Düşük Özkütleli Fe-Mn-Al-C Çelikleri ve Deformasyon Mekanizmaları

Öz

Otomotivde yakıt tasarrufu anlamına gelen ağırlık azaltılması, kullanılan yapısal çeliklerde sadece mukavetmet artışıyla belirli ölçüde sağlanabilir. Yeni nesil çeliklerde, yüksek mukavemete ilaveten, özkütledeki azalma yolu ile de ağırlık tasarrufu mümkündür. Çeliklerin 7.8 g/cm3 konvansiyonel özkütle değeri, L-IP (Lightweight Steel with Induced Plasticity) çeliklerde 6.4g/cm3’e kadar düşürülebilmiştir. Fe-Mn-Al-C esaslı alaşım sistemi yarattığı düşük özkütle sayesinde, otomotiv uygulamaları için çok çekici yeni nesil bir malzeme grubu oluşturmaktadır. Bu alaşım sistemi içinde bulunan çelikleri daha önemli kılan bir diğer özellikleri gösterdikleri yüksek deformasyon sertleşmesi ve söz konusu deformasyon sertleşmesini, çarpışma gibi yüksek deformasyon hızlarında bile sunabilmeleridir. Sıra dışı mühendislik özelliklerini Fe-Mn-Al-C çeliklerinde mümkün kılan mekanizmalar, yüksek deformasyon koşullarında ileri yüksek dayanımlı çeliklerde sıklıkla gözlenen ikizlenme kaynaklı plastisite (TWIP) ve dönüşüm kaynaklı plastisiteye (TRIP) ek olarak kayma bandı kaynaklı plastisite, mikrobant kaynaklı plastisite ve dinamik kayma bandı incelmesi gibi mekanizmalardır. Söz konusu sistem, içerdigi fazlar açısından da konvansiyonel çeliklerden çok farklıdır. Sistemin östenit ve ferrit gibi bilindik ana bileşenleri, kappa karbür, B2, DO3 gibi deformasyon mekanizmalarında yer alan fazları, β-Mn gibi kırılganlık yaratan intermetalik bileşenleri vardır. Temelde Fe-Mn-Al sistemi üzerine kurulu bu yeni sınıf çeliklerde bilindik çeliklerdeki kavramlar yeterli değildir. Bu yazı, Fe-Mn-Al-C esaslı çeliklerinin özelliklerini ve deformasyon sırasında oluşan deformasyon mekanizmalarını sunmaktadır.

Anahtar Kelimeler

• L-IP çelikleri
• kappa karbür
• istif hatası enerjisi
• kısa mesafeli düzen
• deformasyon mekanizmaları

Low Density Fe-Mn-Al-C Steels and Deformation Mechanisms

Öz

Owing to their low density, Fe–Mn–Al–C-based alloys constitute an attractive class of advanced materials for automotive applications. Another important feature of these steels is their high work-hardening capability, which can be maintained even under high strain-rate conditions such as crash loading. The exceptional engineering properties of Fe–Mn–Al–C steels arise from deformation mechanisms including twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), as well as shear-band-induced plasticity, microband-induced plasticity, and dynamic shear-band thinning. This alloy system also differs considerably from conventional steels in terms of its constituent phases. In addition to the well-known austenite and ferrite matrix phases, Fe–Mn–Al–C steels may contain phases such as κ-carbide, B2, and DO3, which influence their deformation behavior, as well as brittle intermetallic phases such as β-Mn. Therefore, conventional concepts used for traditional steels are insufficient to fully describe this emerging class of Fe–Mn–Al-based steels. This review presents the distinctive properties of Fe–Mn–Al–C steels and the deformation mechanisms that govern their mechanical response.

Anahtar Kelimeler

• L-IP Steels
• kappa carbide
• stacking fault energy
• short range order
• deformation mechanism

Kaynakça

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