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Experimental Characterization and Testing

3D Atom Probe Tomography Reconstructions

 

Fine-scale alpha in a commercial Ti-alloy

 

Fine-scale alpha in a commercial Ti-alloy

 

Ellipsoidal omega in Ti-Mo binary alloy

 

Cuboidal omega in Ti-V binary alloy

 

Plate-like alpha precipitates nucleated from cuboidal omega in Ti-V binary alloy

 

Exploration of Various Ultrafine and Uniform α+β Microstructures with Little or No Micro Texture

In our previous research program (DMR1309270), attention in microstructure engineering in metastable β titanium (Ti) alloys has shifted to the problem of refining α+β microstructure via non-conventional transformation mechanism to promote intragranular α nucleation and to suppress intergranular nucleation via direct nucleation and growth of α precipitate from β matrix [1, 2]. Within current DMREF program, three different size scales intragranular α microstructures with little or no micro texture, named refined α, more-refined α and super-refined α microstructure are achieved in a metastable β titanium alloy (Ti-5Al-5Mo-5V-3Cr or Ti-5553), upon the influence of compositional and/or structural instabilities following various non-conventional transformation pathways [3-6]. 

Refined α microstructure is obtained on the condition that the β solutionized sample is step-quenched from above β transus temperature to isothermal aging temperature (e.g. 600°C) [1] or up-quenched from room temperature to isothermal aging temperature (e.g. 600°C) [3] (schematically drawn in Fig. 1(a)) due to thermal compositional fluctuation induced compositional instability within β matrix by the mechanism of pseudo-spinodal decomposition [2, 7], as shown in Fig. 1(b) and 1(d). It was determined that the precipitation of a refined distribution of the α phase involved activation of the pseudo-spinodal mechanism, i.e., as a result of compositional fluctuations in the solid state, where such fluctuations cause the compositions of pockets of the solid solution to be smaller than that of the Co(T) composition such that a congruent β-α transformation may occur in the regions defined by the compositional pockets and therefore produce refined α microstructure [1-3, 7]. In this way, a refined distribution of the α phase, of which the number density quantified using image processing software MIPAR is in the range ~5 to 10 ppts/mm2 (Fig. 1(c)) depending on the isothermal aging time and temperature [3, 6], is produced in the absence of influence from instabilities such as the ω phase. The interfacial structure between refined α precipitate and parent β matrix has been investigated using high resolution HAADF-STEM technique (Fig. 1(e-f)). Clear terrace and ledge structure at α/β interface can be observed [3, 8]. 

 

Fig. 1 Refined α microstructure produced in Ti-5553 being up-quenched to 600°C and isothermally held for 2 hours after β solutionized: (a) Schematic drawing of heat treatment conditions; (b-c) SEM BSE image and processed SEM image by MIPAR showing the number density ~5-10 ppts/mm2; (d) HAADF image showing the size and distribution of refined α microstructure; (e-f) high resolution HAADF-STEM images showing α/β interfacial structure.

 

But upon different specific designed heat treatment, pre-formed metastable phases such as isothermal ω phase with a hexagonal structure are able to alter the local concentration and structure [9] and therefore affect subsequent α precipitation [4, 5]. More-refined α microstructure and super-refined α microstructure are formed in such way, depending on indirect and direct contribution from the uniformly distributed pre-formed metastable ω phase in β phase matrix [4, 5].

More-refined α microstructure, the number density quantified in the range ~20 to 25 ppts/mm2, is produced in Ti-5553 (Fig. 2(b-c)) during slowly heated to 350°C, isothermally aged at 350°C for 3 hours and up-quenched to 600°C for another 2 hours subsequent isothermal aging, shown in Fig. 2(a). The scale of the final α distribution formed in the heat-treatments described above was indeed influenced by the ω phase formed during slowing heated to 350°C (Fig. 2(d)), but in an indirect way. Thus, the isothermal ω precipitates developed during the initial heat-treatment influence directly the scale of the intermediate α distribution, and subsequently, this latter distribution influences the scale of the final distribution of α precipitates by creating the solute lean pocked in parent β phase matrix while being up-quenched to ultimate aging temperature (Fig. 2(e-f)) [4]. 

 

Fig. 2 More-refined α microstructure produced in Ti-5553, slowly heated to 350°C, isothermally aged at 350°C for 3 hours and up-quenched to 600°C for another 2 hours subsequent isothermal aging: (a) Schematic drawing of heat treatment conditions; (b-c) SEM BSE image and processed SEM image by MIPAR showing the number density ~20-25 ppts/mm2; (d) TEM dark field image showing the size and distribution of α microstructure formed at 350°C for 3 hours; (e-f) TEM dark field image and 3DAP result showing the dissolution of α precipitates and the create of solute lean pocket in parent β phase matrix while being up-quenched to 600°C from 350°C.

 

Super-refined α phase precipitates, whose number density quantified is approximately 40 ppts/mm2 (Fig. 3(b-c)), are formed in Ti-5553 during slow continuous heating of samples that are initially solution treated and quenched to room temperature, shown in Fig. 3(a). The critical heating rate to obtain super-refined α precipitates in Ti-5553 is equal to or lower than ≈ 20°C/min (5°C/min shown as example in Fig. 3(a)). Experimental characterization shows the clear contact of super-refined α precipitates and pre-formed isothermal ω particles (Fig. 3(d-f)), revealing that the isothermal ω phase particles are able to serve as preferred sites for heterogeneous intragranular nucleation of the α phase, resulting in the super-refined dispersion of the α phase as is observed [5]. 

 

Fig. 3. Super-refined α microstructure produced in Ti-5553, slowly heated to 600°C, isothermally aged at 600°C for 2 hours before being quenched to room temperature: (a) Schematic drawing of heat treatment conditions; (b-c) SEM BSE image and processed SEM image by MIPAR showing the number density ~40 ppts/mm2; (d) TEM dark field image showing contact of super-refined α precipitates and pre-formed isothermal ω particles; (e-f) High resolution HAADF-STEM images showing pre-formed isothermal ω phase particles serving as preferred sites for subsequent heterogeneous intragranular nucleation of the α phase.

 

References:

[1] S. Nag, Y. Zheng, R.E.A. Williams, A. Devaraj, A. Boyne, Y. Wang, P.C. Collins, G.B. Viswanathan, J.S. Tiley, B.C. Muddle, R. Banerjee, H.L. Fraser, Acta Materialia, 60 (2012) 6247-6256.

[2] A. Boyne, D. Wang, R.P. Shi, Y. Zheng, A. Behera, S. Nag, J.S. Tiley, H.L. Fraser, R. Banerjee, Y. Wang, Acta Materialia, 64 (2014) 188-197.

[3] Y. Zheng, R.E.A. Williams, J.M. Sosa, Y. Wang, R. Banerjee, H.L. Fraser, Scripta Materialia, 111 (2016) 81-84.

[4] Y. Zheng, R.E.A. Williams, J.M. Sosa, T. Alam, Y. Wang, R. Banerjee, H.L. Fraser, Acta Materialia, 103 (2016) 165-173.

[5] Y. Zheng, R.E.A. Williams, D. Wang, R. Shi, S. Nag, P. Kami, J.M. Sosa, R. Banerjee, Y. Wang, H.L. Fraser, Acta Materialia, 103 (2016) 850-858.

[6] Y. Zheng, J.M. Sosa, R.E.A. Williams, Y. Wang, R. Banerjee, H.L. Fraser, in: V. Venkatesh, A.L. Pilchak, J.E. Allison, S. Ankem, R.R. Boyer, J. Christodoulou, H.L. Fraser, M.A. Imam, Y. Kosaka, H.J. Rack, A. Chatterjee, A. Woodfield (Eds.) Proceedings of The 13th World Conference on Titanium, Wiley, San Diego, 2015, pp. 523-528.

[7] D. Wang, R. Shi, Y. Zheng, R. Banerjee, H.L. Fraser, Y. Wang, JOM, 66 (2014) 1287-1298.

[8] Y. Zheng, R.E.A. Williams, G.B. Viswanathan, W.A.T. Clark, H.L. Fraser, in: V. Venkatesh, A.L. Pilchak, J.E. Allison, S. Ankem, R.R. Boyer, J. Christodoulou, H.L. Fraser, M.A. Imam, Y. Kosaka, H.J. Rack, A. Chatterjee, A. Woodfield (Eds.) Proceedings of The 13th World Conference on Titanium, Wiley, San Diego, 2015, pp. 419-424.

[9] Y. Zheng, T. Alam, R.E.A. Williams, S. Nag, R. Banerjee, H.L. Fraser, in: V. Venkatesh, A.L. Pilchak, J.E. Allison, S. Ankem, R.R. Boyer, J. Christodoulou, H.L. Fraser, M.A. Imam, Y. Kosaka, H.J. Rack, A. Chatterjee, A. Woodfield (Eds.) Proceedings of The 13th World Conference on Titanium, Wiley, San Diego, 2015, pp. 559-564.