Tensile Uniform Elongation Research Articles

Influence of microstructure on tensile properties of spheroidized ultrahigh-carbon (1.8 Pct C steel

Ultrahigh-carbon steel (UHCS) containing 1.8 pct carbon was processed to create microstructures consisting of fine-spheroidized carbide particles (0.2- to 1.5-μm size range) within a fine-grained ferrite matrix (0.3- to 5-μm range) through a variety of thermomechanical processing and heat-treatment combinations. Tensile ductility, yield, and fracture strengths, and strain-hardening behavior were evaluated at room temperature. Yield strengths ranged from 640 to 1450 MPa, and uniform tensile elongation ranged from 3 to 23 pct. Quantitative analyses revealed that a Hall-Petch type relationship exists between the yield strength and the ferrite grain size and carbide particle size within grain interiors. The fracture strength, on the other hand, was found to be uniquely dependent on the coarse carbide particle size typically found at grain boundaries. Data from other investigators on spheroidized carbon steels were shown to correlate well with the data for the UHCS (1.8 pct C) material. It was shown that the tensile ductility will increase when the difference between the fracture strength and the yield strength is increased and when the strain-hardening rate is decreased. The basis for the trends observed is that the tensile ductility is limited by the fracture process that appears to be dictated by the nucleation of cracks at large carbide particles. The results obtained indicate that UHCSs have significant potential for sheet applications where high strength and good ductility are primary requirements.

An assessment of tensile, irradiation creep, creep rupture, and fatigue behavior in austenitic stainless steels with emphasis on spectral effects

A review of mechanical properties of austenitic stainless steels is made to assess their behavior in fusion reactors. Since the first walls of fusion devices are expected to range in temperature from 100 to over 500°C, behavior over a wide range of temperatures is reviewed. For tensile properties, the neutron spectrum has little effect on strength, but appears to influence plastic deformation and ductility. The effect is apparent at temperatures above 400°C where ductility begins to increase. Ductility appears to be higher for alloys irradiated in fast reactors where little helium is produced, but even for fast reactor irradiation, ductility begins to drop above 700°C. This behavior, which is enhanced by higher helium concentrations, is believed to be a result of helium embrittlement. Irradiation creep is very weakly dependent on temperature, with nearly constant creep rates from room temperature to half the melting point. There are not yet sufficient data to determine the effect, if any, of helium on irradiation creep. However, creep rupture is exacerbated by helium since bubble formation at grain boundaries is the primary mechanism of in-reactor failure, especially at high temperatures. Fatigue is also dependent upon high temperature helium embrittlement. High temperatures, high helium concentrations, and low strain rates enhance reduction in fatigue life. At temperatures as high as 550°C, there is no apparent effect of helium even at concentrations of 500 appm. at strain rates of 10 −3 s −1 but either a decrease in strain rate by a factor of 100 or an increase in helium concentration by a factor of 6 produces a degradation in fatigue life. In general, helium affects mechanical properties of austenitic stainless steels; however, it is mostly a high temperature phenomenon. Caution must be exercised at temperatures below 250°C, where uniform tensile elongation is extremely low in irradiated alloys. Low-temperature embrittlement must be investigated further to determine if incremental hardening by helium contributes.

The Interplay of Geometric and Materials Variables in Energy Absorption

The specific energy absorbed during uniaxial tension and during axial compression of cylindrical tubes with various wall thicknesses and diameters has been measured for 1015 steel in two heat treatment conditions and for 6061 aluminum alloy in four heat treatment conditions. For axial compression of tubes, the energy absorbed/unit weight, Esc, is a function of the thickness to diameter ratio and the present work shows that for 0.02 < t/D < 0.1, the dependence is well described by a power law of the form Esc = A(t/D)m where m varies between 0.5 and 1.0 for different materials. A salient finding is that the ranking of materials for specific energy absorption depends upon the testing mode. In tension, it is shown that density ρ, ultimate tensile strength σult and the uniform elongation, εu are significant in the ranking of materials. Specifically, the present and previous results show that the energy absorbed/unit weight, depends upon both the ultimate tensile strength σult and the corresponding true (tensile) strains εu, EsT = σult εu/ρ(1 + εu). In axial compression, however, the measured variations in Esc (for a fixed geometry) with the different materials show that Esc is simply proportional to the specific ultimate tensile strength (σult/ρ). The magnitude of the tensile uniform elongation is unimportant in ranking materials for this compression collapse mode because the geometric instability present in tension tests does not arise in the plastic buckling process that occurs during the axial compression of cylinders.

Sheet Thermoforming of a Superplastic Alloy

Ordinary metals and alloys have not heretofore been shaped by the sheet thermoforming techniques so advantageously used by the plastics industry. Although a polymeric material may undergo a manifold increase in surface area during vacuum or pressure forming, metal or alloy sheet invariably fails by localized deformation and thinning at relatively low strains when subjected to tensile plastic deformation. Recent reports of “superplastic” behavior in metals and alloys <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,2</sup> include examples of extraordinary amounts of uniform tensile elongation. This communication reports the initial results of efforts to utilize a superplastic alloy in a laboratory vacuum-forming operation.