Steels for bearings

High-entropy alloys (HEAs) consisting of multiple principle elements provide an avenue for realizing exceptional mechanical, physical and chemical properties. We report a novel strategy for designing a new class of HEAs incorporating the additional interstitial element carbon. This results in joint activation of twinning- and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase's instability in a metastable TRIP-assisted dual-phase HEA. Besides TWIP and TRIP, such alloys benefit from massive substitutional and interstitial solid solution strengthening as well as from the composite effect associated with its dual-phase structure. Nanosize particle formation and grain size reduction are also utilized. The new interstitial TWIP-TRIP-HEA thus unifies all metallic strengthening mechanisms in one material, leading to twice the tensile strength compared to a single-phase HEA with similar composition, yet, at identical ductility.

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Interstitial atoms enable joint twinning and transformation induced plasticity in strong and ductile high-entropy alloys.

Interstitial atoms enable joint twinning and transformation induced plasticity in strong and ductile high-entropy alloys

Experiments are presented which show that the eutectoid transformation in steel can occur by two different modes for temperatures just slightly below A
 1. In the normal mode, the transformation product is lamellar pearlite. The second mode occurs if the austenite contains cementite particles or nuclei with a spacing on the order of a few microns or less. In this case, the transformation product consists of spheroidal cementite particles in a ferrite matrix. This second mode is here called the divorced eutectoid transformation (DET), after recent work by Sherby and co-workers. A literature survey shows that the faster kinetics of the DET over lamellar pearlite in the presence of inhomogeneous austenite was established before 1940, but has received little attention. The inhomogeneities are generally small cementite particles. Experiments show that the DET does not occur by a shell of one phase (ferrite) forming around the other phase of the eutectoid (cementite), as is the case in divorced eutectic growth. Rather, a fairly planar austenite/ferrite front simply advances into the austenite, with no apparent effect on its shape being produced by the cementite particles. A first-order kinetic model is presented for the growth velocity as a function of undercooling below A
 1 and is compared to the velocity vs undercooling for lamellar pearlite. The simple model indicates that the velocity of the divorced mode should be faster than the lamellar mode at low undercooling for cementite nuclei distributed in the austenite with spacings less than a few microns. This result is consistent with the experimental data.

The divorced eutectoid transformation in steel

A two-phase Mg-9Li alloy (where the composition is in weight per cent) was made fine grained (d≈6–35 μm) by extensive cold rolling into foils and press bonding the foils at low homologous temperatures. The material exhibited superplastic properties in the temperature range 423–523 K ((0.49–0.61)T/Tm) with elongations to failure as high as 460%. The creep rate was determined to be inversely proportional to approximately the square of grain size and to the second power of stress. The activation energy for superplastic flow was equal to that for lattice diffusion in the major b.c.c. β phase. The Mg-9Li laminate alloy was found to be similar in its flow characteristics to MA-21, a multicomponent Mg-8.1Li fine-grained commercial Soviet alloy.

Superplastic behavior of a fine-grained two-phase Mg-9wt.%Li alloy

Application of the divorced eutectoid transformation to the development of fine-grained, spheroidized structures in ultrahigh carbon steels

The Structure dependence of power law creep

Recent studies and results on ultrahigh carbon (UHC) steels suggest that major development efforts on these steels are timely and that programs to evaluate prototype structural components should be initiated. These recent results include: the development of economical processing techniques incorporating a divorced eutectoid transformation, the improvement of room temperature strength and ductility by heat treatment, the enhancement of superplastic properties through dilute alloying with silicon, and the attainment of exceptional notch impact strength in laminated UHC steel composites manufactured through solid state bonding. The unique mechanical properties achieved in UHC steels are due to the presence of micron-size fer-rite grains and ultrafine spheroidized carbides.

Ultrahigh Carbon Steel.

: This program has centered on the thermal-mechanical processing of steels containing 1 to 2.3%C and their properties. These ultrahigh carbon (UHC) steels have been commercially neglected because of a traditional belief that they are brittle. The processing however leads to the formation of extremely fine cementite particles of about 1/10 micron (0.0001 mm) in a matrix of extremely fine ferrite grains of about 1 micron (0.001 mm) or less. This fine structure lends itself to several unique and exciting characteristics which revolutionize the traditional attitudes to these materials. In the first place these fine grained steels are superplastic at warm temperatures and elongations to failure of over 1500% have now been obtained. This means that these materials can be very easily formed. Second, they are strong and ductile at room temperature and the strength can be simply controlled by the final processing temperature or subsequent heat treatment. Third, they can be given a simple transformation heat treatment to form a structure of cementite and martensite which is about as hard as tungsten carbide.

B. Walser

Papers

Application of the divorced eutectoid transformation to the development of fine-grained, spheroidized structures in ultrahigh carbon steels

The Structure dependence of power law creep

Ultrahigh Carbon Steel.

Fine Structure and Superplasticity in Ultrahigh Carbon Steels.