Effect of Interrupted Ageing and Retrogression-Reageing Treatments on Fatigue Crack Growth with a Single Applied Overload in 7050 Aluminum Alloy

Carvalho, André Luís Moreira de; Martins, Juliana de Paula

doi:10.1590/1980-5373-MR-2017-0526

André Luís Moreira de Carvalho ^**e-mail: andrelmc@uepg.br

Departamento de Engenharia de Materiais, Universidade Estadual de Ponta Grossa, 84030-900, Ponta Grossa, PR, Brasil

https://orcid.org/0000-0003-0086-7627

Juliana de Paula Martins

Departamento de Engenharia Química, Universidade Tecnológica Federal do Paraná, 84016-210, Ponta Grossa, PR, Brasil

*e-mail: andrelmc@uepg.br

Thumbnail

Figure 1
Schematic crack length versus fatigue life curve of a hypothetic material

Thumbnail

Figure 2
Schematic diagram of ageing heat treatment procedure from the a) T6I4-65 and b) RRA conditions

Thumbnail

Figure 3
Geometry of the compact tension aluminum alloy specimen (dimensions in mm)

Thumbnail

Figure 4
COD taken at distance of 150 µm from the behind the crack tip by subtracting the vertical displacements of the top flank

(U_{y}^{top})

and bottom flank

(U_{y}^{top})

of the crack tip

Thumbnail

Figure 5
Fatigue crack growth rate versus ΔK curves for the three conditions

Thumbnail

Figure 6
Fatigue crack growth rate versus ΔK(√Eσys)√m curves for AA7050 alloys after the three ageing treatments

Thumbnail

Figure 7
Fracture surface analysis by SEM at the near-threshold region of the AA7050 alloy in the three conditions (a) T7451 and (b) T6I4-65 and (c) RRA (crack propagation direction is from left to right)

Thumbnail

Figure 8
Fracture surface SEM images in the Paris growth regime (low ΔK region) of the AA7050 alloy for the three conditions (a) T7451, (b) T6I4-65, and (c) RRA. Arrows highlight flat facets (crack propagation direction is from left to right)

Thumbnail

Figure 9
SEM fractographs in the Paris growth regime (low ΔK region) of the AA7050 alloy for the three conditions: (a) T7451 showing large void formation at the coarse grain boundary, (b) T6I4-65 showing small void formation at the coarse grain boundary, and (c) RRA showing a lack of void formation. Arrows highlight secondary cracks (crack propagation direction is from left to right)

Thumbnail

Figure 10
Fracture surface SEM images in the Paris growth regime (high ΔK region) of the AA7050 alloy for the three conditions showing: (a) the absence of facets for the T7451, and the difference between striation arrangement orientation for both (b) T6I4-65 and (c) RRA (crack propagation direction is from left to right)

Thumbnail

Figure 11
Fracture surface SEM images from the rupture region of the AA7050 alloy for the three conditions showing the predominance of the ductile fracture surface (a) T7451, (b) T6I4-65, and (c) RRA (crack propagation direction is from left to right)

Thumbnail

Figure 12
SEM fractographs in the low ΔK region, showing the measured striation spacing from the three conditions investigated (a) T7451, (b) T6I4-65, and (c) RRA (crack propagation direction is from left to right)

Thumbnail

Figure 13
The COD at a crack length of a = 150 µm behind the crack tip for the T7451, T614-65, and RRA conditions with 57,800, 117,155 and 425,115 cycles, respectively

Thumbnail

Figure 14
COD in the function stress intensity factor, ΔK, from the T7451, T6I4-65, and RRA conditions

Thumbnail

Figure 15
Crack length versus number of cycles with applied overload for the three conditions

Thumbnail

Figure 16
The COD versus cycle fraction curves for the T7451, T6I4-65, and RRA conditions at distance of a = 150 µm behind the crack tip after applied overload of 57,864 cycles, 128,098 cycles, and 426,713 cycles

Thumbnail

Figure 17
COD versus ΔK curves with achieved COD values at the moment of applied overload for the T7451, T6I4-65, and RRA conditions

Thumbnail

Figure 18
SEM fractographs of the striation region after overload of the alloy in three conditions (a) T7451, (b) T6I4-65, and (c) RRA (crack propagation direction is from left to right)

Thumbnail

Figure 19
Surface roughness parameter versus fracture surface sections from the three conditions investigated

Thumbnail

Table 1
Ageing heat treatment schedule for the T7451, T6I4-65 and RRA conditions

Thumbnail

Table 2
The tensile properties of the investigated T7451, T6I4-65 and RRA conditions.

Thumbnail

Table 3
Paris constants and the da/dN values corresponding to various ΔK regions for the three conditions.

Thumbnail

Table 4
Experimental results of the measured striation spacing from fractured surface at the low and high ΔK region of the T7451, T6I4-65, and RRA conditions.

Thumbnail

Table 5
Results from the fatigue test with single applied overload for the T7451, T6I4-65, and RRA conditions. The calculation of monotonic plastic zone was performed according to equation:

r_{y} = \frac{1}{6 π} {(\frac{K_{\max}}{σ_{yield}})}^{2} [μ m]

Thumbnail

(1)

Condition	Solution heat treatment	Ageing	Reversion treatment	Re-ageing treatment
T7451	758 K (485°C) for 4h	As received
RRA		Ageing at 403 K (130°C) for 24h	Reversion at 458 K (185°C) for 20 min	Re-ageing at 338 K (65°C) for two months
T6I4-65		Ageing interrupted at 403 K (130°C) for 15 min		Ageing at 338 K (65°C) for two months

AA7050 Condition	YS (MPa)	UTS (MPa)	R.A. (%)
T7451	492 ± 10.5	592 ± 23	3.7 ± 1.2
T6I4-65	452 ± 10.3	534 ± 14.5	38.2 ± 2.9
RRA	540 ± 15.5	613.20 ± 4.8	22.8± 3.2

Aging treatment condition	C	n	da/dN=C(ΔK)ⁿ (m/cycle)
Aging treatment condition	C	n	ΔK = 8MPa $\sqrt{m}$	ΔK = 10MPa $\sqrt{m}$	ΔK = 15MPa $\sqrt{m}$	ΔK = 20MPa $\sqrt{m}$	ΔK = 22MPa $\sqrt{m}$
T7451	1.688 × 10^-10	2.96	0.80 × 10^-7	1.54 × 10^-7	5.11 × 10^-7	11.98 × 10^-7	15.88 × 10^-7
T6I4-65	0.028 × 10^-10	4.46	0.30 × 10^-7	0.81 × 10^-7	4.93 × 10^-7	17.77 × 10^-7	27.19 × 10^-7
RRA	0.003 × 10^-10	5.14	0.13 × 10^-7	0.41 × 10^-7	3.33 × 10^-7	14.60 × 10^-7	23.83 × 10^-7

Condition	Average striation (µm)
	Low ΔK	High ΔK
T7451	0.191	0.595
T6I4-65	0.121	0.490
RRA	0.097	0.485

Condition	Crack length at overload (mm)	Plastic zone at overload r_y (µm)	Cycles at overload application (Nol)	Delay cycles due to overload (Nd)	Fatigue life (Nf)	Accumulation life (Nf – Nol)
T7451	5.7	75.2	57,581	6,148	108,888	51,307
T6I4-65	5.8	97.6	117,190	218,080	377,088	259,898
RRA	5.9	78.9	425,877	274,471	737,577	311,700

[1] *e-mail: andrelmc@uepg.br