Cyclic Heat Treatment Induced Spheroidization of α Phase in Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe Alloy

doi:10.1007/s40195-025-01903-4

Abstract

Abstract:

In the directed energy deposition (DED) process with high heat input, repeated heating and cooling cycles in the deposited layers have a significant effect on the microstructure. Because of the differences in the cyclic numbers and peak temperatures from the lower layer to the upper layer, inhomogeneous microstructures are formed in the as-built components. In this work, a cyclic heat treatment (CHT) with gradual cooling was used to simulate the thermal process during the DED process of Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe (Ti5321) near-β Ti alloy. The effect of CHT on the microstructural evolution, especially the spheroidization of α phase, was investigated. As the CHT cycle increased, the volume fraction of α phase gradually increased from 35.9% after 1 cycle to 60.9% after 100 cycles, and the length of α phase first increased and then gradually decreased, while the width of α phase increased slowly. The aspect ratio of α phase decreased from 9.90 ± 3.39 after 1 cycle to 2.37 ± 0.87 after 100 cycles, implying that CHT induced α phase spheroidization. This phenomenon resulted from both the boundary splitting mechanism and the termination migration mechanism during CHT. The evolution of microstructure affects its mechanical properties. As the CHT cycles increased, the hardness increased overall, from 342.8 ± 5.3 HV after 1 cycle to 400.3 ± 3.4 HV after 100 cycles. This work provides a potential method to tailor the microstructure of near-β Ti alloys by heat treatment alone, especially for non-deformable additively manufactured metal components.

Key words: Titanium alloy, Cyclic heat treatment, Microstructure, Spheroidization, Additive manufacturing

Yuan Jiang, Baizhi Liang, Shewei Xin, Lei Shi, Siyuan Zhang, Kai Zhang, Hao Wang, Yi Yang, Lai-Chang Zhang. Cyclic Heat Treatment Induced Spheroidization of α Phase in Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe Alloy[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(10): 1827-1838.

Figures/Tables 13

Fig. 1 Optical microstructure of Ti5321 alloy after solution-treated at 900 °C for 90 min followed by water quenching

Table 1 Detailed holding temperatures in different cycles of CHT process

CHT cycles	Holding temperature (°C)	CHT cycles	Holding temperature (°C)	CHT cycles	Holding temperature (°C)	CHT cycles	Holding temperature (°C)
1	870	26	693	51	576	76	524
2	861	27	687	52	573	77	523
3	852	28	681	53	570	78	522
4	843	29	675	54	567	79	521
5	834	30	670	55	564	80	520
6	826	31	665	56	561	81	519
7	818	32	659	57	558	82	518
8	810	33	653	58	555	83	517
9	802	34	647	59	553	84	516
10	795	35	642	60	551	85	515
11	788	36	637	61	549	86	514
12	781	37	632	62	547	87	513
13	774	38	627	63	545	88	512
14	767	39	622	64	543	89	511
15	760	40	617	65	541	90	510
16	753	41	612	66	539	91	509
17	747	42	608	67	537	92	508
18	741	43	604	68	535	93	507
19	735	44	600	69	533	94	506
20	729	45	596	70	531	95	505
21	723	46	592	71	529	96	504
22	717	47	588	72	528	97	503
23	711	48	585	73	527	98	502
24	705	49	582	74	526	99	501
25	699	50	579	75	525	100	500

Fig. 2 Detailed holding temperatures of CHT cycles

Fig. 3 Schematic illustration of CHT process

Fig. 4 a Volume fraction of α phase after different CHT cycles; b length, width and aspect ratio of α phase after different CHT cycles

Fig. 5 a XRD profiles of the solution-treated, CHT-9, CHT-37, CHT-47 and CHT-100 specimens; b an enlarged view of 38°-42° in a

Fig. 6 SEM micrographs of Ti5321 after different CHT cycles: a CHT-9; b CHT-37; c CHT-47; d CHT-100

Fig. 7 EBSD analysis of Ti5321 after different CHT cycles: a1-d1 inverse pole figure (IPF) maps of α phase; a2-d2 aspect ratio frequency distribution plots and cumulative distribution curves of α phase. a CHT-9; b CHT-37; c CHT-47; d CHT-100. GB means grain boundary

Fig. 8 EDS analyses of Ti5321 after different CHT cycles: a1-a4 SEM images of CHT-9, CHT-37, CHT-47 and CHT-100 specimens, respectively. b-i EDS line scanning results of different elements along the direction of the white arrows in a1-a4

Fig. 9 TEM-EDS analysis of Ti5321 in CHT-47 specimen: a bright-field (BF) TEM images; b selected area electron diffraction (SAED) pattern of the cycled region b in a; c dark-field TEM image of α phase; d EDS mapping corresponding to the squared region d in a. Highlighted colors represent element enrichment

Fig. 10 Micro-hardness of the specimens after different CHT cycles

Fig. 11 Schematic diagrams of α phase spheroidization during CHT: a α phase spheroidization process with boundary splitting mechanism; b α phase spheroidization process with termination migration mechanism

Fig. 12 TEM-EDS analysis of Ti5321 in CHT-9 specimen: a, c bright-field (BF) TEM images; b EDS mapping corresponding to the squared region b in a; d EDS mapping corresponding to the squared region d in c. Highlighted colors represent element enrichment

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