B I O G R A P H YWei Sha obtained a BEngat Tsinghua University in1986. In 1992 he wasawarded a DPhil byOxford University. Hehas worked at ImperialCollege, Cambridge Uni-versity and Queen’s University Belfast. He ispresently a professor of materials science,with research interests in phase transitions,mathematical models and reaction kinetics.

A B S T R A C TGas nitriding is one of the thermochemicalconversion methods used to improve thewear performance of a metal. Nitrogen gasis applied to the surface of the metal form-ing a nitrided layer such as titanium nitride.This method of improving the wear perfor-mance and increasing the hardness of metalwas applied to titanium b21s alloy at 1040and 1160oC for 1, 3 or 5 hours. Themicrostructure of the nitrided layers wasinvestigated using scanning electronmicroscopy. From an X-ray diffraction analy-sis, new phases (TiN and TiO2) were detectedin samples nitrided at both 1040 and1160oC. The thickness of the nitrided layerincreased with nitriding time. However thispattern is not seen with increasing temper-ature.

K E Y W O R D Sscanning electron microscopy, X-ray diffrac-tion, microstructure, microhardness, heattreating, phase transformation

A U T H O R D E TA I L SProfessor Wei Sha, Metals Research Group, School of Planning, Architecture and CivilEngineering, Queen’s University Belfast, Belfast BT7 1NN, UKTel: +44 (0) 28 9097 4017Email: w.sha@qub.ac.ukWeb: http://space.qub.ac.uk:8077/cber/Sha

Microscopy and Analysis 23(1):5-8 (EU),2009

SEM O F Ti AL L O Y

I N T R O D U C T I O NTitanium and titanium alloys have becomeimportant materials for many industries in therelatively short period of time since they wereintroduced in the early 1950s. The combina-tion of high strength-to-weight ratio, excel-lent mechanical properties and corrosion resis-tance makes them strong contenders for manycrucial applications. They are broadly used inaerospace [1] and marine applications. Otherindustries, such as the chemical and automo-tive sectors, as well as medical instrumentsmanufacturers [2], also make use of the attrac-tive properties of titanium alloys.

In spite of their excellent properties, the useof titanium and titanium alloys is lessfavourable in mechanical engineering applica-tions due to their low wear resistance, highcoefficient of friction, susceptibility to fatigueand fracture and low modulus of elasticity [1,3]. Many solutions have been developed toenhance the wear performance of titaniumalloys, which include physical vapour deposi-tion, thermal spray and thermochemical con-version treatments. Gas nitriding is one of thethermochemical conversion treatments. In thisprocess, nitrogen gas is applied to the surfaceof a metal forming a thin nitrided layer such astitanium nitride. The characteristics of the tita-nium alloys microstructure determine themechanical properties of the alloys after theapplication of nitrogen to the surface. An ear-lier study had found an increased surface hard-ness on titanium alloys Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si after gas nitriding [4].

Assessment of the near surface hardnessafter gas nitriding of the titanium alloy b21s at850 and 950oC for 1, 3 and 5 hours was carriedout as described previously [3]. The purpose of

the present study was to further assess andinvestigate the surface and cross-sectionalhardness of the titanium alloy b21s afternitriding at higher temperatures but for thesame nitriding times.

TitaniumTitanium was first discovered in 1791 byWilliam Gregor. Four years later, Martin Hein-rich Klaproth successfully isolated titaniumoxide. In 1910, more than 100 years later,Matthew Albert Hunter from Rensselaer Poly-technic Institute in Troy, New York, was able toisolate the metal by heating titanium tetra-chloride with sodium in a steel bomb. Tita-nium was first produced commercially in 1948by the DuPont Company [5].

Titanium is usually found in nature at a lowconcentration and in an impure state. Puretitanium, as well as the majority of titaniumalloys, has a hexagonal close-packed structureat low temperature. This is called the a-tita-nium. However, at high temperature, b-tita-nium is more stable. b-titanium has a body-centred cubic structure.

The transformation from a to b titanium iscalled an allotropic transformation. This is thecomplete transformation from one crystalstructure to another. The transformationoccurs at a transus temperature. For pure tita-nium, the b-transus temperature is 88262oC.The existence of the two different crystal struc-tures and the allotropic transformation tem-perature are very important as they are thebasis for the variety of properties achieved bytitanium alloys [5].

The crystal structure is usually related toboth plastic deformation and diffusion rate.The diffusion rate depends on the lattice

Gas Nitriding of High Strength TitaniumAlloy b21s and Its Microstructure Wei Sha, Haji Muhd Syamaizar Haji Mat Daud, Xiaomin WuSchool of Planning, Architecture and Civil Engineering, Queen’s University Belfast, UK

Figure 1a: Scanning electron microscopeimage of the microstructure ofb21s nitrided for one hour at1040oC.

MICROSCOPY AND ANALYSIS JANUARY 2009 5

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microstructure. The characteristic anisotropyof mechanical behaviour for the a-titanium iscaused by the hexagonal crystal lattice [5].Titanium alloys can be classified into five cate-gories: a, near a, a + b, metastable b and stable b [3].

bb21s Titanium Alloyb21s is a metastable b-alloy with a b-transusof 807oC and has a higher strength-to-weightratio over other engineering materials [3]. Themain features of this alloy are that it is resis-tant to oxidation, and has high strength, creepresistance, hydrogen resistance and thermalstability. The composition of the alloy used inthis study was Ti-14.22Mo-3.48Nb-3Al-0.14Si-0.32Fe-0.016C-0.024-N-0.0963H-0.15O (wt.%),with molybdenum and niobium interactingtogether to raise corrosion resistance to veryhigh level. This alloy was initially developed asan oxidation-resistant alloy for aerospacematerials and as metal-matrix composites, as itcan be rolled to foil, which is compatible withmost fibres, and is satisfactorily stable up to816oC.

Gas Nitriding ProcessGas nitriding is one of the methods used forthermochemical surface treatment. It was firstdeveloped in the early 1900s by Adolph Mach-let [6]. It involves the diffusion of nitrogen intothe surface of metal to form a nitrided layere.g. titanium nitride. This diffusion process isbased on the solubility of nitrogen in metal.

In this study, a differential scanningcalorimeter (DSC) was used for this process.Nitrogen gas was supplied to the DSC chamberat a constant flow rate and at a specific tem-perature. A thin nitrided compound layer, TiN,was formed at the top surface layer of thesample after gas nitriding.

This method of nitriding can improve thesurface hardness, corrosion resistance, fatigueand fracture performance, wear performanceand friction coefficient. It can easily form aharder layer on the surface of the material.High temperatures of 650o to 1000oC and along period of nitriding time between 1-100

hours are required in this process [3]. Zhechevaet al. have been successful in increasing thesurface hardness for both Ti-6Al-4V (a + b) andTi-6Al-2Sn-4Zr-2Mo-0.08Si (near a) alloys. Thiswas due to the mixture of TiN, TiN0.3 (similarstructure to a-Ti ), Ti2N, TiO2 and nitrogen-enriched titanium formed at the surface [4].

Nitrided LayerThe nitrided layer is formed on the surface ofthe titanium alloy as a result of the diffusionprocess. The surface differs from that achievedthrough oxidising in that a useful, hard com-pound layer structure is created [7]. Thenitrided alloy consists of several layers, includ-ing compound layer, diffusion zone and corematerial.

The phases of the transition on the surfaceof the nitrided titanium alloy can be written as[3]: Ti → Ti(N) → Ti2N → TiN where Ti(N)denotes titanium with nitrogen in solid solu-tion. The grain growth and thickness of thenitrided layer depend on two main parame-ters: nitriding temperature and time.

A previous study has found that there was aslight increase in the near surface hardness ofthe titanium alloy b21s. The main objective ofthis study was to further investigate the sur-face and cross-sectional hardness of titaniumalloy b21s after gas nitriding at 1040o and1160oC for the same time interval as the previ-ous study. At the end of the gas nitridingprocess, it was predicted that there will be anincrease in the surface hardness of the tita-nium alloy b21s. We also expected that therewould be a change in the microstructure ofthe alloy.

E X P E R I M E N TA LThe whole process from nitriding to the mea-surement of the cross-sectional hardness pro-file involves several stages. Each stage usedspecific equipment:1. A differential scanning calorimeter for gasnitriding; 2. A diffractometer for obtaining anX-ray diffraction pattern of the alloy to iden-tify the phases present; 3. A scanning electronmicroscope for examining the microstructure

of the alloy; and 4. A hardness machine formeasuring the surface and cross-sectionalhardness.

Preparation Of SamplesBefore the gas nitriding process was started,the samples were first sawn into small piecesapproximately 43433 mm. The cut sampleswere rinsed with methanol and then the sam-ples were ready for gas nitriding.

Gas Nitriding Using Differential ScanningCalorimeterTwo temperatures were chosen for the gasnitriding process, 1040oC and 1160oC. Thenitriding durations for both temperatureswere 1, 3 and 5 hours.

The samples were nitrided using differentialscanning calorimeters (DSC): a Netzsch DSC404at 1040oC and a Netzsch STA 449C at 1160oC.The samples were put inside a crucible andthen put inside the DSC chamber. Nitrogen gaswas connected to flow through the chamberat a rate of 100 ml min-1. The heating and cool-ing rate to and from both temperatures was50oC min-1.

Hardness TestingA Vickers indenter was used to make an inden-tation on the surface of the samples. A Knoopindenter was used to make an indentation onthe cross-sections of the samples.

Scanning Electron MicroscopyAfter determining the hardness profiles of thesamples, the samples were treated with etch-ing solution (nitric acid) and dried usingmethanol. This was to make sure clearerimages of the microstructure could be seenunder the microscope.

The samples were carbon-coated beforebeing placed in a JEOL 6500 field-emissionscanning electron microscope operated at 5 or15 kV to examine their surface microstructure.

The scanning electron microscope was con-nected to a computer and images of themicrostructure of the samples were collectedusing the EOS 6500F software package.

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MICROSCOPY AND ANALYSIS JANUARY 20096

Figure 1 b, c: Scanning electron microscope images of the microstructure of b21s nitrided at 1040oC for 3 hours (b) and 5 hours (c).

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SEM O F Ti AL L O Y

R E S U LT S A N D D I S C U S S I O NSurface AppearanceAfter gas nitriding, the obvious change inphysical appearance of the samples was theircolour. Nitrided layer can be seen on the sur-face of all the samples. The colour of all thesamples is greyish white after gas nitriding at1040oC, and golden after gas nitriding at1160oC. The initial colour of the samples wassilver-grey.

Referring to a previous publication, thenitrided layer should be golden in colour if TiNis formed [4]. Thus, in the case of the samplesnitrided at 1160oC, it can be safely assumedthat TiN was formed on the surface of the sam-ples due to the colouration of the samplesfrom the original silver-grey to a nitridedgolden colour. Further confirmation is madeduring the X-ray diffraction analysis. In thecase of the samples nitrided at 1040oC, theother phases, TiO2 in particular, whichemerged at the surface layer of the samples,might contribute to the greyish-white colour.

There was no dimensional change in thesamples after gas nitriding [6].

X-ray DiffractionThe X-ray diffraction patterns of samplesnitrided at 1040oC for 1, 3 and 5 hours showedthat TiN and TiO2 were present on the surfacelayer of the samples. This result was obtainedwhen comparing the X-ray diffraction pat-terns with patterns in the previous study [3].The changes in the amount of TiO2 can beobserved by the changes in the intensity(counts) of the peaks [3]. The intensity (counts)of TiO2 is higher for samples nitrided for 3 and5 hours. The amount of TiO2 is almost the samefor samples nitrided for 3 and 5 hours. Theamount of TiN is also higher after nitriding for3 and 5 hours .

For samples nitrided at 1160oC, the X-ray dif-fraction patterns showed that TiN and TiO2

were again present, the latter with itsstrongest peak at a 2u angle of 42.8o for all thethree samples. Higher amounts of nitridingreaction phases were detected at this temper-ature. Comparing the two sets of samplesnitrided at 1040o and 1160oC, respectively,samples nitrided at 1160oC have an additionalcompound, TiO0.34N0.74. The new detectedphase was identified using the X’Pert HighScore software. A higher amount of TiO2 wasformed on samples nitrided at 1040oC com-pared with samples nitrided at 1160oC.

Microstructure of Nitrided LayerFigure 1 shows the microstructures of the b21snitrided at 1040oC observed using scanningelectron microscopy. The phase transforma-tion at the surface can be seen clearly. For thepurpose of consistency in comparing the sam-ples at all stages, the magnification was keptthe same. The needle-like structure at thecross-section of the samples represents part ofthe nitrided layer formed under the exposedtop surface and is the acicular a phase formedfrom the high temperature b phase uponcooling [3]. By comparing Figures 1 a, b and c,it can be seen that the thickness of the com-pound layer increases with nitriding duration,

MICROSCOPY AND ANALYSIS JANUARY 2009 7

Figure 2: Scanning electron microscope images of the microstructure of b21s nitrided at 1160oC for 1 hour (a) 3 hours (b) and 5 hours (c).

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because it provided a longer time for thenitrogen to diffuse into the sample and alonger time for the layer to grow. Samplesnitrided for 5 hours have the thickest com-pound layer, followed by samples nitrided for3 and 1 hours, respectively.

Figure 2 shows the SEM images of themicrostructure of samples nitrided at 1160oC.The same accelerating voltage was used forthese samples as for the 1040oC samples. Aneedle-like structure can be seen on the sam-ples nitrided at 1160oC. As with the samplesnitrided at 1040oC, the thickness of the com-pound layer increased with nitriding time.

However, when comparing the two sets ofsamples by temperature, samples nitrided at1040oC had a thicker compound layer thansamples nitrided at 1160oC. Theoretically, sam-ples nitrided at 1160oC should have thickercompound layers than samples nitrided at1040oC due to faster growth at the highertemperature [3].

Figure 3 shows the unetched surface mor-phology; compare this to the etched cross-sec-tional microstructures in Figures 1 and 2.

Microhardness TestingSurface MicrohardnessThe average surface microhardnesses of theb21s samples nitrided at 1040oC were7746152, 6286117, and 11266166 HK 0.5,for 1, 3, and 5 hour samples, respectively. Thesample nitrided for 5 hours had the highestsurface hardness among the three samples.This is reasonable because hardness increaseswith nitriding time. For samples nitrided at1160oC, the surface microhardness could notbe assessed due to some difficulties encoun-tered during the test. The indents made on thesamples were not clear. The samples could notbe focused properly under the microscope,because of the uneven surface, as shown inFigure 3. This was also the case, albeit to alesser extent, for the 1040oC nitrided samples.This made reading the measurement of theindents rather difficult, which was the maincause for the large testing error. Anotherproblem encountered during surface hardnesstesting was that the indents made were notsymmetrical which also made the measure-ments inaccurate.

Cross-sectional Microhardness ProfileThe microhardness (HK 0.2) profiles of thesamples nitrided at 1040oC showed that thedifference between the samples nitrided for1, 3 and 5 hours was not statistically signifi-cant. The hardness fluctuates moving towardsthe centre of the samples. The hardness of allthe three samples decreases moving towardsthe centre of the samples, again not alwayssignificantly due to the large standard devia-tion of each data point.

In the case of b21s samples nitrided at1160oC, the microhardness profile showedthat the samples nitrided for 3 and 5 hourshave higher hardness values in general com-pared to the 1 hour sample. The microhard-ness of samples nitrided at 1160oC is similar tothat of samples nitrided at 1040oC movingtowards the centre of the sample, with a

MICROSCOPY AND ANALYSIS JANUARY 20098

Figure 3: Scanning electron microscope images of the surface morphology microstructure of b21s nitrided at 1160oC for 1 hour (a) and 5 hours (b).

decreasing trend. The reason for this is thatthe concentration of nitrogen decreases mov-ing towards the centre of the samples. Themicrohardness of the three samples nitridedat 1160oC at 40 µm below the sample surfaceshows that the microhardness increases as thenitriding time increases, but not significantly.The hardness of the b21s increases withincreasing nitriding temperature, though notsignificantly.

C O N C L U S I O N SThis study shows that we have been successfulin increasing the cross-sectional microhardnessof titanium alloy b21s with gas nitriding. How-ever, the extent of the effect of gas nitridingon the surface hardness of the alloy is still inquestion due to some unavoidable problemsencountered during the assessment of the sur-face hardness of the alloy. The thickness of thenitrided layer increases with nitriding time.However this pattern is not seen with increas-ing temperature. From the X-ray diffraction

analysis, TiN and TiO2 phases were detected onthe surface layers of both sets of samplesnitrided at 1040 and 1160°C, respectively.

R E F E R E N C E S1. Boyer R. R. Titanium for aerospace: Rationale and

applications. Adv. Perform. Mater. 2:349-368, 1995.2. Feofilov, R. N., Chirkov, V. K., Levin, M. V. Application of

titanium alloys in medical instruments. BioMed. Eng.111:44-47, 1977.

3. Sha, W. et al. X-ray diffraction, optical microscopy, andmicrohardness studies of gas nitrided titanium alloys andtitanium aluminide. Mater. Charact. 59:229-240, 2008.

4. Zhecheva, A., Malinov, S., Sha, W. Surface gas nitriding of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.08Si alloys. Z. Metallkd.94:19-24, 2003.

5. Leyens, C., Peters, M., editors. Titanium and titanium alloys:Fundamental and applications. Wiley-VCH, 1-55, 2003.

6. Pye, D. Practical nitriding and ferritic nitrocarburizing,Materials Park, OH: ASM International, 1-11, 2003.

7. ASM Handbook, Vol. 5, Surface engineering, Materials Park,OH: ASM International, 1994.

©2009 John Wiley & Sons, Ltd

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