High-strength aluminum alloys are important for lightweighting vehicles and are extensively used in aircraft and, increasingly, in automobiles. The highest-strength aluminum alloys require a series of high-temperature “bakes” (120° to 200°C) to form a high number density of nanoparticles by solid-state precipitation. We found that a controlled, room-temperature cyclic deformation is sufficient to continuously inject vacancies into the material and to mediate the dynamic precipitation of a very fine (1- to 2-nanometer) distribution of solute clusters. This results in better material strength and elongation properties relative to traditional thermal treatments, despite a much shorter processing time. The microstructures formed are much more uniform than those characteristic of traditional thermal treatments and do not exhibit precipitate-free zones. These alloys are therefore likely to be more resistant to damage.
Martensite is a key constituent in advanced high strength steels and plays an important role in providing the high strength. While the strength of martensite has been extensively studied in the past, its low elastic limit and extremely high strain hardening rate remain a puzzle for the steel community. Composite models proposed recently can successfully reproduce these features as result of gradual yielding of microstructural constituents with either variations in intrinsic yield strengths or transformation induced residual stresses. Although these composite models can explain certain observations associated with the deformation of as-quenched martensite, neither can self-consistently describe all the key characteristics in the tension-compression behaviour of asquenched martensite. Attempts to extend these composite models to tempered martensite have been limited.In this contribution, we conduct a systematic experimental study on the strain hardening of as-quenched and tempered martensite with mechanical testing (e.g. monotonic tension and tension-compression) and interrupted X-ray diffraction. It is shown that the high strain hardening rate, large Bauschinger effect and diffraction line narrowing found in as-quenched martensite during straining can be sustained in tempered martensite tempered up to 400°C. These phenomena can be understood by considering martensite as a multi-constituent composite having both variations in intrinsic yield strengths and relaxation of transformation induced residual stresses during straining.
Recent theories consider as-quenched martensite as a composite which strain hardens by the gradual yielding of constituents. An underlying hypothesis is that hardening comes primarily from athermal hardening contributions. In this contribution, we conducted strain-rate jump and tension-compression tests to quantify the athermal and kinematic hardening contributions in martensite. It is shown that athermal hardening accounts for ~75% of the total strength of as-quenched martensite. The magnitudes of athermal and kinematic hardening decrease as a function of tempering. A correlation between the athermal and kinematic hardening contributions is identified and shown to be independent of chemistry and tempering condition.
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