Superelastic anisotropy and meso-scale interface regions in textured NiTi sheets

Öz

Typical manufacturing processes of NiTi shape memory alloys (SMAs) involve drawing and rolling operations leading to crystallographic texture and superelastic anisotropic behavior. Therefore, an in-depth understanding of directional material properties and underlying mechanisms may help to guide the production process to create suitable textures for applications requiring different mechanical properties. It is well known that superelastic NiTi SMAs exhibit localized deformation during uniaxial tensile loading. During the stress-induced martensitic transformation (SIMT), macroscopic deformation occurs only in the meso-scale interface regions between the fully martensitic and the fully austenitic regions. In case of localized deformation, the detailed characterization of these transition regions is therefore of crucial practical importance. In this study, the effects of crystallographic texture and specimen geometry on mechanical behavior and local deformation characteristics of polycrystalline NiTi superelastic sheets were systematically analyzed. Local surface strain fields are recorded during deformation by digital image correlation (DIC) in order to characterize interface regions in detail. The results show that the mechanical material behavior has a clear directional dependency and that the orientation/texture effect is reduced with decreasing width/thickness ratio of specimen. Furthermore, it is shown that the martensite/austenite interface formed in localized deformation has different angles to the load direction for different sample geometries and orientations and that the interface angle changes continuously during SIMT. The results provide new insights into the interaction of crystallographic texture, SIMT characteristics, superelastic anisotropy and specimen geometry which is essential for engineering applications of NiTi.

Anahtar Kelimeler

• Crystallographic texture,superelastic anisotropy,SMAs,DIC,localization of SIMT

Tekstüre NiTi levhalarda süperelastik anizotropi ve mezo ölçekli arayüzey bölgeleri

Öz

NiTi şekil hafızalı alaşımların tipik üretim yöntemleri arasında, malzemede kristalografik tekstüre ve süperelastik anizotropik davranışa yol açan çekme ve haddeleme işlemleri bulunmaktadır. Bu nedenle, yöne bağlı malzeme özelliklerinin ve altta yatan mekanizmaların derinlemesine araştırılarak anlaşılması; farklı mekanik özellikler gerektiren uygulamalar için uygun malzeme tekstürü oluşturabilmek adına üretim prosesisin dizayn edilmesine yardımcı olacaktır. Süperelastik NiTi alaşımların, çekme yüklenmesi altında lokalize deformasyon sergilediği bilinmektedir. Gerilim kaynaklı indüklenmiş martenzitik faz dönüşümü (SIMT) esnasında, makroskobik deformasyon sadece, tamamıyla martenzitik ve tamamıyla östenitik bölgeler arasında oluşan mezoscale arayüzey bölgelerinde gerçekleşmektedir. Bu yüzden, lokalize deformasyonda bu geçiş bölgelerinin detaylı karakterizasyonu teknik açıdan kritik bir öneme sahiptir. Bu çalışma kapsamında, kristalografik tekstürün ve numune geometrisinin polikristalin NiTi süperelastik sac levhaların mekanik davranışı ve lokal deformasyon karakteristiği üzerine etkisi sistematik bir şekilde analiz edilmiştir. Dijital görüntü korelasyonu (DIC) tekniği ile lokal yüzey gerinim alanları deformasyon sırasında ölçülerek, arayüzey geçiş bölgeleri detaylıca karakterize edilmiştir. Malzemenin mekanik davranışının belirgin şekilde yöne bağlı olduğu ve numune genişliği/kalınlığı oranının düşmesiyle oryantasyon/tekstür etkisinin azaldığı saptanmıştır. Ayrıca, farklı numune geometrileri ve oryantasyonlarında, lokalize deformasyonda oluşan martenzit/östenit arayüzeyinin numune yüklenme eksenine göre farklı açılarda oluştuğu ve faz dönüşümü süresince sürekli değiştiği gözlenmiştir. Elde edilen sonuçlar, faz dönüşüm özellikleri, kristalografik tekstür, süperelastik anizotropi ve numune geometrisi arasındaki, NiTi malzemenin mühendislik uygulamaları açısından büyük öneme sahip etkileşime dair yeni anlayışlar kazandırmaktadır.

Anahtar Kelimeler

• Kristalografik tekstür,süperelastik anizotropi,Şekil hafızalı alaşımlar,DIC,SIMT lokalizasyonu

Kaynakça

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