Unravelling the competitive effect of microstructural features on the fracture toughness and tensile properties of near beta titanium alloys

doi:10.1016/j.jmst.2021.04.032

Abstract

Abstract:

The competitive effect of microstructural features including primary α (α_p), secondary α (α_s), grain boundary α (α_GB) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes. The relative effect of β grain size and STA (solution treatment and ageing) processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method. These results were further explained via correlating microstructure with the fracture toughness and tensile properties. It was found that large numbers of fine α_s precipitates and continuous α_GB played greater roles than other features, resulting in a high strength and very low ductility (<2%) of STA process samples. The β grain size had a negative correlation with fracture toughness. In the samples prepared by BASCA (β anneal slow cooling and ageing) process, improved ductility and fracture toughness were obtained due to a lower density of α_s precipitates, a basket-weave structure and zigzag morphology of α_GB. For this heat treatment, an increase in prior β grain size had an observable positive effect on fracture toughness. The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained. It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties. This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties. A novel table was proposed to quasi-quantitatively unravel these competitive effects.

Key words: Near β titanium alloys, Microstructural features, Competitive effect, Fracture toughness, β grain size effect

Yang Liu, Samuel C.V. Lim, Chen Ding, Aijun Huang, Matthew Weyland. Unravelling the competitive effect of microstructural features on the fracture toughness and tensile properties of near beta titanium alloys[J]. J. Mater. Sci. Technol., 2022, 97: 101-112.

Figures/Tables 18

Table 1 Chemical composition of the as-forged Ti-5Al-5Mo-5V-1Cr-1Fe alloy (wt.%).

Al	Mo	V	Cr	Fe	Zr	Si	C	O	H	Ti
5.39	5.13	5.06	1.04	1.00	<0.002	0.026	0.005	0.111	0.0011	Bal.

Fig. 1. Optical micrographs of etched Ti-55511 of (a) as-forged, showing globular alpha phase (white area) and beta matrix (dark area), with the elongated prior beta grain outlined by red dotted lines; (b) fully equiaxed beta grains after recrystallization.

Table 2 STA process design with orthogonal array L9 (34) for 4 factors and 3 levels.

Experiment no.	β grain size (μm)	Solution temperature (°C)	Ageing temperature (°C)	Coolingrate
STA#1	200	720	500	WQ
STA#2	200	770	550	AC
STA#3	200	820	600	FC
STA#4	400	720	550	FC
STA#5	400	770	600	WQ
STA#6	400	820	500	AC
STA#7	800	720	600	AC
STA#8	800	770	500	FC
STA#9	800	820	550	WQ

Fig. 2. Schematic of (a) STA and (b) BASCA process for Ti-5Al-5Mo-5V-1Cr-1Fe alloy.

Table 3 Mechanical properties with B-STA and BASCA process.

Samples	Fracture toughness(MPa m^1/2)	σ_0.2(MPa)	UTS(MPa)	Elongation(%)
B-STA#1	65 ± 2.2	1065 ± 14.5	1141 ± 15.9	5.3 ± 0.86
B-STA#2	53 ± 3.6	1129 ± 13.4	1189 ± 18.1	3.2 ± 0.23
B-STA#3	40 ± 1.9	n.a	1285 ± 32.9*	1.4 ± 0.20
B-STA#4	58 ± 0.7	1144 ± 7.5	1158 ± 8.2	1.3 ± 0.17
B-STA#5	67 ± 1.3	1201 ± 15.0	1205 ± 14.3	1.5 ± 0.09
B-STA#6	30 ± 0.1	n.a	1086 ± 45.3*	1.1 ± 0.04
B-STA#7	65 ± 5.3	n.a	1132 ± 12.5*	1.1 ± 0.07
B-STA#8	42 ± 2.3	n.a	1068 ± 30.6*	1.0 ± 0.04
B-STA#9	36 ± 0.1	n.a	1216 ± 28.4*	1.2 ± 0.04
BASCA-200µm	87 ± 0.9	1030 ± 5.3	1108 ± 2.5	9.4 ± 1.11
BASCA-400µm	96 ± 3.5	1002 ± 0.5	1074 ± 0.6	7.9 ± 0.75
BASCA-800µm	97 ± 3.1	998 ± 7.0	1056 ± 6.6	5.4 ± 0.65

Fig. 3. Engineering stress-strain curves of (a, b) B-STA and (c) BASCA processed samples.

Table 4 Orthogonal analysis of four parameters effect with standard deviation.

Standard deviation	Variable 1 - β grain size	Variable 2 - Solution T	Variable 3 - Ageing T	Variable 4 - Cooling rate
Fracture toughness (MPa m^1/2)	2.89	13.97	5.83	4.90
Ultimate tensile strength (MPa)	35.43	27.73	57.99	26.27
Tensile elongation (%)	1.23	0.68	0.58	0.73

Fig. 4. Secondary electron micrographs showing the fractography: (a, b) B-STA#1 with mixed fracture mode, (c) B-STA#4 with intergranular fracture mode. The dashed line circles in (a) and (b) shows flat areas with no obvious dimples.

Fig. 5. Secondary electron micrographs showing the fractography of (a) 200 µm BASCA sample, (d) 400 µm BASCA sample; (b) the rough areas with crack and dimples; (e) the flat areas with no obvious dimples. The BSE micrographs showing the cross section of (c) the rough areas with arrows indicating the voids beneath the tortuous crack surface; (f) the flat areas caused by fracture along the grain boundaries.

Fig. 6. XRD patterns of B-STA#1, BASCA-200µm comparing with the as-forged sample showing only alpha and beta phase exist after ageing.

Fig. 7. Backscattered electron micrographs of B-STA processed samples showing primary alpha and continuous grain boundary alpha morphology. From left to right, the three columns of images are the microstructure corresponding to solution temperatures 720°C, 770°C, 820°C. All in same magnification.

Table 5 Size of primary α and secondary α precipitates in B-STA and BASCA samples.

Samples	α_p length (nm)	α_p width (nm)	α_p aspect ratio	α_s length (nm)	α_s width (nm)	α_s aspect ratio
B-STA#1	853 ± 453	198 ± 63	4.3	119 ± 73	19.2 ± 5.7	6.2
B-STA#2	1342 ± 195	322 ± 112	4.2	172 ± 113	25.2 ± 10.6	6.8
B-STA#3	1764 ± 793	484 ± 155	3.6	261 ± 161	52.3 ± 21.7	5.0
B-STA#4	516 ± 251	182 ± 82	2.8
B-STA#5	1376 ± 592	371 ± 104	3.7	245 ± 143	55.1 ± 19.3	4.4
B-STA#6	991 ± 531	261 ± 67	3.8	156 ± 96	19.3 ± 8.3	8.1
B-STA#7	431 ± 199	146 ± 61	3.0
B-STA#8	696 ± 277	344 ± 116	2.0	128 ± 74	20.1 ± 7.3	6.3
B-STA#9	1099 ± 539	398 ± 111	2.8	180 ± 112	27.5 ± 11.2	6.5
BASCA	5141 ± 1839	556 ± 186	9.2	535 ± 246	150 ± 57.5	3.6

Fig. 8. Backscattered electron micrographs of B-STA processed samples showing nano-scale secondary α precipitates. From left to right, the three columns of images are the microstructure corresponding to solution temperatures 720°C, 770°C, 820°C. All is in the same magnification.

Fig. 9. Backscattered electron micrographs of BASCA processed samples showing zigzag grain boundary alpha morphology and basket-weave primary alpha morphology with varying β grain size: (a, d) 200 µm; (b, e) 400 µm and (c, f) 800 µm. All is in same magnification.

Fig. 10. Trends of changing processing variables on fracture toughness, elongation and tensile strength. For each property, same Y axis are normalised to clearly show their relative importance.

Fig. 11. Ageing temperature effect on secondary alpha length (a) and width (b) of B-STA samples solutioned at various temperatures.

Fig. 12. Backscattered electron micrographs comparing secondary α precipitates number density and size between (a) B-STA#3 and (b) BASCA sample, which both have 200 μm β grain size.

Table 6 The competitive effect of microstructural features on mechanical properties of near β alloys.

Microstructural Features	Strength	Ductility	Fracture toughness (intrinsic)	Fracture toughness (geometric)
Beta grain
• fine → coarse	o	-	-	++
Primary α (α_p)
• volume fraction ↑	-	+	+	o
• globular →elongated	o	-	-	++
• fine → coarse	-	+	+	+
Secondary α (α_s)
• number density ↑	++	- -	- -	o
• size ↑	-	+	+	o
Grain boundary α (α_GB)
• continuous	o	-	o	- -
• zigzag	o	o	o	+
• discontinuous / broken	o	+	o	++

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