GAS NITRIDING INFLUENCE ON FATIGUE AND CREEP BEHAVIORS IN TITANIUM GRADE 2

The present work aims nitriding to inves�gate the eﬀect on �tanium grade 2 with rota�ng bending fa�gue tests at room temperature and short-term creep tests at 500°C. The nitriding treatment was carried out at 850°C for 9 hours. A�er thermochemical treatment, XRD analysis revealed the forma�on layer composed of TiN and Ti 2 N. Microscopy analysis revealed the presence of a uniform layer with a mean thickness of 3.6 m M and 50% increase in surface roughness. Fa�gue life (10 7 cycles) was reduced by 42% by increasing surface roughness. On the other hand, nitriding treatment increased creep life by at least two �mes due to oxida�on rate reducion and blocking disloca�ons movement by nitrogen in solid solu�on during the plas�c deforma�on process.

The material used in this study was �tanium grade 2 with composi�on equal to 0.05 w.% Fe, 0.03 w.% C, 0.22 w.% O, 0.03 w.% Ni, 0.001 w.% H, and 99,669 w.% Ti. The thermochemical treatment was carried out in a furnace at 850 °C for nine (9) hours. The specimens were encapsulated in quartz tubes with pure nitrogen-rich atmosphere (99.99%). Samples with (nitride) and without (asreceived) surface treatment were characterized by XRD, op�cal microscopy, profilometry and microhardness for the evalua�on of the surface treatment effec�veness and surface compounds characteriza�on. The fa�gue tests were performed for all experimental condi�ons in rota�ng bending condi�ons (R = -1), at room temperature, and conducted un�l rupture or 10 7 cycles. The creep tests were conducted at 500°C, the stress ranged between 55 and 100 MPa, in the as-received and nitride condi�ons. The creep rupture test was conducted based on ASTM 139-11 [19]. Figure 1 shows an op�cal micrograph of �tanium grade 2 before surface treatment. Titanium grade 2 microstructure, in the as-received condi�on, has an average grain size of 12 μm and α phase presence, which has hexagonal crystal structure and equiaxed morphology along the longitudinal and transverse sec�ons. A�er thermochemical treatment, XRD analysis detected on the sample surface TiN phase with cubic structure, and the tetragonal structure phase, Ti 2 N [20].

Results
Op�cal microscopy showed a uniform layer with average thickness equal to 3.6 μm. The mean values of surface microhardness of �tanium grade 2 in as-received and nitrided condi�ons are shown in Table 1. The mean roughness of �tanium grade 2 in the as-received and nitrided condi�ons are shown in Table 2.
(2) Based on Equa�ons 1 and 2, for a life�me of 10 7 cycles, the nitriding treatment decreased the life�me of �tanium grade 2, in the as-received condi�on, during fa�gue process in 42%. Table 3 shows the life�me (t f ) and final deforma�on (ε f ) parameters obtained during the creep tests. The rela�on between the absolute life�me values of nitrided and as-received specimens (t fN /t f AR ) shows that nitriding efficiency increases gradually with the applied stress. Thus, the life�me for nitrided condi�on is, at least, twice higher than the one for as-received condi�on.

Discussion
Titanium and its alloys are known to exhibit heterogeneous behavior during plas�c deforma�on. This behavior is related to its microstructure, which has few slip systems. The crack nuclea�on, on �tanium surface during fa�gue process, is related to disloca�on slipping in prisma�c and basal planes. However, the slipping ac�vity is lower in the basal plane. Thus, the deforma�on is concentrated in the neighboring regions, imposing a higher strain level to the basal planes. In addi�on to external applied stress, the stress level on the basal plans exceeds the elas�c limit and, therefore, cracks nucleate and spread more quickly along the basal planes [21] A ceramic layer should increase �tanium resistance to fa�gue, because the it has different deforma�on mechanisms, higher microhardness and consequently resistance to crack nuclea�on [22]. Farokhzadeh and Edrisy [23] have studied the fa�gue behavior of Ti-6Al-4V alloy nitrided at 600°C for 24 h. According to the authors, the treatment produced on surface a nitride layer with 2 μm thickness, plus a nitrogen diffusion zone with 44 μm of thickness. Fa�gue life decreased due to the ceramic bri�leness and surface roughness (4 �mes higher than the roughness of the untreated material) [23]. In this present work, the fa�gue resistance decreasing was related to the nitride layer thickness and to the roughness increasing. The higher values of surface roughness increased the number of stress concentrator sites on surface, increasing the chances of crack nuclea�on. Once nucleated, the crack has no propaga�on resistance in coa�ng due to its bri�le nature [25]. Regarding creep behavior, the nitriding of �tanium grade 2 decreased the surface oxida�on effects, since nitrides have more stability at 500°C, protec�ng the substrate reac�on with the environment. Titanium reacts instantly with inters��al elements, propor�onal to the temperature increase [11]. At room temperature, �tanium reacts with oxygen forming on surface a thin, uniform and adherent layer, TiO 2 based. The increase in temperature decreases TiO 2 stability, which grows in thickness and lose adhesion, exposing the substrate to atmosphere and lowering the support area strength [11,25]. In this present study, the increase in creep resistance by nitriding treatment is related to the nitrates stability, which protect the substrate from the atmosphere, retarding the creep deforma�on mechanisms. The analysis of creep parameters indicates that creep deforma�on increasing with applied stress may be related to the increased movement of mobile disloca�ons in pure metal. For the nitrided condi�on, it is possible that at higher stress, nitrogen in solid solu�on restricted disloca�on ac�vity and reduced duc�lity.

Conclusion
This study examined the effects of nitriding on the creep and fa�gue behaviors of �tanium grade 2. The main results are as follows: