بررسی رفتار خزش فروروندگی آلیاژهای منیزیم AZ61‒0.7Si‒xCa
محورهای موضوعی : فصلنامه علمی - پژوهشی مواد نوین
نیکو برنجی طهرانی
1
(گروه مهندسی مواد و متالورژی، واحد تهران جنوب، دانشگاه آزاد اسلامی، تهران، ایران)
رضا محمودی
2
(دانشکده مهندسی متالورژی و مواد، پردیس دانشکدههای فنی ، دانشگاه تهران، تهران، ایران)
عبدالرضا گرانمایه
3
(گروه مهندسی مکانیک، واحد تهران جنوب، دانشگاه آزاد اسلامی، تهران، ایران)
کلید واژه: خزش فروروندگی, آلیاژ AZ61‒0.7Si, مورفولوژی Mg2Si, مکانیزم خزش,
چکیده مقاله :
در این تحقیق رفتار خزش فرورندگی با فرورنده هرمی آلیاژهای AZ61‒0.7Si‒xCa, ( x = 0.2, 0.5, 1.0 wt%) در شرایط ریختگی و تحت نیروی ثابت مورد بررسی قرار گرفت. نتایج بررسی‌های ریز ساختاری با استفاده از میکروسکوپ الکترونی روبشی (SEM) و پراش پرتو ایکس (XRD)، نشان دادند که افزودن کلسیم، با تشکیل فازهای پایدار در برابر حرارت CaMgSi و CaSi2 و از طریق مکانیزم جوانه‌زنی ناهمگن، سبب تغییر در مورفولوژی فاز Mg2Siاز شکل حروف چینی به شکل چند وجهی و در نهایت سبب بهبود خواص مکانیکی آلیاژ شده است. رفتار خزش فروروندگی این آلیاژها در محدوده دمایی 523-448 کلوین تحت نیروهای ثابت 5 و 15 نیوتن بررسی شد. در کلیه‌ی آلیاژها، در رژیم تنشی پایین، توان تنشی بین 7-4 و انرژی فعالسازی نزدیک به انرژی فعالسازی نفوذ از طریق هسته نابجایی‌های منیزیم، مکانیزم خزش را لغزش ویسکوز نابجایی‌ها معرفی می‌نماید. در رژیم تنشی بالا، توان تنشی بین 14-11 و انرژی فعالسازی بسیار نزدیک به انرژی فعالسازی نفوذ در خود منیزیم، مکانیزم غالب را، صعود نابجایی‌ها به همراه مقادیری تنش برگشتی، مشابه آنچه در آلیاژهای سخت شونده رسوبی مشاهده شده است، پیش‌بینی می‌کند. همچنین در تحقیق حاضر نشان داده شده است که آزمون خزش فروروندگی با نیروی ثابت می‌تواند در بررسی رفتار خزشی آلیاژهای ریختگی منیزیم، مورد استفاده قرار گیرد.
The effect of 0.2, 0.5 and 1.0 wt% Ca additions on the microstructure and creep behavior of an Mg‒6Al‒1Zn‒0.7Si cast alloy was investigated by indentation creep test under constant loads of 5 and 15 N and at temperatures in the range of 448–523 K. The microstructural examination of alloys was conducted using electron microscopy (SEM), and X‒ray diffraction (XRD). Results showed that addition of Ca enhances hardness and the thermal stability of the alloys. It was found that AZ61‒0.7Si‒1.0Ca had the best creep resistance among three tested alloys. The main cause of enhancing mechanical properties of Ca-containing alloys was the change in the morphology of Mg2Si phase from Chinese script to polygonal shape, with formation of CaSi2 and CaMgSi phases. Stress exponents of all alloys showed two different regimes in creep tests. In the low stress regime, n-values of about 4–7 and activation energies of about 95 kJ/mol, introduce pipe-diffusion-controlled dislocation viscous glide as the controlling creep mechanism. In the high stress regime, however, stress exponents of about 11–14 and activation energies of about 135 kJ/mol, suggest that deformation mechanism is dislocation climb with some sort of back stress, similar to those noted in dispersion strengthening alloys.
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