Magnezyum lityum aluminyum alaşımlarına alaşım elementi ilavesi ve özelliklerinin incelenmesi
Effects of alloying elements on the properties of Mg-Li-al alloy
Yazar: ŞEHZAT AÇIKGÖZ
Danışman: PROF. DR. SÜLEYMAN CAN KURNAZ
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Doktora, 2022
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Alaşım elementleri = Alloy elements ; Alüminyum alaşımları = Aluminum alloys ; Lityum = Lithium ; Magnezyum alaşımları = Magnesium alloys

Özet:
Bu çalışmada, metal kalıba döküm yöntemi ile Mg-8Li-2Al, Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd ve Mg-8Li-2Al-1,5Ca alaşımları üretilmiştir. Sn, Nd ve Ca elementlerinin ayrı ayrı eklenmesinin Mg-8Li-2Al alaşımının mikroyapısı, mekanik özellikleri ve korozyon davranışı üzerindeki etkileri incelenmiştir.
Alaşımları üretmek için yüksek saflıkta metaller ve master alaşımları ergitme işlemi için gerekli kütle ve boyutlara küçültülmüştür. Ergitme ve alaşımlama işlemi, SF6+CO2 koruyucu gaz karışımları, paslanmaz çelik potalar kullanılarak gerçekleştirilmiştir. Döküm işleminde ise koruyucu gaz atmosfer destekli döküm kutusu ve 120 X ɸ110mm ebatlarında çelik kalıplar kullanılmıştır.
Döküm sonrası üretilen alaşımların kimyasal analiz sonuçları değerlendirilip, mikroyapı incelemeleri yapılmıştır. Mikroyapı incelemelerinin yanısıra X-ışını ve EDS analizleri de yapılmıştır. Mg-8Li-2Al alaşımı %8 Li elementi içerdiğinden dolayı alaşımın mikroyapısı çift fazlı (α-Mg ve β-Li) olduğu, ayrıca mikroyapıda AlLi intermetalik fazlarının ortaya çıktığı gözlemlenmiştir. Mikroyapı incelemeleri sonucunda Mg-8Li-2Al alaşımı ile kıyaslandığında, ilave edilen Sn, Nd ve Ca elementlerinin etkisi ile α-Mg ve β-Li faz boyutlarının küçüldüğü, mikroyapının iyi yönde geliştiği sonucuna varılmıştır. Üretilen alaşımların X-ışını ve EDS analizinden sonra, incelenen alaşımlarda AlLi, Mg2Sn, Al2Nd, Al11Nd3 ve Al2Ca'nın intermetalik bileşikleri tespit edilmiştir. Deneysel alaşımların mekanik özelliklerinin, bu intermetalik bileşikler tarafından geliştirildiği sonucuna varılmıştır. 
Alaşımların mekanik özelliklerini araştırmak için sertlik testi ve çekme testi yapılmıştır. Test sonuçlarına göre, alaşım elementlerinin eklenmesi baz alaşımın sertliğini arttırmıştır. Mg-8Li-2Al, Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd ve Mg-8Li-2Al-1,5Ca alaşımlarının Brinell sertlik değerleri sırasıyla 52, 60, 56 ve 59 HB olarak ölçülmüştür. Mg-8Li-2Al Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd ve Mg-8Li-2Al-1,5Ca alaşımlarının çekme mukavemetleri ve uzama değerleri sırasıyla 115MPa ve %12,8, 136 MPa ve %10,4, 129 MPa ve %10, 125 MPa ve %8 olarak ölçülmüştür.  Mekanik özellikler göz önüne alındığında, Mg-8Li-2Al-1,5Sn alaşımı ile Mg-8Li-2Al alaşımına göre çekme gerilmesinde %18.3 ve basma gerilmesinde %34 artış ile en iyi sonuçlara ulaşılmıştır. Ayrıca diğer deneysel alaşımlardan daha yüksek olan Mg-8Li-2Al-1,5Sn alaşımının çekme uzama değeri %10,4 olarak ölçülmüştür. Elektrokimyasal korozyon test sonuçları, 10,49 mpy ile en düşük korozyon hızı değerine Mg-8Li-2Al-1,5Sn alaşımının sahip olduğunu göstermiştir.

Abstract:
In this study, Mg-8Li-2Al, Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd, and Mg-8Li-2Al-1,5Ca alloys were produced by the gravity die casting method. The effects of individual addition of Sn, Nd, and Ca on the microstructure, mechanical properties, and corrosion behavior of the Mg-8Li-2Al alloy were studied. 
Four experimental alloys were produced by melting pure Magnesium (99.9 wt%), pure Lithium (99,9 wt%), pure Aluminum (99,9 wt%), pure Tin (99,9 wt%), Mg – Neodymium (20 wt%), and pure Calcium (99,9 wt%) using resistance melting furnace. Target alloys were melted at 770 °C under SF6 and CO2 protective gas atmosphere in a steel crucible in an electric resistance furnace. The mold was preheated at 200 °C for 2 hours prior to pouring, and the mold was filled with protective gas for 60 seconds. After the molten metal in the crucible was taken into the casting box by using tongs under a protective gas atmosphere, it was poured into metal cylinder molds having 120mm X ɸ110mm under gravity.
Inductively coupled plasma–optical emission spectrometry (ICP-OES) was used to determine the chemical composition of the alloys. Chemical analysis results of experimental alloys were measured and then microstructure studies were carried out. In addition to microstructure examinations, X-ray and EDS analyzes were also performed.
A wire electrical discharge machining (EDM) machine was used to cut metallographic and other experimental samples. Grinding was completed with 1200 grit silicon carbide (SiC) grinding papers. After rinsing with ethanol, samples were polished with a 0,05 µm alumina polishing suspension. Chemical etching of optical microscopy (OM) specimens was done in a solution of 3% HNO3 + 97% ethanol. To check the distribution of alloying elements in the structure, scanning electron microscopy (SEM) equipment (JEOL 6060LV) with an energy-dispersive spectrometer (EDS) was used. A Rigaku D-Max 1000 X-ray diffractometer with Cu K radiation was used to conduct X-ray diffraction (XRD) examination on the alloys. Analysis of area fraction of α-Mg, β-Li, and intermetallic phases with image processing method in the microstructure was analyzed using Tescan VEGA-II Electron Microscopes.
It was observed that the microstructure of Mg-8Li-2Al alloy was double phases (α-Mg and β-Li) due to the 8% Li metal, moreover, AlLi intermetallic structure appeared in the microstructure. The microstructure photographs showed that the α-Mg and β-Li phase sizes were reduced, and the microstructure was improved with the addition of Sn, Nd, and Ca elements compared to the Mg-8Li-2Al alloy. The size of the β-Li and α-Mg phase reduced with the addition of Sn to the Mg-8Li-2Al alloy. α-Mg phase turned into a strip shape with the addition of Nd and Ca to Mg-8Li-2Al alloys, the amount of β-Li phase and white particles increased.
The XRD patterns show that the Mg-8Li-2Al alloy was composed of the α-Mg phase, β-Li phase, and AlLi phases. EDS results demonstrate that Mg, Al, Sn, Nd, and Ca elements were present in the alloys. Still, because of its characteristic X-ray diffraction line energy, EDS cannot detect Li elements. In addition, AlLi phases were located at the α/β boundaries with a size of approximately 1-1,5 μm. According to the EDS and XRD analyses, Mg-8Li-2Al-1,5Nd alloy consisted of α-Mg, β-Li, Al11Nd3, and Al2Nd phases. XRD results show that the Mg-8Li-2Al-1,5Ca alloy composed of β-Li, α-Mg, and Al2Ca phases (approximately 8-12 μm in length and 1-2 μm width). α-Mg size decreased due to the presence of Al2Ca particles in the structure. With the addition of Ca, the α-Mg size became smaller than the α-Mg size in the Mg-8Li-2Al alloy. However, similar to the Mg-8Li-2Al-1.5Nd alloy, β-Li phase became coarser in the Mg-8Li-2Al-1,5Ca alloy. After X-ray and EDS analysis of experimental alloys, intermetallic compounds of AlLi, Mg2Sn, Al2Nd, Al11Nd3, and Al2Ca were found in the investigated alloys. The mechanical properties of experimental alloys were enhanced by these intermetallic compounds. 
The relationship between the area fraction of the α-Mg, β-Li, and intermetallic phases in the microstructure of the alloys was measured by using Tescan VEGA-II Electron Microscopes. It was observed that the area fraction of α-Mg, β-Li, and intermetallic phases in Mg-8Li-2Al alloy were 85%, 15%, and <1%, respectively. The area fraction of the intermetallic phases occurring in Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd, and Mg-8Li-2Al-1,5Ca alloys was determined as 5%, 15%, and 8%, respectively. After the individual addition of Sn, Nd, and Ca alloying elements, the amount of α-Mg decreased while the amount of β-Li phase increased. The experimental results concluded that the changes in the amounts of α-Mg and β-Li depend on the alloying elements and intermetallic compounds. In comparing Mg-8Li-2Al alloy with Mg-8Li-2Al-1,5Sn alloy, the amount of β-Li phase increased from 15% to 26%, while α-Mg phase decreased from 85% to 69%, and Mg2Sn formed 5%. With the individual addition of Nd and Ca, the α-Mg phase in Mg-8Li-2Al-1,5Nd, and Mg-8Li-2Al-1,5Ca alloys decreased from 65% to 40% and 36%, respectively. The area fraction of Al2Nd and Al11Nd3 formed in Mg-8Li-2Al-1,5Nd alloy was about 15%, while the area fraction of Al2Ca phase was determined as 8%.
To investigate the mechanical properties of alloys, hardness test and tensile test were performed.  Brinell hardness (HB) of the alloys and microhardness (HV) tests of the β-Li and α-Mg phase tests were carried out. Application times and loads for HB and HV were 15 seconds / 62,5 kg and 15 seconds / 10 g, respectively. At least ten measurements were taken to avoid the influence of any alloying element segregation and to determine the average hardness value at various locations. With respect to the test results, the addition of alloying elements increased the hardness of the base alloy. Brinell hardness values of Mg-8Li-2Al, Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd, and Mg-8Li-2Al-1,5Ca alloys were measured as 52, 60, 56 and 59 HB, respectively. 
The gauge dimensions of Compression and Tensile specimens were 15 x ɸ10mm and 32 x 6 x 4 mm respectively. At room temperature, tensile and compression tests were performed using an Instron 3367 universal testing equipment at a strain rate of 103 s-1. The elongation of samples was measured by using a mechanical extensometer. The tensile strength and elongation values of Mg-8Li-2Al Mg-8Li-2Al-1,5Sn, Mg-8Li-2Al-1,5Nd and Mg-8Li-2Al-1,5Ca alloys were measured as 115 MPa and 12,8%, 136 MPa and 10,4%, 129 MPa and 10%, 125 MPa and 8%. Considering the mechanical properties, the best outcomes were achieved with Mg-8Li-2Al-1,5Sn alloy, compared to Mg-8Li-2Al alloy, with an increase of 18,3% in tensile stress and 34% in compression stress. Also, the tensile elongation value of Mg-8Li-2Al-1,5Sn alloy, higher than the other experimental alloys, was measured as 10,4%. 
Fracture surfaces of all four experimental alloys seemed to belong to brittle fracture regions. The addition of Sn and Nd to Mg-8Li-2Al alloy did not cause a significant decrease in the tensile elongation of the alloy. Intermetallic compounds like Mg2Sn, Al2Nd, and Al11Nd3 dispersed in α-Mg and β-Li phases, unlike brittle Al2Ca phases mostly collected at the α-Mg/β-Li grain boundaries. Therefore, tensile elongation of the Mg-8Li-2Al-1,5Ca alloy decreased.
Electrochemical corrosion tests were performed at room temperature using a computer-controlled Gamry Reference 600+ system. The pH of the solution in the system was adjusted to 5,76 with 0.1 M NaCl solution. Open circuit potential (OCP) was measured after 10 min immersion of the tested samples with an active area of 3,54 cm2 in the solution for potential stabilization. Potentiodynamic polarization measurements were made at a potential scan rate of 1 mV/s, and the scanning range was set from –1 to + 2,2 V. Electrochemical corrosion test results showed that Mg-8Li-2Al-1,5Sn alloy has the lowest corrosion rate value with 10,49 mpy.