High Strength of Mg-9%Al-1%Zn Alloys Achieved by Severe Working

Mitsuhiro Okayasu, Takuya Muranaga

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)

Abstract

To obtain the excellent mechanical properties of AZ91 magnesium alloy (Mg-8.9%Al-0.6%Zn-0.2%Mn), the microstructural characteristics of AZ91 alloys are modified by various forging and heating processes. High tensile properties (ultimate tensile strength σUTS = 420 MPa and fracture strain εf = 3%) are obtained for the alloy made by the following process: solution treatment (ST) at 410 °C for 24 h plus water quenching, multidirectional forging (MDF) with 5% strain applied in 15 forgings at room temperature, and warm unidirectional forging (WUF) at a forging rate of 75% at 225 °C. The high tensile strength is a reflection of improved microstructural characteristics, namely a fine α-Mg phase and a high dislocation density. Moreover, brittle β-phase is significantly attributed to the mechanical properties of AZ91 alloy. Because of the severe deformation undergone by the alloy during the MDF process, the solution treatment is important to achieve high ductility with low internal strain, i.e., normalization. In fact, the εf value for the ST sample is as high as 10%, leading to severe work hardening during the tensile test, with deformation twins and slips. The WUF process is conducted immediately after the sample has been heated to 225 °C, for less than 5 min, to avoid material softening. A relatively high tensile strength (σUTS = 305 MPa) is also achieved using the WUF process (with a forging rate of 75% at 200 °C) after the ST and aging process (200 °C for 12 h) although low ductility is found (εf = 0.7%), with hard and brittle β-phases being precipitated around the grain boundaries.

Original languageEnglish
Pages (from-to)1-13
Number of pages13
JournalJournal of Materials Engineering and Performance
DOIs
Publication statusAccepted/In press - Sep 22 2017

Keywords

  • forging
  • heating
  • magnesium alloy
  • mechanical property
  • microstructural characteristic
  • precipitation
  • work hardening

ASJC Scopus subject areas

  • Materials Science(all)
  • Mechanics of Materials
  • Mechanical Engineering

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