Accepted Manuscript
Microstructure, phase composition and mechanical properties of new, low cost Ti-Mn-Nb alloys for biomedical applications
Shima Ehtemam-Haghighi, Hooyar Attar, Matthew S. Dargusch, Damon Kent
PII: S0925-8388(19)30556-0
DOI: https://doi.org/10.1016/j.jallcom.2019.02.116
Reference: JALCOM 49548
To appear in: Journal of Alloys and Compounds
Received Date: 31 August 2018
Revised Date: 8 February 2019
Accepted Date: 9 February 2019
Please cite this article as: S. Ehtemam-Haghighi, H. Attar, M.S. Dargusch, D. Kent, Microstructure,phase composition and mechanical properties of new, low cost Ti-Mn-Nb alloys for biomedicalapplications, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.02.116.
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Microstructure, phase composition and mechanical properties of new, low cost
Ti-Mn-Nb alloys for biomedical applications
Shima Ehtemam-Haghighi a,b,c*, Hooyar Attar b, Matthew S. Dargusch b,c, Damon Kent a,b,c
aSchool of Science and Engineering, University of the Sunshine Coast, Maroochydore DC, QLD 4558, Australia
bCentre for Advanced Materials Processing and Manufacturing, School of Mechanical and Mining Engineering,
The University of Queensland, St Lucia, Queensland, 4072, Australia
cARC Research Hub for Advanced Manufacturing of Medical Devices
* Corresponding author
Email address: [email protected]
Abstract
A group of new Ti-7Mn-xNb alloys with Nb contents varying from 0-10 wt% were fabricated
by conventional press and sinter powder metallurgy processing from blended elemental
powders. The effect of Nb content on the sintering and phase stability as well as
microstructure and mechanical characteristics of the alloys was investigated. Microstructural
studies and phase analysis showed a two-phase microstructure composed of α and β phases in
all alloys. However, addition of Nb up to 10 wt% enhanced the stability of the β phase
hindering the transformation to α during furnace cooling and increasing the proportion of
retained β. Sintering and densification of the Ti-7Mn-xNb alloys was enhanced by the fast
diffusing Mn and negatively impacted by the slow diffusing Nb due to the reliance of solid
sintering processes on diffusion. Nonetheless, the relative densities of the Ti-7Mn-xNb alloys
with up to 10 wt% Nb were higher than that of the sintered CP-Ti. The compressive strength
and hardness of the alloys varied in the range of (1842-2127 MPa) and (341-375 Hv)
respectively. It was also observed that with increasing Nb contents, the elastic modulus
decreased, while compressive strain increased due to stabilization of a greater proportion of
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the β phase. The results showed that the low-cost Ti-7Mn-xNb alloys possess superior
properties to those of CP-Ti and a number of other Ti based alloys developed for biomedical
implant applications.
Keywords: Titanium alloy; Phase stability; Microstructure; Mechanical property; Powder
metallurgy
1. Introduction
Titanium (Ti) and its alloys have been used extensively for biomedical applications due to
their outstanding combination of properties. In comparison to other conventional metallic
biomaterials such as 316L stainless steel and cobalt-chromium alloys, they possess higher
specific strength, lower elastic modulus, better corrosion resistance and superior
biocompatibility [1-4]. Among Ti-based alloys, commercially pure Ti (CP-Ti) and Ti-6Al-4V
are the most commonly used biomaterials. However, they present some concerns such as the
release of Al and V ions from the Ti-6Al-4V alloy which may cause long-term health
problems including neurological disorders and osteomalacia [5]. Additionally, the elastic
modulus mismatch between implants made from CP-Ti and Ti-6Al-4V (104 and 110 GPa,
respectively) and that of the surrounding bone (10-30 GPa) can lead to stress shielding, where
the bone is not subject to normal loads essential for maintaining its strength, density and a
healthy structure, leading to bone resorption and premature failure of the implant [6-10].
Therefore, in recent years, attention has been directed toward development of new, more
biocompatible Ti alloys with mechanical properties better suited to implant applications for
replacement of hard tissue, particularly lower elastic moduli in combination with higher
strength [11]. These new Ti alloys are predominantly β-type alloys which typically include
significant proportions of expensive and scarce alloying elements such as Ta, Zr, Nb and Hf
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with high melting points and high densities such as Ti-70Ta, Ti-29Nb-13Ta-4.6Zr and Ti-
42Nb [12-14]. The high melting points of these alloying elements introduces increased
difficulty to the processing of biocompatible Ti alloys and make the alloys more prone to
compositional segregation which is detrimental to their mechanical properties and
performance [15, 16]. Additional post-fabrication processes such as high temperature
homogenising heat treatments and/or thermoplastic processing are required to alleviate or
eliminate segregation [16] which adds to their expense. Therefore, due to the high content of
costly alloying elements and difficulty in alloy fabrication, many of the recently developed Ti
alloys are not competitive with current commercial Ti alloys [15]. Thus, increasing attention
has been paid to design and development of Ti alloys containing greater proportions of
common low-cost, low melting points elements such as Mn, Fe, Sn and reduced content of
expensive, scarce metals [15, 17].
As stated, Mn is a low cost and strong β-stabilising alloy element which can be used to
develop high strength Ti alloys [18]. Mn is also an essential element for maintenance of
regular body and brain function. In addition, the cytotoxicity and cell viability of Ti-Mn
alloys with Mn content up to 13 wt% are comparable to that of CP-Ti [18]. Therefore, a Mn
concentration of less than 13 wt% can be used to produce biocompatible Ti alloys. Nb is
another important biocompatible β-stabiliser and its addition to Ti alloys contributes to
reducing their elastic modulus as well as enhancing their corrosion resistance due to
development of stable bio-inert Nb2O5 surface oxide layer which suppresses the dissolution
of Nb ions [19, 20]. Therefore, there is potential to utilise Mn in conjunction with Nb to
develop more biocompatible, lower cost Ti alloys for biomedical implant applications.
To the authors’ best knowledge, there are limited studies addressing the properties of Ti-Mn
based alloys [18, 21]. Therefore, this research aims to study a series of new Ti-Mn-Nb alloys
prepared by conventional powder metallurgy (PM) from blended elemental powders. As a
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near-net shape process, PM offers considerable cost advantages, providing in the order of a
50% reduction in processing costs compared to other conventional techniques such as ingot
based processing routes due to fewer processing steps, limited need for post-fabrication
machining and less material wastage [22]. Additionally, this method is suited to preparation
of complex alloy compositions, containing alloying elements with significantly different
melting points (such as Mn and Nb in this study), which are otherwise difficult to produce by
conventional liquid metallurgy based approaches [23]. In the present study, a group of Ti-
7Mn-xNb alloys is fabricated using the most flexible and cheapest PM route (i.e. cold
pressing of the blended elemental powders in conjunction with furnace sintering) [24] and the
effect of Nb contents on the phase stability, sintering response, microstructure and
mechanical properties of the alloys is investigated. This study demonstrates that the Ti-Mn-
Nb based alloys are suited to press and sinter PM fabrication and possess a good combination
of mechanical properties for biomedical implant applications.
2. Experimental
Ti powder (average particle size < 50 µm), Mn powder (average particle size ≤ 10 µm) and
Nb powder (average particle size < 50 µm) were used as raw materials. Fig. 1 shows the
typical morphology of the Ti, Mn and Nb elemental powders. Powder mixtures with nominal
compositions of Ti-7Mn-xNb (x = 0, 3, 7 and 10 wt%) were blended in a GlenMills Turbula
T2F mixer for 2 h. The mixed powders were then uniaxially cold pressed under a fixed
pressure of 550 MPa using a Carver 12 Ton Manual Hydraulic press to prepare green
cylindrical compacts. The green components were subsequently sintered in a Carbolite high
temperature tubular furnace (STF 15) under protection of high purity argon atmosphere.
Samples were heated up to 1170 ºC with a heating rate of 4 ºC/min and held isothermally for
8 h before furnace cooling at the rate of 4 ºC/min. For comparison compacts were also
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prepared from the Ti powder only (CP-Ti) using the same compaction and sintering
conditions.
Fig. 1. SEM images showing the morphology of the starting powders: (a) Ti, (b) Nb and (c) Mn.
The relative density of the sintered compacts was calculated as below [25]:
� = �
��× 100% (1)
where is the density of the sintered sample measured using Archimedes’ principal, � is the
alloys nominal theoretical density estimated using the below equation [25]:
� =�
(�%��)�(
�%��)�(
�%��) (2)
where �%, �% and �% are the weight fraction of the constituent alloying elements and �,
� and � are their corresponding theoretical densities. The density measurements were
repeated three times for each alloy composition.
For microstructural studies, specimens were prepared using standard metallographic
procedures and then etched with Kroll’s solution (5 vol% HF, 30 vol% HNO3 and 65 vol%
H2O). The microstructure of the specimens was characterised with a Hitachi Tabletop
Scanning Electron Microscope (SEM, TM3030). The phase constitutions of the sintered
alloys were identified by X-Ray diffraction (XRD) using Bruker D8 Advance X-Ray
Diffractometer with Cu Kα radiation (λ = 1.5406 Å) operated at 40 kV and 40 mA. The
integrated areas for α and β peaks in the XRD patterns were determined using the peak-fitting
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program, Fityk. Based on the integrated areas, the volume fraction (��) of these phases was
estimated in the similar manner to Ref. [26] as below:
��,� =��
����� (3)
��, =��
����� (4)
where ��,� and ��, are the volume fractions and �� and � are the total integrated areas
corresponding to α and β phases, respectively [11].
From the XRD patterns, the lattice parameter, !, of the β phase was estimated using the
following equation [18]:
! = "
# $%&'× √ℎ# + +# + ,# (5)
where - is the X-ray wavelength, . is Brag angle and (ℎ, +, ,) is the miller indices of the
diffraction plane. ! was determined by extrapolation of the variation of the lattice parameter
as a function of the Nelson-Riley parameter (/0$#1$%&'
+/0$#1'
) [27].
To determine the mechanical properties, compression and hardness tests were carried out at
room temperature. Cylindrical specimens with the height-to-diameter ratio of 1.8 were
prepared from the sintered alloys. Compression tests were conducted using an Instron 5584
mechanical tester with a crosshead speed of 0.001 mm/s according to the standard DIN
50106. Mechanical properties including elastic modulus, yield strength, compressive strength
and strain were determined from tests performed in triplicate for each alloy composition.
Vickers hardness measurements on the polished samples was carried out using a Struers
Duramin hardness tester with a load and dwell time of 981 mN and 10 s, respectively. The
Vickers hardness values reported represent the average of at least 10 measurements.
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3. Results and discussion
3.1. Phase analysis and microstructural studies
The morphological characteristics of the initial powders have considerable influence on the
properties of PM processed Ti alloys [28]. The starting powders consist of blended elemental
mixtures of Ti, Mn and Nb powders with irregular shapes as shown in Fig. 1. The irregular
powder morphologies facilitate mechanical interlocking during uniaxial cold pressing
resulting in enhanced green strengths for the compacted powders. Typically higher green
strengths result in better sintering and higher strengths for the final sintered part [29].
Fig. 2 shows XRD spectra obtained from the as-sintered Ti-7Mn-xNb alloys. No obvious
diffraction peaks from remnant elemental Mn and Nb were detected in the XRD patterns,
indicating their complete diffusion into the Ti matrix during sintering. As can be seen, all of
the Ti-7Mn-xNb alloys are comprised of mixtures of the body-centred cubic (bcc) β and
hexagonal closed-packed (hcp) α phases. This is a desirable outcome as biphasic Ti alloys
generally present a good balance of properties including high yield strength, good fracture
toughness and desirable corrosion resistance [30]. Based on the XRD profiles, the volume
fraction of the phases present in the alloys were estimated and are summarised in Table 1. Ti-
7Mn alloy contains the lowest proportion of the β phase (��, = 39%) and the highest amount
of α phase (��,� = 61%). When 3 or 7 wt% Nb is added, the intensity of β peaks, and hence
the proportions of the β phase are increased, whereas the intensity of the α peaks and
correspondingly the α phase volume fraction is decreased. Further increase in the Nb content
to 10 wt% leads to further reduction in the intensity of α peaks and retention of the highest
proportion of the β phase (��, = 70%) in the as-sintered alloy.
Table 1 also presents the values for lattice parameter (a) of the β phase. The results show that
increasing levels of Nb addition lead to proportionate increases in the lattice parameter of the
β phase from a = 3.219 Å for the Ti-7Mn alloy up to 3.250 Å for the Ti-7Mn-10Nb alloy.
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This can ascribed to the substitutional alloying of Nb with a larger atomic radius (2.08 Å)
than that of Ti (2 Å) [25], so that the lattice parameter increases due to substitution of Ti
atoms by Nb in the β solid solution. The increase in the lattice parameter is evident in the
slight shift of the XRD peaks to lower angles with increasing levels of Nb addition (Fig. 2).
Fig. 2. XRD patterns of the sintered Ti-7Mn-xNb alloys.
Table 1. Volume fraction (Vf) of α and β phases, lattice parameter (a) of β phase and relative density of the sintered Ti–7Mn–xNb alloys and CP-Ti
Alloy α phase 23 (%)
β phase 23 (%)
β phase a (Å)
Relative density (%)
Ti-7Mn 61 39 3.219 96.31 ± 0.07 Ti-7Mn-3Nb 52 48 3.234 96.23 ± 0.06 Ti-7Mn-7Nb 39 61 3.246 96.12 ± 0.07 Ti-7Mn-10Nb 30 70 3.250 95.90 ± 0.11 CP-Ti 100 - - 95.16 ± 0.10
The relative density of the sintered Ti-7Mn-xNb alloys and CP-Ti is summarized in Table 1.
As evident, the relative density of the Ti-7Mn is higher than that of CP-Ti and slightly
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reduces (by around 0.4%) with increment of the Nb content from 0 to 10 wt%. It has been
reported that different alloying elements diffuse into the Ti matrix at different rates [23]. Mn
is a fast diffusing element and its diffusion rate is higher than that of the self-diffusion of Ti
in the β phase [31], thus Mn can improve the solid state sintering response of Ti alloys, which
is reliant on diffusion, promoting enhanced densification [32]. In contrast, Nb is a slow
diffuser and has a lower rate of diffusion than that of the self-diffusion of Ti in the β-Ti
matrix [31], so the addition of Nb tends to reduce densification of the Ti alloys [33].
Therefore, the degree of densification of the Ti-7Mn-xNb alloys is dependent on two
counteracting effects induced by the relative diffusivities of the Mn and Nb alloying
additions. In this study, when 7 wt% Mn is added to Ti, the highest final sintered relative
density of 96.31% was achieved. However, as the Nb content was increased to 10 wt%, the
relative density of the alloys was slightly decreased and reaches the minimum of 95.90% in
Ti-7Mn-10Nb alloy, although this is still higher than the relative density obtained for the CP-
Ti. The size of the solute elemental powders also influences the final density of the sintered
alloys. Finer alloying powders require shorter diffusion distances for alloying elements to
dissolve into the Ti matrix, thereby leading to more rapid dissolution of the particles and
enhanced sintering [33]. Hence, to accelerate sintering and enhance the densification of the
alloys, Mn with an average particle size of less than 10 µm was selected to partly compensate
for the effects of the larger and lower diffusivity Nb particles. It should be noted that the Nb
particle size (< 50 µm) used in this work is smaller than that used in other studies [33, 34].
Additionally, it was previously reported that the addition of 10 wt% Nb to CP-Ti reduced the
relative density of the alloy by around 2% [33]. In contrast, the addition of the same
proportion of Nb (10 wt%) to the Ti-7Mn alloy in this study led to reductions in the relative
density by ~0.4%.
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SEM images of the microstructures of the Ti-7Mn-xNb alloys are presented in Fig. 3. In
accordance with the XRD results, all of the alloys display a two-phase composition consisting
of the α and β phases which appear as dark and lightly contrasting grey areas in the
microstructures, respectively. There are also residual pores represented by regions of black
contrast which are typically located along the β phase grain boundaries. Additionally, no
remnant Nb and Mn powder particles can be distinguished indicating their complete
dissolution during sintering. In fact, homogenous distributions of the alloying elements is a
key factor to guarantee reliable and consistent properties [29]. As can be seen in Fig. 3(a), the
Ti-7Mn alloy consists of large irregularly shaped primary α plates, mainly located at prior β
grain boundaries, as well as α and β lamellae which are observed within light grey contrasting
regions in the microstructure. With the addition of 3 wt% Nb (Fig. 3(b)), the α phase becomes
finer and its proportion decreases. Increasing the Nb content to 7 wt% enhances the β phase
stability and leads to further reductions in the α phase volume fraction as evident in Fig. 3(c).
At this composition, the prior β grain boundaries, which are covered by finer α (αGB), become
clearly observable. In addition, some finer α lamellae in the vicinity of prior β grain
boundaries known as intergranular α as well as α lamellae within the β grains known as
intragranular α can be seen. The αGB formed on β grain boundaries in the early stages of
cooling (to temperatures below the β transus), act as a precursor for nucleation and growth of
intergranular α lamella [35, 36]. These lamella side plates are produced by branching from
the αGB. The intragranular α lamellae nucleate and grow inside the β grains [36]. Finally,
when the Nb content reaches 10 wt%, the β phase is more stabilised and the α phase
concentration and size is further reduced as can be seen in Fig. 3(d).
Based on the above results it can be deduced that increasing levels of Nb to the base Ti-7Mn
alloy promotes β phase stabilisation and makes the β to α phase transformation during
furnace cooling more difficult, resulting in a decrease in the proportion and size of the α
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plates and an increased β phase fraction. The hampering effect of the β-stabilising elements
which causes fewer α nuclei to form during cooling has also been reported in other studies
[37-39].
Fig. 3. SEM images showing the microstructure of the sintered Ti-7Mn-xNb alloys: a) Ti-7Mn, b) Ti-7Mn-3Nb, c) Ti-7Mn-7Nb and d) Ti-7Mn-10Nb.
Fig. 4 presents the microstructure of the sintered Ti-7Mn-10Nb alloy at a higher
magnification and the elemental Ti, Mn and Nb distributions are revealed in corresponding
energy dispersive spectroscopy (EDS) maps. Dark areas in the Mn and Nb EDS maps clearly
reveal a deficit of these elements in these regions. In contrast, the same regions are richer in
Ti as revealed by the Ti EDS map. On this basis and from additional EDS spot analysis, it
was found that β phase is enriched with Mn (9.1 ± 0.9 wt%) and Nb (10.0 ± 0.6 wt%)
elements, whilst the α phase contains higher Ti content (97.2 ± 0.4 wt%) and less Mn and Nb
(0.4 ± 0.3 wt% and 2.4 ± 0.5 wt%, respectively). This observation confirms that the Mn and
Nb act as β-stabilisers and have higher solubility in β than α.
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Fig. 4. SEM image of the sintered Ti-7Mn-10Nb alloy at a higher magnification and EDS maps indicating Mn and Nb enrichment in the β phase.
Further microstructural investigations also revealed the existence of some lighter contrasting
regions in the microstructure of the Ti-7Mn-10Nb alloy typically surrounded by the β phase.
An SEM image of one of these brighter contrasting features and the corresponding EDS line
scan analysis across the region are shown in Fig 5. As can be seen in Fig. 5(a), there is no
clear boundary between the bright zone and the surrounding β phase. Nonetheless, the EDS
line scan (Fig. 5(b)) along the yellow line indicates a higher content of Nb in the brightly
contrasting region with the bright contrast due to a relatively higher concentration of heavier
atomic weight Nb, suggesting that diffusion of Nb into the Ti matrix is incomplete. However,
Mn shows a homogeneous distribution, signifying its complete diffusion into the matrix. EDS
spot analyses revealed that the concentration of Nb in the brightly contrasting Nb rich region
is almost 27 wt% which is higher than the nominal 10 wt% Nb content of the alloy.
Nevertheless, the bright regions detected in our study do not contain a predominant
proportion of Nb in contrast to similar brightly contrasting features detected in a Ti-10Nb-
3Mo PM alloy microstructure [38]. This can be related to the lower sintering time (3 h) used
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in the other study and also to an absence of the high diffusivity Mn which led to the higher
concentrations of the undissolved Nb in the Ti-10Nb-3Mo alloy microstructure.
Fig. 5. Typical SEM image of an area with higher Nb content observed in the (a) sintered Ti-7Mn-10Nb alloy and b) corresponding EDS line scan analysis.
3.2. Mechanical properties
Compressive strength is a key factor in determining the suitability of implant materials since
bone tissues are primarily exposed to compressive stresses [40]. Fig. 6 displays the typical
compressive engineering stress-strain curves for the as-sintered Ti-7Mn-xNb and CP-Ti
alloys. The plots show that addition of Nb influences the mechanical properties of the alloys.
Furthermore, all alloys exhibit a combination of high strength and large strain indicating they
do not have major defects which could cause premature failure.
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Fig. 6. Compressive engineering stress-strain curves of the sintered Ti-7Mn-xNb alloys and CP-Ti.
The compression properties of the alloys and CP-Ti are presented in Fig. 7. It can be
observed that addition of 7 wt% Mn to CP-Ti increases the yield strength (45 = 1005 MPa)
and ultimate compressive strength (46�7 = 2127 MPa) of the alloy through solid solution
strengthening. Conversely, with increasing levels of Nb additions, the 45 and 46�7 of the
alloys gradually decreases. It has been reported that with increasing Nb content in Ti alloys,
the bonding force of the β phase crystal lattice decreases [19] and thus, the strength of the
alloys reduces accordingly. Therefore, the Nb both increases the proportion of the β phase
and acts to reduce its lattice bonding strength. This contributes to decreases in 45 and 46�7
with increasing levels of Nb in the Ti-7Mn-xNb alloys, with the lowest values observed for
the Ti-7Mn-10Nb (45 = 842 MPa, 46�7 = 1842 MPa). However, it should be noted that all of
the Ti-7Mn-xNb alloys present 45 and 46�7 values higher than that of the sintered CP-Ti
(602 MPa and 1647 MPa respectively). A high 45 is desirable for implant materials as it
improves their capacity to resist permanent deformation and prevent premature failure [41].
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Fig. 7. Compressive mechanical properties of the sintered Ti-7Mn-xNb and CP-Ti alloys.
Fig 7(a) also displays the compressive strain (8) of the Ti-7Mn-xNb alloys as well as that of
the sintered CP-Ti. As can be seen, 8 of the alloys are lower than that of the CP-Ti (8= 43%)
which is likely due to the solid solution hardening effect of Mn. However, upon increasing
the Nb content, 8 slightly increases and is highest for the Ti-7Mn-10Nb alloy (8 = 34%). This
is presumably due to increased stabilisation and volume fractions of the β phase which
contributes to increase 8 [42].
Fig. 7(b) displays the elastic modulus (E) of the alloys. Ti-7Mn alloy possesses lower E than
that of CP-Ti. Upon increasing the Nb content, E slightly reduces and reaches 87 GPa in Ti-
7Mn-10Nb alloy due to an increased β phase content [43].
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Fig. 7(b) also shows the variation in Vickers hardness (9:) values of the alloys. The 9: of
the Ti-7Mn-xNb alloys shows the same trend to that observed for 45 and 46�7. The highest
9: was observed for Ti-7Mn alloy (9: = 375) due to the solid solution hardening induced by
Mn. It also has the lowest β phase content compared to the Ti-7Mn-xNb alloys. However, as
the Nb is increased, the 9: of the alloys decreases, although they remain higher than that of
CP-Ti. The decrease in 9: with increasing Nb additions is ascribed to increased proportions
of the softer β phase [39].
Classical theories of wear state that hardness is a key parameter in controlling wear resistance
and usually a hard material presents better wear resistance [44]. Therefore, based on the
hardness values, it is anticipated that all of the Ti-7Mn-xNb alloys will exhibit better wear
resistance than CP-Ti. For biomedical implants, a higher wear resistance reduces the
propensity for generation of wear particles during service which can otherwise cause
inflammatory responses and/or loosening of the implant necessitating further surgical
intervention [44-46].
It has been reported that the presence of porosity reduces the effective elastic modulus of a
component. It can also lead to local stress concentrations, which thereby decrease the
strength, hardness and ductility of the alloys [18]. However, in this study, due to only limited
reductions in the relative density (Table 1) or minor increases in the porosity with increasing
Nb additions, this influence is fairly consistent across the range of Ti-7Mn-xNb alloys. It
should be noted that the observed trends in the mechanical properties of the studied alloys are
similar to those reported for other similar alloy systems [19, 38, 42], whereby increasing Nb
contents in Ti-Nb and Ti-Nb-Mo alloys resulted in reduced strength, hardness and elastic
modulus and increased ductility.
Examination of the failed samples after compression testing shows that they fracture into two
distinct pieces after reaching 46�7. Fig. 8 shows SEM fractographs of the Ti-7Mn-10Nb
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alloy. It reveals that the fracture surface is oriented at about 45º to the compressive loading
axis which is characteristic of compressive shear failure in ductile materials (Fig. 8(a)) [47].
A typical fracture surface of the alloy at higher magnification (Fig. 8(b)) shows that the
former interface between powder particles cannot be distinguished indicating that the sintered
sample achieved full metallurgical bonding. In addition, a mixture of smooth and rough zones
are observable across the fracture surface. The presence of a high proportion of rough regions
is indicative of predominantly ductile fracture [48]. Also, some micro-cracks can be seen
which may have originated from the pores in the alloy microstructure acting as stress
concentrators during compression testing.
Fig. 8. Typical SEM fractographs of the Ti-7Mn-10Nb alloy after compression testing; (a) macroscopic view of fracture profile and (b) microscopic view of fracture interface.
The Ti-7Mn-xNb alloys exhibit yield strengths (45 = 842-1005 MPa) comparable to those of
other conventional Ti alloys (758-1117 MPa) used for biomedical application [49]. In
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addition, the Ti-7Mn-xNb alloys exhibit comparable or lower elastic moduli than some β-
type Ti alloys produced by PM techniques such as Ti-12Mo-6Zr-xFe (x = 1, 2, 3, 4 wt%) (E
= 93-105 GPa) [50], Ti-6Mn-3Mo (E = 87 GPa) [21] and Ti-35Nb-10Ta-3Fe (E = 98 GPa)
[51]. Furthermore, although the strain to failure of the Ti-7Mn-xNb alloys is lower than that
of CP-Ti, the values are higher than other Ti alloys such as Ti-10Nb-3Mo (8 = 29%) [38], Ti-
6Al-4V ELI (8 = 24%) [18], Ti-13Nb-(0-6)Zr (8 = 20-26%) [52] and Ti-(2-25)Cu (8 = 15-
28%) [53]. Therefore, this study demonstrates that by using β-stabilising Mn which is a
widely available and low cost element which imparts considerable strengthening to the alloy,
along with Nb, also an effective β-stabiliser which reduces the β phase bond strength and
increases the corrosion resistance of the Ti alloys, a good combination of high strength and
ductility as well as relatively low elastic modulus can be achieved. Moreover, considering the
high melting temperature of Nb which makes the fabrication of Ti alloys by traditional
casting methods difficult, the PM process used in this study offers a suitable, cost-effective
route for production of the net-shape Ti-Mn-Nb alloy components for biomedical implant
applications.
4. Conclusions
In this work, a series of new Ti-Mn-Nb alloys were fabricated using a press and sinter
powder metallurgy approach from blended elemental powders. The effect of Nb content on
the sintering, phase stability, microstructure and mechanical properties of Ti-7Mn-xNb alloys
were evaluated. The XRD and microstructural examinations revealed that all alloys consisted
of β and α phases, the proportions of which depend on the levels of Nb. Increasing Nb
contents enhanced the stability of β phase and decreased the proportion and size of the α
plates. The alloys exhibited higher relative densities than that of CP-Ti due to the fast
diffusion of Mn in Ti which aids solid state sintering. The mechanical properties obtained
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from hardness and compression tests showed that the Ti-7Mn-xNb alloys present higher
hardness, compressive and yield strengths as well as lower elastic moduli than the widely-
used CP-Ti. It is concluded that the press and sinter powder metallurgy approach can be
employed to successfully fabricate Ti-Mn-Nb alloys and the newly-developed alloys are
promising materials for biomedical implant applications.
Acknowledgements
The Authors acknowledge the support of Queensland Centre for Advanced Materials
Processing and Manufacturing (AMPAM). The Authors would like also to acknowledge the
support of the Australian Research Council through the ARC Research Hub for Advanced
Manufacturing of Medical Devices (IH150100024) and the ARC Hub for transforming
Australian industry through Additive Manufacturing (IH130100008).
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Highlights
• Low-cost Ti-Mn-Nb alloys were produced by conventional press and sinter.
• Phase and microstructural evolution induced by Nb addition is elucidated.
• Relationship between microstructure and mechanical properties is established.
• Ti-Mn-Nb alloys show desirable mechanical properties for biomedical applications.