Sha W., Malinov S. Titanium Alloys: Modelling of Microstructure, Properties and Applications
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Surface gas nitriding: mechanical properties, morphology 463
higher than those of the substrate because of the possible different nitrogen
content, as was discussed earlier in this chapter.
17.1.5 β21s
In the case of nitriding at 850 °C (Fig. 17.12a), the microhardness is not
significantly different for each nitriding time and with the variation of depth.
Gas nitriding at 850 °C is not effective in increasing the hardness near the
surfaces. After gas nitriding at 950 °C (Fig. 17.12b), the microhardness
increases compared to 850 °C. There is evidence of a limited increase in
hardness near the outer surface.
17.1.6 Timetal 205
Somewhat surprisingly, there is no significant difference in the hardness
values through the cross-sections of this alloy (Fig. 17.13). This is contrary
to previous sections on other titanium alloys. The hardness next to the surface
after 1, 3 and 5 hours nitriding is similar to the hardness 335 µm into the
substrate. There is also no significant difference in the hardness between the
624
412
474
690
100 µm
17.11
Microhardness values (HV0.1) of different phases in
Ti-10V-2Fe-3Al nitrided at 1050 °C for 5 hours.
Titanium alloys: modelling of microstructure464
different nitriding temperatures and lengths of time. The microstructure images
in Section 16.7.2 show that the depth of the nitriding-affected layer is
approximately 50 µm. Therefore, the apparent uniformity of the hardness
through the cross-sections might be explained by the majority of the indents
1 h with vacuum
1 h
3 h
5 h
0 50 100 150 200 250 300 350
Distance from the surface (µm)
(a)
Hardness (HK0.2)
480
460
440
420
400
380
360
340
320
300
1 h
3 h
5 h
0 50 100 150 200 250 300 350
Distance from the surface (µm)
(b)
Hardness (HK0.2)
500
480
460
440
420
400
380
360
340
17.12
Microhardness profiles of β21s after gas nitriding at (a) 850
and (b) 950 °C for different periods of time. ‘With vacuum’ refers
to evacuation of the nitriding chamber prior to nitriding
commencement.
Surface gas nitriding: mechanical properties, morphology 465
being situated at depths larger than 50 µm from the very surface. However,
the first indents of each sample are situated within 50 µm, but the hardness
values for the first indents are not higher than the hardness values in the rest
of the samples.
17.1.7 Ti–Al
The hardness of the nitrided Ti–Al samples increases near the surface of the
alloy (Fig. 17.14). The maximum and minimum hardness values of 1050 °C
nitrided Ti–Al alloy are 608 HK0.2 and 449 HK0.2, respectively.
1 h
3 h
5 h
0 50 100 150 200 250 300 350
Distance from the surface (µm)
(a)
Hardness (HK0.2)
460
440
420
400
380
360
340
320
300
1 h
3 h
5 h
0 50 100 150 200 250 300 350
Distance from the surface (µm)
(b)
Hardness (HK0.2)
440
420
400
380
360
340
320
300
17.13
Microhardness profiles of Timetal 205 after gas nitriding at
(a) 730 and (b) 830 °C for different periods of time.
Titanium alloys: modelling of microstructure466
All the hardness increases shown in cross-sectional microhardness profiles
are due to the sub-surface solid solution strengthening, because the first
indent below the surface is already beyond the thickness of the compound
layer. The error bars are standard deviations of three to five measurements.
They are large relative to the hardness increases because of the possible
sample inhomogeneity, limited equipment accuracy with the small load, and
the small number of measurements in the cases of averaging from three data
only.
17.1.8 Influence of the alloy type
Literature sources revealed that in more than 80% of reported surface
hardening of titanium alloys by means of nitriding, the Ti-6Al-4V alloy was
used. This is not surprising because this is the most widely used titanium
alloy. However, over recent years, Ti-6Al-4V has been ‘force fitted’ in many
applications, and there might be better alloy compositions for different particular
applications.
Following this thought, the nitriding response of different types and classes
of titanium alloys is compared, in order to reveal the influence of the nitriding
on the microhardness evolution with respect to their alloy composition.
Microhardness profiles of these alloys after nitriding at 850, 950 and
1050 °C for 5 hours show that the maximal microhardness values are obtained
for Ti-6Al-2Sn-4Zr-2Mo, after nitriding for 5 hours at various temperatures.
They also show that Ti-6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-1V have stronger
nitriding response in terms of hardness and thickness of the hardened nitrided
layer as compared with the other alloys. The conclusion is that the near-α
1 h
3 h
5 h
0 50 100 150 200 250 300 350
Distance from the surface (µm)
Hardness (HK0.2)
700
650
600
550
500
450
400
17.14
Microhardness profiles of the Ti–Al alloy after gas nitriding at
1050 °C for different periods of time.
Surface gas nitriding: mechanical properties, morphology 467
alloys give better results than the α + β (Ti-6Al-4V), near-β (Ti-10V-2Fe-
3Al) and β (β21s and Timetal 205) alloys. Two reasons are suggested for
this:
(i) The ratio between the α and β phases at the same temperature (e.g.
950 °C) is different in the different alloys. There is more α phase in Ti-
6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-1V as compared to the other alloys.
This difference in the phase composition probably results in differences
in the diffusion conditions.
(ii) Some alloying elements that are β-stabilisers (e.g. vanadium) may have
a negative effect on the diffusion of nitrogen in titanium. One evidence
for this is a comparison between the nitriding response of Ti 6-2-4-2
and Ti 8-1-1 alloys. The Ti 6-2-4-2 alloy (without vanadium) gives a
better result than the Ti 8-1-1 alloy (with vanadium).
On the basis of the above, for practical applications, near-α titanium alloys
such as Ti-6Al-2Sn-4Zr-2Mo and Ti-8Al-1Mo-1V are recommended to be
used for nitriding.
17.1.9 Summary of the influence of gas nitriding
on the hardness of titanium alloys
Surface gas nitriding significantly increases the hardness of titanium
alloys, due to the new phase formation on the surface of these materials
during nitriding, such as TiN and TiO
2
. The titanium nitride formed on the
surface of the alloys has a very high hardness that can reach values above
2000 HV. The evolution of the surface hardness of the near-α, α + β and
near-β titanium alloys in response to the nitriding temperature and time is
similar.
The microhardness increases with the increase of temperature and time of
nitriding. A maximal value of 1027 HK0.05 is recorded for Ti-6Al-2Sn-4Zr-
2Mo after nitriding at 1050 °C for 5 hours. One of the main purposes of the
nitriding is to increase the surface hardness of the materials in order to
improve their tribological properties. The increase of the surface hardness
does not necessarily improve significantly the wear properties of titanium
alloys, which is one of the main problems of titanium alloys for mechanical
applications.
The microhardness profiles of the titanium alloys after nitriding, prepared
on cross-sections, give information not only about the microhardness behaviour
of the materials but also about the thickness of the nitrided layer, because it
can be assumed that the nitrided layer ends where the microhardness values
reach the microhardness of the unsaturated core of the sample. The thickness
of the nitrided layer varies between 25 and 350 µm, depending on the processing
parameters of nitriding and the chemical composition of the alloy.
Titanium alloys: modelling of microstructure468
Nitriding at temperatures below the β-transus for titanium alloys is
recommended for practical applications. However, with gas nitriding, β21s
shows marginal success in achieving higher value of hardness along the
cross-section of the near surface layer. Nitriding Timetal 205 below and
above the β-transus temperature of 780 °C does not cause any significant
increase in hardness at all. The gas nitriding technique is not effective in
increasing the hardness of near surface regions in β-titanium alloys because
of the low β-transus temperatures. Normally, it is most effective to nitride
titanium alloys at temperatures just below their β-transus temperatures, but
for β alloys, this necessitates a very low nitriding temperature. Because of
the lower nitriding temperatures required compared with α or α + β alloys,
the diffusion activity of nitrogen during the process is much lower, resulting
in less nitrogen entering the substrate, thus the low hardness increase. It is
also possible that nitrogen solid solution in the bcc β-phase is not so effective
in increasing its strength compared to that in the hcp α-phase.
Gas nitriding is not overly effective in increasing the hardness of Ti–Al
alloy. However, the explanation for this is not clear, because there is no α to
β transformation occurring at around the nitriding temperatures.
17.2 Tensile properties and fatigue
performance after nitriding
Gas nitriding is seen to be a promising method to improve the surface hardness,
wear resistance and, in general, the tribological properties of titanium alloys.
However, it is still not quite clear in what direction and to what extent the
nitriding can influence the bulk mechanical properties of these materials.
This section examines the effect of nitriding on the tensile properties and
fatigue performance of titanium alloys.
17.2.1 Nitriding and testing conditions
Two materials are used for this purpose, commercially pure titanium (CP-Ti)
and the widely used Ti-6Al-4V alloy. Tensile and fatigue samples are surface
gas nitrided in a pure nitrogen atmosphere for periods of 1 and 5 hours, at
830 °C for CP-Ti and at 950 °C for Ti-6Al-4V alloy. The temperatures are
chosen to be 50 °C below the β-transus for each material. Reference samples
are also prepared by heat treatment in vacuum, at the same temperatures for
5 hours. Thus, the reference samples and the samples after nitriding for 5
hours will have the same bulk microstructure and properties, and only the
effect of the surface layer on the tensile and fatigue performance will be
extracted.
Surface gas nitriding: mechanical properties, morphology 469
17.2.2 Properties and micro- and nano-structure of the
nitrided layer
The surface hardness of the tensile and fatigue samples after nitriding is 780
and 870 HV, for CP-Ti and Ti-6Al-4V, respectively. The thickness of the
hardened surface layer is about 75 µm for CP-Ti and about 200 µm for Ti-
6Al-4V. The difference in the thickness is due to the difference in the
temperature of nitriding for the two materials.
The surface of the nitrided CP-Ti is a light gold colour. The surface of the
nitrided Ti-6Al-4V is a deeper golden colour. Colonies of grown titanium
nitride are observed on the surface of Ti-6Al-4V alloy nitrided for 5 hours
(Fig. 17.15). These colonies result in a rougher surface, which will be discussed
in Section 17.3. Between the colonies, there are microcracks. The image at
the highest magnification (Fig. 17.15c) reveals the presence of fine lamella
crystals with widths in the nano-scale.
17.2.3 Tensile strength and elongation
Nitriding influences the tensile behaviour. Fig. 17.16 compares the average
values of the tensile strength of four types of samples, where ET refers to
tensile testing at 230 °C. The tensile strength at room temperature is not
affected very much by nitriding. For Ti-6Al-4V alloy, there is a small decrease
of 2% after nitriding for 1 hour, and 4% after nitriding for 5 hours. The
tensile strength of CP-Ti is decreased by 6% after nitriding for 5 hours. The
elongation is significantly affected by the formation of the nitrided surface
layer. The elongation of Ti-6Al-4V alloy decreases after nitriding for 1 hour,
and further after nitriding for 5 hours. The fracture type also changes (Fig.
17.17). The reference sample breaks after necking and reduction of the cross-
section (Fig. 17.17a). The nitrided samples break along a 45° angle across
the section of the specimens, with a very small reduction of the cross-section
(Fig. 17.17c). The reduction of the elongation after nitriding of Ti-6Al-4V
alloy can be explained by the formation of a surface hard layer with very low
ductility. The trend in the elongation change of CP-Ti after nitriding is the
opposite. The elongation after nitriding is slightly higher as compared to the
reference sample. Indeed, for CP-Ti, the nitrided surface layer is very thin
and should not have much influence on the bulk properties as is the case for
Ti-6Al-4V alloy.
Naturally, nitrided samples have lower tensile strength and higher elongation
when tested at elevated temperature, as compared to the same samples tested
at room temperature. The tensile strength is decreased by 26% for Ti-6Al-4V
and by 55% for CP-Ti. A neural network model for the prediction of mechanical
properties (see Chapter 15) can calculate the change of the tensile properties
from RT to 230 °C of titanium alloys without nitriding, which gives similar
Titanium alloys: modelling of microstructure470
(a)
(b)
17.15
Scanning electron microscopy images, at different
magnifications, of the surface of Ti-6Al-4V alloy after nitriding.
(c)
Surface gas nitriding: mechanical properties, morphology 471
values to the percentage decrease of the tensile strength and increase of the
elongation. Hence, formation of a surface nitrided layer does not have any
influence on the degree of change of tensile properties with temperature
increase.
17.2.4 Fatigue strength
Nitriding decreases the low cycle fatigue (Fig. 17.18). The reason for this is
the formation of a rougher surface after nitriding, consisting of titanium
nitride colonies and microcracks (see Fig. 17.15). The microcracks serve as
ready nucleation sites for the formation of macrocracks. There is some
indication for improvement of the fatigue performance in the high cycle
region after nitriding. Probably, at lower stress, the stresses acting on the
sample surface are not high enough for formation of macrocracks. On the
other hand, the formation of the nitrided surface layer creates residual stresses
in the surface, which could result in an increase of the fatigue life in the high
cycle region.
17.3 Surface morphology and roughness of
Ti-6Al-2Sn-4Zr-2Mo after nitriding
Ti-6Al-2Sn-4Zr-2Mo has shown a good nitriding performance in comparison
with the other alloys, especially after nitriding at 950 °C for 5 hours. This
section studies how the surface morphology of this alloy changes after the
thermo-chemical treatment of gas nitriding and how this change influences
its surface properties. The influence of the processing parameters of nitriding
on the surface roughness, phase modifications and the microstructure of the
nitrided layer will be shown.
Reference material
Nitriding for 1 hour, Test
at RT
Nitriding for 5 hours, Test
at RT
Nitriding for 5 hours, Test
at ET
Ti-6Al-4V cp-Ti
Tensile strength (MPa)
1000
900
800
700
600
500
400
300
200
100
0
17.16
Influence of nitriding on the tensile strength of titanium alloys.
Titanium alloys: modelling of microstructure472
17.3.1 Surface morphology
For quantifying the surface roughness, the most frequently used parameter is
R
a
. This value is the arithmetic mean of the deviations of the height from the
image mean value. R
a
can be written as:
R
N
ZZ
i
N
ia
=1
=
1
–
Σ
||
[17.1]
where the image mean value
()Z
is defined as the sum of all the height
values (Z) divided by the number of data points (N) in the image:
(f) CP-Ti, nitrided and tested at elevated temperature
(e) Ti-6Al-4V, nitrided and tested at elevated
temperature
(d) CP-Ti, nitrided and tested at room temperature
(c) Ti-6Al-4V, nitrided and tested at room temperature
(b) CP-Ti, reference condition
(a) Ti-6Al-4V, reference condition
17.17
Samples after tensile tests under different conditions.