Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part II: Influence of Processing Conditions on Densification and Properties

Abstract

In this study, nanostructured Al 5083 powders, which were prepared via cryomilling, were consolidated using spark plasma sintering (SPS). The influence of processing conditions, e.g., the loading mode, starting microstructure (i.e., atomized vs cryomilled powders), sintering pressure, sintering temperature, and powder particle size on the consolidation response and associated mechanical properties were studied. Additionally, the mechanisms that govern densification during SPS were discussed also. The results reported herein suggest that the morphology and microstructure of the cryomilled powder resulted in an enhanced densification rate compared with that of atomized powder. The pressure-loading mode had a significant effect on the mechanical properties of the samples consolidated by SPS. The consolidated compact revealed differences in mechanical response when tested along the SPS loading axis and radial directions. Higher sintering pressures improved both the strength and ductility of the samples. The influence of grain size on diffusion was considered on the basis of available diffusion equations, and the results show that densification was attributed primarily to a plastic flow mechanism during the loading pressure period. Once the final pressure was applied, power law creep became the dominant densification mechanism. Higher sintering temperature improved the ductility of the consolidated compact at the expense of strength, whereas samples sintered at lower temperature exhibited brittle behavior. Finally, densification rate was found to be inversely proportional to the particle size.

Appendices

APPENDIX A. Parameters Used in this Paper

Table AI. Parameters Used in this Paper

Material Property	Aluminum	Reference
Crystallographic and thermal data
Atomic volume Ω (m3)	1.66 × 10−29	30
Burgers vector b (m)	2.86 × 10−10	30
Melting temperature T M (K)	933	30
Modulus
Shear modulus at 300 K (27 °C), U 0 (MPa)	2.54 × 104	30
Temperature dependence of modulus, $\frac{{T_{M} dU}}{{U_{0} dt}}$	–0.5	30
Lattice diffusion
Preexponential, D 0V (m2/s)	1.7 × 10−4	30
Activation energy, Q V (kJ/mol)	142	30
Boundary diffusion
Preexponential, δD 0b (m2/s)	5.0 × 10−14	30
Activation energy, Q b (kJ/mol)	84	30
Power-law creep
Exponent n	4.4	30
Dorn constant A	3.4 × 106	30
Applied load P A (MPa)	50
Radius of particle, B (μm)	10
Fractional green density ρg	0.64	16
Yield stress σ y of Al 5083 at 673 K (400 °C) (MPa)*	24
Diameter of the sample ϕ (mm)	20
Electric current flowing through the sample I s (A)	800
Radius of the particle r p (μm)	10
Specific heat of the material C v (J/(kg K))	900	5
Mass density of the particle ρ m (g/cm3)	2.66	5
Electric resistivity of the material ρ e (nΩ m)	59.5	5
A duration of the pulsed current Δt (ms)	2.7 × 12 = 32.4
Bulk melting entropy S vib (J mol−1 K−1)	42.931	31
Radius of spherical nanocyrstal r (nm)	20
Atomic diameter h (nm)	0.286	15
${\text{U}} = {\text{U}}_{0} [1 + \frac{{({{\rm T}} - 300)}}{{{{\rm T}}_{M} }}\frac{{{{\rm T}}_{M} dU}}{{{{\rm U}}_{0} dt}}]$		30
$D_{V} = D_{0V} { \exp }( - \frac{{Q_{V} }}{\text{RT}}),\,\delta D_{b} = D_{0b} { \exp }( - \frac{{Q_{b} }}{\text{RT}})$		30
$P = \frac{{P_{A} (1 - \rho_{g} )}}{{\rho^{2} (\rho - \rho_{g} )}} + \frac{{2\gamma_{SV} }}{{r_{pore} }} - \frac{{(1 - \rho_{c} )\rho }}{{(1 - \rho )\rho_{c} }}P_{0}$		32
$r_{\text{pore}} = B(\frac{1 - \rho }{6})^{1/3}$ is the pore radius, P 0 is the outgassing pressure, and ρ c is the density at which pores close		32

1. *Extrapolated by polynomial fitting of the yield stress of Al 5083 at an increased temperature[5]

APPENDIX B. Calculation of the Theoretical Density of Al 5083 Consolidated by SPS

The O atoms exist in the form of Al₂O₃. So the content of alumina can be calculated by:

$$ 0.56 {\text{ pct}} \times \left( {101.96/48} \right) = 1.1896 {\text{ pct}} $$

The N atoms exist in the form of interstitial atoms or nitride, AlN. The content of the former is about 0.1 wt pct and the latter is 0.42 wt pct.[29] Consequently, the percentage of AlN can be estimated by:

$$ 0. 1 {\text{ pct}} \times \left( { 40. 9 9/ 1 4.0 1} \right) = 0. 2 9 2 6 {\text{ pct}} $$

The density of the sample is:

$$ \frac{100}{{\frac{4.5}{1.74} + \frac{0.57}{7.43} + \frac{0.25}{7.86} + \frac{1.1896}{3.95} + \frac{0.29263}{3.26} + \frac{0.2}{2.62} + \frac{92.57778}{2.7}}} = 2. 6 7 {\text{ g}}/{\text{cm}}^{ 3} $$

Table BI. Composition of the Consolidated Al 5083

Elements	Al	Mg	Mn	Fe	O	C	N	H
wt pct	bal	4.5	0.57	0.25	0.56	0.2	0.52	0.02