Sains Malaysiana 42(12)(2013): 1769–1773

 

Phase Transformation Temperatures (PPTs) and Microstructure of

Moulded NiTi Alloy Using a Water Soluble Binder System

(Suhu Perubahan Fasa dan Mikrostruktur Aloi NiTi Teracuan Menggunakan

Sistem Bahan Pengikat Larut Air)

 

 

MUHAMMAD HUSSAIN ISMAIL¹*,  MARTIN BRAM², ANA PAULA CYSNE BARBOSA²,  MANUEL KÖHL², HYWEL A. DAVIES³& IAIN TODD³

 

¹Faculty of Mechanical Engineering, Universiti Teknologi MARA (UiTM)

40450 Shah Alam, Selangor, Malaysia

 

²Institute of Energy Research (IEF-1), Jülich, 52425, Jülich, Germany

 

³Department of Material Science & Engineering, University of Sheffield

Sir Robert Hadfield Building, Mappin Street, S 1 3J Sheffield, United Kingdom

 

Received: 25 April 2012/Accepted: 31 May 2012

 

ABSTRACT

In this work, the effects of backbone polymer in the binder system mixed with pre-alloyed NiTi powder, on impurity contents, phase transformation temperatures and microstructures were investigated. A spherical gas-atomised pre-alloyed NiTi powder (50.3 at. %Ni) with a mean particle size of less than 22 μm and powder loading of 69.5 vol. % was used. The binder consisted of a water soluble binder system, mainly polyethylene glycol (PEG), with two different backbone binders, namely polyethylene 520 (PE 520) and poly-methyl- methacrylate (PMMA). The latter was used in the form of a powder and as an emulsion. Green parts were prepared by warm-press the feedstock into a cylindrical shape. The samples were then leached in warm water, thermally debound in Argon and finally, vacuum sintered at 1240°C for 10 h. The experimental results indicate that the oxygen content in the as-sintered condition increased to almost double than that of the powder state (from 0.08 to 0.14 - 0.16 wt. %) and the carbon increased by one third to half (from 0.06 to 0.08 - 0.09 wt. %). This consequently resulted in a shift of the phase transformation temperature to lower values and consequently broadened the reversible austenite to martensite transformation. The uptake of oxygen and carbon during the process led to the formation of the well-known Ti₄Ni₂O_x  and TiC precipitate phases which were evident from grey-scale images of back-scattered SEM.

 

Keywords: Metal injection moulding (MIM); NiTi alloy; phase transformation temperatures (PTTs); pseudo-elasticity; shape memory

 

ABSTRAK

Dalam kajian ini, kesan polimer sekunder di dalam campuran bahan suapan pengacuan suntikan logam yang terdiri daripada serbuk logam pra-aloi NiTi terhadap kandungan bendasing, suhu penjelmaan fasa dan mikrostruktur yang terbentuk telah dikaji. Serbuk logam NiTi yang digunakan mempunyai komposisi kimia 50.3 at. %Ni dengan saiz purata serbuk kurang daripada 22 μm dan telah dihasilkan secara pembutiran gas. Sebanyak 3 formulasi bahan suapan telah dikaji dengan menggunakan isi padu serbuk yang sama (69.5 vol. %). Komposisi bahan pengikat dalam bahan suapan adalah terdiri daripada polimer primer yang sama iaitu poli-etilena-glikol (PEG) dan polimer sekunder pula terdiri daripada polietilena (PE 520) dan poli-metil-meta-akrilik (PMMA). Untuk kes PMMA, 2 bentuk telah digunakan, iaitu pertamanya dalam bentuk serbuk dan keduanya dalam bentuk emulsi. Bahan suapan yang dihasilkan seterusnya diproses menjadi bentuk silinder dengan menggunakan kaedah padatan hangat. Sampel kemudiannya direndam di dalam air yang hangat untuk mengeluarkan bahan pengikat primer, iaitu PEG. Sampel kemudiannya dikeringkan sebelum dibawa ke relau vakum untuk proses penyahikatan terma dan persinteran. Proses penyahikatan terma dijalankan dalam persekitaran Argon dan diakhiri dengan persinteran dalam vakum pada suhu 1240°C selama 10 jam. Hasil uji kaji menunjukkan kandungan oksigen dalam sampel yang disinter meningkat hampir dua kali ganda berbanding dalam keadaan serbuk (daripada 0.08 ke 0.14-0.16 wt. %), manakala kandungan karbon pula meningkat antara separuh ke satu pertiga (daripada 0.06 ke 0.08-0.09 wt. %). Keadaan ini seterusnya menyebabkan pengurangan pada suhu penjelmaan fasa dan meluaskan julat perubahan suhu boleh balik daripada fasa austenite ke martensite. Peningkatan kandungan oksigen dan karbon pada sampel tersinter juga menunjukkan wujudnya fasa sekunder seperti Ti₄Ni₂O_x dan TiC yang dibuktikan daripada perbezaan warna pada setiap fasa pada gambarajah SEM dan seterusnya melalui bacaan EDX.

 

Kata kunci: Aloi NiTi; memori bentuk; pengacuan suntikan logam; pseudo-keanjalan; suhu penjelmaan fasa

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*Corresponding author; email: hussain305@salam.uitm.edu.my