The Effect of Fe-Rich Intermetallics on the Weibull Distribution of Tensile Properties in a Cast Al-5 Pct Si-3 Pct Cu-1 Pct Fe-0.3 Pct Mg Alloy

Abstract

A comparison was made between the tensile properties of an Al-5 pct Si-3 pct Cu-1 pct Fe-0.3 pct Mg (319-type) alloy in the as-cast condition. The alloy was either unmodified or modified with Mn or Sr. The additions of 0.6 wt pct Mn, or 0.9 wt pct Mn, or 300 ppm Sr were made. All modifying additions increased the average and decreased the scatter in tensile strength, and increased elongation, with the Sr addition being most effective among the three. Fractographic examinations showed that the b phase, the polyhedral a phase, and porosity appear to be the main defects responsible for failure. The distribution of tensile strength has been quantitatively assessed using both two-and three-parameter Weibull distribution functions. In the twoparameter Weibull analysis, the high Weibull modulus (46) achieved by Sr modification is due to its binary effect on eutectic silicon and b platelets. Reservations about potentially misleading implications of the two-parameter approach are explored. The three-parameter Weibull analysis provides new information. In particular, minimum values of strength below which the material is extremely unlikely to fail are found. These values again show the clear benefit of Sr addition. The results are explained by the effects of modifying agents on the microstructure.

Tensile elongation of unidirectional or laminated composites combining a brittle reinforcement with a ductile strain and strain-rate hardening matrix

“…(16)) and therefore decreases the uniform strain to failure. The highest influence of strain hardening parameters is observed for ρ=2; however, the effect is in practice smaller than that, since most realistic values for brittle metals and engineering ceramics give ρ between 5 and 20 (roughly, ρ=3-20 for ceramics, ρ=8-20 for powder metallurgy steels, ρ>30 for wrought steels) [20][21][22][28][29][30][31][32][33][34][35][36]. We now examine more systematically the influence exerted by the five dimensionless parameters governing the problem: n, m, ρ, γ and φ, taking n between 0 and 0.5 and m between 0 and 0.1, the Weibull modulus ρ between 5 and 10 (above 10 the ductile phase influence becomes negligible), and γ between 0 and 1.…”

“…Where ρ plays (at fixed φ) a more important role is where elongations are low: there, its influence can be considerable, as shown by the high slope of the iso-deformation lines or by the increase in tensile elongation that is, for example, brought by going from ρ = 7 to ρ = 15 when φ equals 10 12 : the ductility then goes roughly from 1 to 10%. Table 2 gives relevant physical properties of a few brittle metals in which fracture statistics have been reported in the form of two-parameter Weibull statistics (often, three-parameter Weibull statistics are given instead [31,32,35,37,[46][47][48][49][50][51][52][53][54]); note that these are often for deformation at temperatures well below ambient. Let us consider a "typical" medium-strength brittle steel, with E = 210 GPa, σ 0 = 1,500 MPa and L 0 = 1 m, leaving the Weibull parameter ρ as a free-floating variable.…”

Simulation of mechanical behaviour of cast aluminium components

“…the presence of structural defects such as porosity, oxide films, macro-and micro-porosity. While yield strength is relatively unaffected by porosity, [25][26][27] both UTS and elongation to fracture decrease with increased amount of porosity. 25,27 Defects generally affect fracture strength, elongation to fracture and fatigue life of cast aluminium alloys 28,29 and cause variations in properties achieved during the casting process.…”

“…The mechanical properties are also reduced by the presence of iron-rich intermetallic phases, most importantly α-, β-and π-Fe phases, 7,23,24 and by the structural integrity [25][26][27][28][29][30][31][32] of the component, i.e. the presence of structural defects such as porosity, oxide films, macro-and micro-porosity.…”

The Effects of Fe-Particles on the Tensile Properties of Al-Si-Cu Alloys

“…However, Fe-rich particles have different morphologies and sizes depending upon chemical composition and thermal history [2] and different morphologies will affect properties in different ways. The plate-like β-particles in complex 3D structures [3] (Al 5 FeSi) are the most studied and their morphology has been shown to degrade mechanical properties [4,5]. Wang et al [6] states that the β-particles are the most detrimental Fe-rich particles because they act as stress concentrations due to their sharp edges.…”