

respectively This improved resistance to indentation with the

decreased grain size is in agreement with the HallePetch relation[36]

3.2.2 Tensile properties

The stressestrain plots of the unmodiﬁed and Ni modiﬁed

CuZnAl alloys are presented inFig 6, while the ultimate tensile

strength and percentage elongation plots are presented inFigs 7 and 8, respectively

As it can be seen inFig 7, the ultimate tensile strength (UTS) of the unmodiﬁed and the 0.4% Ni modiﬁed CuZnAl alloys show values

of 450.85 MPa and 452.62 MPa, respectively; which are lower than the UTS values of the other Ni modified CuZnAl alloys Compared to the unmodified CuZnAl alloy, tensile strength increases of 9.8, 12.1, 13.7, and 0.4% were obtained with the use of 0.1, 0.2, 0.3, and 0.4 wt.% Ni, respectively The low tensile strength of the unmodified and 0.4 wt.% Ni modified CuZnAl alloys can be linked to the rela-tively larger width of the elongated grains in these samples Also the sharp edges of the grain structure for both samples is another important factor Sharp tip grain edges usually serve as stress concentration sites which would suggest that the nominal stress acting on the material is considerably amplified to values exceeding its maximum stress bearing capacity at the grain tips [37] Accordingly, the improved tensile strength of the 0.1, 0.2, and 0.3 wt.% Ni modified CuZnAl alloys, is linked to the finer elongated grain width coupled with the change in the grain morphology to the granular structure (for the 0.1% Ni modified alloy), and the round/elliptical grain edges (for the 0.3% Ni modified alloy) The finer grain structure, round/elliptical grain edges and granular structures observed in these Ni modified CuZnAl alloys, reduce the Fig 2 Representative SEM secondary electron mode micrographs of (a) unmodified CueZneAl alloy, (b) 0.2 wt.% Ni-, and (c) 0.4 wt.% Ni-modified CueZneAl alloys.

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tendency of the grain edges to serve as stress concentration sites.

Consequently, the nominal applied stress on the alloys must be

high to attain the maximum stress bearing capacity values for the

process of micro-crack formation and fracture to be heralded in the

alloys The signiﬁcance of the results is that the addition of between

0.1 and 0.3 wt.% Ni to CuZnAl alloy can enhance the stress trans-mission/bearing capacity of the alloy

The percentage elongation of the unmodified and Ni modified CuZnAl alloys are presented inFig 8 It is observed that the elon-gation values of the unmodified and 0.4 wt.% Ni modified CuZnAl

Fig 3 Representative SE mode images and EDS profiles of (a) unmodified CueZneAl alloy, and (b) 0.4 wt.% Ni modified CueZneAl alloy.

Fig 4 X-ray diffractograms of the unmodiﬁed and Ni modiﬁed CueZneAl alloys.

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alloys are almost at the same level (8.5e8.6%) But there is an

improvement in the elongation values for the other Ni modiﬁed

CuZnAl alloys ranging between 10.7e14.3% The CuZnAl alloy

modiﬁed with 0.3 wt.% Ni, shows the highest elongation value of

14.3% This implies that the addition of 0.1e0.3 wt.% Ni can result in

improved ductility in CuZnAl alloys and hence enhanced plastic

workability of these The reasons for the lower percentage

elon-gation of the unmodiﬁed and 0.4% Ni modiﬁed CuZnAl alloys, is tied

to the sharp tip grain edges, which can restrain plastic deformation

due to the triaxial stress state created at such sites The plasticity

restraint makes the alloys more resistant to yielding and hence

exhibit less ductility[36] In the case of the 0.1, 0.2 and 0.3 wt.% Ni modiﬁed CuZnAl alloys, the development of a granular structure, fewer sharp edged grains, and curved/elliptical grain edges, respectively; are responsible for the high plastic strain sustaining capacity of these alloys

3.2.3 Fracture toughness The fracture toughness of the unmodified and the Ni modified CuZnAl alloys are presented inFig 9 It is clearly seen that all the Ni modified CuZnAl alloys except the 0.4% Ni modified composition, show fracture toughness values that are higher than that of the unmodified one The fracture toughness increase of 13.4, 28, and 12% are obtained for the 0.1, 0.2 and 0.3% Ni modified CuZnAl alloys, respectively, while a 4% decrease in fracture toughness is observed for the 0.4% Ni modified one These observations imply that the 0.1e0.3% Ni microalloying addition in the CuZnAl alloy improves its resistance to crack propagation, while the 0.4% Ni microalloying addition, is detrimental to the toughness of the CuZnAl alloys The same reasons attributed to the improved ductility are valid for the improvement in the fracture toughnesse that is, the change in the grain edge shape from sharp edged to round/elliptical shape for the 0.1 and 0.3 Ni modified CuZnAl alloys and to the fewer elongated grain structure in the 0.2% Ni modified one A preponderance of sharp edge grains is known to facilitate the triaxial stress state at the grain tip which suppresses yielding and accentuates brittle fracture susceptibility[21]

Fig 5 Hardness values of the unmodiﬁed and Ni modiﬁed CueZneAl alloys.

Fig 6 Stressestrain curves of the unmodiﬁed and Ni modiﬁed CueZneAl alloys.

Fig 7 Ultimate tensile strength of unmodiﬁed and Ni modiﬁed CueZneAl alloys.

Fig 8 Percentage elongation of the unmodiﬁed and Ni modiﬁed CueZneAl alloys.

Fig 9 Fracture toughness of the unmodiﬁed and Ni modiﬁed CueZneAl alloys K.K Alaneme, S Umar / Journal of Science: Advanced Materials and Devices 3 (2018) 371e379 377

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3.3 Damping properties

Fig 10shows the damping capacity, storage modulus, and loss

modulus of the unmodiﬁed and Ni modiﬁed CuZnAl alloys As it is

seen inFig 10a, at 1 Hz test frequency only the 0.2% Ni modiﬁed

CuZnAl alloy shows higher damping capacity values than the

un-modiﬁed one for the test temperature range of 25Ce250C The

0.4% Ni modiﬁed CuZnAl alloy exhibits the least damping capacity

of all the alloys under investigation The Peak internal friction of

0.43 was obtained at 75C for the 0.2% Ni modiﬁed CuZnAl alloy,

while the unmodiﬁed, the 0.1 and 0.3% Ni modiﬁed CuZnAl alloys

show less obvious damping peaks A peak internal friction of 0.026

was observed for the 0.4% Ni modiﬁed CuZnAl alloy at 225C; but

this alloy maintains a constant low value of 0.001 from room temperature to about 200C

The same trend is observed for 2 Hz frequency (Fig 10b) where also the 0.2% Ni modiﬁed CuZnAl alloy shows the highest damping capacity for the test temperature range of 25Ce250C However, a

peak internal friction of 0.034, which is lower than 0.043 at 1 Hz is obtained at 50C for this 0.2% Ni modiﬁed CuZnAl composition The unmodiﬁed alloy exhibits the next highest damping capacity among other alloy compositions, while the 0.4% Ni sample again exhibits the least damping capacity Damping capacity is associated with the movement and reorientation of martensite variants and

Fig 10 (a) Damping capacity of the unmodified and Ni modified CueZneAl alloys at 1 Hz test frequency; (b) Damping capacity of the unmodified and Ni modified CueZneAl alloys

at 2 Hz test frequency; (c) Storage modulus of the unmodified and Ni modified CueZneAl alloys at 1 Hz test frequency; (d) Storage modulus of the unmodified and Ni modified CueZneAl alloys at 2 Hz test frequency; (e): Loss modulus of the unmodified and Ni modified CueZneAl alloys at 1 Hz test frequency; and (f) Loss modulus of the unmodified and

Ni modiﬁed CueZneAl alloys at 2 Hz test frequency.

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interfaces[14] The results described above, thus, imply that such

movement and orientation of the martensite variants are better

facilitated in the 0.2% Ni modiﬁed CuZnAl alloy than in others The

low damping capacity of the 0.4% Ni modiﬁed CuZnAl alloy will

accordingly be linked to the restriction of the movement of the

parent phase/martensite interfaces and the martensite variants

[18] This may be on the account of the population of the Ni solute

atoms which can wield a pinning effect on the boundaries and

in-terfaces This is why smaller internal friction peaks are observed for

this composition It should be noted that at 2 Hz, the 0.1 and 0.3% Ni

modiﬁed alloys show peak internal frictions of about 0.015 and

0.013, respectively, at 225C

The E’ “storage modulus” (seeFig 10c and d) as a measure of the

capacity of a material to absorb and to store energy induced by

vibrations[29]is observed to be stable for the temperature range of

25Ce250C for all alloy compositions, with the exception of the

0.4% Ni modiﬁed one, which is observed to exhibit a drastic drop in

storage modulus (energy absorption capacity) at about 200C The

0.1% Ni modiﬁed CuZnAl alloy shows the highest storage modulus

of 130,300 MPa, while the 0.2% modiﬁed one has the least storage

modulus of averagely 4500 MPa It is noted that the storage

modulus was not affected by the frequency (either 1 or 2 Hz)

The E” “loss modulus” (seeFig 10e and f) meaning the energy

dissipation capacity in form of heat of the material[29]shows an

intermittent variability in the temperature range of 25Ce200C,

but beyond this temperature, there is continuous increase in the

loss modulus with the increase in temperature The 0.1 and 0.2% Ni

modiﬁed compositions exhibit higher loss modulus values

compared with the others above 200C It is noted that at about

200C, there is a dramatic and signiﬁcant drop in the loss modulus

of the 0.4% Ni modiﬁed CuZnAl alloy This suggests that this

composition may not be suitable for damping applications

4 Conclusion

The microstructure, mechanical behaviour and damping

prop-erties of unmodiﬁed and 0.1e0.4 Ni modiﬁed Cue18Zne7Al alloys

were investigated Sharp edged elongated grain structures

synon-ymous with the directional solidiﬁcations were observed in the

unmodiﬁed and the 0.4% Ni modiﬁed CuZnAl alloys The grain

structure was, however, signiﬁcantly altered in the 0.1, 0.2 and 0.3 %

Ni modiﬁed CuZnAl alloys, where granular structure, small grain

width with fewer sharp edge grains, and curved/round grain edges,

respectively, were observed The mechanical properties of the

un-modiﬁed and the 0.4% Ni modiﬁed CuZnAl alloys were generally

lower than those of the 0.1 and 0.3% Ni modiﬁed ones The 0.4%

modiﬁed CuZnAl alloy showing the lowest damping capacity, does

not seem, thus, suitable to serve as a damping material, while the

0.2% Ni modiﬁed one exhibits the highest damping capacity among

all the CuZnAl alloy compositions

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