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RELATIONSHIPS BETWEEN PROCESSING PARAMETERS, MICROSTRUCTURE AND PROPERTIES AFTER GAS NITRIDING OF COMMERCIAL TITANIUM ALLOYS

S. MALINOV, A. ZHECHEVA, W. SHA

The Queen's University of Belfast, School of Civil Engineering, Belfast, BT7 1NN, UK

ABSTRACT Titanium alloys are unique materials with excellent combination of properties. However in many cases their application is limited because of their low surface hardness. In this work a process of gas nitriding has been attempted to improve the surface properties of commercial titanium alloys. Four most widely used titanium alloys are studied, namely Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, Ti-8Al-1Mo-1V and Ti-10V-2Fe-3Al. The process of gas nitriding is performed in N2 atmosphere at two temperatures (950 and 1050°C) for different processing times (1, 3 and 5 hours). The results obtained showed a significant increase (from 3 to 5 times) of the surface hardness of the titanium alloys. This is due to the new phase compositions formed on the sample surface involving solid solutions of nitrogen in -Ti phase, TiN and TiO2. The influences of the processing parameters and alloy composition on the phases present, microstructure, microhardness and the layer thickness are analysed. For and + alloys at 1050°C the surface layers are inhomogeneous and irregular while at 950°C homogenous layers with good properties are obtained. Practical recommendations of processing parameters for different applications and alloys are given. KEYWORDS Surface treatment, Nitriding, Titanium alloys, Diffusion, Surface hardness, Processing parameters.

INTRODUCTION

Over the past some ten years there is a clear tendency for increase of the application of titanium alloys in many industries for production of variety of components and workpieces. It is believed that the expansion of their usage in different areas will continue for the forthcoming years. The competitiveness of the titanium alloys for different applications is largely based on the excellent combination of properties. Notable of these are their good strength to weight ratio, high

temperature capabilities and excellent corrosion behaviour in many environments. These properties and advantages of titanium and its alloys are discussed elsewhere [1-3]. In the same time, however, titanium alloys possess some disadvantages. The most significant of these are their cost as well as low surface hardness and wear resistance. The low surface hardness and wear resistance limit the application of titanium alloys for structures working in conditions of friction and contact loading. Different technologies for surface treatment aiming at improving the tribological properties of titanium alloys are being attempted. Some of the recent works include plasma assisted surface treatments [4-13], laser assisted surface treatments [1419], ion implantation [10,20-22], PVD and CVD technologies [8,23-25]. Some of these treatments mainly aim to change the microstructure of the surface layers (without change of the chemistry) using thermal influence with extraordinarily high heating and cooling rates. The second group of methods can be classified as formation of coatings on the top of the base material. Finally, the third and probably the largest group consists of technologies for thermo-chemical surface treatments of titanium alloys - nitriding, carburising, oxidation, nitrooxidation, etc. Recently, duplex surface engineering (combining two or more of the above groups) of titanium alloys is becoming very popular [8,23-30]. One of the most widely used technologies for thermo-chemical treatments of titanium alloys is nitriding. Nitriding is a surface treatment by which nitrogen is incorporated into Ti-based alloys at elevated temperature via the surface. This process can achieve various combinations of improved material performances. In principle, surface hardness, wear performance, fatigue performance as well as aesthetic appeal can be improved. The microstructure and the properties of surface nitrided layers in titanium alloys have been studied by several research groups [8-9,1213,18-19,23,25-27,31-41]. Different technologies of nitriding of titanium alloys exist including plasma nitriding, laser nitriding, ion nitriding, salt-bath nitriding, gas nitriding, etc. Comparing to the other technologies the gas nitriding is a promising method for many applications because of its simplicity in terms of equipment and independency in terms of geometry and dimensions of the workpieces to be treated. In the general case gas nitrided layers consist of (i) the diffusion zone, which is composed of titanium and alloying-element nitrides in the nitrogen saturated -titanium matrix and (ii) the compound layer on the top of the diffusion zone, which is mainly composed of the titanium nitrides TiN and Ti2N. In some cases there is simultaneous presence of titanium oxides. The change of the phase composition and the morphology of the surface layers is the main reason for surface hardening after gas nitriding. For the past few years we have been working on a project aiming at optimisation and application of surface gas nitriding to improve the surface properties of titanium alloys. This work presents experimental results on nitriding of some most commonly used commercial titanium alloys at different thermodynamic conditions. The main aim of the work is to quantify the influences of different processing parameters and alloy composition on the phase composition, microstructure and properties of the surface layers after gas nitriding of titanium alloys. The aim was motivated by the desire to optimise processing parameters for gas nitriding of different titanium alloys.

EXPERIMENTAL PROCEDURE

Four commercial titanium alloys are used as experimental materials to study their response to gas nitriding at different processing conditions, namely Ti-6Al-4V (Ti 6-4), Ti-6Al-2Sn-4Zr-2Mo (Ti 6-2-4-2), Ti-8Al-1Mo-1V (Ti 8-1-1) and Ti-10V-2Fe-3Al (Ti 10-2-3). The alloy compositions are given in Table 1. Most of the materials are provided by Timet UK Ltd. The as received alloys were in the following processing conditions: - The Ti 6-4 alloy was rolled with a reduction degree not less than 60% at temperatures in the + field. Thereafter recrystallization annealing at 800°C for 2h was employed. - The Ti 6-2-4-2 alloy was processed from an ingot at a temperature above the beta transus and then successively alpha/beta rolled to the final thickness. - The Ti 8-1-1 alloy was processed at 1125°C to small bloom (square section). Then it was alpha/beta rolled at about 1010°C.

-

The processing conditions of Ti 10-2-3 were unknown, but from the results after phase and structural analysis in "as received" conditions we concluded that it was after heat treatment classical for this alloy, namely solution treated and aged.

TABLE 1. Compositions and other characteristics of the titanium alloys used for gas nitriding

Alloy Ti 8-1-1 Ti 6-2-4-2 Ti 6-4 Ti 10-2-3 Chemical composition (wt. %) Al=8.02, Mo=1.04, V=0.98, Fe=0.08, C=0.01, H=0.0035, N=0.004, O=0.085 Al=6.13, Sn=1.93, Zr=3.97, Mo=1.95, Si=0.11, Fe=0.07, C<0.01, H=0.0045, N=0.002, O=0.065 Al=6.59, V=4.1, Fe=0.18, C<0.01, H=0.002, N=0.005, O=0.19 V=9.95, Fe=1.93, Al=3.06, C=0.02, H=0.002, N=0.009, O=0.09 Aluminium equivalent (wt.%) 8 7.4 6 3 Molybdenum equivalent (wt.%) 1.7 2 2.7 11.7 Beta transus (°C) 1040±15 1000±15 1000±15 790±15

There were two reasons for selecting these alloys: · these alloys are the most commonly used commercial titanium alloys · these alloys are typical representatives of the different classes of titanium alloys - , near-, + and (see Fig. 1).

Fig. 1. A schematic pseudobinary -isomorphous phase diagram [1] and the alloys used for nitriding in this work mapped onto it.

By studying the nitriding response in typical representatives of the different classes of titanium alloys we believe that we can obtain systematic information that can be used for practical recommendations for application on the nitriding in these materials. It should be noted that the positions of Ti 8-1-1 and Ti 6-2-4-2 alloys in the pseudobinary phase diagram are questionable and it highly depends on the presence of impurities and the oxygen level. For some oxygen levels the places of these two alloys could be opposite. All alloys were nitrided at two temperatures, 950 and 1050°C, for three different periods, or lengths, of time - 1, 3 and 5 hours. Specimens with dimensions approximating 4 x 4 x 1.5 mm and weight of about 120 mg were cut, ground and polished for the nitriding experiments. The nitriding was performed in pure N2 atmosphere ("oxygen free nitrogen" according to the specification of

BOC gases, UK) using differential scanning calorimetry (DSC) apparatus. Details of the DSC equipment and experimentation are given elsewhere [42-43]. In some cases technique for simultaneous DSC and thermo-gravimetry (TG) was used for the nitriding. In this way DSC and TG signals were recorded "in situ" during the nitriding experiments. These results were used to develop and verify model for simulation of the kinetics of layer growth during nitriding of titanium alloys [41]. After the nitriding samples were cooled down to room temperature using a cooling rate of 50°C/min. Subsequently samples from all four alloys nitrided at different temperatures and for different times were studied at room temperature by microscopy (both optical and scanning electron microscopy (SEM)), X-ray diffraction and hardness/microhardness measurements. The microstructure of the nitrided layers was investigated by microscopy study of cross sections, using Eclipse ME600D optical microscope at different magnifications (x50, x100, x200, x500, x1000). Details for this equipment and the procedures are given elsewhere [44]. X-ray diffraction analyses were carried out on the sample surface in order to detect the phases present. Measurements were performed with a Siemens diffractometer using Cu K radiation in the angular range of 30 ­ 75° (2) with a step size 0.05° (2) and a counting time of 8 sec/step. The diffraction patterns were fitted and analysed using X'Pert HighScore and X'Pert Plus software. The surface hardness was measured using Vickers method on a hardness testing machine Mitutoyo HM-124 with load of 0.5 kg for 10 sec. The microhardness profiles of the nitrided layers were obtained by applying a Knoop indenter with a load of 0.05 kg for 10 sec. The measurements were performed on cross sections from the surface to the middle of the sample with step of 25 µm. The microhardness profiles were prepared based on at least 100 test readings per sample, at least 5 of which at the same distance from the sample surface.

RESULTS AND DISCUSSION

The main aim of this work was to study and analyse the influence of the most important processing parameters on the microstructure and properties for different titanium alloys in order to optimise these parameters for different applications.

Temperature of nitriding

The nitriding experiments were performed at two different temperatures, namely 950 and 1050°C. These temperatures were purposely chosen. For three of the alloys used (Ti 6-4, Ti 6-24-2 and Ti 8-1-1) these temperatures were below and above their beta transus temperatures (see Table 1). This means that when the nitriding is performed at 1050°C these alloys will be in homogeneous -phase state whereas when the nitriding is performed at 950°C these alloys will be in + mixture phase state. Hence, one can expect different kinetics as well as microstructure morphologies and properties for nitriding at these different phase compositions conditions. For Ti 10-2-3 alloy both temperatures are above beta transus temperatures, implying that at both temperatures of nitriding this alloy would be in -phase state. It should also be taken into account that the nitrogen has a dramatic effect on the phase equilibria in titanium alloys. According to the binary Ti-N phase diagram [45] very small amounts of nitrogen cause significant increase of the beta transus temperature. For example 1 wt.% N will increase the -transus temperature by more than 150°C. The nitrogen presence and different nitrogen levels would result in different equilibrium amounts between the and phases at same temperature. In order to estimate this effect we performed thermodynamic calculations for the alloys used. The thermodynamic calculations were performed with Thermo- Calc and using the Ti database. Some details on these can be found in Refs. 3, 46-48. Though the Ti database is not validated for high nitrogen contents the results we obtained were consistent with the theory and the thermodynamics of titanium alloys. Some of the calculation results regarding the equilibrium phase amounts and chemical compositions of both and phases as functions of the nitrogen content at 950 and 1050°C in Ti 6-4 alloy are plotted in Fig. 2.

Vanadium in beta phase, wt.%

70

Alpha phase, mole %

950 C 1050 C

o

o

9 8 7 6 5 4 3 2 0.0

60 50 40 30 20 10 0 0.0

N in hcp (alpha phase) V in bcc (beta phase)

1050 C

o

4

Nitrogen in alpha phase, wt.%

3

2

950 C

o

1

(a)

0.2

(b) Nitrogen content, wt.%

0.2 0.4 0.6 0.8 1.0

0

Nitrogen content, wt.%

0.4

0.6

0.8

1.0

Fig. 2. Thermodynamic calculations for the phase equilibria in Ti-6Al-4V alloy at 950 and 1050°C as functions of the nitrogen content. (a) Equilibrium amounts of alpha phase; (b) vanadium concentration in beta phase and nitrogen concentration in alpha phase.

The calculation results undoubtedly showed that the increase of the nitrogen content in the surface layers during nitriding would result in change of the equilibrium phase composition. The amount of equilibrium phase is increased and therefore the ratio between and phase is changed. This process continues with further increase of the nitrogen content. At 950°C and usual nitrogen content of 0.05 wt.% the amount of the alpha phase is about 15-20%. This amount increases significantly with the nitrogen content increase to 70% of -phase at 1.0 wt.% N (Fig. 2a). At 1050°C and usual nitrogen content (0.05 wt.%), as earlier stated, the alloy is in homogeneous -phase state. However, the increase of the nitrogen content changes the phase equilibria and the alpha phase should appear even at this high temperature, which is above the beta transus for this alloy (Table 1). The amount of the equilibrium -phase increases and reaches value of 25% at nitrogen content of 1.0 wt.% (Fig. 2a). These phase transformations would naturally influence the morphology and properties of the diffusion zone formed during nitriding. One may argue whether in reality there is a constant change of the / ratio in the surface layers during saturation with nitrogen because the above calculations show the equilibrium states and the process of continuous change of the / phase ratio could be limited by kinetics reasons. Our previous studies showed that the phase transformation at isothermal conditions is rather fast [46,47]. For the Ti 8-1-1, Ti 6-2-4-2 and Ti 6-4 alloys it completes within seconds (up to 60 sec.) depending on the alloy composition, temperature and the amount to be transformed. This time is negligible as compared to the times of nitriding. Hence, we suggest that that there is a constant change of the / ration in the surface layers during the process of nitriding. This suggestion is valid only for the diffusion zone, where there is a solid solution of nitrogen in the titanium matrix, and it is not relevant to the compound layer. The calculation results also showed that the process of / ratio change in Ti 6-4 alloy is concerned with vanadium redistribution between the two phases with the increase of the vanadium in the phase (Fig. 2b). The nitrogen is mainly solved in the phase that is expected from crystallographic reasons. The X-ray results of the samples after nitriding at different temperatures and time showed: (i) Presence of + phases in the initial samples in different ratios depending on the alloy; (ii) phase with increased lattice parameters due to the enrichment with nitrogen ((N)) for all temperatures and time of nitriding. The phase was not observed in any of the samples after nitriding. This is consistent with the above calculations; (iii) TiN phase was formed in all samples for all times of nitriding. In general its amount increased with time prolongation; (iv) TiO2 phase was observed in most of the cases especially at longer time of nitriding (5 hours). Apparently, there were small amounts of oxygen in the nitrogen used, or, the level of air tightness of the system was not adequate. The oxide phase was not formed in Ti 6-2-4-2 alloy nitrided at 950°C for 1, 3 and 5 hours and at 1050°C for 1 and 3 hours and in some cases in Ti 6-4 alloy; More detail information for the X-ray results and analysis are given in Refs. 40.

200 µm a/ Ti 8-1-1, 950°C, 5 hours

200 µm e/ Ti 8-1-1, 1050°C, 5 hours

200 µm b/ Ti 6-2-4-2, 950°C, 5 hours

200 µm f/ Ti 6-2-4-2, 1050°C, 5 hours

100 µm c/ Ti 6-4, 950°C, 5 hours

100 µm g/ Ti 6-4, 1050°C, 5 hours

100 µm d/ Ti 10-2-3, 950°C, 5 hours

100 µm h/ Ti 10-2-3, 950°C, 5 hours

Fig. 3. Morphology of the microstructure after nitriding at 950 (a-d) and 1050°C (e-h) of different alloys for 5 hours.

There was a significant difference between the microstructures of the surface layers formed at different temperatures. Nitriding at 950°C for Ti 8-1-1, Ti-6-2-4-2 and Ti 6-4 alloys results in formation of uniform and homogeneous layers with expected gradient of the microstructure change from the sample surface to the core (see Fig. 3a-c). When the same alloys were nitrided at 1050°C for the same time the microstructure of the surface layers was coarse, inhomogeneous and irregular (Fig. 3e-g). This significant difference is because the two temperatures used for nitriding, 950 and 1050°C, are below and above the beta transus temperatures of these alloys (see Table 1). The alloys are in different phase conditions during nitriding at 950 and 1050°C. At 950°C the diffusion zone is mainly in phase state (see Fig. 2a) while at 1050°C the phase is still the predominant phase. The mechanisms and kinetics of diffusion would be different, which would result in the difference in the microstructure morphology. Some difference might be caused by precipitation and transformation during cooling (from different temperatures) after nitriding. For Ti 10-2-3 alloy the morphology was coarse and irregular for both temperatures of nitriding (Fig. 3d,h). For this alloy both 950 and 1050°C are above its beta transus (Table 1). Further work is in progress for this alloy on nitriding at lower temperatures. The microhardness profiles after nitriding at different temperatures are compared in Fig. 4. The increase of the temperature of nitriding from 950 to 1050°C does not result in significant increase of the layer thickness for the same time of nitriding. The increases of the surface microhardness were comparable for the two temperatures of nitriding. It was observed that for all alloys, the scattering in the data for microhardness readings at 1050°C was much larger as compared to 950°C. The reason is the inhomogeneous microstructure at the higher temperature.

900

Ti 8-1-1, 5 hours

900

950°C 1050°C

Ti 6-4, 5 hours

Hardness, (HK0.05)

800 700 600 500 400 300 0

800

950°C 1050°C

Hardness, HK0.05

700 600 500 400

(a)

500 Distance from the surface,(µm m) 100 200 300 400 600

300 0

(b)

500 µm Distance from the surface, um 100 200 300 400 600

Fig. 4. Microhardness profiles of Ti 8-1-1 (a) and Ti 6-4 (b) alloys after gas nitriding for 5 hours at 950 and 1050°C.

Considering all the above observations we recommend that the gas nitriding of titanium alloys should be performed at temperatures lower than their beta transus. Work is in progress for more precise optimisation of the nitriding temperatures for different titanium alloys.

Time of nitriding

Naturally the layer thickness increased with the time prolongation for all alloys. An example of the layer microstructure evolution with the time at 950°C for Ti 6-2-4-2 alloy is demonstrated in Fig. 5. The speed of the layer growth was different for the different titanium alloys used. The layer growth with the time was faster in Ti 6-2-4-2 and Ti 8-1-1 alloys. Fig. 6 presents smoothed profiles of the layer microhardness after different times of nitriding at 950°C for these two alloys. Smoothing was performed by polynomial regression using all microhardness readings (five readings at one same distance from the surface). In the figure only the smoothed lines and the average values are given. In the Ti 6-4 alloy the speed of the layer growth at 950°C was lower (see Fig. 4b). The difference in the layer growth speed is mainly due to the different diffusion coefficients of nitrogen in and phases and different ratios between the two phases in the different alloys. There might

be also influence of the compound layers formed since these were different in the different alloys. For the Ti 10-2-3 alloy, as earlier stated, even 950°C results in formation of irregular microstructure. Hence, averaging and smoothing in this case are meaningless.

200 µm

200 µm

200 µm

a/ 1 hour b/ 3 hours c/ 5 hours Fig. 5. Microstructure evolution of the surface layer with time for Ti-6-2-4-2 alloy during nitriding at 950°C.

1000

Ti 6-2-4-2

Hardness, HK0.05

900 800 700 600 500 400 300 0

Hardness, HK0.05

1 h - average 1 h - smoothed 3 h - average 3 h - smoothed 5 h - average 5 h - smoothed

800 700 600 500 400 300 0

Ti 8-1-1

1 h - average 1 h - smoothed 3 h - average 3 h - smoothed 5 h - average 5 h - smoothed

400 Distance from the surface, µm um

100

200

300

(a)

(b)

400 Distance from the surface, µm um 100 200 300 500

500

Fig. 6. Smoothed microhardness profiles for Ti 6-2-4-2 (a) and Ti 8-1-1 (b) alloys after nitriding at 950°C for 1, 3 and 5 hours.

Smoothed microhardness profiles were used for precise determination of the layer thickness after different times of nitriding. A linear correlation between the layer thickness and square root of time was found out that is usual for diffusion processes. The results were fitted to the classical equation x = k t (where x is layer thickness in µm and t is the time in hours) and the k coefficient was derived. For nitriding at 950°C values of k = 156.7 and k = 149.9 for Ti-6-2-4-2 and Ti 8-1-1 alloys, respectively were obtained. These values can be used for calculation of the time of gas nitriding required in order to obtain desirable thickness of the surface hardened layer.

Alloy composition

Our literature search revealed information that in more than 80% of surface hardening of titanium alloys by means of nitriding the Ti-6Al-4V alloy is used. This is not surprising because it is well known that this is the most widely used titanium alloy. However, over the last few years experts in the titanium industry suggested that Ti-6Al-4V is "force fitted" in many applications and there might be better alloy compositions for different particular applications. Following these thought we studied the nitriding response in different types and classes of titanium alloys (see Fig. 1). In Fig. 7 the microhardness profiles after nitriding for 5 hours at 950°C of different alloys are compared. Our results showed that Ti 6-2-4-2 and Ti 8-1-1 have better nitriding behaviour as compared to the Ti 6-4 and Ti 10-2-3. The conclusion is that the alloys to the left side of the pseudobinary isomorphous phase diagram (Fig. 1) give better results than the + (Ti-6Al-4V) and (Ti-10V2Fe-3Al). Two reasons are suggested for this:

1000

Hardness, HK0.05

900 800 700 600 500 400 300 0 100 200 300

Ti 6-2-4-2 Ti 6-4

400

500

Fig. 7. Comparison of the microhardness profiles of Ti 6-2-4-2 and Ti 6-4 alloys after nitriding at 950°C for 5 hours.

Distance from the surface, µm

(i) The ratio between the alpha and beta phases at the same temperature (e.g. 950°C) is different in the different alloys. There is more alpha phase in the Ti 6-2-4-2 and Ti 8-1-1 as compared to the Ti 6-4 and Ti 10-2-3. This difference in the phase composition will result in difference in the diffusion conditions. (ii) Some alloying elements that are beta stabiliser (e.g. vanadium) may have negative effect on the diffusion of nitrogen in titanium. One evidence for this is the 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 better result than the Ti 8-1-1 alloy (with vanadium). On the base of the above for the practical application we recommend that and near- titanium alloys, such as Ti 6-2-4-2 and Ti 8-1-1 are used for nitriding.

SUMMARY AND CONCLUSIONS

This work has led to the following suggestions for the optimisation of the processing parameters and alloy composition for gas nitriding of titanium alloys: - Gas nitriding of titanium alloys should be performed at temperatures below the beta transus temperature for the corresponding alloys; - The derived are quantitative correlations between the thickness of the surface hardened layer and the time of nitriding should be used in determining the time of nitriding for targeted properties; Alpha and near-alpha titanium alloys have better hardening behaviour as compared to alpha + beta and beta titanium alloys. Acknowledgements Dr Sha's contribution to this work is being sponsored under The Royal Academy of Engineering's Global Research Award Scheme.

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