Quantitative analysis on friction stress of hot-extruded AZ31 magnesium alloy at room temperature

doi:10.1016/j.jmst.2018.02.011

Abstract

Abstract:

Mg alloy AZ31 with ~79% (volume fraction of scattering less than 30°) basal-fiber texture through hot extrusion exhibits strong grain-size dependent yield strength. Samples with grain sizes varying from 4.5 to 22.3 μm were obtained by altering annealing time durations. The Hall-Petch relations of tension and compression are σ_0.2 = 86+200d^-1/2 and σ_0.2 = 17 + 327d^-1/2, respectively. Considering the correlation between grain orientation and deformation modes, a novel weighted average method of calculating friction stress σ₀ was proposed, and results of calculation agreed with the experimental ones, which can reasonably understand the yielding behavior in tension and compression.

Key words: Mg alloy AZ31, Hot extrusion, Basal-fiber texture, Hall-Petch relation

Ling Wang, Yiquan Zhao, Jing Zhang, Ru Ma, Yandong Liu, Yinong Wang, Qun Zhang, Weigang Li, Yuan Zhang. Quantitative analysis on friction stress of hot-extruded AZ31 magnesium alloy at room temperature[J]. J. Mater. Sci. Technol., 2018, 34(10): 1765-1772.

Figures/Tables 12

Table 1 H-P parameters for AZ31 magnesium alloy at room-temperature.

Processing condition	Grain size (μm)	(MPa)σ0	k (MPa μm^-1/2)	Refs
FSP	1.4-10.2	145	157	[11]
FSP	1.4-10.2	103	236	[11]
FSP	2.6-6.1	10	160	[12]
ECAP	4.5-32	30	170	[16]
ECAP	2.5-6.3	85	205	[15]
Extrusion	2.5-8.0	80	303	[12]
Rolling	16-35	70	348	[14]
Rolling	13-140	115	272	[17]
Rolling	13-140	88	281	[17]

Fig. 1. Optical microstructures of as-extruded AZ31 after different annealing conditions: (a) at 350 °C for 15 min, (b)-(e) at 450 °C for 1 h, 5 h, 15 h and 25 h respectively.

Table 2 Summary of grain size in AZ31 samples.

Annealing conditions	Grain size (μm)
at 350 °C for 15 min	4.5
at 450 °C for 1 h	9.8
at 450 °C for 5 h	16.1
at 450 °C for 15 h	18.5
at 450 °C for 25 h	22.3

Fig. 2. Microstructures obtained using EBSD for samples underwent (a) initial annealing (at 350 °C for 15 min) and (b) final annealing (at 450 °C for 25 h).

Fig. 3. {0001} Pole figures for the cross-sectional in the extruding direction of the (a) initial annealed sample(at 350 °C for 15 min) and (b) final annealed sample(at 450 °C for 25 h).

Fig. 4. Typical engineering compressive and tensile stress-strain curves of the initial annealed AZ31 specimen (annealed at 350 °C for 15 min) loaded parallel to the extrusion direction.

Fig. 5. Optical microstructures of the samples with different grain sizes: (a) 4.5 μm, (b) 9.8 μm, (c) 16.1 μm, (d) 18.5 μm and (e) 22.3 μm, compressed to strain of ～2%, indicating that twinning was the dominant deformation mode in the process of yielding.

Fig. 6. Optical microstructures of the samples with different grain sizes: (a) 4.5 μm, (b) 9.8 μm, (c)16.1 μm, (d) 18.5 μm and (e) 22.3 μm, at a strain of ～2%, indicating that dislocation slip was the dominant deformation mode in the process of yielding.

Table 3 Summary of mechanical properties in AZ31 samples.

	Tension			Compression
Grain size, d (μm)	σTYS (MPa)	σUTS (MPa)	El (%)	σCYS (MPa)	σUCS (MPa)	El (%)	Yield asymmetry
4.5	181	256	23	171	431	16	1.06
9.8	147	211	21	119	435	15	1.24
16.1	139	214	20	101	422	13	1.38
18.5	133	209	18	93	404	13	1.43
22.3	127	171	16	84	384	12	1.51

Fig. 7. Hall-Petch relations for the tension and compression specimens at room temperature.

Fig. 8. Statistics of the basal poles distribution: (a) 0-90°, (b) 0°-15°, (c) 15°-30°, (d) 30-45°, (e) 45°-60°, (f)60°-75°, (g) 75°-90°.

Table 4 Summary of Schmid factors for different deformation systems obtained from EBSD data for tension and compression.

		Tension		Compression
Range of basal pole	Volume fraction	Prismatic slip	Basal slip	Twinning	Basal slip
0°-90°	100%	0.398	0.277	0.413	0.277
75°-90°	38%	0.394	0.118	0.419	0.118
60°-75°	41%	0.456	0.336	0.437	0.336
45°-60°	18%	0.410	0.446	0.357	0.446
30°-45°	3%	0.382	0.466	0.345	0.466
0°-30°	0	-	-	-	-

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